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A: New Tools and Methods in Experiment and Theory

KiMoPack: A python Package for Kinetic Modeling of the Chemical Mechanism
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Cite this: J. Phys. Chem. A 2022, 126, 25, 4087–4099
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https://doi.org/10.1021/acs.jpca.2c00907
Published June 14, 2022

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Abstract

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Herein, we present KiMoPack, an analysis tool for the kinetic modeling of transient spectroscopic data. KiMoPack enables a state-of-the-art analysis routine including data preprocessing and standard fitting (global analysis), as well as fitting of complex (target) kinetic models, interactive viewing of (fit) results, and multiexperiment analysis via user accessible functions and a graphical user interface (GUI) enhanced interface. To facilitate its use, this paper guides the user through typical operations covering a wide range of analysis tasks, establishes a typical workflow and is bridging the gap between ease of use for less experienced users and introducing the advanced interfaces for experienced users. KiMoPack is open source and provides a comprehensive front-end for preprocessing, fitting and plotting of 2-dimensional data that simplifies the access to a powerful python-based data-processing system and forms the foundation for a well documented, reliable, and reproducible data analysis.

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This paper was published ASAP on June 14, 2022, with errors in eq 1 and in the level of the headings “Optimization Routines” and “Collecting and Plotting Routine”. The corrected version was posted on June 15, 2022.

Introduction

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In many chemical disciplines, kinetic modeling is used to extract and understand the mechanisms underlying time-resolved spectroscopic data. It is extensively used in the analysis of time-resolved spectra from optical spectroscopy, (1−3) but also in, e.g., X-ray spectroscopy, (4,5) spectroelectrochemistry, (6−10) and photocatalysis. (8,11,12) Common to all techniques is that a spectrum is distorted by one, or multiple processes. The evolution of the distortion can be used to, e.g., disentangle processes or/and to extract additional information from the system. In the study of photoinduced dynamics including energy and electron transfer, transient/time-resolved spectroscopy can be an essential tool. (1) One form of time-resolved spectroscopy is to follow the temporal evolution after a pulsed excitation by monitoring changes in optical properties such as absorption, emission or scattering of light. The challenge in working with these inherently multivariate data sets is the need to extract the information in a controlled and reproducible way, not using the proverbial black box approach.
In this paper, we present KiMoPack (kinetic modeling package), a software package designed to model photoinduced chemical kinetic measurements, which is also capable to model modulated data obtained by other initiation methods. We have used KiMoPack successfully to analyze datasets obtained from transient X-ray spectroscopy, (5) photocatalysis, (12) and spectroelectrochemistry. (6,7,13) However, in the following discussions, we will focus on transient optical spectroscopy and the associated data handling. The essential design criteria for KiMoPack are to provide a reproducible workflow tool in which preprocessing, model-free analysis, and fast and advanced modeling are combined with a powerful set of plotting and comparison methods. The data work up can be divided into five general stages:
(A)

Preprocessing. Perform background subtraction, arrival time correction, combine and filter multiple measurements, define limits, suppress scattered light, and visually inspect/compare data.

(B)

Kinetic Model-Free Analysis. Employ model free analysis methods such as, e.g., singular value decomposition (SVD) (14−19) to gain insights into the number of processes/states (e.g., chemical species) contributing to the data set.

(C)

Global Analysis. Express the dynamics using independent first-order exponential decays with guessed parameters and use global analysis (2,20−22) to optimize the parameters and decompose the time-resolved spectra into kinetic traces and transient spectra, assuming a bi-linearity of the data.

(D)

Target Analysis. Express different chemical mechanisms through parameter dependent temporally changing concentrations. Often this is achieved through numerically integrating differential rate equations. Optimize the used set of parameter through global analysis and extract the species associated spectra (SAS). (2,20−22) In KiMoPack, all fitting steps may include external spectra (e.g., spectro-electroctochemical data) and external kinetic information (e.g., measured laser pulse profiles) and can be simultaneously performed on multiple data sets. A confidence interval of the variable parameter set can then be evaluated using an F-test, comparing the parameter induced variations (under reoptimization) to the statistical variations.

(E)

Visualization of results. In KiMoPack, powerful visualization routines are provided to facilitate the refinement and comparison of the results from different kinetic models, create informative report files or publication ready plots. Flexible reporting, data extraction, and postprocessing capabilities simplify interaction with other software packages, e.g., as input in other modeling tools or specialized scientific plotting software.

KiMoPack is an open source python package that uses customizable Jupyter notebooks as workflow tools to provide a powerful, but still user-friendly interface for a well documented, flexible, and reproducible analysis of transient data. Its modularity also allows the combination with other tools like the recently introduced DeepSKAN, (23) which suggests probable kinetic models from time-resolved spectra by artificial intelligence algorithms but does not perform the target analysis. However, we have found that at the current state of development, human intuition and experience often leads faster to the right conclusions. To the best of our knowledge, there is not yet such a comprehensive open source program available for analyzing data in more complicated models than the standard parallel and sequential approaches that simultaneously integrate a comprehensive data pre- and postprocessing and statistically robust error estimation. (24−27)
The program that comes closest to the desired functionality is Glotaran, (24,28,29) which however is lacking in some flexibility of model building and fitting control, and has significant limitations due to primarily relying on a graphical framework. (24) In contrast, the herein presented KiMoPack is designed as a very flexible python framework that triggers the analysis tasks through flexible and user-friendly functions, acting as a frontend to the programming language. The source code of KiMoPack is available on Github (30) and as frozen releases on zenodo. (31) Typically it is installed through one of the two common package managers conda (32) or pypi. (33) The code is documented comprehensively on ReadtheDocs. (34) A few introductory movies are available on the KiMoPack tutorial youtube channel. (35) The workflow tools and a series of working-along tutorials are available on the Github page. (30)
The aim of this contribution is to present the general workflow and advanced features of KiMoPack that outperform state-of-the-art open-source and commercial programs. (24,36) This contribution also provides Jupyter notebooks that guide through several typical analysis tasks (workflow tools) making KiMoPack easier accessible to users with less programming background while offering the full flexibility of python for more experienced user. Both the tutorials and the following sections focus on the analysis and discussion of transient absorption (TA) data.

Results

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We will demonstrate the application of KiMoPack for analyzing typical TA data sets using the widely studied ruthenium complex [(tbbpy)2Ru(dppz)]2+ (Ru-dppz, tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, and dppz = dipyrido[3.2-a:2′,3′-c]phenazine) as model substance. (37−41) Ru-dppz is interesting due to its so-called light-switch effect in which the relative population of a bright (emissive) and a dark (non-emissive) 3MLCT excited state is governed by the local solvent environment (see Figure 1). (38−40,42−46) The general relaxation models in various solvents are well-known from literature. (42,43,45,47−49) However, the analysis is challenging in the sense that it requires the handling of multiple data sets, different kinetic models, and the comparison functions. This makes Ru-dppz a suitable reference system to demonstrate the workflow in KiMoPack. The respective data sets are available as Supporting Information (e.g., TA_Ru-dppz_400 nm_ACN.SIA) as are a series of tutorials that allow the reader to follow the analysis of TA data of Ru-dppz step-by-step. The sample data shown here was collected upon 400 nm excitation in dichloromethane (DCM), acetonitrile (ACN) and water (H2O) in various levels of complexity.

Figure 1

Figure 1. Schematic Jablonski diagrams for the photoinduced relaxation schemes of Ru-dppz in dichloromethane (DCM), acetonitrile (ACN), and water (H2O) upon MLCT excitation between 390 and 450 nm.

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The provided Jupyter notebooks contain the functions and parameters that are employed and adjusted during a representative analysis session. This allows early users to easily change the values or extend the comments and the experienced users to have a customizable, fast, and reproducible workflow. Typically, a new Jupyter notebook is created from the templates/previous analysis for each new analysis session, which then documents the procedures, parameters, and results in a visible way. This procedure also enables very fast and repeated data processing as the entire notebook can be run repeatedly. In the tutorials, several of these analysis sessions are extensively documented, and their individual functionality is explained. An even more comprehensive documentation of all functions and their parameters can be found on ReadtheDocs. (34) In the following, we will guide the reader through the key analysis steps: reading, preprocessing, kinetic modeling, plotting, and saving of data, for the example of TA data from Ru-dppz. These analysis steps can be divided into three working routines: the input, optimization, and collecting routine of KiMoPack (see Figure 2). This modular structure allows not only flexible workflows but also the concept that future extensions can be made without significant changes to the program core and prior analysis notebooks can be reused and extended.

Figure 2

Figure 2. Schematic representation of the processing structure and the three main workflow routines, i.e., the input (blue box) and standard optimization (green box), as well as collecting and plotting routines (orange box) KiMoPack. Individual operations (functions) and their returns are indicated by bright gray and colored boxes, respectively.

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Input Routine

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In the input routine, the experimental data are loaded and converted into the form of a matrix Aij (, m = number of times, n = number of wavelengths). Subsequently, these data can be preprocessed, e.g., chirp-corrected, and a range of parameter is defined that defines, e.g., a spectral or temporal range of interest or certain masks for the subsequent routines of analysis and plotting.

Import of Experimental Data

KiMoPack supports the work with various formats of time-resolved spectroscopic data. The internal format is based on a pandas DataFrame(s) (50) that assembles n probe wavelengths {λ1, λ2, ..., λn} and at m times relative to the instant of excitation {t1, t2, ..., tm} into a matrix Aij with the times and wavelength as index. Noteworthy, the import functions support many file formats; i.e., the measured data can be provided transposed, with separate files for temporal and wavelength (or energy) information or can be a single wavelength measured kinetics to name just a few. The file(s) to be processed are selected either via a GUI (pf.TA(’gui’) or pf.TA(’recent’) for a single file or pf.GUI_open() for multiple files) or the filename (e.g., pf.TA(’filename.SIA’)) with a wide range of further importing options. For further details see the Opening of data section in the documentation. (51)

Single Scan Handling

Often spectroscopic data are collected over multiple separate measurements (scans). In this case a separate inspection and selection of the scans prior to the analysis is often beneficial. The Summarize_scans function displays the sum of the measured intensities in one or two user definable time-spectral windows for each scan and allows an interactive selection of scans with damaged/changed samples regions or scans with particularly strong disturbances. The tutorial 04_KiMoPack_ScanHandling.ipynb is a guide through this procedure. In the tutorial, one of the windows represents the excited state absorption (ESA) and the second the ground state bleach (GSB). In this example, fluctuations in the GSB indicate fluctuations of the laser pump intensity, while changes in the ESA indicate sample degradation, which can be selected and excluded from the average.

Data Preprocessing

Preprocessing of data typically includes a variety of tasks, including the reading (of many formats), binning, baseline subtraction, filtering, and arrival-time (chirp) correction as well as the suppression of one or multiple (temporal or spectral) regions containing scattered excitation light or artifacts. (52,53) One important concept of KiMoPack is to apply the shaping during each action, such as plotting or fitting anew to a fresh copy of the unaltered data set. This requires that all necessary parameters are passed to each function. For the user this is simplified by packaging the data, functions, and parameters into a TA object that contains all the parameters. A detailed description of the functions and their arguments can be found in the documentation in the Shaping of data section. (54) Typically, the initial task of inspecting the measured data consists (all optional) of: (i) Importing of a file (creating the TA object), (ii) filtering of obvious spikes (Filter_data-function), (iii) baseline subtraction (Background function), (iv) arrival time correction (time-zero of each probe wavelength, Cor_Chirp function), and (v) plotting of the so processed data (Plot_RAW or Plot_Interactive function). Of these, all but the import are optional and triggered by calling the respective functions.

Arrival-Time (Chirp) Correction

An essential preprocessing step is the correction of the temporal axis for different wavelength, often called arrival time or chirp correction. This is initially accomplished by selecting a series of points via a GUI in the 2D-map of the TA data in a selected time-window (Cor_Chirp). Those points are then approximated with a 4th order polynomial and the original data (stored in ds_ori) are interpolated and corrected along this so defined dispersion curve and stored as copy (in the variable ds). This selection can be refined manually at any time or automatically during the fitting procedure. Moreover, correction curves from prior or similar measurements can easily be employed.

Optimization Routines

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In physics, chemistry, and biology, much effort and research are devoted to understanding the dynamics of various processes. To unravel and characterize the processes that contribute to changes in system parameters (e.g., absorption or vibration properties) with time, it is necessary to analyze the respective time-resolved data within physically meaningful models. Modeling of the kinetic development of spectral species is one of the main tasks of KiMoPack (see orange box in Figure 2).
In the following section, the different kinetic models included in KiMoPack and the standard optimization routines for these models are described (see green box in Figure 2). Thereafter we describe how to create target models and present selected advanced models. Finally, we present a typical workflow including the optimization routine. Using the TA data set of Ru-dppz as an example, we show how the optimization settings are made and which options are available in addition to the standard optimization of the kinetic model, such as chirp or multiexperiment fit.

Built-In Models and Global Lifetime Analysis

In KiMoPack, the time-resolved spectra (Aab) can be analyzed within the bi-linear approach, treating the profiles of the probe wavelength {λ1, λ2, ..., λn} and the times relative to the initial excitation (delay times) {t1, t2, ..., tm} independently. (22) The time-resolved spectra are described by N independent spectral components with spectra and their Φ parameter dependent, temporally developing concentration profiles (see also the Supporting Information, Sections S1.1 and S1.2):
(1)
The measured intensities (Aab) are described by the product of time-dependent concentration Cac(t,Φ) of each component and their corresponding (differential) spectral responses Scb(λ). The refined parameter together with the model function (rate equations) form the temporally developing concentration profiles (Cac) and describe the kinetic development of the system.
For this purpose, KiMoPack provides three internal kinetic models, namely exponential, consecutive, and full_consecutive (see dashed box in Figure 2), which allow a flexible adaption of the number of parameters (Φ) and the inclusion of background and non-decaying components (see documentation section Fitting (55)). In the first step of the optimization routine, time-dependent concentration profiles Cac (a ∈ [m] and c ∈ [N]) are created depending on the kinetic model (see green box in Figure 2).
exponential: In this model all components N are taken to decay independent to each other in parallel. Hence, this model approximates the data by N independent exponential decays, namely Cac′ = exp(−kc × ta). For each component, the exponential decay is convoluted with a symmetric Gaussian-shape response function (IRF), which reads as Cac = Cac′ ⊗ IRF (see dashed green box in Figure 2).
consecutive and full_consecutive: These models assume that initially one excited state is populated, which decays unbranched and unidirectional (A → B → ··· → N). Thus, the decay of the initial component causes the population of a next component that turns to the following and so on. That is why this kinetic model is often referred as sequential. As a result, the concentration matrix is composed of both single exponential decays (describing the concentration profile of the initial state A) and weighted sum of exponential functions for the other N – 1 components that account for the population caused from the preceding component and reads as Cac = (see definition of bcd in the Supporting Information Section S1.2). The full_consecutive model is formed by this stepwise integrated differential equation. In the consecutive model the rate parameters are refined using the exponential model. In the final step, the consecutive differential equations are then formed with these optimized parameters and integrated for the extraction of the spectra. This approach is significantly faster and in many cases results in near equivalent solutions.
It should be emphasized that only the creation of Cac depends on the particular kinetic model (kinetic parameters, such as rates, start-time, and resolution), while the subsequent steps of the optimization routine are identical for all models (built-in and user-defined target models). Once the concentration profiles (Cac) are created, the spectral components (Scb), i.e., decay associated spectra (DAS) for exponential models or species associated spectra (SAS) for all other models, are calculated unless they are externally provided. The calculated spectra Scb are the solution of the linear matrix equation Cac × Scb = Aab, where Cac × Scb corresponds to the calculated fit data set (ACab). In the last step, the measured and calculated data sets are compared (AEab = Aab – ACab) and the metric ∥AEab∥2 is used as input in the optimization algorithm to determine if the termination conditions are reached. For each optimization step the parameters are updated and all previously described steps, calculation of Cab, Scb, AEab, and the fit-error (∥AEab∥2) are repeated. The optimization uses primarily the Nelder–Mead parameter optimization (20,56) implemented in the minimize function of Scipy (57) as a standard setting. Some advanced optimization routines that can overcome the trapping of the minimization in local minima (e.g., AmpGo (58)) are implemented and can be used in addition to the very important careful selection of starting parameter and flexible use of constraints to achieve a global minimum.

Target and Advanced Kinetic Models

For more complex decay cascades, a target analysis strategy is required, where a specific model that combines, e.g., parallel and sequential reaction steps, is used to fit the data. (59−64) As such kinetic schemes are commonly based on a priori knowledge of the system, this type of analysis is referred to as target analysis, where the target is to describe the real concentrations of the single components. In KiMoPack such target models are typically defined via kinetic rate equations (differential equations) or through other suitable functions (e.g., a measured instrument response function). For creating the time-dependent concentration profiles Cac (a ∈ [m] and c ∈ [N]) for each component, which is the first step in the optimization routine (see global analysis section), a pulse profile and the differential equations are numerically sampled. Subsequently, the SAS (Scb), the fit data set (ACab), and the difference between the measured and calculated data set AEab = Aab – ACab) are computed as described above (see section global analysis and green box in Figure 2).
KiMoPack was developed during continuous active use in several research groups and projects. (6,37,65−77) This motivated the development of advanced modeling capabilities as are required by challenging samples and complex mechanisms, as well as the need to demonstrate the validity of the kinetic modeling approaches. Many analysis approaches simply use the implemented functionality in unexpected ways. Examples that might be of interest to the reader are, e.g., that single wavelength kinetics can be concatenated to a matrix or fitted separately or that multiple data sets (of the same or different techniques) can be fitted simultaneously using the same or a freely varying SAS. Other good analysis practices include testing of the (locked) initial guess parameter before refinement, the careful selection of the to be optimized parameters and limits, and the estimation of confidence intervals. In the following, we will present and discuss some scenarios that we have encountered in our analysis that go beyond the combination of consecutive and parallel processes.

Analysis of Nonlinear (Higher Order) Processes

When studying solid materials or molecules in sufficiently high concentration or at high excitation densities using time-resolved spectroscopy, nonlinear processes are often found. Typical examples for such processes are two photon or excited state absorption and annihilation/recombination processes. The observed rates or models do then depend on, e.g., the fluence of the excitation. In the provided examples the excited molecule fraction generated by sampling a pulse shape, which is given a fluence dependent amplitude. A nonlinear process would then be expressed in the usual way as, e.g., where [Y] represents the concentration of excited units and α is an exponent that represents the order of a process. This approach is particularly powerful if multiple data sets with different fluences/concentrations are analyzed simultaneously using the multi_project option in the global analysis.

Distributed-Rate Model Analysis

Distributed-rate models are an important approach for the description of natural systems that have a distribution of activation energies. Examples for systems like this are light induced polymer-folding or dynamics in polymer-based solar cells. (78,79) The idea behind the modeling is that a distributed species is created and subdivided into smaller units. Then the dynamics of each unit is tracked separately in an independent matrix. In the returned concentration matrix these units are recombined into a single species with the same spectral feature. The system is then described (and optimized) with the parameters of the distribution. In the case of the example, the Gaussian distribution has a central rate constant and a width. We found that this approach often not only replaces several exponential decays (that would be required to explain the observed rates) but also represents a much more realistic description of the system. (78,79)

Spectroelectrochemistry

In spectroelectrochemistry, a fraction of the substances measured in the beampath are changed by a potential applied between two electrodes. Depending on the precise conditions, the absolute spectral changes that are induced can be very small, making the measurement challenging. In a recent experiment, we were unable to use a standard spectroelectrochemistry cell and instead focused a conventional lightsource onto a small round gold electrode and collected the reflected light using a fiber spectrometer. (13) The electrode was part of a three electrode measurement system with which we performed repeated cyclovoltaic measurements. The measured current (compensated for a number of effects) was then read into KiMoPack as an indicator for the number of oxidized/reduced molecules and used as a model for the global analysis. Using the information obtained from the potentiostat allowed us to extract the spectrum of the transient species and perform further analysis based upon these spectra.
The example file, namely function_library.py, is provided in the Supporting Information and contains documented code for a simple sequential model, a model that includes a (laser) power-dependent dynamics and a distributed-rate model. Moreover, these models are included and documented in the advanced modeling workflow tool that can be found on the Github page (30) of KiMoPack and as a locked release on zenodo. (31)

Kinetic Modeling Workflow

In the following, a typical workflow for the global analysis of TA data with different kinetic models is described, using the TA data of Ru-dppz collected in DCM, ACN, and H2O as example. After preprocessing of the data we determine a first guess on the number and position of the main temporal components in the spectra. This visual inspection using the functions Plot_RAW() or Plot_Interactive() is often combined with a model-free matrix factorization technique, such as SVD, to explore the number of spectrally and temporally independent components. (22) The number and magnitude of the singular values is an indication about the importance of the independent components in the data and often a cutoff value can be chosen. Other useful tools include 2D-correlation spectroscopy (2D-COS) (37) that can use the preprocessed data as input.
In the example of TA data of Ru-dppz collected in ACN at 400 nm excitation shown in Figure 3, the singular values and the shape of the singular vectors of the SVD indicate that the TA matrix most likely can be described by three main components, with one being dominant, and the characteristic times lie in the range between 1 and 2 ps and between 100 and  200 ps and contain one long-lived component (>2 ns). These initial insights are consistent with a priori knowledge about the photoinduced dynamics of Ru-dppz in ACN, indicating that the excited state dynamics can be described with three characteristic rate-constants. (46) The next step is to assume three independent decay processes with the initial characteristic rates k0, k1, and k2. (note: the standard numbering starts at “0” in python, so k0 is the first decay component and the rates are linked to the characteristic times in the usual manner as k = 1/time). The fitting function (Fit_Global) uses the parameter control options of lmfit. (80) For this purpose, each individual time constant is created as a parameter with starting value (e.g., ’k0′, value=1/2, vary=True) and passed into the corresponding lmfit parameter object (ta.par=lmfit.Parameters()). The optional flags vary=True/False or the setting of constraints using e.g., max=0.5 allows a flexible and controlled refinement, see the documentation section Setting of Fit parameter for more details. (55)

Figure 3

Figure 3. Singular value decomposition of the TA-matrix of Ru-dppz collected upon 400 nm excitation in acetonitrile. The plots show the singular values and respective spectral and temporal singular vectors for the first six singular components.

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Commonly, the analysis starts by employing the exponential model to refine the set of initial parameters. The respective model definition and global analysis read as ta.mod=’exponential’ and ta.Fit_Global(). This usually serves as a useful starting point for subsequent modeling with more complex models, such as consecutive or target models.
In the tutorial we assume as target model that two excited states (X and Y) are populated upon photoexcitation. In this model we sample a Gaussian peak function to generate X and Y and thus generate a broadened instrument response function. Those states independently and irreversibly decay with the rate constants k0 and k1, respectively, populating the excited state Z. Ultimately, Z decays back to the ground state with k2. The respective model sketch and rate equations for X, Y, and Z are summarized in Figure 4. The example code for the definition of the mentioned kinetic model and its application can be found in the tutorial notebook (cf. 02_KiMoPack_Fitting-2.ipynb). In all manual models the column labels of each species N can have arbitrary names (e.g., ’MLCT-hot’ or ’IL’), which are then used as plot labels.

Figure 4

Figure 4. Schematic illustration of an user-defined (tutorial) model. In this model, the states X and Y are initially populated and the decay independently and irreversibly with the decay rate k0 and k1, respectively, finally populating state Z. This state irreversibly decays back to the ground state with the rate-constants k2. The corresponding rate equations for the concentrations of X, Y, and Z are displayed.

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The optimized parameter set can be evaluated by performing a confidence interval estimation using the switch confidence_level. This evaluation is different from the usual covariance based arguments in that it compares the continuously reoptimized model against the statistical fluctuation from the measured data. (20) The confidence interval for each parameter is estimated by first calculating a statistically significant χ2 variation based on the scaled variance of the measured matrix, the number of parameters and the targeted confidence interval using a F-test (cf. Supporting Information, Section S1.3). (20) Then each variable parameter is increased and decreased until the calculated χ2 reaches the so defined threshold, all other flexible parameters are simultaneously reoptimized for each step. The calculated confidence interval is thus the maximum variation on the multidimensional parameter surface and a narrow confidence interval is an indication of a model in which none of the other parameters cancompensate for this variation.
At any time, the preprocessing and model parameter can be changed, limited, or adapted to find a stable solution. We define and discuss criteria of a stable solution a little bit later in this paper. Tools that can help in this search are, e.g., the refinement of the arrival time correction (fit_chirp), or the use of advanced global optimizing algorithms. Currently, the adaptive memory programming for constrained global optimization (81) (AMPGO, use_ampgo) is employed in KiMoPack to automatically explore the data for different local minima.

Collecting and Plotting Routine

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Data can be (interactively) plotted and exported (in form of images or tabular data) at any time during the analysis. The usual examples are prior or after preprocessing or optimization of the fit parameters. As each analysis problem might require a different view on the data, we use a function to generate a number of plots automatically but provide options to select and shape a favorite representation. Typical examples are illustrated in the orange box in Figure 2.

Plotting of Raw- and Preprocessed Data

Generally, three different types of plots are used to visualize the data: (i) 2D-contour plots, (ii) kinetic traces, and (iii) transient spectra. In the 2D-map, the value of the measurement (e.g., differential absorption or fluorescence intensity) is depicted as a function of the time delay (abscissa) and the probe wavelengths (ordinate). The kinetic traces and transient spectra are horizontal or vertical slices of this data. The function Plot_Interactive allows the interactive visualization of a transient spectrum and its kinetics by cursor placement on the 2D-map. The Plot_RAW-function uses specified wavelength and times to generate an overview of the interesting information and generate publication ready plots. A wide range of arguments in the TA object can alter the appearance of the plots and the shape of the data. The example and template Jupyter notebooks include a selection of the commonly used options that, e.g., suppress the pump scattered light or the artifacts that stem from cross-phase modulation around time-zero (t = 0). Commonly, these parameters are adjusted at the beginning of an analysis project for a specific purpose, refined later if needed and duplicating for similar analysis projects. For further information, see the examples and the documentation section Shaping of data. (54)

Plotting of Fit Results

The quality of the fit can be evaluated based on the fit parameters returned by the Fit_Global function as well as the plot(s) that are created by the Plot_fit_output or Plot_Interactive function. Plot_fit_output generates six plots that contain the most often used indicators. (i) 2D-contour plots of the measured, modeled and difference matrix (see Figure 5), (ii) the integrated kinetic traces, (iii) the selected kinetic traces, (iv) the selected spectra, and (v) the transient spectra (DAS/SAS) as well as (vi) the temporal profiles of each component. A step-by-step guide is shown in the tutorial notebook and further information in documentation section Plotting functions. (82)

Figure 5

Figure 5. Contour plots of the preprocessed (measured: Aab) and modeled (calculated: ACab) fs-TA data of Ru-dppz in acetonitrile (ACN) in the time-delay range of 350 fs to 1800 ps and probe wavelength range of 330 to 780 nm upon photoexcitation at 400 nm. The bottom plot shows the 10-fold amplified difference of the measured and calculated data (AEab) enabling an inspection of the overall fit quality.

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Evaluation of the Quality of a Model Representation (Fit Quality)

We will try to formulate in this section which parameter we employ to evaluate the quality of a model representation. The reader will note that the reduced χ2 is consciously not included. It includes the number of parameters and is as such mainly useful in comparing descriptions, but challenged through the continuous optimization of the spectra (and the large number of parameters in the spectra). Internally the similar metric ∥Aab – ACab∥2 is used to evaluate the optimization progression. Instead, several indicators should be combined to define a sufficiently good description of a data set and we recommend the following criteria:
(1)

Closeness of Fit: The coefficient of determination, also called the R2 parameter, is close to 1.0. The R2 compares the residual between model and fit to the variance of the data. If a residual matrix (see bottom panel in Figure 5) only shows values close to zero in all regions, without any visible structure, this is a good description and will lead to an R2 value close to 1.0. In this example periodic structure or specific spectral regions that are not well described (also note the different color scale to estimate the magnitude). In the optimization process, all plots from Plot_fit_output should be used to estimate this criteria in addition to the R2 value.

(2)

Precision: A narrow(er) confidence interval often indicates a good/better model.

(3)

DAS/SAS: None of the DAS/SAS are mirror images of each other (indicating linear compensating), and specific features are verifiable with other spectroscopic techniques (e.g., spectrophotometric, acid/base titration, or spectroelectrochemistry).

(4)

Sensitivity: The fit-parameters are insensitive to small changes in the preprocessing parameters.

(5)

Stability: The initial guesses of the starting parameters do not strongly influence the fitted parameters.

(6)

Global Minimum: Other minima using the same model and a feasible parameter space represent the data worse (under consideration of the error margins)

(7)

Defensible Model: The proposed model must be physically correct and consistent with other techniques and chemical principles. To verify or constrain a model is good scientific praxis.

(8)

Simpler Models: In general, all models that fulfill the criteria 1–7 should be discussed, and external arguments should be used to disregard other feasible descriptions. Finding/defining external constraints often helps to minimize the number.

Data Export and Project Saving

The export of preprocessed data (Save_Data) and fit results in the form of images, tabular files and as report summary are important for extending the functionality to other tools. All graphs can be saved as single files or via the function Save_Plots all at once. Particularly useful for creating fast reports is the function Save_Powerpoint that creates one summary file as *.pptx, *.pdf, *.svg or two *.png files, containing all relevant graphs and fit results. (Figure 6). The whole analysis project including all relevant data and fit results can be saved and recalled as hdf5 file. These summary files are particularly useful for the comparison of several data sets, e.g., those collected in different solvents, at different excitation wavelengths, and various quencher concentrations or the comparison of different analysis strategies. A detailed description can be found in the documentation sections Plotting functions (82) and Data Export and Project Saving (83)

Figure 6

Figure 6. KiMoPack-generated summary of the preprocessed TA data and the fit results of Ru-dppz in acetonitrile (ACN) upon 400 nm excitation.

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Comparison of Data Sets and Fit Results

To compare data sets and fit results is an important point of most analysis. It does contribute to understand the influence of different conditions onto the processes. During the comparison of data often the need for flexible normalization of different data sets is challenging. Three functions were designed to simplify the comparison of data. Transient spectra at defined delay times are shown with Compare_at_time that also allows the inclusion of external spectra. Kinetic traces at selected wavelengths are displayed with Compare_at_wave and the modeled spectra (DAS or SAS) are shown with Compare_DAC. The first two functions include a flexible normalization procedure that integrates the intensity in a defined spectral–temporal window and divides all spectra by this value. This permits, for example, normalization to the intensity of the ground state bleach signals to compare measurements with, i.e., different sample concentrations, solvents, or excitation intensities or the normalization to an excited state absorption to extract the relative efficiency of photo product formation. Spectra (DAS, SAS, or TA spectra) can be compared to external data, such as the absorption spectra of electrochemically modified species, photoproducts or simulated spectra, which has been proven to be tremendously helpful in evaluating complex data. (5,84,85) The Jupyter notebook 03_KiMoPack_CompareFit.ipynb demonstrates the comparison procedure using the TA data of Ru-dppz collected in DCM, ACN, and H2O.

Interpretation of Fit Results

Unfortunately there is no universal procedure/recipe for the interpretation of fit results. The particular challenges often lay in the assignment of photochemical and photophysical processes occurring with a characteristic rate-constant kc. The combination of lifetimes together with the process oriented DAS and the species selective SAS can however often be used to indicated a particular process, particularly if they can be compared to, e.g., spectro-electrochemical results or if the interpretation is supported by multiple techniques.
We found that new user of transient absorption find the difference between DAS and SAS particularly challenging to understand and would like to use the analysis of one process in the model complex Ru-dppz to highlight the difference. A detailed description of the excited state processes of the model complex Ru-dppz can be found in the literature (see also Figure 1). (38−40,42−46) We use exemplary the interligand hopping process to highlight the difference and rely on that certain feature of the SAS have been identified as characteristic ligand-to-metal charge-transfer or ππ*-absorption bands in prior work. In this hopping process the electron density is shifted from the tbbpy ligand to the dppz ligand sphere, (46) and this shift is manifested in the DAS and SAS (see Figure 7). Considering the DAS, the interligand hopping process is reflected in the DAS associated with k1 as follows: The excited state absorption (ESA) centered around 380 nm, which is assigned to states with excess electron density on the tbbpy ligands, decreases (positive signal region), while the ESA for phz-centered states at 340 and 590 nm increases (negative signal region). Similarly, this interligand hopping process is manifested in the SAS when the spectra associated with the species A and B are compared: The latter spectrum (A) stems from a phenazine-centered MLCT state (3MLCTphz) showing prominent ESA maxima at 340 and 590 nm, which can be assigned, e.g., via comparison with UV–vis spectroelectrochemical data. (86) The A-associated spectrum exhibits the characteristics typically observed for Ru(II) polypyridine-type complexes, namely comparably strong ground state bleach (GSB) between 400 and 500 nm accompanied by a strong and broad as well as a unstructured and flat ESA band between 340 and 400 nm and above 500 nm, respectively. Consequently, the first species can be assigned to a state with excess electron density in the proximal ligand sphere (3MLCTprox: 3MLCTtbbpy, 3MLCTphen).

Figure 7

Figure 7. Schematic representation of a parallel and sequential model involving three different species, namely A (3MLCTprox), B (3MLCTdist), and C (3MLCTphen). Decay associated spectra (DAS, a) and species associated spectra (SAS, b) obtained from global analysis of the transient absorption (TA) data of Ru-dppz collected upon 400 nm excitation in acetonitrile (ACN) within the exponential (giving DAS) and consecutive model (giving SAS).

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Conclusion

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The deeper analysis of spectrally- and time-resolved data usually required commercial software or extended programming knowledge, as many open-source software just provide fitting routines for simple kinetic schemes. We developed a flexible python-based toolbox, named KiMoPack, which allows global and target modeling of transient data with implemented and user-defined models without the prerequisite of extensive simulation/programming knowledge. Through this introduction, the tutorials and workflow sheets the intuitive functions of KiMoPack were demonstrated at the example of transient absorption data of Ru-dppz recorded in different solvents. Generally, the tool can be used for the analysis of any transient/time-resolved spectra, i.e., optical or X-ray emission or Raman spectroscopy data sets, and can show through its open-source and modular nature the potential for further extensions by the community. Recently, KiMoPack was used for the modeling transient absorption, X-ray, time-resolved emission, spectroelectrochemistry, and photocatalysis data.
In summary, the work demonstrates the use of KiMoPack to analyze complex spectroscopic data, and we believe that this tool offers ways to simplify and combine the standard analysis routine with the improved objectivity of performing all preprocessing, fitting, plotting, reporting, and more analysis steps in a single powerful analysis tool.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.2c00907.

  • Detailed explanation of the model construction, error analysis, data export, and a summary of the tutorials (PDF)

  • File containing Jupyter notebook demonstrating the general workflow of KiMoPack to analyze TA data of Ru-dppz collected in DCM, ACN and H2O employing two built-in kinetic models (parallel and sequential model), Jupyter notebook showing how to define and implement a user-defined kinetic model in the global fit using Ru-dppz in acetonitrile as an example, Jupyter notebook using the example of Ru-dppz to show how TA data recorded in different solvents, namely DCM, ACN, and H2O, can be (visually) compared to each other and external spectra, i.e., steady-state and spectroelectrochemical absorption data, Jupyter notebook showing how individual TA scans of Ru-dppz (in ACN, DCM, or H2O) can be selected and averaged using an interactive plot, a python module file that contains the documented definitions of three function for the generation of a consecutative model, a non linear power dependent model and a model with distributed rate, folder containing the raw data used for the tutorials, where the data are subdivided into a specific folder per tutorial, and a folder containing the images rendered in the tutorial notebooks (ZIP)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • Carolin Müller - Institute for Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany;  Leibniz Institute of Photonic Technology (IPHT) Jena, Albert-Einstein-Strasse 9, 07745 Jena, Germany;  Email: [email protected]
    • Jens Uhlig - Department of Chemical Physics, Lund University, SE-22100 Lund, Sweden;  Orcidhttps://orcid.org/0000-0002-0528-0422;  Email: [email protected]
  • Authors
    • Torbjörn Pascher - Department of Chemical Physics, Lund University, SE-22100 Lund, Sweden
    • Axl Eriksson - Department of Chemical Physics, Lund University, SE-22100 Lund, Sweden
    • Pavel Chabera - Department of Chemical Physics, Lund University, SE-22100 Lund, Sweden;  Orcidhttps://orcid.org/0000-0002-0531-5138
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge the workgroup of Prof. Benjamin Dietzek for providing transient absorption data of the model substance Ru-dppz. C.M. gratefully acknowledges funding from the German Research Foundation. J.U. acknowledges funding from the Crafoordska stiftelsen and the Vetenskåpsrådet under contract 2020-04995. We acknowledge the diligent testing of KiMoPack and suggestions from Linnea Lindh, Sebastian Bold, Julien Klaus, Arkady Yartsev and Rasmus Ringström.

References

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This article references 86 other publications.

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    Chemistry - A European Journal (2009), 15 (31), 7678-7688, S7678/1-S7678/4CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Photoinduced electron-transfer processes within a precatalyst for intramol. hydrogen evolution [(tbbpy)2Ru(tpphz)PdCl2]2+ (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)2Ru(tpphz)]2+ subunit Ru with that of the supramol. catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the mol. ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramol. complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-detg. steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-sepd. states.
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  12. 12
    Pfeffer, M.; Müller, C.; Kastl, E. T. E.; Mengele, A. K.; Bagemihl, B.; Fauth, S.; Habermehl, J.; Petermann, L.; Wächtler, M.; Schulz, M. Active repair of a dinuclear photocatalyst for visible light-driven hydrogen production. Nat. Chem. 2022, 14, 500– 506,  DOI: 10.1038/s41557-021-00860-6
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    12
    Active repair of a dinuclear photocatalyst for visible-light-driven hydrogen production
    Pfeffer, Michael G.; Mueller, Carolin; Kastl, Evelyn T. E.; Mengele, Alexander K.; Bagemihl, Benedikt; Fauth, Sven S.; Habermehl, Johannes; Petermann, Lydia; Waechtler, Maria; Schulz, Martin; Chartrand, Daniel; Laverdiere, Francois; Seeber, Phillip; Kupfer, Stephan; Graefe, Stefanie; Hanan, Garry S.; Vos, Johannes G.; Dietzek-Ivansic, Benjamin; Rau, Sven
    Nature Chemistry (2022), 14 (5), 500-506CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
    The mol. app. behind biol. photosynthesis retains its long-term functionality through enzymic repair. However, bioinspired mol. devices designed for artificial photosynthesis, consisting of a photocentre, a bridging ligand and a catalytic center, can become unstable and break down when their individual modules are structurally compromised, halting their overall functionality and operation. Here we report the active repair of such an artificial photosynthetic mol. device, leading to complete recovery of catalytic activity. We have identified the hydrogenation of the bridging ligand, which inhibits the light-driven electron transfer between the photocentre and catalytic center, as the deactivation mechanism. As a means of repair, we used the light-driven generation of singlet oxygen, catalyzed by the photocentre, to enable the oxidative dehydrogenation of the bridging unit, which leads to the restoration of photocatalytic hydrogen formation.
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  13. 13
    Tasić, M.; Ivković, J.; Carlström, G.; Melcher, M.; Bollella, P.; Bendix, J.; Gorton, L.; Persson, P.; Uhlig, J.; Strand, D. Electro-mechanically switchable hydrocarbons based on [8]annulenes. Nat. Commun. 2022, 13, 860– 869,  DOI: 10.1038/s41467-022-28384-8
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    Electro-mechanically switchable hydrocarbons based on [8]annulenes
    Tasic, Magdalena; Ivkovic, Jakov; Carlstroem, Goeran; Melcher, Michaela; Bollella, Paolo; Bendix, Jesper; Gorton, Lo; Persson, Petter; Uhlig, Jens; Strand, Daniel
    Nature Communications (2022), 13 (1), 860CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Abstr.: Pure hydrocarbons with shape and conjugation properties that can be switched by external stimuli is an intriguing prospect in the design of new responsive materials and single-mol. electronics. Here, we develop an oligomeric [8]annulene-based material that combines a remarkably efficient topol. switching upon redox changes with structural simplicity, stability, and straightforward synthesis: 5,12-alkyne linked dibenzo[a,e]cyclooctatetraenes (dbCOTs). Upon redn., the structures accommodate a reversible reorganization from a pseudo-conjugated tub-shape to a conjugated arom. system. This switching in oligomeric structures gives rise to multiple defined states that are deconvoluted by electrochem., NMR, and optical methods. The combination of stable electromech. responsivity and ability to relay electrons stepwise through an extended (pseudo-conjugated) π-system in partially reduced structures validate alkyne linked dbCOTs as a practical platform for developing new responsive materials and switches based on [8]annulene cores.
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  14. 14
    Satzger, H.; Zinth, W. Visualization of transient absorption dynamics – towards a qualitative view of complex reaction kinetics. Chem. Phys. 2003, 295, 287– 295,  DOI: 10.1016/j.chemphys.2003.08.012
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    14
    Visualization of transient absorption dynamics - towards a qualitative view of complex reaction kinetics
    Satzger, H.; Zinth, W.
    Chemical Physics (2003), 295 (3), 287-295CODEN: CMPHC2; ISSN:0301-0104. (Elsevier Science B.V.)
    Photochem. reactions may involve complex reaction schemes and their study requires spectroscopic techniques extending over a broad spectral range and over several orders of magnitude in time. Two commonly used numerical procedures to evaluate such data sets - global fitting and singular value decompn. - are discussed and a method for the qual. visualization is proposed: differentiation of the transient spectra on a logarithmic scale allows to ext. special kinetic components and to obtain reasonable starting information for subsequent fitting procedures. The proposed method should be well adapted to situations where e.g. due to unstable samples the data cannot be recorded at the precision required for unambiguous anal. by std. data handling procedures. The method is applied to synthetic data sets as well as to exptl. data taken from femtosecond absorption expts. on the laser dye DCM in DMSO.
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  15. 15
    Henry, E. R. The Use of Matrix Methods in the Modeling of Spectroscopic Data Sets. Biophys. J. 1997, 72, 652– 673,  DOI: 10.1016/S0006-3495(97)78703-4
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    The use of matrix methods in the modeling of spectroscopic data sets
    Henry, Eric R.
    Biophysical Journal (1997), 72 (2, Pt. 1), 652-673CODEN: BIOJAU; ISSN:0006-3495. (Biophysical Society)
    We describe a general approach to the model-based anal. of sets of spectroscopic data that is built upon the techniques of matrix anal. A model hypothesis may often be expressed by writing a matrix of measured spectra as the product of a matrix of spectra of individual mol. species and a matrix of corresponding species populations as a function of exptl. conditions. The modeling procedure then requires the simultaneous detn. of a set of species spectra and a set of model parameters (from which the populations are derived), such that this product yields an optimal description of the measured spectra. This procedure may be implemented as an optimization problem in the space of the (possibly nonlinear) model parameters alone, coupled with the efficient soln. of a corollary linear optimization problem using matrix decompn. methods to obtain a set of species spectra corresponding to any set of model parameters. Known species spectra, as well as other information and assumptions about spectral shapes, may be incorporated into this general framework, using parametrized anal. functional forms and basis-set techniques. The method by which assumed relationships between global features (e.g., peak positions) of different species spectra may be enforced in the modeling without otherwise specifying the shapes of the spectra will be shown. We also consider the effect of measurement errors on this approach and suggest extensions of the matrix-based least-squares procedures applicable to situations in which measurement errors may not be assumed to be normally distributed. A generalized anal. procedure is introduced for cases in which the species spectra vary with exptl. conditions.
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    Henry, E.; Hofrichter, J. [8] Singular value decomposition: Application to analysis of experimental data. Methods in Enzymology 1992, 210, 129– 192,  DOI: 10.1016/0076-6879(92)10010-B
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    16
    Singular value decomposition: application to analysis of experimental data
    Henry, E. R.; Hofrichter, J.
    Methods in Enzymology (1992), 210 (Numer. Comput. Methods), 129-92CODEN: MENZAU; ISSN:0076-6879.
    The title method is illustrated by the anal. of spectral data of, e.g., modified Hb.
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    Hendler, R. W.; Shrager, R. I. Deconvolutions based on singular value decomposition and the pseudoinverse: a guide for beginners. Journal of Biochemical and Biophysical Methods 1994, 28, 1– 33,  DOI: 10.1016/0165-022X(94)90061-2
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    Deconvolutions based on singular value decomposition and the pseudoinverse: a guide for beginners
    Hendler R W; Shrager R I
    Journal of biochemical and biophysical methods (1994), 28 (1), 1-33 ISSN:0165-022X.
    Singular value decomposition (SVD) is deeply rooted in the theory of linear algebra, and because of this is not readily understood by a large group of researchers who could profit from its application. In this paper, we discuss the subject on a level that should be understandable to scientists who are not well versed in linear algebra. However, because it is necessary that certain key concepts in linear algebra be appreciated in order to comprehend what is accomplished by SVD, we present the section, 'Bare basics of linear algebra'. This is followed by a discussion of the theory of SVD. Next we present step-by-step examples to illustrate how SVD is applied to deconvolute a titration involving a mixture of three pH indicators. One noiseless case is presented as well as two cases where either a fixed or varying noise level is present. Finally, we discuss additional deconvolutions of mixed spectra based on the use of the pseudoinverse.
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    Shrager, R. I. Chemical transitions measured by spectra and resolved using singular value decomposition. Chemometrics and Intelligent Laboratory Systems 1986, 1, 59– 70,  DOI: 10.1016/0169-7439(86)80026-0
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    Chemical transitions measured by spectra and resolved using singular value decomposition
    Shrager, Richard I.
    Chemometrics and Intelligent Laboratory Systems (1986), 1 (1), 59-70CODEN: CILSEN; ISSN:0169-7439.
    A sequence of optical or other spectra, generated by overlapping chem. transitions, may be more easily analyzed after a data redn. step involving singular value decompn. (SVD). The method is described, and its development since 1965 is summarized. A new SVD-based method described which corrects some theor. and practical flaws in past methods.
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    Oang, K. Y.; Yang, C.; Muniyappan, S.; Kim, J.; Ihee, H. SVD-aided pseudo principal-component analysis: A new method to speed up and improve determination of the optimum kinetic model from time-resolved data. Structural Dynamics 2017, 4, 044013,  DOI: 10.1063/1.4979854
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    SVD-aided pseudo principal-component analysis: A new method to speed up and improve determination of the optimum kinetic model from time-resolved data
    Oang, Key Young; Yang, Cheolhee; Muniyappan, Srinivasan; Kim, Jeongho; Ihee, Hyotcherl
    Structural Dynamics (2017), 4 (4), 044013/1-044013/13CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
    Detn. of the optimum kinetic model is an essential prerequisite for characterizing dynamics and mechanism of a reaction. Here, we propose a simple method, termed as singular value decompn.-aided pseudo principal-component anal. (SAPPA), to facilitate detn. of the optimum kinetic model from time-resolved data by bypassing any need to examine candidate kinetic models. We demonstrate the wide applicability of SAPPA by examg. three different sets of exptl. time-resolved data and show that SAPPA can efficiently det. the optimum kinetic model. In addn., the results of SAPPA for both time-resolved X-ray soln. scattering (TRXSS) and transient absorption (TA) data of the same protein reveal that global structural changes of protein, which is probed by TRXSS, may occur more slowly than local structural changes around the chromophore, which is probed by TA spectroscopy. (c) 2017 American Institute of Physics.
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    Johnson, M. L.; Faunt, L. M. [1] Parameter estimation by least-squares methods. Methods in Enzymology 1992, 210, 1– 37,  DOI: 10.1016/0076-6879(92)10003-V
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    Parameter estimation by least-squares methods
    Johnson M L; Faunt L M
    Methods in enzymology (1992), 210 (), 1-37 ISSN:0076-6879.
    There is no expanded citation for this reference.
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    Beechem, J. M. [2] Global analysis of biochemical and biophysical data. Methods in Enzymology 1992, 210, 37– 54,  DOI: 10.1016/0076-6879(92)10004-W
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    21
    Global analysis of biochemical and biophysical data
    Beechem, Joseph M.
    Methods in Enzymology (1992), 210 (Numer. Comput. Methods), 37-54CODEN: MENZAU; ISSN:0076-6879.
    Global anal. of data represents the simultaneous anal. of multiple expts. in terms of internally consistent sets of fitting parameters. What has been presented is a motivation for performing global types of analyses and the importance of rigorous error anal.
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    Ruckebusch, C.; Sliwa, M.; Pernot, P.; de Juan, A.; Tauler, R. Comprehensive data analysis of femtosecond transient absorption spectra: A review. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2012, 13, 1– 27,  DOI: 10.1016/j.jphotochemrev.2011.10.002
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    Comprehensive data analysis of femtosecond transient absorption spectra: A review
    Ruckebusch, C.; Sliwa, M.; Pernot, P.; de Juan, A.; Tauler, R.
    Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2012), 13 (1), 1-27CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
    A review. Nowadays, time-resolved spectroscopy data can be routinely and accurately collected in UV-visible femtosecond transient absorption spectroscopy. However, the data anal. strategy and the postulation of a phys. valid model for this kind of measurements may be tackled with many different approaches ranging from pure soft-modeling (model-free) to hard-modeling, where the elaboration of a parametric spectro-temporal model may be required. This paper reviews methods that are used in practice for the anal. of femtosecond transient absorption spectroscopy data. Model-based methods, common in photochem., are revisited, and soft-modeling methods, which originate from the chemometrics field and that recently disseminated in the photo(bio)chem. literature, are presented. These soft-modeling methods are designed to suit the intrinsic nature of the multivariate (or multi-way) measurement. Soft-modeling tools do not require a priori phys. or mechanistic models to provide a decompn. of the data on the time and wavelength dimensions, the only requirement being that these 2 (or more) dimensions are separable. Addnl., Bayesian data anal., which provides a probabilistic framework for data anal., is considered in detail, since it allows uncertainty quantification and validation of the model selection step.
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    Kollenz, P.; Herten, D.-P.; Buckup, T. Unravelling the Kinetic Model of Photochemical Reactions via Deep Learning. J. Phys. Chem. B 2020, 124, 6358– 6368,  DOI: 10.1021/acs.jpcb.0c04299
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    Unravelling the Kinetic Model of Photochemical Reactions via Deep Learning
    Kollenz, Philipp; Herten, Dirk-Peter; Buckup, Tiago
    Journal of Physical Chemistry B (2020), 124 (29), 6358-6368CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    Time-resolved spectroscopies have been playing an essential role in the elucidation of the fundamental mechanisms of light-driven processes, particularly in exploring relaxation models for electronically excited mols. However, the detn. of such models from exptl. obtained time-resolved and spectrally resolved data still demands a high degree of intuition, frequently poses numerical challenges, and is often not free from ambiguities. Here, we demonstrate the anal. of time-resolved laser spectroscopy data via a deep learning network to obtain the correct relaxation kinetic model. In its current design, the presented Deep Spectroscopy Kinetic Anal. Network (DeepSKAN) can predict kinetic models (involved states and relaxation pathways) consisting of up to five states, which results in 103 possible different classes, by estg. the probability of occurrence of a given kinetic model class. DeepSKAN was trained with synthetic time-resolved spectra spanning over 4 orders of magnitude in time with a unitless time axis, thereby demonstrating its potential as a universal approach for analyzing data from various time-resolved spectroscopy techniques in different time ranges. By adding the probabilities of each pathway of the top-k models normalized by the total probability, we can det. the relaxation pathways for a given data set with high certainty (up to 99%). Due to its architecture and training, DeepSKAN is robust against exptl. noise and typical preanal. errors like time-zero corrections. Application of DeepSKAN to exptl. data is successfully demonstrated for three different photoinduced processes: transient absorption of the retinal isomerization, transient IR spectroscopy of the relaxation of the photoactivated DRONPA, and transient absorption of the dynamics in lycopene. This approach delivers kinetic models and could be a unifying asset in several areas of spectroscopy.
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    Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M. Glotaran: A Java -Based Graphical User Interface for the R Package TIMP. Journal of Statistical Software 2012, 49, 1– 22,  DOI: 10.18637/jss.v049.i03
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    Dorlhiac, G. F.; Fare, C.; van Thor, J. J. PyLDM - An open source package for lifetime density analysis of time-resolved spectroscopic data. PLOS Computational Biology 2017, 13, e1005528  DOI: 10.1371/journal.pcbi.1005528
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    PyLDM - an open source package for lifetime density analysis of time-resolved spectroscopic data
    Dorlhiac, Gabriel F.; Fare, Clyde; van Thor, Jasper J.
    PLoS Computational Biology (2017), 13 (5), e1005528/1-e1005528/15CODEN: PCBLBG; ISSN:1553-7358. (Public Library of Science)
    Historically, the dual approach of global anal. and target modeling has been used to elucidate kinetic descriptions of the system, and the identity of transient species resp. While global anal. approximates the data to the sum of a small no. of exponential decays, typically on the order of 2-4, LDA uses a semi-continuous distribution of 100 lifetimes. This allows for the elucidation of lifetime distributions, which may be expected from investigation of complex systems with many chromophores, as opposed to avs. Furthermore, the inherent assumption of linear combinations of decays in global anal. means the technique is unable to describe dynamic motion, a process which is resolvable with LDA. The technique was introduced to the field of photosynthesis over a decade ago by the Holzwarth group. The anal. has been demonstrated to be an important tool to evaluate complex dynamics such as photosynthetic energy transfer, and complements traditional global and target anal. techniques. Although theory has been well described, no open source code has so far been available to perform lifetime d. anal. Therefore, we introduce a python (2.7) based package, PyLDM, to address this need. We furthermore provide a direct comparison of the capabilities of LDA with those of the more familiar global anal., as well as providing a no. of statistical techniques for dealing with the regularization of noisy data.
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    Beckwith, J. S.; Rumble, C. A.; Vauthey, E. Data analysis in transient electronic spectroscopy - an experimentalist’s view. Int. Rev. Phys. Chem. 2020, 39, 135– 216,  DOI: 10.1080/0144235X.2020.1757942
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    Data analysis in transient electronic spectroscopy - an experimentalist's view
    Beckwith, Joseph S.; Rumble, Christopher A.; Vauthey, Eric
    International Reviews in Physical Chemistry (2020), 39 (2), 135-216CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
    Time-resolved electronic spectroscopy has grown into a technique that provides hundreds to thousands of electronic spectra with femtosecond time resoln. This enables complex questions to be interrogated, with an obvious cost that the data are more detailed and thus require accurate modeling to be properly reproduced. Anal. of these data comes in a variety of forms, starting with a variety of assumptions about how the data may be decompd. Here, four different types of anal. commonly used are discussed: band-shape anal., global kinetic anal., lifetime distribution models, and soft-modeling. This review provides a 'user's guide' to these various methods of data anal., and attempts to elucidate their successes, domains in which they may be useful, and potential pitfalls in their usage.
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    Kandoth, N.; Pérez Hernández, J.; Palomares, E.; Lloret-Fillol, J. Mechanisms of photoredox catalysts: the role of optical spectroscopy. Sustainable Energy & Fuels 2021, 5, 638– 665,  DOI: 10.1039/D0SE01454K
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    Mechanisms of photoredox catalysts: the role of optical spectroscopy
    Kandoth, Noufal; Perez Hernandez, Javier; Palomares, Emilio; Lloret-Fillol, Julio
    Sustainable Energy & Fuels (2021), 5 (3), 638-665CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)
    A review. Photoinduced org. transformations have stimulated the org. chem. community to develop light-driven renewed reaction methodologies, which in many cases are complementary to std. thermal catalysis. This revitalization of photoinduced transformations is in part due to the straightforward access to powerful photosensitizers. Among them, Ru(II) and Ir(III) polypyridyl complexes have been extensively utilized as prototypical photoredox catalysts. Despite the flourishing of new org. reactivity, studies of photocatalytic cycles are still scarce. The current mechanistic proposal mostly relies on luminescence quenching studies, redox potentials, and bond-dissocn. energy values, which provide an essential but partial picture of the catalytic processes. Besides, the quantum efficiency and overall energy efficiency of photoredox org. transformations are not usually considered merit yet. On the other hand, during the last few decades, the photochem. community has studied the energy and electron transfer mechanism of transition metal complexes from the ground and the excited state extensively, with a partial address of the catalytic photoredox cycles probably due to their complexity. Those studies are needed to develop new photoredox org. transformations further and make them more sustainable and energy-efficient. In this review, we outline an overview of selected basic concepts of photophysics and photochem. encountered in the photocatalytic cycles. Selected examples are detailed to illustrate how steady-state and time-resolved optical spectroscopy can be employed to elucidate catalytic intermediates and photocatalytic mechanisms. As such, this review aims to motivate mechanistic studies on photoredox catalysis and serves as a guide to perform them to develop more sustainable and energy-efficient chem. transformations.
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    Dedecker, P. Software review: Glotaran. Journal of Chemometrics 2014, 28, 137– 138,  DOI: 10.1002/cem.2596
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    Software review: Glotaran
    Dedecker, Peter
    Journal of Chemometrics (2014), 28 (3), 137-138CODEN: JOCHEU; ISSN:0886-9383. (John Wiley & Sons Ltd.)
    There is no expanded citation for this reference.
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    Mullen, K. M.; van Stokkum, I. H. M. TIMP: An R Package for Modeling Multi-Way Spectroscopic Measurements. Journal of Statistical Software 2007, 18, 1– 46,  DOI: 10.18637/jss.v018.i03
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    Uhlig, J. KiMoPack GitHub project page , release 6.6.2. 2022; https://github.com/erdzeichen/KiMoPack.
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    Uhlig, J. KiMoPack - Open source tool for the analysis of transient spectral data, version 6.6.2. Zenodo 2022,  DOI: 10.5281/zenodo.6049186
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    Uhlig, J. KiMoPack Anaconda installation (using conda package manager) version 6.6.2. 2022; https://conda.anaconda.org/erdzeichen.
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    Uhlig, J. KiMoPack PyPi installation (using pip package manager) version 6.6.2. 2022; https://pypi.org/project/KiMoPack/.
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    Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io . 2022; https://kimopack.readthedocs.io/en/latest.
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    Uhlig, J. KiMoPack Youtube Tutorial Channel . 2022; https://www.youtube.com/channel/UCmhiK0P9wXXjs_PJaitx8BQ.
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    Weißborn, J.; Snellenburg, J.; Weigand, S.; Van Stokkum, I. H. pyglotaran: a Python library for global and target analysis, version v0.5. Zenodo 2021,  DOI: 10.5281/zenodo.4534043
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    Hniopek, J.; Müller, C.; Bocklitz, T.; Schmitt, M.; Dietzek, B.; Popp, J. Kinetic-Model-Free Analysis of Transient Absorption Spectra Enabled by 2D Correlation Analysis. J. Phys. Chem. Lett. 2021, 12, 4148– 4153,  DOI: 10.1021/acs.jpclett.1c00835
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    37
    Kinetic-Model-Free Analysis of Transient Absorption Spectra Enabled by 2D Correlation Analysis
    Hniopek, Julian; Mueller, Carolin; Bocklitz, Thomas; Schmitt, Michael; Dietzek, Benjamin; Popp, Juergen
    Journal of Physical Chemistry Letters (2021), 12 (17), 4148-4153CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Here, we present, to the best of our knowledge for the first time, a systematic study of utilizing 2D correlation anal. in the field of femtosecond transient absorption (fs-TA) spectroscopy. We present that the application of 2D correlation spectroscopy (2DCOS) to fs-TA spectroscopy enables a model-free means to analyze excited state kinetics, which is demonstrated on the model system [(tbbpy)2Ru(dppz)]2+ in different solvents. We show that TA-2DCOS is able to det. the no. of processes contributing to the time-resolved spectral changes in fs-TA data sets, as well as ext. the spectral response of these components. Overall, the results show that TA-2DCOS leads to the same results as obtained with methods relying on global lifetime anal. or multivariate curve resoln. but without the need to specify a predetd. kinetic model. The work presented therefore highlights the potential of TA-2DCOS as a model-free approach for analyzing fs-TA spectral data sets.
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  38. 38
    Véry, T.; Ambrosek, D.; Otsuka, M.; Gourlaouen, C.; Assfeld, X.; Monari, A.; Daniel, C. Photophysical Properties of Ruthenium(II) Polypyridyl DNA Intercalators: Effects of the Molecular Surroundings Investigated by Theory. Chem. Eur. J. 2014, 20, 12901– 12909,  DOI: 10.1002/chem.201402963
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    38
    Photophysical Properties of Ruthenium(II) Polypyridyl DNA Intercalators: Effects of the Molecular Surroundings Investigated by Theory
    Very, Thibaut; Ambrosek, David; Otsuka, Miho; Gourlaouen, Christophe; Assfeld, Xavier; Monari, Antonio; Daniel, Chantal
    Chemistry - A European Journal (2014), 20 (40), 12901-12909CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The environmental effects on the structural and photophys. properties of [Ru(L)2(dppz)]2+ complexes (L=bpy=2,2'-bipyridine, phen=1,10-phenanthroline, tap=1,4,5,8-tetraazaphenanthrene; dppz=dipyrido[3,3-a:2',3'-c]phenazine), used as DNA intercalators, have been studied by means of DFT, time-dependent DFT, and quantum mechanics/mol. mechanics calcns. The electronic characteristics of the low-lying triplet excited states in water, acetonitrile, and DNA have been investigated to decipher the influence of the environment on the luminescent behavior of this class of mols. The lowest triplet intra-ligand (IL) excited state calcd. at λ≈800 nm for the three complexes and localized on the dppz ligand is not very sensitive to the environment and is available for electron transfer from a guanine nucleobase. Whereas the lowest triplet metal-to-ligand charge-transfer (3MLCT) states remain localized on the ancillary ligand (tap) in [Ru(tap)2(dppz)]2+, regardless of the environment, their character is drastically modified in the other complexes [Ru(phen)2(dppz)]2+ and [Ru(bpy)2(dppz)]2+ upon going from acetonitrile (MLCTdppz/phen or MLCTdppz/bpy) to water (MLCTdppz) and DNA (MLCTphen and MLCTbpy). The change in the character of the low-lying 3MLCT states accompanying nuclear relaxation in the excited state controls the emissive properties of the complexes in water, acetonitrile, and DNA. The light-switching effect has been rationalized on the basis of environment-induced control of the electronic d. distributed in the lowest triplet excited states.
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  39. 39
    Friedman, A. E.; Chambron, J. C.; Sauvage, J. P.; Turro, N. J.; Barton, J. K. A molecular light switch for DNA: Ru(bpy)2(dppz)2+. J. Am. Chem. Soc. 1990, 112, 4960– 4962,  DOI: 10.1021/ja00168a052
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    A molecular light switch for DNA: Ru(bpy)2(dppz)2+
    Friedman, Alan E.; Chambron, Jean Claude; Sauvage, Jean Pierre; Turro, Nicholas J.; Barton, Jacqueline K.
    Journal of the American Chemical Society (1990), 112 (12), 4960-2CODEN: JACSAT; ISSN:0002-7863.
    The application of a novel transition metal complex as a sensitive spectroscopic probe for DNA is reported. The complex, Ru(bpy)2(dppz)2+ (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2: a-2',3':c]phenazine), shows no detectable photoluminescence in aq. soln. at ambient temps., but in the presence of double helical DNA, to which the complex binds avidly, intense photoluminescence is obsd. In the presence of 100 μM calf thymus DNA, single photon counting expts. at 25° reveal a biexponential decay of emission with a short lived component of 75 ns and a longer lived component of 259 nsec. With B-form poly d(GC)·d(GC), emission is centered at 628 nm (λexc = 482 nm) and is comparable in intensity to that found in isopropanol (where Φ ≥ 0.02) in the absence of DNA. With Z-form poly d(GC)·d(GC) the relative intensity of steady state luminescence is even greater than that found with B-DNA and the emission max. is shifted to 640 nm. Only weak emission is apparent in the presence of A-form poly r(AU)·r(AU) with emission centered at 650 nm. The enhancement factor in aq. soln. at ambient temp. for photoluminescence of Ru(bpy)2(dppz)2+ in the presence of B-DNA is estd. to be >104. The complex should be useful as a sensitive, non-radioactive, luminescent DNA probe in both heterogenous and homogeneous assays.
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    Smith, J. A.; George, M. W.; Kelly, J. M. Transient spectroscopy of dipyridophenazine metal complexes which undergo photo-induced electron transfer with DNA. Coord. Chem. Rev. 2011, 255, 2666– 2675,  DOI: 10.1016/j.ccr.2011.04.007
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    Transient spectroscopy of dipyridophenazine metal complexes which undergo photo-induced electron transfer with DNA
    Smith, Jayden A.; George, Michael W.; Kelly, John M.
    Coordination Chemistry Reviews (2011), 255 (21-22), 2666-2675CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    This review considers transient spectroscopic studies of electron transfer reactions between nucleic acids and the excited states of transition metal complexes contg. dipyridophenazine or related ligands and focuses mainly on complexes of ruthenium, chromium and rhenium. Particular emphasis is placed on systems where transient UV/visible and/or IR absorption spectroscopy have been employed.
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    Keane, P. M.; Kelly, J. M. Transient absorption and time-resolved vibrational studies of photophysical and photochemical processes in DNA-intercalating polypyridyl metal complexes or cationic porphyrins. Coord. Chem. Rev. 2018, 364, 137– 154,  DOI: 10.1016/j.ccr.2018.02.018
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    Transient absorption and time-resolved vibrational studies of photophysical and photochemical processes in DNA-intercalating polypyridyl metal complexes or cationic porphyrins
    Keane, Paraic M.; Kelly, John M.
    Coordination Chemistry Reviews (2018), 364 (), 137-154CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Recent advances in the use of transient absorption (TA) and time-resolved vibrational spectroscopies (TRIR and TR3) to study both excited states and reaction intermediates in metal complexes and porphyrins which intercalate into DNA are reviewed. A particularly well-studied class of compds., which nicely illustrates the comparative advantages of these techniques, is that of ruthenium dppz complexes where the complex might show light-switching or photo-oxidising behavior depending on the nature of the ancillary ligand. Comparative data on Re- and Cr-dppz complexes are also considered. In the second part of this review transient studies of porphyrins, which are known to intercalate into DNA, are considered with particular emphasis on tetramethyl-pyridiniumporphyrins, where the photophys. behavior of the metal-free and various metal derivs. are compared.
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    Brennaman, M. K.; Meyer, T. J.; Papanikolas, J. M. [Ru(bpy) 2 dppz] 2+ Light-Switch Mechanism in Protic Solvents as Studied through Temperature-Dependent Lifetime Measurements. J. Phys. Chem. A 2004, 108, 9938– 9944,  DOI: 10.1021/jp0479670
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    [Ru(bpy)2dppz]2+ Light-Switch Mechanism in Protic Solvents as Studied through Temperature-Dependent Lifetime Measurements
    Brennaman, Matthew K.; Meyer, Thomas J.; Papanikolas, John M.
    Journal of Physical Chemistry A (2004), 108 (45), 9938-9944CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Temp.-dependent excited-state lifetime measurements have been performed on four different Ru(II)-based dppz compds. in protic and aprotic solvents. This work supports the existence of a dynamic equil. between two MLCT states assocd. with the dppz ligand: one is a bright state with a ligand orbital similar in size to that assocd. with the 3MLCT state of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridyl), and the other is a dark phenazine-like state. The authors results are consistent with a light-switch mechanism involving a competition between energetic factors that favor the dark (phz) state and entropic factors that favor the bright (bpy) state. This paper explores the photophysics of these light-switch compds. through a systematic variation of the equil. energetics. This is accomplished by (1) varying the dielec. strength of the solvent and (2) making chem. substitutions on the dipyridophenazine (dppz) ligand. Observations obtained from all four compds. in six different solvents can be explained using this equil. model.
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    Brennaman, M. K.; Alstrum-Acevedo, J. H.; Fleming, C. N.; Jang, P.; Meyer, T. J.; Papanikolas, J. M. Turning the [Ru(bpy) 2 dppz] 2+ Light-Switch On and Off with Temperature. J. Am. Chem. Soc. 2002, 124, 15094– 15098,  DOI: 10.1021/ja0279139
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    Turning the [Ru(bpy)2dppz]2+ Light-Switch On and Off with Temperature
    Brennaman, Matthew K.; Alstrum-Acevedo, James H.; Fleming, Cavan N.; Jang, Paul; Meyer, Thomas J.; Papanikolas, John M.
    Journal of the American Chemical Society (2002), 124 (50), 15094-15098CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors report temp.-dependent excited-state lifetime measurements on [Ru(bpy)2dppz]2+ in both protic and aprotic solvents. These expts. yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Measurements support the notion of bpy-like and phz-like states assocd. with the dppz ligand and show that the ligand orbital assocd. with the bright state is similar in size to the corresponding orbital in the 3MLCT state of [Ru(bpy)3]2+. In contrast to the current thinking, the expts. presented here indicate that the light-switch effect is not driven by a state reversal. Rather, probably the dark state is always lowest in energy, even in aprotic solvents, and the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state.
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    Olson, E. J. C.; Hu, D.; Hörmann, A.; Jonkman, A. M.; Arkin, M. R.; Stemp, E. D. A.; Barton, J. K.; Barbara, P. F. First Observation of the Key Intermediate in the “Light-Switch” Mechanism of [Ru(phen) 2 dppz] 2+. J. Am. Chem. Soc. 1997, 119, 11458– 11467,  DOI: 10.1021/ja971151d
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    First Observation of the Key Intermediate in the "Light-Switch" Mechanism of [Ru(phen)2dppz]2+
    Olson, E. J. C.; Hu, D.; Hoermann, A.; Jonkman, A. M.; Arkin, M. R.; Stemp, E. D. A.; Barton, J. K.; Barbara, P. F.
    Journal of the American Chemical Society (1997), 119 (47), 11458-11467CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    [Ru(phen)2dppz]2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and closely related complexes have previously been obsd. to have an undetectably small quantum yield of photoluminescence in water but a moderate emission yield when bound to DNA. This so-called "light-switch" effect is a crit. factor in the utility of these complexes as spectroscopic probes for DNA. Here we describe a detailed investigation of the photophysics of [Ru(phen)2dppz]2+ in aq. soln., and in mixts. of acetonitrile and water, by time-resolved absorption and emission spectroscopies. The emission of the complex in water has been measured for the first time. A prompt initial emission, derived from a metal-to-ligand charge-transfer (MLCT) excited state typical for polypyridyl-ruthenium complexes, is obsd. along with a delayed emission attributed to a novel MLCT species. The small quantum yield of photoluminescence for [Ru(phen)2dppz]2+ in water, and in water/acetonitrile, depends upon efficient formation of a novel MLCT species, followed by its rapid radiationless decay. The MLCT interconversion is assigned to an intramol. charge-transfer process that is induced by the polarity and proton donating ability of the solvent.
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    Olofsson, J.; Önfelt, B.; Lincoln, P. Three-State Light Switch of [Ru(phen) 2 dppz] 2+: Distinct Excited-State Species with Two, One, or No Hydrogen Bonds from Solvent. J. Phys. Chem. A 2004, 108, 4391– 4398,  DOI: 10.1021/jp037967k
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    45
    Three-State Light Switch of [Ru(phen)2dppz]2+: Distinct Excited-State Species with Two, One, or No Hydrogen Bonds from Solvent
    Olofsson, Johan; Oenfelt, Bjoern; Lincoln, Per
    Journal of Physical Chemistry A (2004), 108 (20), 4391-4398CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The ruthenium complexes of dppz (dipyrido[3,2-a:2',3'-c]phenazine) have found wide interest due to their environment-sensitive luminescence and are used, for example, as spectroscopic probes for DNA. The deactivation process for the excited state of the "light-switch" complex [Ru(phen)2dppz]2+ (phen = 1,10-phenanthroline) has been studied in water, glycerol, ethylene glycol, and 1,2- and 1,3-propanediol by using fluorescence spectroscopy and single photon counting. In all solvents anomalous temp. dependence is found (increasing quantum yield and excited-state lifetime with increasing temp.). Model-independent anal. shows that only two emissive species, with solvent- and temp.-invariant emission spectral profiles, are sufficient to account for all the data in the polyol solvents. Van't Hoff plots of the ratio of the two species are linear at higher temps. in all solvents, indicating rapid thermal equilibration of the two species, except for lower temps. in the most viscous solvent glycerol. Kinetic modeling of the system with microscopic rate consts. with pos. Arrhenius activation energies requires a third nonemissive species, which is assigned to an excited state with two hydrogen bonds from the solvent, whereas the first two species are assigned to the mono-hydrogen-bonded and non-hydrogen-bonded excited-state species. This assignment is supported by the observation of a growing luminescence intensity as temp. is increased, but no wavelength shift, of high-purity [Ru(phen)2dppz]2+ in water soln.
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    Kuhnt, C.; Karnahl, M.; Tschierlei, S.; Griebenow, K.; Schmitt, M.; Schäfer, B.; Krieck, S.; Görls, H.; Rau, S.; Dietzek, B. Substitution-controlled ultrafast excited-state processes in Ru-dppz-derivatives. Phys. Chem. Chem. Phys. 2010, 12, 1357– 1368,  DOI: 10.1039/B915770K
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    Substitution-controlled ultrafast excited-state processes in Ru-dppz-derivatives
    Kuhnt, Christian; Karnahl, Michael; Tschierlei, Stefanie; Griebenow, Kristin; Schmitt, Michael; Schaefer, Bernhard; Krieck, Sven; Goerls, Helmar; Rau, Sven; Dietzek, Benjamin; Popp, Juergen
    Physical Chemistry Chemical Physics (2010), 12 (6), 1357-1368CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Ru-dppz (dppz = dipyrido[3,2-a:2',3,3'-c]phenazine) complexes play an important role as environmentally sensitive luminescence sensors and building blocks for larger supramol. compds. Their photophys. properties are known to be highly sensitive to intermol. solvent-solute interactions and solvent bulk-properties. The synthesis and characterization of a novel Ru-dppz deriv. is reported. The potential of drastically tuning the photophys. properties of such complexes is exemplified, by introducing very simple structural modifications, namely bromine, into the dppz-ligand scaffold. The photophysics i.e. nature of excited states and the excited-state relaxation pathway of the various complexes has been investigated by means of electrochem. measurements, steady-state emission expts. and femtosecond time-resolved spectroscopy. It could be shown that the location of bromine substitution influences the relative energy between a luminescent and a non-luminescent metal-to-ligand charge-transfer state and therefore quenches or facilitates transitions between both. Hence it is illustrated that the luminescent properties and the underlying ultrafast excited-state dynamics of the complexes can be controlled by structural variations, i.e. by intramol. interactions as opposed to changes in the intermol. interactions.
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    Pourtois, G.; Beljonne, D.; Moucheron, C.; Schumm, S.; Kirsch-De Mesmaeker, A.; Lazzaroni, R.; Brédas, J.-L. Photophysical Properties of Ruthenium(II) Polyazaaromatic Compounds: A Theoretical Insight. J. Am. Chem. Soc. 2004, 126, 683– 692,  DOI: 10.1021/ja034444h
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    Photophysical Properties of Ruthenium(II) Polyazaaromatic Compounds: A Theoretical Insight
    Pourtois, Geoffrey; Beljonne, David; Moucheron, Cecile; Schumm, Stephan; Kirsch-De Mesmaeker, Andree; Lazzaroni, Roberto; Bredas, Jean-Luc
    Journal of the American Chemical Society (2004), 126 (2), 683-692CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Quantum-chem. methods are applied to study the nature of the excited states relevant in the photophys. processes (absorption and emission) of a series of polyazaarom.-ligand-based ruthenium(II) complexes. The electronic and optical properties of the free polyazaarom. ligands and their corresponding ruthenium(II) complexes are detd. on the basis of correlated Hartree-Fock semiempirical approaches. While the emission of complexes contg. small-size ligands, such as 1,10-phenanthroline or 2,2'-bipyridine, arises from a manifold of metal-to-ligand charge-transfer triplet states (3MLCTs), an addnl. ligand-centered triplet state (3L) is identified in the triplet manifold of complexes contg. a π-extended ligand such as dipyrido[3,2-a:2',3'-c]phenazine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene. Recent exptl. data are interpreted in light of these theor. results; namely, the origin for the abnormal solvent- and temp.-dependent emission measured in π-extended Ru complexes is revisited.
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    Olofsson, J.; Wilhelmsson, L. M.; Lincoln, P. Effects of Methyl Substitution on Radiative and Solvent Quenching Rate Constants of [Ru(phen) 2 dppz] 2+ in Polyol Solvents and Bound to DNA. J. Am. Chem. Soc. 2004, 126, 15458– 15465,  DOI: 10.1021/ja047166a
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    Effects of Methyl Substitution on Radiative and Solvent Quenching Rate Constants of [Ru(phen)2dppz]2+ in Polyol Solvents and Bound to DNA
    Olofsson, Johan; Wilhelmsson, L. Marcus; Lincoln, Per
    Journal of the American Chemical Society (2004), 126 (47), 15458-15465CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Me substituents on the distant benzene ring of the dppz ligand in the "light switch" complex [Ru(phen)2dppz]2+ have profound effects on the photophysics of the complexes in water as well as in the polyol solvents ethylene glycol, glycerol, and 1,2- and 1,3-propanediol. Whereas 11,12-dimethyl substitution decreases the rate of quenching by diminishing hydrogen bonding by solvent, the 10-Me substituent in addn. also decreases both the radiative and the nonradiative rate const. for decay to the ground state of the non-hydrogen-bonded excited state species. For both the 10-Me and the 11,12-dimethyl derivs., the effect of Me substitution on the equil. of solvent hydrogen bonding to the excited state is due to changes in the entropy terms, rather than in the enthalpy, indicating that the effect is a steric perturbation of the solvent cage around the mol. When intercalated into DNA, the effects of Me substitution is smaller than those in polyol solvent or water, suggesting that the water mols. that quench the excited state by hydrogen bonding to the phenazine aza nitrogens mainly access them from the same groove as in which the Ru(II) ion resides. Since the Δ-enantiomer of [Ru(phen)210-methyl-dppz]2+ has an abs. quantum yield of up to 0.23 when bound to DNA, a value 7000 times higher than in pure water soln., it is promising as a new luminescent DNA probe.
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    Önfelt, B.; Olofsson, J.; Lincoln, P.; Nordén, B. Picosecond and Steady-State Emission of [Ru(phen) 2 dppz] 2+ in Glycerol: Anomalous Temperature Dependence. J. Phys. Chem. A 2003, 107, 1000– 1009,  DOI: 10.1021/jp0269266
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    Dietzek, B.; Pascher, T.; Sundström, V.; Yartsev, A. Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design. Laser Physics Letters 2007, 4, 38– 43,  DOI: 10.1002/lapl.200610070
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    Dobryakov, A. L.; Kovalenko, S. A.; Ernsting, N. P. Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution. J. Chem. Phys. 2005, 123, 044502,  DOI: 10.1063/1.1948383
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    Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution
    Dobryakov, A. L.; Kovalenko, S. A.; Ernsting, N. P.
    Journal of Chemical Physics (2005), 123 (4), 044502/1-044502/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    A unified description is presented of sequential and coherent contributions to optical transient absorption measured by femtosecond pump-supercontinuum probe spectroscopy. All inherent transient terms are taken into account. The "coherence spike" seen during pump-probe overlap is thereby decompd. into its components. The method is demonstrated with rhodamine 110 in methanol. Pure homogeneous dephasing times are obtained from a simultaneous fit of all pertinent measurements. Vibronic structure in the coherence spectrum is assigned to stimulated Raman scattering between vibrational levels in the first excited electronic state. The time-zero spectrum for stimulated emission and the solvation relaxation function are also obtained.
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    Uhlig, J. KiMoPack v.6.6.2 Documentation on “ReadTheDocs.io”, Section Shaping ; 2022; https://kimopack.readthedocs.io/en/latest/Shaping.html.
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    Virtanen, P.; Gommers, R.; Oliphant, T. E.; Haberland, M.; Reddy, T.; Cournapeau, D.; Burovski, E.; Peterson, P.; Weckesser, W.; Bright, J. SciPy 1.0: Fundamental Algorithms for Scientific Computing in Python. Nat. Methods 2020, 17, 261– 272,  DOI: 10.1038/s41592-019-0686-2
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    SciPy 1.0: fundamental algorithms for scientific computing in Python
    Virtanen, Pauli; Gommers, Ralf; Oliphant, Travis E.; Haberland, Matt; Reddy, Tyler; Cournapeau, David; Burovski, Evgeni; Peterson, Pearu; Weckesser, Warren; Bright, Jonathan; van der Walt, Stefan J.; Brett, Matthew; Wilson, Joshua; Millman, K. Jarrod; Mayorov, Nikolay; Nelson, Andrew R. J.; Jones, Eric; Kern, Robert; Larson, Eric; Carey, C. J.; Polat, Ilhan; Feng, Yu; Moore, Eric W.; Vander Plas, Jake; Laxalde, Denis; Perktold, Josef; Cimrman, Robert; Henriksen, Ian; Quintero, E. A.; Harris, Charles R.; Archibald, Anne M.; Ribeiro, Antonio H.; Pedregosa, Fabian; van Mulbregt, Paul
    Nature Methods (2020), 17 (3), 261-272CODEN: NMAEA3; ISSN:1548-7091. (Nature Research)
    Abstr.: SciPy is an open-source scientific computing library for the Python programming language. Since its initial release in 2001, SciPy has become a de facto std. for leveraging scientific algorithms in Python, with over 600 unique code contributors, thousands of dependent packages, over 100,000 dependent repositories and millions of downloads per yr. In this work, we provide an overview of the capabilities and development practices of SciPy 1.0 and highlight some recent tech. developments.
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    Henrich, J. D.; Zhang, H.; Dutta, P. K.; Kohler, B. Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand. J. Phys. Chem. B 2010, 114, 14679– 14688,  DOI: 10.1021/jp102776r
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    Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand
    Henrich, Joseph D.; Zhang, Haoyu; Dutta, Prabir K.; Kohler, Bern
    Journal of Physical Chemistry B (2010), 114 (45), 14679-14688CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The excited-state dynamics of two mixed-ligand mononuclear Ru(II) complexes, [(bpy)2RuLDQ]4+ (bpy = 2,2'-bipyridine, LDQ = 1-[4-(4'-methyl)-2,2'-bipyridyl]-2-[4-(4'-N,N'-tetramethylene-2,2'-bipyridinium)]), and [(bpy)2RuL]2+ (L = 1, 2-bis[4-(4'-methyl)-2,2'-bipyridyl]ethene), were investigated by femtosecond transient absorption spectroscopy. Photoexcitation of the [(bpy)2RuLDQ]4+ complex at three sep. pump wavelengths leads to a common charge-sepd. state consisting of Ru3+ and an excited electron delocalized over the extended π-system centered on the ethenyl moiety of the LDQ ligand. In [(bpy)2RuL]2+, the excited electron is unable to delocalize throughout the π system and remains on the bipyridyl end of ligand L closest to the ruthenium atom. Vibrational cooling in the charge-sepd. state of [(bpy)2RuLDQ]4+ indicates that this state is formed faster than excess energy can be dispersed to the solvent and orders of magnitude more rapidly than in previously studied ruthenium-diquat or Ru-viologen dyads with nonconjugated linkers.
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  60. 60
    Greenough, S. E.; Roberts, G. M.; Smith, N. A.; Horbury, M. D.; McKinlay, R. G.; Żurek, J. M.; Paterson, M. J.; Sadler, P. J.; Stavros, V. G. Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine) 2 (nicotinamide) 2 ] 2+: experimental and theoretical insight into its photoactivation mechanism. Phys. Chem. Chem. Phys. 2014, 16, 19141– 19155,  DOI: 10.1039/C4CP02359E
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    60
    Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine)2(nicotinamide)2]2+: experimental and theoretical insight into its photoactivation mechanism
    Greenough, Simon E.; Roberts, Gareth M.; Smith, Nichola A.; Horbury, Michael D.; McKinlay, Russell G.; Zurek, Justyna M.; Paterson, Martin J.; Sadler, Peter J.; Stavros, Vasilios G.
    Physical Chemistry Chemical Physics (2014), 16 (36), 19141-19155CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Mechanistic insight into the photo-induced solvent substitution reaction of cis-[Ru(bipyridine)2(nicotinamide)2]2+ (1) is presented. Complex 1 is a photoactive species, designed to display high cytotoxicity following irradn., for potential use in photodynamic therapy (photochemotherapy). In Ru(II) complexes of this type, efficient population of a dissociative triplet metal-centered (3MC) state is key to generating high quantum yields of a penta-coordinate intermediate (PCI) species, which in turn may form the target species: a mono-aqua photoproduct [Ru(bipyridine)2(nicotinamide)(H2O)]2+ (2). Following irradn. of 1, a thorough kinetic picture is derived from ultrafast UV/visible transient absorption spectroscopy measurements, using a 'target anal.' approach, and provides both timescales and quantum yields for the key processes involved. We show that photoactivation of 1 to 2 occurs with a quantum yield ≥0.36, all within a time-frame of ∼400 ps. Characterization of the excited states involved, particularly the nature of the PCI and how it undergoes a geometry relaxation to accommodate the water ligand, which is a keystone in the efficiency of the photoactivation of 1, is accomplished through state-of-the-art computation including complete active space SCF methods and time-dependent d. functional theory. Importantly, the conclusions here provide a detailed understanding of the initial stages involved in this photoactivation and the foundation required for designing more efficacious photochemotherapy drugs of this type.
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    Oviedo, P. S.; Baraldo, L. M.; Cadranel, A. Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry. Proc. Natl. Acad. Sci. U. S. A. 2021, 118, e2018521118  DOI: 10.1073/pnas.2018521118
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    Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry
    Oviedo, Paola S.; Baraldo, Luis M.; Cadranel, Alejandro
    Proceedings of the National Academy of Sciences of the United States of America (2021), 118 (4), e2018521118CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The concept that differential wave function overlap between excited states can be engineered within a mol. chromophore is explored. The aim is to control excited state wave function symmetries, so that symmetry matches or mismatches result in differential orbital overlap and define low-energy trajectories or kinetic barriers within the excited state surface, that drive excited state population toward different reaction pathways. Two donor-acceptor assemblies were explored, where visible light absorption preps. excited states of different wave function symmetry. These states could be resolved using transient absorption spectroscopy, thanks to wave function symmetry-specific photoinduced optical transitions. One of these excited states undergoes energy transfer to the acceptor, while another undertakes a back-electron transfer to restate the ground state. This differential behavior is possible thanks to the presence of kinetic barriers that prevent excited state equilibration. This strategy can be exploited to avoid energy dissipation in energy conversion or photoredox catalytic schemes.
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  62. 62
    Berera, R.; van Stokkum, I. H. M.; Kodis, G.; Keirstead, A. E.; Pillai, S.; Herrero, C.; Palacios, R. E.; Vengris, M.; van Grondelle, R.; Gust, D. Energy Transfer, Excited-State Deactivation, and Exciplex Formation in Artificial Caroteno-Phthalocyanine Light-Harvesting Antennas. J. Phys. Chem. B 2007, 111, 6868– 6877,  DOI: 10.1021/jp071010q
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    Energy transfer, excited-state deactivation and exciplex formation in artificial carotene-phthalocyanine light-harvesting antennas
    Berera, Rudi; van Stokkum, Ivo H. M.; Kodis, Gerdenis; Keirstead, Amy E.; Pillai, Smitha; Herrero, Christian; Palacios, Rodrigo E.; Vengris, Mikas; van Grondelle, Rienk; Gust, Devens; Moore, Thomas A.; Moore, Ana L.; Kennis, John T. M.
    Journal of Physical Chemistry B (2007), 111 (24), 6868-6877CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Results from transient absorption spectroscopy on artificial light-harvesting dyads, made up of a Zn phthalocyanine (Pc) covalently linked to carotenoids with 9, 10 or 11 conjugated double bonds, referred to as dyads 1, 2 and 3, resp., are presented. Energy transfer and excited-state deactivation pathways, following excitation of the strongly allowed carotenoid S2 state as a function of the conjugation length, were studied. The S2 state rapidly relaxes to the S* and S1 states. In all systems, there was a new pathway of energy deactivation in the carotenoid manifold in which the S* state acts as an intermediate state in the S2 → S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state was obsd. that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only obsd. upon direct carotenoid excitation and is not fluorescent. In dyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet-singlet energy transfer to Pc, making the process efficient (> 90%), but for dyads 2 and 3, the S1 energy transfer channel is prohibited and only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a quencher of the phthalocyanine excited-state.
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    Venkatesh, Y.; Venkatesan, M.; Ramakrishna, B.; Bangal, P. R. Ultrafast Time-Resolved Emission and Absorption Spectra of meso -Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy. J. Phys. Chem. B 2016, 120, 9410– 9421,  DOI: 10.1021/acs.jpcb.6b05767
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    Ultrafast Time-Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy
    Venkatesh, Yeduru; Venkatesan, M.; Ramakrishna, B.; Bangal, Prakriti Ranjan
    Journal of Physical Chemistry B (2016), 120 (35), 9410-9421CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    A comprehensive study of ultrafast mol. relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and THF solvent mixt. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate consts. assocd. with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is obsd. at low-frequency mode, ca. 26 cm-1 region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful anal. of time profiles of up-converted signals at different emission wavelengths helps to reveal detail mol. dynamics. The obsd. lifetimes are as indicated: A very fast decay component with 80 ± 20 fs obsd. at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time consts. are due to intramol. vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, resp. The weighted av. of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An addnl. ∼20 ± 2 ps rise component is obsd. in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model anal. of TA data exclusively generate the true spectra of each excited species/state with their resp. lifetimes along with microscopic rate consts. assocd. with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are obsd. which are assocd. with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, resp., when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), resp., as expected. The TA results parallel the fluorescence up-conversion studies, and both the results not only compliment each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx, and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.
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    Müller, C.; Schwab, A.; Randell, N. M.; Kupfer, S.; Dietzek-Ivansic, B.; Chavarot-Kerlidou, M. A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents. Chem. Eur. J. 2022, 28, e202103882  DOI: 10.1002/chem.202103882
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    64
    A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents
    Mueller, Carolin; Schwab, Alexander; Randell, Nicholas M.; Kupfer, Stephan; Dietzek-Ivansic, Benjamin; Chavarot-Kerlidou, Murielle
    Chemistry - A European Journal (2022), 28 (18), e202103882CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophys. properties of the ruthenium photosensitizer [(bpy)2Ru(oxim-dppqp)]2+ (Ru), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent d. functional theory; the results are discussed in comparison to the parent [(bpy)2Ru(dppz)]2+ and [(bpy)2Ru(oxo-dppqp)]2+ complexes. In addn., this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equiv. on the reactive excited states.
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    Kaufmann, M.; Müller, C.; Cullen, A. A.; Brandon, M. P.; Dietzek, B.; Pryce, M. T. Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands. Inorg. Chem. 2021, 60, 760– 773,  DOI: 10.1021/acs.inorgchem.0c02765
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    Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands
    Kaufmann, Martin; Muller, Carolin; Cullen, Aoibhin A.; Brandon, Michael P.; Dietzek, Benjamin; Pryce, Mary T.
    Inorganic Chemistry (2021), 60 (2), 760-773CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradn. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophys. characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent d. functional theory calcns. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (1MLCT) and thiazole (tz)-induced intraligand charge-transfer (1ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered 3MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived 3ILCT state (τ ≈ 40-300 ns) with excess charge d. localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τEm,1 ≈ 40-300 ns and τEm,2 ≈ 100-200 ns, which are attributed to emission from the 3ILCT and 3MLCT manifolds, resp. A single-bond extension of the dipyrido[3,2-a:2',3'-c]phenazine ligand with thiazole units leads to changes in the electrochem. and photophys. properties of the corresponding ruthenium complexes. Dual emission is obsd., which is based on two possible pathways following photoexcitation; the first leads to a metal-based 3MLCT emission, and the second relaxes through a ligand-based 3ILCT into the ground state.
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    Müller, C.; Isakov, D.; Rau, S.; Dietzek, B. Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands. J. Phys. Chem. A 2021, 125, 5911– 5921,  DOI: 10.1021/acs.jpca.1c03856
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    Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands
    Mueller, Carolin; Isakov, Dajana; Rau, Sven; Dietzek, Benjamin
    Journal of Physical Chemistry A (2021), 125 (27), 5911-5921CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Ruthenium(II) complexes, like [(tbbpy)2Ru(dppz)]2+ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there was ongoing interest in the design of π-extended Ru-dppz systems with red shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, the authors explore the photophys. properties of protonation isomers of the linearly π-extended [(tbbpy)2Ru(L)]2+-type complexes bearing a dppz ligand with directly fused imidazole (Im) and methyl-imidazole units (mim) as L. Steady-state UV-visible absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-Im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the 3ILCT and 3MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-) imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.
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    Chábera, P.; Lindh, L.; Rosemann, N. W.; Prakash, O.; Uhlig, J.; Yartsev, A.; Wärnmark, K.; Sundström, V.; Persson, P. Photofunctionality of iron(III) N-heterocyclic carbenes and related d transition metal complexes. Coord. Chem. Rev. 2021, 426, 213517,  DOI: 10.1016/j.ccr.2020.213517
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    Photofunctionality of iron(III) N-heterocyclic carbenes and related d5 transition metal complexes
    Chabera, Pavel; Lindh, Linnea; Rosemann, Nils W.; Prakash, Om; Uhlig, Jens; Yartsev, Arkady; Waernmark, Kenneth; Sundstroem, Villy; Persson, Petter
    Coordination Chemistry Reviews (2021), 426 (), 213517CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A Review. Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d5 electronic configuration, the photophysics and photochem. of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophys. and photochem. properties assocd. with low-spin (doublet) ligand-to-metal charge transfer (2LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidn. and photo-redn. reactions. These encouraging results are at first sight surprising in light of the general scarcity of known photofunctional complexes of any transition metal complexes with a d5 electronic configuration, including 1st, 2nd and 3rd row transition metal complexes of Mn(II), Tc(II), Re(II), Fe(III), Ru(III) and Os(III). Here, we review the photophys. and photochem. properties of the new Fe(III) NHC complexes together with related d5 transition metal complexes as a basis for a broader understanding of the unorthodox photophys. and photochem. properties assocd. with this open-shell electronic configuration. This includes considerations of the role of charge and spin effects on the ground state electronic structure, as well as discussions of charge transfer (CT) and metal centered (MC) excited state properties.
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    Rein, C.; Uhlig, J.; Carrasco-Busturia, D.; Khalili, K.; Gertsen, A. S.; Moltke, A.; Zhang, X.; Katayama, T.; Lastra, J. M. G.; Nielsen, M. M. Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution. Structural Dynamics 2021, 8, 024501,  DOI: 10.1063/4.0000055
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    Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution
    Rein, Christian; Uhlig, Jens; Carrasco-Busturia, David; Khalili, Khadijeh; Gertsen, Anders S.; Moltke, Asbjoern; Zhang, Xiaoyi; Katayama, Tetsuo; Lastra, Juan Maria Garcia; Nielsen, Martin Meedom; Adachi, Shin-Ichi; Haldrup, Kristoffer; Andreasen, Jens Wenzel
    Structural Dynamics (2021), 8 (2), 024501CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
    Ultrafast, light-induced dynamics in copper-zinc-tin-sulfide (CZTS) photovoltaic nanoparticles are investigated through a combination of optical and x-ray transient absorption spectroscopy. Laser-pump, x-ray-probe spectroscopy on a colloidal CZTS nanoparticle ink yields element-specificity, which reveals a rapid photo-induced shift of electron d. away from Cu-sites, affecting the MO occupation and structure of CZTS. We observe the formation of a stable charge-sepd. and thermally excited structure, which persists for nanoseconds and involves an increased charge d. at the Zn sites. Combined with d. functional theory calcns., the results provide new insight into the structural and electronic dynamics of CZTS absorbers for solar cells. (c) 2021 American Institute of Physics.
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    Kaufhold, S.; Rosemann, N. W.; Chábera, P.; Lindh, L.; Bolaño Losada, I. B.; Uhlig, J.; Pascher, T.; Strand, D.; Wärnmark, K.; Yartsev, A. Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene–Co(III) Complex. J. Am. Chem. Soc. 2021, 143, 1307– 1312,  DOI: 10.1021/jacs.0c12151
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    Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex
    Kaufhold, Simon; Rosemann, Nils W.; Chabera, Pavel; Lindh, Linnea; Bolano Losada, Iria; Uhlig, Jens; Pascher, Torbjoern; Strand, Daniel; Waernmark, Kenneth; Yartsev, Arkady; Persson, Petter
    Journal of the American Chemical Society (2021), 143 (3), 1307-1312CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The photofunctionality of the cobalt-hexacarbene complex [Co(III)(PhB(MeIm)3)2]+ (PhB(MeIm)3 = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ~ 10-4) but remarkably long-lived (τ ~ 1μs) orange photoluminescence at 690 nm in soln. at room temp. following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chem. calcns. as a rare case of luminescence from a metal-centered state in a 3d6 complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.
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  70. 70
    Lindh, L.; Gordivska, O.; Persson, S.; Michaels, H.; Fan, H.; Chábera, P.; Rosemann, N. W.; Gupta, A. K.; Benesperi, I.; Uhlig, J. Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality. Chemical Science 2021, 12, 16035– 16053,  DOI: 10.1039/D1SC02963K
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    70
    Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality
    Lindh, Linnea; Gordivska, Olga; Persson, Samuel; Michaels, Hannes; Fan, Hao; Chabera, Pavel; Rosemann, Nils W.; Gupta, Arvind Kumar; Benesperi, Iacopo; Uhlig, Jens; Prakash, Om; Sheibani, Esmaeil; Kjaer, Kasper S.; Boschloo, Gerrit; Yartsev, Arkady; Freitag, Marina; Lomoth, Reiner; Persson, Petter; Waernmark, Kenneth
    Chemical Science (2021), 12 (48), 16035-16053CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A new generation of octahedral iron(II)-N-heterocyclic carbene (NHC) complexes, employing different tridentate Ĉ N̂ C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(II) center, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochem., and quantum chem. calcns. demonstrate the improved mol. excited state properties in terms of a broader absorption spectrum compared to the ref. complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I-/I3- redox mediator and std. operating conditions, outcompeting the corresponding DSSC based on the homoleptic ref. complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.
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  71. 71
    Lindh, L.; Chábera, P.; Rosemann, N. W.; Uhlig, J.; Wärnmark, K.; Yartsev, A.; Sundström, V.; Persson, P. Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis. Catalysts 2020, 10, 315,  DOI: 10.3390/catal10030315
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    Photophysics and photochemistry of iron carbene complexes for solar energy conversion and photocatalysis
    Lindh, Linnea; Chabera, Pavel; Rosemann, Nils W.; Uhlig, Jens; Warnmark, Kenneth; Yartsev, Arkady; Sundstrom, Villy; Persson, Petter
    Catalysts (2020), 10 (3), 315CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
    A review. Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compds. based on iron are esp. interesting, as iron is the most common transition metal element in the Earth's crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compds. with significantly improved photophys. and photochem. properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochem. of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochem. properties of iron carbenes and related complexes for photovoltaic, photoelectrochem. and photocatalytic applications.
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  72. 72
    Kjær, K. S.; VanDriel, T. B.; Harlang, T. C. B.; Kunnus, K.; Biasin, E.; Ledbetter, K.; Hartsock, R. W.; Reinhard, M. E.; Koroidov, S.; Li, L. Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy. Chemical Science 2019, 10, 5749– 5760,  DOI: 10.1039/C8SC04023K
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    72
    Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy
    Kjaer, Kasper S.; Van Driel, Tim B.; Harlang, Tobias C. B.; Kunnus, Kristjan; Biasin, Elisa; Ledbetter, Kathryn; Hartsock, Robert W.; Reinhard, Marco E.; Koroidov, Sergey; Li, Lin; Laursen, Mads G.; Hansen, Frederik B.; Vester, Peter; Christensen, Morten; Haldrup, Kristoffer; Nielsen, Martin M.; Dohn, Asmus O.; Papai, Matyas I.; Moeller, Klaus B.; Chabera, Pavel; Liu, Yizhu; Tatsuno, Hideyuki; Timm, Cornelia; Jarenmark, Martin; Uhlig, Jens; Sundstom, Villy; Warnmark, Kenneth; Persson, Petter; Nemeth, Zoltan; Szemes, Dorottya Sarosine; Bajnoczi, Eva; Vanko, Gyorgy; Alonso-Mori, Roberto; Glownia, James M.; Nelson, Silke; Sikorski, Marcin; Sokaras, Dimosthenis; Canton, Sophie E.; Lemke, Henrik T.; Gaffney, Kelly J.
    Chemical Science (2019), 10 (22), 5749-5760CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Light-driven mol. reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, exptl. approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 fs time-resoln. provide a solid exptl. foundation for detg. the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such expts., it becomes possible to identify the dominant trajectory followed during the excited state cascade and to det. the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2'-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered mol. systems involving 3d transition metals.
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  73. 73
    Skov, A. B.; Ree, N.; Gertsen, A. S.; Chabera, P.; Uhlig, J.; Lissau, J. S.; Nucci, L.; Pullerits, T.; Mikkelsen, K. V.; Brøndsted Nielsen, M. Excited-State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity. ChemPhotoChem. 2019, 3, 619– 629,  DOI: 10.1002/cptc.201900088
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    73
    Excited-State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity
    Skov, Anders B.; Ree, Nicolai; Gertsen, Anders S.; Chabera, Pavel; Uhlig, Jens; Lissau, Jonas S.; Nucci, Luigi; Pullerits, Tonu; Mikkelsen, Kurt V.; Brondsted Nielsen, Mogens; Solling, Theis I.; Hansen, Thorsten
    ChemPhotoChem (2019), 3 (8), 619-629CODEN: CHEMYH ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The gain and loss of aromaticity plays a key role in org. chem. and in the prediction of rate-detg. steps. Herein, we explore the concept of aromaticity in photoisomerization reactions. Benzannulated derivs. of the dihydroazulene-vinylheptafulvene (DHA-VHF) photoswitch were investigated using transient absorption spectroscopy and time-dependent d. functional theory to elucidate the effect of built-in aromaticity on the switching properties. We found that benzannulation hampered the switching ability by enhancing an already existing barrier on the excited state surface. This enhancement was found to arise from a significant loss of aromaticity in the DHA-to-VHF transition state on the excited state potential energy surface. The VHF was found to be highly arom. on the excited state surface, showing a reversal of aromaticity compared to the ground state. The barrier was found to be dependent on the position of benzannulation, since one deriv. was found to switch as fast as the non-benzannulated mol. although with lower efficiency, whereas another deriv. completely lost the ability to undergo reversible photoswitching. The findings herein provide novel principles for the design of mol. photoswitches, shedding new light on excited state aromaticity, as previous discussions have mainly considered excited state aromaticity to be beneficial to switching. Our findings show that this view must be reconsidered.
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  74. 74
    Chábera, P.; Kjær, K. S.; Prakash, O.; Honarfar, A.; Liu, Y.; Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundström, V. FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime. J. Phys. Chem. Lett. 2018, 9, 459– 463,  DOI: 10.1021/acs.jpclett.7b02962
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    FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime
    Chabera, Pavel; Kjaer, Kasper S.; Prakash, Om; Honarfar, Alireza; Liu, Yizhu; Fredin, Lisa A.; Harlang, Tobias C. B.; Lidin, Sven; Uhlig, Jens; Sundstroem, Villy; Lomoth, Reiner; Persson, Petter; Waernmark, Kenneth
    Journal of Physical Chemistry Letters (2018), 9 (3), 459-463CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN soln. that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidn. states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, resp., demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.
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  75. 75
    Kjær, K. S.; Kunnus, K.; Harlang, T. C. B.; Van Driel, T. B. V.; Ledbetter, K.; Hartsock, R. W.; Reinhard, M. E.; Koroidov, S.; Li, L.; Laursen, M. G. Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2’-bipyridine)(CN)4]2–. Phys. Chem. Chem. Phys. 2018, 20, 4238– 4249,  DOI: 10.1039/C7CP07838B
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    Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2'-bipyridine)(CN)4]2-
    Kjaer, Kasper S.; Kunnus, Kristjan; Harlang, Tobias C. B.; Van Driel, Tim B.; Ledbetter, Kathryn; Hartsock, Robert W.; Reinhard, Marco E.; Koroidov, Sergey; Li, Lin; Laursen, Mads G.; Biasin, Elisa; Hansen, Frederik B.; Vester, Peter; Christensen, Morten; Haldrup, Kristoffer; Nielsen, Martin M.; Chabera, Pavel; Liu, Yizhu; Tatsuno, Hideyuki; Timm, Cornelia; Uhlig, Jens; Sundstom, Villy; Nemeth, Zoltan; Szemes, Dorottya Sarosine; Bajnoczi, Eva; Vanko, Gyorgy; Alonso-Mori, Roberto; Glownia, James M.; Nelson, Silke; Sikorski, Marcin; Sokaras, Dimosthenis; Lemke, Henrik T.; Canton, Sophie E.; Warnmark, Kenneth; Persson, Petter; Cordones, Amy A.; Gaffney, Kelly J.
    Physical Chemistry Chemical Physics (2018), 20 (6), 4238-4249CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and x-ray emission transient spectroscopies, the authors have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 ps lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as DMSO and MeCN. The authors use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in H2O, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in <100 fs, forming a quasi-stable metal centered excited state with a 13 ps lifetime. This MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(II)-centered coordination compds., which form MC quintet (5MC) states. The solvent dependent changes in excited state nonradiative relaxation for [Fe(bpy)(CN)4]2- allows the authors to infer the influence of the solvent on the electronic structure of the complex. Also, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.
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    Kjær, K. S.; Kaul, N.; Prakash, O.; Chábera, P.; Rosemann, N. W.; Honarfar, A.; Gordivska, O.; Fredin, L. A.; Bergquist, K.-E.; Häggström, L. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 2019, 363, 249– 253,  DOI: 10.1126/science.aau7160
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    Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime
    Kjaer, Kasper Skov; Kaul, Nidhi; Prakash, Om; Chabera, Pavel; Rosemann, Nils W.; Honarfar, Alireza; Gordivska, Olga; Fredin, Lisa A.; Bergquist, Karl-Erik; Haeggstroem, Lennart; Ericsson, Tore; Lindh, Linnea; Yartsev, Arkady; Styring, Stenbjoern; Huang, Ping; Uhlig, Jens; Bendix, Jesper; Strand, Daniel; Sundstroem, Villy; Persson, Petter; Lomoth, Reiner; Waernmark, Kenneth
    Science (Washington, DC, United States) (2019), 363 (6424), 249-253CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Iron's abundance and rich coordination chem. are potentially appealing features for photochem. applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is phenyl[tris(3-methylimidazol-1-ylidene)]borate-, exhibits strong, visible, room temp. photoluminescence with a 2.0-ns lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were obsd. for the 2LMCT state of [Fe(phtmeimb)2]+ in bimol. quenching studies with methylviologen and diphenylamine.
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    ElNahhas, A.; Shameem, M. A.; Chabera, P.; Uhlig, J.; Orthaber, A. Synthesis and characterization of cyclopentadithiophene heterofulvenes - Design tools for light activated processes. Chem. Eur. J. 2017, 23, 5673– 5677,  DOI: 10.1002/chem.201700917
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    Pascher, T.; Chesick, J. P.; Winkler, J. R.; Gray, H. B. Protein Folding Triggered by Electron Transfer. Science 1996, 271, 1558– 1560,  DOI: 10.1126/science.271.5255.1558
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    Protein folding triggered by electron transfer
    Pascher, Torbjorn; Chesick, John P.; Winkler, Jay R.; Gray, Harry B.
    Science (Washington, D. C.) (1996), 271 (5255), 1558-60CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Rapid photochem. electron injection into unfolded ferricytochrome c titrated with 2.3 to 4.6 M guanidine hydrochloride (GuHCl) at pH 7 and 40° produced unfolded ferrocytochrome, which then converted to the folded protein. Two folding phases were obsd.: a fast process with a time const. of 40 μs (4.6 M GuHCl), and a slower phase with a rate const. of 90 per s (2.3 M GuHCl). The activation free energy for the slow step varied linearly with GuHCl concn.; the rate const., extrapolated to aq. soln., was 7600 per s. Electron-transfer methods can bridge the nanosecond to millisecond measurement time gap for protein folding.
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    De, S.; Pascher, T.; Maiti, M.; Jespersen, K. G.; Kesti, T.; Zhang, F.; Inganäs, O.; Yartsev, A.; Sundström, V. Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends. J. Am. Chem. Soc. 2007, 129, 8466– 8472,  DOI: 10.1021/ja068909q
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    Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends
    De, Swati; Pascher, Torbjoern; Maiti, Manisankar; Jespersen, Kim G.; Kesti, Tero; Zhang, Fengling; Inganaes, Olle; Yartsev, Arkady; Sundstroem, Villy
    Journal of the American Chemical Society (2007), 129 (27), 8466-8472CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    By measuring excited state and charge dynamics in blends of an alternating polyfluorene copolymer and fullerene deriv. over nine orders in time and two orders in light intensity, the light-induced processes were monitored from ultrafast charge photogeneration to much slower decay of charges by recombination. At low light intensities relevant to solar cell operation, relatively fast (∼30 ns) geminate recombination is the dominating charge decay process, while nongeminate recombination has a negligible contribution. Under solar illumination conditions, geminate recombination of charges may be directly competing with efficient charge collection in polymer/fullerene solar cells.
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    Newville, M.; Stensitzki, T.; Allen, D. B.; Ingargiola, A. LMFIT: Non-Linear Least-Square Minimization and Curve-Fitting for Python (1.0.3). Zenodo 2014,  DOI: 10.5281/zenodo.598352
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    Lasdon, L.; Duarte, A.; Glover, F.; Laguna, M.; Martí, R. Adaptive memory programming for constrained global optimization. Computers. & Operations Research 2010, 37, 1500– 1509,  DOI: 10.1016/j.cor.2009.11.006
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    Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io”, Section Plotting ; 2022; https://kimopack.readthedocs.io/en/latest/Plotting.html.
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    Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io”, Section Saving ; 2022; https://kimopack.readthedocs.io/en/latest/Saving.html.
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    Schindler, J.; Zhang, Y.; Traber, P.; Lefebvre, J.-F.; Kupfer, S.; Demeunynck, M.; Gräfe, S.; Chavarot-Kerlidou, M.; Dietzek, B. A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex. J. Phys. Chem. C 2018, 122, 83– 95,  DOI: 10.1021/acs.jpcc.7b08989
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    A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex
    Schindler, Julian; Zhang, Ying; Traber, Philipp; Lefebvre, Jean-Francois; Kupfer, Stephan; Demeunynck, Martine; Graefe, Stefanie; Chavarot-Kerlidou, Murielle; Dietzek, Benjamin
    Journal of Physical Chemistry C (2018), 122 (1), 83-95CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The π-extended dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand of the Ru(II) complex [Ru(bpy)2(oxo-dppqp)](PF6)2 (oxo-dppqp = dipyrido[3,2-a:2',3'-c]pyrido[2'',3''-4,5,6]quinolino[2,3-h]phenazin-15-one, bpy = 2,2'-bipyridine) enables the mononuclear complex for visible-light-driven accumulation of two electrons on a single ligand structure. Although this has been shown before, the excited-state physics underlying this promising feature are exploited in this work. The photophysics of the complex was investigated by excitation-wavelength-dependent resonance Raman and transient absorption spectroscopy in combination with time-dependent d. functional theory. The results show that excitation with visible light leads to the population of the two excited-state branches: (i) the population of a short-lived 3MLCT state in which the excess electronic d. is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and (ii) the population of a more long-lived 3ππ* state (τ = 9 ns). Notably, the long-lived 3ππ* state rather than a 3MLCT state is prone to reductive quenching by the sacrificial electron donor and, hence, presents the crit. excited-state intermediate in the photochem. charge accumulation expts.
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    Mengele, A. K.; Müller, C.; Nauroozi, D.; Kupfer, S.; Dietzek, B.; Rau, S. Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes. Inorg. Chem. 2020, 59, 12097– 12110,  DOI: 10.1021/acs.inorgchem.0c01022
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    85
    Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes
    Mengele, Alexander K.; Mueller, Carolin; Nauroozi, Djawed; Kupfer, Stephan; Dietzek, Benjamin; Rau, Sven
    Inorganic Chemistry (2020), 59 (17), 12097-12110CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Bi(benz)imidazoles (b(b)i.m.) acting as N,N-chelates in ruthenium complexes represent a unique class of ligands. They do not harbor MLCT excited states in Ru polypyridyl complexes upon visible light excitation provided that no substitution is introduced at the N atoms. Hence, they can be used to steer light-driven electron transfer pathways into a desired direction. Nonetheless, the free N atoms are susceptible to protonation and hence introduce highly pH-dependent properties into the complexes. Previous results for ruthenium complexes contg. R2bbim ligands with alkylic or arylic N,N'-substitution indicated that although pH insensitivity was accomplished unexpected loss of spectator ligand features incurred simultaneously. Here, the authors report the synthesis and photophys. characterization of a series of differently N,N'-alkylated b(b)i.m. ligands along with their corresponding [(tbbpy)2Ru(R2b(b)i.m.)](PF6)2 complexes (tbbpy = 4,4'-tert-butyl-2,2'-bipyridine). The data reveal that elongation of a rigid ethylene bridge by just one methylene group drastically increases the emission quantum yield, emission lifetime and photostability of the resultant complexes. Quantum chem. calcns. support these findings and allow the authors to rationalize the obsd. effects based on the energetic positions of the resp. excited states. The authors suggest that N,N'-propylene protected 1H,1'H-2,2'-biimidazole (prbim) is a suitable spectator ligand as it stabilizes sufficiently long-lived MLCT excited states exclusively localized at auxiliary bipyridine ligands. This ligand represents therefore a vital building block for next generation photochem. mol. devices in artificial photosynthesis. A spectator ligand appears on the horizon: Detailed spectroscopic techniques on a series of bi(benz)imidazole-based Ru polypyridine model compds. reveal a propylene protected biimidazole as suitable spectator ligand which allows to evade the dilemma of intertwined pH-sensitive photophysics and excited state localization.
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    Zhang, Y.; Traber, P.; Zedler, L.; Kupfer, S.; Gräfe, S.; Schulz, M.; Frey, W.; Karnahl, M.; Dietzek, B. Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system. Phys. Chem. Chem. Phys. 2018, 20, 24843– 24857,  DOI: 10.1039/C8CP04595J
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    Cu(I) vs. Ru(II) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system
    Zhang, Ying; Traber, Philipp; Zedler, Linda; Kupfer, Stephan; Graefe, Stefanie; Schulz, Martin; Frey, Wolfgang; Karnahl, Michael; Dietzek, Benjamin
    Physical Chemistry Chemical Physics (2018), 20 (38), 24843-24857CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Heteroleptic Cu(I) complexes are a promising alternative towards traditional Ru(II) photosensitizers. In particular, Cu(I) complexes of the type [Cu(P̂ P)(N̂ N)]+, where N̂ N represents a diimine and P̂ P a bulky diphosphine ligand, are already successfully applied for photocatalysis, org. light-emitting diodes or dye-sensitized solar cells. Therefore, this study aims for the systematic comparison of three novel heteroleptic Cu(I) compds., composed of xantphos (xant) as P̂ P ligand and different diimine ligands with an extended π-system in the backbone, with their structurally related Ru(II) analogs. In these Ru(II) photosensitizers [Ru(bpy)2(N̂ N)]2+ (bpy = 2,2'-bipyridine) the same N̂ N ligands were used, namely, dipyrido[3,2-f:2',3'-h]quinoxaline (dpq) and dipyrido[3,2-a:2',3'-c]phenazine (dppz). To gain an in-depth understanding of the photoinduced charge transfer processes, the photophys. features of these complexes and their electrochem. oxidized/reduced species were studied by a combination of UV-visible absorption, resonance Raman and spectroelectrochem. (TD)DFT calcns. were applied to qual. analyze these measurements. As a result, the heteroleptic Cu(I) complexes exhibit comparable charge transfer properties to their Ru(II) analogs, i.e. upon visible light excitation they undergo a metal-to-ligand charge transfer to the diimine ligand(s). In contrast, the reduced Cu(I)- and Ru(II)-dppz complexes show considerably different electronic transitions. The singly reduced Cu(I)-dppz complexes are able to accumulate an addnl. electron at the phenanthroline moiety upon blue-light excitation, which is beneficial for multi-electron-transfer reactions. Upon low-energy light irradn. electronic transitions from the dppz- anion to the xant ligand are excited, which could shorten the lifetime of the photosensitizer intermediates in an unwanted way.
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  • Abstract

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    Figure 1

    Figure 1. Schematic Jablonski diagrams for the photoinduced relaxation schemes of Ru-dppz in dichloromethane (DCM), acetonitrile (ACN), and water (H2O) upon MLCT excitation between 390 and 450 nm.

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    Figure 2

    Figure 2. Schematic representation of the processing structure and the three main workflow routines, i.e., the input (blue box) and standard optimization (green box), as well as collecting and plotting routines (orange box) KiMoPack. Individual operations (functions) and their returns are indicated by bright gray and colored boxes, respectively.

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    Figure 3

    Figure 3. Singular value decomposition of the TA-matrix of Ru-dppz collected upon 400 nm excitation in acetonitrile. The plots show the singular values and respective spectral and temporal singular vectors for the first six singular components.

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    Figure 4

    Figure 4. Schematic illustration of an user-defined (tutorial) model. In this model, the states X and Y are initially populated and the decay independently and irreversibly with the decay rate k0 and k1, respectively, finally populating state Z. This state irreversibly decays back to the ground state with the rate-constants k2. The corresponding rate equations for the concentrations of X, Y, and Z are displayed.

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    Figure 5

    Figure 5. Contour plots of the preprocessed (measured: Aab) and modeled (calculated: ACab) fs-TA data of Ru-dppz in acetonitrile (ACN) in the time-delay range of 350 fs to 1800 ps and probe wavelength range of 330 to 780 nm upon photoexcitation at 400 nm. The bottom plot shows the 10-fold amplified difference of the measured and calculated data (AEab) enabling an inspection of the overall fit quality.

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    Figure 6

    Figure 6. KiMoPack-generated summary of the preprocessed TA data and the fit results of Ru-dppz in acetonitrile (ACN) upon 400 nm excitation.

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    Figure 7

    Figure 7. Schematic representation of a parallel and sequential model involving three different species, namely A (3MLCTprox), B (3MLCTdist), and C (3MLCTphen). Decay associated spectra (DAS, a) and species associated spectra (SAS, b) obtained from global analysis of the transient absorption (TA) data of Ru-dppz collected upon 400 nm excitation in acetonitrile (ACN) within the exponential (giving DAS) and consecutive model (giving SAS).

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      Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis
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      Sherman, Benjamin D.; Ashford, Dennis L.; Lapides, Alexander M.; Sheridan, Matthew V.; Wee, Kyung-Ryang; Meyer, Thomas J.
      Journal of Physical Chemistry Letters (2015), 6 (16), 3213-3217CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      An electrochem. procedure for prepg. chromophore-catalyst assemblies on oxide electrode surfaces by reductive vinyl coupling is described. On core/shell SnO2/TiO2 nanoparticle oxide films, excitation of the assembly with 1 sun (100 mW cm-2) illumination in 0.1 M H2PO4-/HPO42- at pH 7 with an applied bias of 0.4 V vs. SCE leads to water splitting in a DSPEC (dye-sensitized photoelectrosynthesis cell) with a Pt cathode. Over a 5 min photolysis period, the core/shell photoanode produced O2 with a faradaic efficiency of 22%. Instability of the surface bound chromophore in its oxidized state in the phosphate buffer leads to a gradual decrease in photocurrent and to the relatively modest faradaic efficiencies.
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    10. 10
      Kranz, C.; Wächtler, M. Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes. Chem. Soc. Rev. 2021, 50, 1407– 1437,  DOI: 10.1039/D0CS00526F
      10
      Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes
      Kranz, Christine; Waechtler, Maria
      Chemical Society Reviews (2021), 50 (2), 1407-1437CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Research on light-driven catalysis has gained tremendous importance due to the ever-increasing power consumption and the threatening situation of global warming related to burning fossil fuels. Significant efforts have been dedicated to artificial photosynthesis mimicking nature to split H2O into H2 and O2 by solar energy. Novel semiconductor und mol. photocatalysts focusing on one-step excitation processes via single component photocatalysts or via two-step excitation processes mimicking the Z-scheme of natural photosynthesis are currently developed. Anal. and physicochem. methods, which provide information at different time and length scales, are used to gain fundamental understanding of all processes leading to catalytic activity, i.e., light absorption, charge sepn., transfer of charges to the reaction centers and catalytic turnover, but also understanding degrdn. processes of the photocatalytic active material. Esp., mol. photocatalysts still suffer from limited long-term stability due to the formation of reactive intermediates, which may lead to degrdn. Although there is an overwhelming no. of research articles and reviews focussing on various materials for photocatalytic water splitting, to date only few reviews have been published providing a comprehensive overview on methods for characterizing such materials. This review will highlight spectroscopic, spectroelectrochem., and electrochem. approaches in respect to their potential in studying processes in semiconductor and (supra)mol. photocatalysts. Special emphasis will be on spectroscopic methods to investigate light-induced processes in intermediates of sequential electron transfer chains. Further, microscopic characterization methods, which are predominantly used for semiconducting and hybrid photocatalytic materials will be reviewed as surface area, structure, facets, defects, and bulk properties such as crystallinity and crystal size are key parameters for charge sepn., transfer processes and suppression of charge recombination. Recent developments in scanning probe microscopy will also be highlighted as such techniques are highly suited for studying photocatalytic active material.
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    11. 11
      Tschierlei, S.; Presselt, M.; Kuhnt, C.; Yartsev, A.; Pascher, T.; Sundström, V.; Karnahl, M.; Schwalbe, M.; Schäfer, B.; Rau, S. Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst. Chem. Eur. J. 2009, 15, 7678– 7688,  DOI: 10.1002/chem.200900457
      11
      Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst
      Tschierlei, Stefanie; Presselt, Martin; Kuhnt, Christian; Yartsev, Arkady; Pascher, Torbjoern; Sundstroem, Villy; Karnahl, Michael; Schwalbe, Matthias; Schaefer, Bernhard; Rau, Sven; Schmitt, Michael; Dietzek, Benjamin; Popp, Juergen
      Chemistry - A European Journal (2009), 15 (31), 7678-7688, S7678/1-S7678/4CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Photoinduced electron-transfer processes within a precatalyst for intramol. hydrogen evolution [(tbbpy)2Ru(tpphz)PdCl2]2+ (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)2Ru(tpphz)]2+ subunit Ru with that of the supramol. catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the mol. ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramol. complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-detg. steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-sepd. states.
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    12. 12
      Pfeffer, M.; Müller, C.; Kastl, E. T. E.; Mengele, A. K.; Bagemihl, B.; Fauth, S.; Habermehl, J.; Petermann, L.; Wächtler, M.; Schulz, M. Active repair of a dinuclear photocatalyst for visible light-driven hydrogen production. Nat. Chem. 2022, 14, 500– 506,  DOI: 10.1038/s41557-021-00860-6
      12
      Active repair of a dinuclear photocatalyst for visible-light-driven hydrogen production
      Pfeffer, Michael G.; Mueller, Carolin; Kastl, Evelyn T. E.; Mengele, Alexander K.; Bagemihl, Benedikt; Fauth, Sven S.; Habermehl, Johannes; Petermann, Lydia; Waechtler, Maria; Schulz, Martin; Chartrand, Daniel; Laverdiere, Francois; Seeber, Phillip; Kupfer, Stephan; Graefe, Stefanie; Hanan, Garry S.; Vos, Johannes G.; Dietzek-Ivansic, Benjamin; Rau, Sven
      Nature Chemistry (2022), 14 (5), 500-506CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
      The mol. app. behind biol. photosynthesis retains its long-term functionality through enzymic repair. However, bioinspired mol. devices designed for artificial photosynthesis, consisting of a photocentre, a bridging ligand and a catalytic center, can become unstable and break down when their individual modules are structurally compromised, halting their overall functionality and operation. Here we report the active repair of such an artificial photosynthetic mol. device, leading to complete recovery of catalytic activity. We have identified the hydrogenation of the bridging ligand, which inhibits the light-driven electron transfer between the photocentre and catalytic center, as the deactivation mechanism. As a means of repair, we used the light-driven generation of singlet oxygen, catalyzed by the photocentre, to enable the oxidative dehydrogenation of the bridging unit, which leads to the restoration of photocatalytic hydrogen formation.
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    13. 13
      Tasić, M.; Ivković, J.; Carlström, G.; Melcher, M.; Bollella, P.; Bendix, J.; Gorton, L.; Persson, P.; Uhlig, J.; Strand, D. Electro-mechanically switchable hydrocarbons based on [8]annulenes. Nat. Commun. 2022, 13, 860– 869,  DOI: 10.1038/s41467-022-28384-8
      13
      Electro-mechanically switchable hydrocarbons based on [8]annulenes
      Tasic, Magdalena; Ivkovic, Jakov; Carlstroem, Goeran; Melcher, Michaela; Bollella, Paolo; Bendix, Jesper; Gorton, Lo; Persson, Petter; Uhlig, Jens; Strand, Daniel
      Nature Communications (2022), 13 (1), 860CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Abstr.: Pure hydrocarbons with shape and conjugation properties that can be switched by external stimuli is an intriguing prospect in the design of new responsive materials and single-mol. electronics. Here, we develop an oligomeric [8]annulene-based material that combines a remarkably efficient topol. switching upon redox changes with structural simplicity, stability, and straightforward synthesis: 5,12-alkyne linked dibenzo[a,e]cyclooctatetraenes (dbCOTs). Upon redn., the structures accommodate a reversible reorganization from a pseudo-conjugated tub-shape to a conjugated arom. system. This switching in oligomeric structures gives rise to multiple defined states that are deconvoluted by electrochem., NMR, and optical methods. The combination of stable electromech. responsivity and ability to relay electrons stepwise through an extended (pseudo-conjugated) π-system in partially reduced structures validate alkyne linked dbCOTs as a practical platform for developing new responsive materials and switches based on [8]annulene cores.
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    14. 14
      Satzger, H.; Zinth, W. Visualization of transient absorption dynamics – towards a qualitative view of complex reaction kinetics. Chem. Phys. 2003, 295, 287– 295,  DOI: 10.1016/j.chemphys.2003.08.012
      14
      Visualization of transient absorption dynamics - towards a qualitative view of complex reaction kinetics
      Satzger, H.; Zinth, W.
      Chemical Physics (2003), 295 (3), 287-295CODEN: CMPHC2; ISSN:0301-0104. (Elsevier Science B.V.)
      Photochem. reactions may involve complex reaction schemes and their study requires spectroscopic techniques extending over a broad spectral range and over several orders of magnitude in time. Two commonly used numerical procedures to evaluate such data sets - global fitting and singular value decompn. - are discussed and a method for the qual. visualization is proposed: differentiation of the transient spectra on a logarithmic scale allows to ext. special kinetic components and to obtain reasonable starting information for subsequent fitting procedures. The proposed method should be well adapted to situations where e.g. due to unstable samples the data cannot be recorded at the precision required for unambiguous anal. by std. data handling procedures. The method is applied to synthetic data sets as well as to exptl. data taken from femtosecond absorption expts. on the laser dye DCM in DMSO.
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    15. 15
      Henry, E. R. The Use of Matrix Methods in the Modeling of Spectroscopic Data Sets. Biophys. J. 1997, 72, 652– 673,  DOI: 10.1016/S0006-3495(97)78703-4
      15
      The use of matrix methods in the modeling of spectroscopic data sets
      Henry, Eric R.
      Biophysical Journal (1997), 72 (2, Pt. 1), 652-673CODEN: BIOJAU; ISSN:0006-3495. (Biophysical Society)
      We describe a general approach to the model-based anal. of sets of spectroscopic data that is built upon the techniques of matrix anal. A model hypothesis may often be expressed by writing a matrix of measured spectra as the product of a matrix of spectra of individual mol. species and a matrix of corresponding species populations as a function of exptl. conditions. The modeling procedure then requires the simultaneous detn. of a set of species spectra and a set of model parameters (from which the populations are derived), such that this product yields an optimal description of the measured spectra. This procedure may be implemented as an optimization problem in the space of the (possibly nonlinear) model parameters alone, coupled with the efficient soln. of a corollary linear optimization problem using matrix decompn. methods to obtain a set of species spectra corresponding to any set of model parameters. Known species spectra, as well as other information and assumptions about spectral shapes, may be incorporated into this general framework, using parametrized anal. functional forms and basis-set techniques. The method by which assumed relationships between global features (e.g., peak positions) of different species spectra may be enforced in the modeling without otherwise specifying the shapes of the spectra will be shown. We also consider the effect of measurement errors on this approach and suggest extensions of the matrix-based least-squares procedures applicable to situations in which measurement errors may not be assumed to be normally distributed. A generalized anal. procedure is introduced for cases in which the species spectra vary with exptl. conditions.
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    16. 16
      Henry, E.; Hofrichter, J. [8] Singular value decomposition: Application to analysis of experimental data. Methods in Enzymology 1992, 210, 129– 192,  DOI: 10.1016/0076-6879(92)10010-B
      16
      Singular value decomposition: application to analysis of experimental data
      Henry, E. R.; Hofrichter, J.
      Methods in Enzymology (1992), 210 (Numer. Comput. Methods), 129-92CODEN: MENZAU; ISSN:0076-6879.
      The title method is illustrated by the anal. of spectral data of, e.g., modified Hb.
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    17. 17
      Hendler, R. W.; Shrager, R. I. Deconvolutions based on singular value decomposition and the pseudoinverse: a guide for beginners. Journal of Biochemical and Biophysical Methods 1994, 28, 1– 33,  DOI: 10.1016/0165-022X(94)90061-2
      17
      Deconvolutions based on singular value decomposition and the pseudoinverse: a guide for beginners
      Hendler R W; Shrager R I
      Journal of biochemical and biophysical methods (1994), 28 (1), 1-33 ISSN:0165-022X.
      Singular value decomposition (SVD) is deeply rooted in the theory of linear algebra, and because of this is not readily understood by a large group of researchers who could profit from its application. In this paper, we discuss the subject on a level that should be understandable to scientists who are not well versed in linear algebra. However, because it is necessary that certain key concepts in linear algebra be appreciated in order to comprehend what is accomplished by SVD, we present the section, 'Bare basics of linear algebra'. This is followed by a discussion of the theory of SVD. Next we present step-by-step examples to illustrate how SVD is applied to deconvolute a titration involving a mixture of three pH indicators. One noiseless case is presented as well as two cases where either a fixed or varying noise level is present. Finally, we discuss additional deconvolutions of mixed spectra based on the use of the pseudoinverse.
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    18. 18
      Shrager, R. I. Chemical transitions measured by spectra and resolved using singular value decomposition. Chemometrics and Intelligent Laboratory Systems 1986, 1, 59– 70,  DOI: 10.1016/0169-7439(86)80026-0
      18
      Chemical transitions measured by spectra and resolved using singular value decomposition
      Shrager, Richard I.
      Chemometrics and Intelligent Laboratory Systems (1986), 1 (1), 59-70CODEN: CILSEN; ISSN:0169-7439.
      A sequence of optical or other spectra, generated by overlapping chem. transitions, may be more easily analyzed after a data redn. step involving singular value decompn. (SVD). The method is described, and its development since 1965 is summarized. A new SVD-based method described which corrects some theor. and practical flaws in past methods.
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    19. 19
      Oang, K. Y.; Yang, C.; Muniyappan, S.; Kim, J.; Ihee, H. SVD-aided pseudo principal-component analysis: A new method to speed up and improve determination of the optimum kinetic model from time-resolved data. Structural Dynamics 2017, 4, 044013,  DOI: 10.1063/1.4979854
      19
      SVD-aided pseudo principal-component analysis: A new method to speed up and improve determination of the optimum kinetic model from time-resolved data
      Oang, Key Young; Yang, Cheolhee; Muniyappan, Srinivasan; Kim, Jeongho; Ihee, Hyotcherl
      Structural Dynamics (2017), 4 (4), 044013/1-044013/13CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
      Detn. of the optimum kinetic model is an essential prerequisite for characterizing dynamics and mechanism of a reaction. Here, we propose a simple method, termed as singular value decompn.-aided pseudo principal-component anal. (SAPPA), to facilitate detn. of the optimum kinetic model from time-resolved data by bypassing any need to examine candidate kinetic models. We demonstrate the wide applicability of SAPPA by examg. three different sets of exptl. time-resolved data and show that SAPPA can efficiently det. the optimum kinetic model. In addn., the results of SAPPA for both time-resolved X-ray soln. scattering (TRXSS) and transient absorption (TA) data of the same protein reveal that global structural changes of protein, which is probed by TRXSS, may occur more slowly than local structural changes around the chromophore, which is probed by TA spectroscopy. (c) 2017 American Institute of Physics.
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    20. 20
      Johnson, M. L.; Faunt, L. M. [1] Parameter estimation by least-squares methods. Methods in Enzymology 1992, 210, 1– 37,  DOI: 10.1016/0076-6879(92)10003-V
      20
      Parameter estimation by least-squares methods
      Johnson M L; Faunt L M
      Methods in enzymology (1992), 210 (), 1-37 ISSN:0076-6879.
      There is no expanded citation for this reference.
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    21. 21
      Beechem, J. M. [2] Global analysis of biochemical and biophysical data. Methods in Enzymology 1992, 210, 37– 54,  DOI: 10.1016/0076-6879(92)10004-W
      21
      Global analysis of biochemical and biophysical data
      Beechem, Joseph M.
      Methods in Enzymology (1992), 210 (Numer. Comput. Methods), 37-54CODEN: MENZAU; ISSN:0076-6879.
      Global anal. of data represents the simultaneous anal. of multiple expts. in terms of internally consistent sets of fitting parameters. What has been presented is a motivation for performing global types of analyses and the importance of rigorous error anal.
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    22. 22
      Ruckebusch, C.; Sliwa, M.; Pernot, P.; de Juan, A.; Tauler, R. Comprehensive data analysis of femtosecond transient absorption spectra: A review. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2012, 13, 1– 27,  DOI: 10.1016/j.jphotochemrev.2011.10.002
      22
      Comprehensive data analysis of femtosecond transient absorption spectra: A review
      Ruckebusch, C.; Sliwa, M.; Pernot, P.; de Juan, A.; Tauler, R.
      Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2012), 13 (1), 1-27CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
      A review. Nowadays, time-resolved spectroscopy data can be routinely and accurately collected in UV-visible femtosecond transient absorption spectroscopy. However, the data anal. strategy and the postulation of a phys. valid model for this kind of measurements may be tackled with many different approaches ranging from pure soft-modeling (model-free) to hard-modeling, where the elaboration of a parametric spectro-temporal model may be required. This paper reviews methods that are used in practice for the anal. of femtosecond transient absorption spectroscopy data. Model-based methods, common in photochem., are revisited, and soft-modeling methods, which originate from the chemometrics field and that recently disseminated in the photo(bio)chem. literature, are presented. These soft-modeling methods are designed to suit the intrinsic nature of the multivariate (or multi-way) measurement. Soft-modeling tools do not require a priori phys. or mechanistic models to provide a decompn. of the data on the time and wavelength dimensions, the only requirement being that these 2 (or more) dimensions are separable. Addnl., Bayesian data anal., which provides a probabilistic framework for data anal., is considered in detail, since it allows uncertainty quantification and validation of the model selection step.
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    23. 23
      Kollenz, P.; Herten, D.-P.; Buckup, T. Unravelling the Kinetic Model of Photochemical Reactions via Deep Learning. J. Phys. Chem. B 2020, 124, 6358– 6368,  DOI: 10.1021/acs.jpcb.0c04299
      23
      Unravelling the Kinetic Model of Photochemical Reactions via Deep Learning
      Kollenz, Philipp; Herten, Dirk-Peter; Buckup, Tiago
      Journal of Physical Chemistry B (2020), 124 (29), 6358-6368CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Time-resolved spectroscopies have been playing an essential role in the elucidation of the fundamental mechanisms of light-driven processes, particularly in exploring relaxation models for electronically excited mols. However, the detn. of such models from exptl. obtained time-resolved and spectrally resolved data still demands a high degree of intuition, frequently poses numerical challenges, and is often not free from ambiguities. Here, we demonstrate the anal. of time-resolved laser spectroscopy data via a deep learning network to obtain the correct relaxation kinetic model. In its current design, the presented Deep Spectroscopy Kinetic Anal. Network (DeepSKAN) can predict kinetic models (involved states and relaxation pathways) consisting of up to five states, which results in 103 possible different classes, by estg. the probability of occurrence of a given kinetic model class. DeepSKAN was trained with synthetic time-resolved spectra spanning over 4 orders of magnitude in time with a unitless time axis, thereby demonstrating its potential as a universal approach for analyzing data from various time-resolved spectroscopy techniques in different time ranges. By adding the probabilities of each pathway of the top-k models normalized by the total probability, we can det. the relaxation pathways for a given data set with high certainty (up to 99%). Due to its architecture and training, DeepSKAN is robust against exptl. noise and typical preanal. errors like time-zero corrections. Application of DeepSKAN to exptl. data is successfully demonstrated for three different photoinduced processes: transient absorption of the retinal isomerization, transient IR spectroscopy of the relaxation of the photoactivated DRONPA, and transient absorption of the dynamics in lycopene. This approach delivers kinetic models and could be a unifying asset in several areas of spectroscopy.
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    24. 24
      Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M. Glotaran: A Java -Based Graphical User Interface for the R Package TIMP. Journal of Statistical Software 2012, 49, 1– 22,  DOI: 10.18637/jss.v049.i03
      There is no corresponding record for this reference.
    25. 25
      Dorlhiac, G. F.; Fare, C.; van Thor, J. J. PyLDM - An open source package for lifetime density analysis of time-resolved spectroscopic data. PLOS Computational Biology 2017, 13, e1005528  DOI: 10.1371/journal.pcbi.1005528
      25
      PyLDM - an open source package for lifetime density analysis of time-resolved spectroscopic data
      Dorlhiac, Gabriel F.; Fare, Clyde; van Thor, Jasper J.
      PLoS Computational Biology (2017), 13 (5), e1005528/1-e1005528/15CODEN: PCBLBG; ISSN:1553-7358. (Public Library of Science)
      Historically, the dual approach of global anal. and target modeling has been used to elucidate kinetic descriptions of the system, and the identity of transient species resp. While global anal. approximates the data to the sum of a small no. of exponential decays, typically on the order of 2-4, LDA uses a semi-continuous distribution of 100 lifetimes. This allows for the elucidation of lifetime distributions, which may be expected from investigation of complex systems with many chromophores, as opposed to avs. Furthermore, the inherent assumption of linear combinations of decays in global anal. means the technique is unable to describe dynamic motion, a process which is resolvable with LDA. The technique was introduced to the field of photosynthesis over a decade ago by the Holzwarth group. The anal. has been demonstrated to be an important tool to evaluate complex dynamics such as photosynthetic energy transfer, and complements traditional global and target anal. techniques. Although theory has been well described, no open source code has so far been available to perform lifetime d. anal. Therefore, we introduce a python (2.7) based package, PyLDM, to address this need. We furthermore provide a direct comparison of the capabilities of LDA with those of the more familiar global anal., as well as providing a no. of statistical techniques for dealing with the regularization of noisy data.
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    26. 26
      Beckwith, J. S.; Rumble, C. A.; Vauthey, E. Data analysis in transient electronic spectroscopy - an experimentalist’s view. Int. Rev. Phys. Chem. 2020, 39, 135– 216,  DOI: 10.1080/0144235X.2020.1757942
      26
      Data analysis in transient electronic spectroscopy - an experimentalist's view
      Beckwith, Joseph S.; Rumble, Christopher A.; Vauthey, Eric
      International Reviews in Physical Chemistry (2020), 39 (2), 135-216CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
      Time-resolved electronic spectroscopy has grown into a technique that provides hundreds to thousands of electronic spectra with femtosecond time resoln. This enables complex questions to be interrogated, with an obvious cost that the data are more detailed and thus require accurate modeling to be properly reproduced. Anal. of these data comes in a variety of forms, starting with a variety of assumptions about how the data may be decompd. Here, four different types of anal. commonly used are discussed: band-shape anal., global kinetic anal., lifetime distribution models, and soft-modeling. This review provides a 'user's guide' to these various methods of data anal., and attempts to elucidate their successes, domains in which they may be useful, and potential pitfalls in their usage.
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    27. 27
      Kandoth, N.; Pérez Hernández, J.; Palomares, E.; Lloret-Fillol, J. Mechanisms of photoredox catalysts: the role of optical spectroscopy. Sustainable Energy & Fuels 2021, 5, 638– 665,  DOI: 10.1039/D0SE01454K
      27
      Mechanisms of photoredox catalysts: the role of optical spectroscopy
      Kandoth, Noufal; Perez Hernandez, Javier; Palomares, Emilio; Lloret-Fillol, Julio
      Sustainable Energy & Fuels (2021), 5 (3), 638-665CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)
      A review. Photoinduced org. transformations have stimulated the org. chem. community to develop light-driven renewed reaction methodologies, which in many cases are complementary to std. thermal catalysis. This revitalization of photoinduced transformations is in part due to the straightforward access to powerful photosensitizers. Among them, Ru(II) and Ir(III) polypyridyl complexes have been extensively utilized as prototypical photoredox catalysts. Despite the flourishing of new org. reactivity, studies of photocatalytic cycles are still scarce. The current mechanistic proposal mostly relies on luminescence quenching studies, redox potentials, and bond-dissocn. energy values, which provide an essential but partial picture of the catalytic processes. Besides, the quantum efficiency and overall energy efficiency of photoredox org. transformations are not usually considered merit yet. On the other hand, during the last few decades, the photochem. community has studied the energy and electron transfer mechanism of transition metal complexes from the ground and the excited state extensively, with a partial address of the catalytic photoredox cycles probably due to their complexity. Those studies are needed to develop new photoredox org. transformations further and make them more sustainable and energy-efficient. In this review, we outline an overview of selected basic concepts of photophysics and photochem. encountered in the photocatalytic cycles. Selected examples are detailed to illustrate how steady-state and time-resolved optical spectroscopy can be employed to elucidate catalytic intermediates and photocatalytic mechanisms. As such, this review aims to motivate mechanistic studies on photoredox catalysis and serves as a guide to perform them to develop more sustainable and energy-efficient chem. transformations.
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    28. 28
      Dedecker, P. Software review: Glotaran. Journal of Chemometrics 2014, 28, 137– 138,  DOI: 10.1002/cem.2596
      28
      Software review: Glotaran
      Dedecker, Peter
      Journal of Chemometrics (2014), 28 (3), 137-138CODEN: JOCHEU; ISSN:0886-9383. (John Wiley & Sons Ltd.)
      There is no expanded citation for this reference.
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    29. 29
      Mullen, K. M.; van Stokkum, I. H. M. TIMP: An R Package for Modeling Multi-Way Spectroscopic Measurements. Journal of Statistical Software 2007, 18, 1– 46,  DOI: 10.18637/jss.v018.i03
      There is no corresponding record for this reference.
    30. 30
      Uhlig, J. KiMoPack GitHub project page , release 6.6.2. 2022; https://github.com/erdzeichen/KiMoPack.
      There is no corresponding record for this reference.
    31. 31
      Uhlig, J. KiMoPack - Open source tool for the analysis of transient spectral data, version 6.6.2. Zenodo 2022,  DOI: 10.5281/zenodo.6049186
      There is no corresponding record for this reference.
    32. 32
      Uhlig, J. KiMoPack Anaconda installation (using conda package manager) version 6.6.2. 2022; https://conda.anaconda.org/erdzeichen.
      There is no corresponding record for this reference.
    33. 33
      Uhlig, J. KiMoPack PyPi installation (using pip package manager) version 6.6.2. 2022; https://pypi.org/project/KiMoPack/.
      There is no corresponding record for this reference.
    34. 34
      Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io . 2022; https://kimopack.readthedocs.io/en/latest.
      There is no corresponding record for this reference.
    35. 35
      Uhlig, J. KiMoPack Youtube Tutorial Channel . 2022; https://www.youtube.com/channel/UCmhiK0P9wXXjs_PJaitx8BQ.
      There is no corresponding record for this reference.
    36. 36
      Weißborn, J.; Snellenburg, J.; Weigand, S.; Van Stokkum, I. H. pyglotaran: a Python library for global and target analysis, version v0.5. Zenodo 2021,  DOI: 10.5281/zenodo.4534043
      There is no corresponding record for this reference.
    37. 37
      Hniopek, J.; Müller, C.; Bocklitz, T.; Schmitt, M.; Dietzek, B.; Popp, J. Kinetic-Model-Free Analysis of Transient Absorption Spectra Enabled by 2D Correlation Analysis. J. Phys. Chem. Lett. 2021, 12, 4148– 4153,  DOI: 10.1021/acs.jpclett.1c00835
      37
      Kinetic-Model-Free Analysis of Transient Absorption Spectra Enabled by 2D Correlation Analysis
      Hniopek, Julian; Mueller, Carolin; Bocklitz, Thomas; Schmitt, Michael; Dietzek, Benjamin; Popp, Juergen
      Journal of Physical Chemistry Letters (2021), 12 (17), 4148-4153CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Here, we present, to the best of our knowledge for the first time, a systematic study of utilizing 2D correlation anal. in the field of femtosecond transient absorption (fs-TA) spectroscopy. We present that the application of 2D correlation spectroscopy (2DCOS) to fs-TA spectroscopy enables a model-free means to analyze excited state kinetics, which is demonstrated on the model system [(tbbpy)2Ru(dppz)]2+ in different solvents. We show that TA-2DCOS is able to det. the no. of processes contributing to the time-resolved spectral changes in fs-TA data sets, as well as ext. the spectral response of these components. Overall, the results show that TA-2DCOS leads to the same results as obtained with methods relying on global lifetime anal. or multivariate curve resoln. but without the need to specify a predetd. kinetic model. The work presented therefore highlights the potential of TA-2DCOS as a model-free approach for analyzing fs-TA spectral data sets.
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    38. 38
      Véry, T.; Ambrosek, D.; Otsuka, M.; Gourlaouen, C.; Assfeld, X.; Monari, A.; Daniel, C. Photophysical Properties of Ruthenium(II) Polypyridyl DNA Intercalators: Effects of the Molecular Surroundings Investigated by Theory. Chem. Eur. J. 2014, 20, 12901– 12909,  DOI: 10.1002/chem.201402963
      38
      Photophysical Properties of Ruthenium(II) Polypyridyl DNA Intercalators: Effects of the Molecular Surroundings Investigated by Theory
      Very, Thibaut; Ambrosek, David; Otsuka, Miho; Gourlaouen, Christophe; Assfeld, Xavier; Monari, Antonio; Daniel, Chantal
      Chemistry - A European Journal (2014), 20 (40), 12901-12909CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The environmental effects on the structural and photophys. properties of [Ru(L)2(dppz)]2+ complexes (L=bpy=2,2'-bipyridine, phen=1,10-phenanthroline, tap=1,4,5,8-tetraazaphenanthrene; dppz=dipyrido[3,3-a:2',3'-c]phenazine), used as DNA intercalators, have been studied by means of DFT, time-dependent DFT, and quantum mechanics/mol. mechanics calcns. The electronic characteristics of the low-lying triplet excited states in water, acetonitrile, and DNA have been investigated to decipher the influence of the environment on the luminescent behavior of this class of mols. The lowest triplet intra-ligand (IL) excited state calcd. at λ≈800 nm for the three complexes and localized on the dppz ligand is not very sensitive to the environment and is available for electron transfer from a guanine nucleobase. Whereas the lowest triplet metal-to-ligand charge-transfer (3MLCT) states remain localized on the ancillary ligand (tap) in [Ru(tap)2(dppz)]2+, regardless of the environment, their character is drastically modified in the other complexes [Ru(phen)2(dppz)]2+ and [Ru(bpy)2(dppz)]2+ upon going from acetonitrile (MLCTdppz/phen or MLCTdppz/bpy) to water (MLCTdppz) and DNA (MLCTphen and MLCTbpy). The change in the character of the low-lying 3MLCT states accompanying nuclear relaxation in the excited state controls the emissive properties of the complexes in water, acetonitrile, and DNA. The light-switching effect has been rationalized on the basis of environment-induced control of the electronic d. distributed in the lowest triplet excited states.
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    39. 39
      Friedman, A. E.; Chambron, J. C.; Sauvage, J. P.; Turro, N. J.; Barton, J. K. A molecular light switch for DNA: Ru(bpy)2(dppz)2+. J. Am. Chem. Soc. 1990, 112, 4960– 4962,  DOI: 10.1021/ja00168a052
      39
      A molecular light switch for DNA: Ru(bpy)2(dppz)2+
      Friedman, Alan E.; Chambron, Jean Claude; Sauvage, Jean Pierre; Turro, Nicholas J.; Barton, Jacqueline K.
      Journal of the American Chemical Society (1990), 112 (12), 4960-2CODEN: JACSAT; ISSN:0002-7863.
      The application of a novel transition metal complex as a sensitive spectroscopic probe for DNA is reported. The complex, Ru(bpy)2(dppz)2+ (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2: a-2',3':c]phenazine), shows no detectable photoluminescence in aq. soln. at ambient temps., but in the presence of double helical DNA, to which the complex binds avidly, intense photoluminescence is obsd. In the presence of 100 μM calf thymus DNA, single photon counting expts. at 25° reveal a biexponential decay of emission with a short lived component of 75 ns and a longer lived component of 259 nsec. With B-form poly d(GC)·d(GC), emission is centered at 628 nm (λexc = 482 nm) and is comparable in intensity to that found in isopropanol (where Φ ≥ 0.02) in the absence of DNA. With Z-form poly d(GC)·d(GC) the relative intensity of steady state luminescence is even greater than that found with B-DNA and the emission max. is shifted to 640 nm. Only weak emission is apparent in the presence of A-form poly r(AU)·r(AU) with emission centered at 650 nm. The enhancement factor in aq. soln. at ambient temp. for photoluminescence of Ru(bpy)2(dppz)2+ in the presence of B-DNA is estd. to be >104. The complex should be useful as a sensitive, non-radioactive, luminescent DNA probe in both heterogenous and homogeneous assays.
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    40. 40
      Smith, J. A.; George, M. W.; Kelly, J. M. Transient spectroscopy of dipyridophenazine metal complexes which undergo photo-induced electron transfer with DNA. Coord. Chem. Rev. 2011, 255, 2666– 2675,  DOI: 10.1016/j.ccr.2011.04.007
      40
      Transient spectroscopy of dipyridophenazine metal complexes which undergo photo-induced electron transfer with DNA
      Smith, Jayden A.; George, Michael W.; Kelly, John M.
      Coordination Chemistry Reviews (2011), 255 (21-22), 2666-2675CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      This review considers transient spectroscopic studies of electron transfer reactions between nucleic acids and the excited states of transition metal complexes contg. dipyridophenazine or related ligands and focuses mainly on complexes of ruthenium, chromium and rhenium. Particular emphasis is placed on systems where transient UV/visible and/or IR absorption spectroscopy have been employed.
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    41. 41
      Keane, P. M.; Kelly, J. M. Transient absorption and time-resolved vibrational studies of photophysical and photochemical processes in DNA-intercalating polypyridyl metal complexes or cationic porphyrins. Coord. Chem. Rev. 2018, 364, 137– 154,  DOI: 10.1016/j.ccr.2018.02.018
      41
      Transient absorption and time-resolved vibrational studies of photophysical and photochemical processes in DNA-intercalating polypyridyl metal complexes or cationic porphyrins
      Keane, Paraic M.; Kelly, John M.
      Coordination Chemistry Reviews (2018), 364 (), 137-154CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Recent advances in the use of transient absorption (TA) and time-resolved vibrational spectroscopies (TRIR and TR3) to study both excited states and reaction intermediates in metal complexes and porphyrins which intercalate into DNA are reviewed. A particularly well-studied class of compds., which nicely illustrates the comparative advantages of these techniques, is that of ruthenium dppz complexes where the complex might show light-switching or photo-oxidising behavior depending on the nature of the ancillary ligand. Comparative data on Re- and Cr-dppz complexes are also considered. In the second part of this review transient studies of porphyrins, which are known to intercalate into DNA, are considered with particular emphasis on tetramethyl-pyridiniumporphyrins, where the photophys. behavior of the metal-free and various metal derivs. are compared.
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    42. 42
      Brennaman, M. K.; Meyer, T. J.; Papanikolas, J. M. [Ru(bpy) 2 dppz] 2+ Light-Switch Mechanism in Protic Solvents as Studied through Temperature-Dependent Lifetime Measurements. J. Phys. Chem. A 2004, 108, 9938– 9944,  DOI: 10.1021/jp0479670
      42
      [Ru(bpy)2dppz]2+ Light-Switch Mechanism in Protic Solvents as Studied through Temperature-Dependent Lifetime Measurements
      Brennaman, Matthew K.; Meyer, Thomas J.; Papanikolas, John M.
      Journal of Physical Chemistry A (2004), 108 (45), 9938-9944CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Temp.-dependent excited-state lifetime measurements have been performed on four different Ru(II)-based dppz compds. in protic and aprotic solvents. This work supports the existence of a dynamic equil. between two MLCT states assocd. with the dppz ligand: one is a bright state with a ligand orbital similar in size to that assocd. with the 3MLCT state of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridyl), and the other is a dark phenazine-like state. The authors results are consistent with a light-switch mechanism involving a competition between energetic factors that favor the dark (phz) state and entropic factors that favor the bright (bpy) state. This paper explores the photophysics of these light-switch compds. through a systematic variation of the equil. energetics. This is accomplished by (1) varying the dielec. strength of the solvent and (2) making chem. substitutions on the dipyridophenazine (dppz) ligand. Observations obtained from all four compds. in six different solvents can be explained using this equil. model.
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    43. 43
      Brennaman, M. K.; Alstrum-Acevedo, J. H.; Fleming, C. N.; Jang, P.; Meyer, T. J.; Papanikolas, J. M. Turning the [Ru(bpy) 2 dppz] 2+ Light-Switch On and Off with Temperature. J. Am. Chem. Soc. 2002, 124, 15094– 15098,  DOI: 10.1021/ja0279139
      43
      Turning the [Ru(bpy)2dppz]2+ Light-Switch On and Off with Temperature
      Brennaman, Matthew K.; Alstrum-Acevedo, James H.; Fleming, Cavan N.; Jang, Paul; Meyer, Thomas J.; Papanikolas, John M.
      Journal of the American Chemical Society (2002), 124 (50), 15094-15098CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors report temp.-dependent excited-state lifetime measurements on [Ru(bpy)2dppz]2+ in both protic and aprotic solvents. These expts. yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Measurements support the notion of bpy-like and phz-like states assocd. with the dppz ligand and show that the ligand orbital assocd. with the bright state is similar in size to the corresponding orbital in the 3MLCT state of [Ru(bpy)3]2+. In contrast to the current thinking, the expts. presented here indicate that the light-switch effect is not driven by a state reversal. Rather, probably the dark state is always lowest in energy, even in aprotic solvents, and the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state.
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    44. 44
      Olson, E. J. C.; Hu, D.; Hörmann, A.; Jonkman, A. M.; Arkin, M. R.; Stemp, E. D. A.; Barton, J. K.; Barbara, P. F. First Observation of the Key Intermediate in the “Light-Switch” Mechanism of [Ru(phen) 2 dppz] 2+. J. Am. Chem. Soc. 1997, 119, 11458– 11467,  DOI: 10.1021/ja971151d
      44
      First Observation of the Key Intermediate in the "Light-Switch" Mechanism of [Ru(phen)2dppz]2+
      Olson, E. J. C.; Hu, D.; Hoermann, A.; Jonkman, A. M.; Arkin, M. R.; Stemp, E. D. A.; Barton, J. K.; Barbara, P. F.
      Journal of the American Chemical Society (1997), 119 (47), 11458-11467CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      [Ru(phen)2dppz]2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and closely related complexes have previously been obsd. to have an undetectably small quantum yield of photoluminescence in water but a moderate emission yield when bound to DNA. This so-called "light-switch" effect is a crit. factor in the utility of these complexes as spectroscopic probes for DNA. Here we describe a detailed investigation of the photophysics of [Ru(phen)2dppz]2+ in aq. soln., and in mixts. of acetonitrile and water, by time-resolved absorption and emission spectroscopies. The emission of the complex in water has been measured for the first time. A prompt initial emission, derived from a metal-to-ligand charge-transfer (MLCT) excited state typical for polypyridyl-ruthenium complexes, is obsd. along with a delayed emission attributed to a novel MLCT species. The small quantum yield of photoluminescence for [Ru(phen)2dppz]2+ in water, and in water/acetonitrile, depends upon efficient formation of a novel MLCT species, followed by its rapid radiationless decay. The MLCT interconversion is assigned to an intramol. charge-transfer process that is induced by the polarity and proton donating ability of the solvent.
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    45. 45
      Olofsson, J.; Önfelt, B.; Lincoln, P. Three-State Light Switch of [Ru(phen) 2 dppz] 2+: Distinct Excited-State Species with Two, One, or No Hydrogen Bonds from Solvent. J. Phys. Chem. A 2004, 108, 4391– 4398,  DOI: 10.1021/jp037967k
      45
      Three-State Light Switch of [Ru(phen)2dppz]2+: Distinct Excited-State Species with Two, One, or No Hydrogen Bonds from Solvent
      Olofsson, Johan; Oenfelt, Bjoern; Lincoln, Per
      Journal of Physical Chemistry A (2004), 108 (20), 4391-4398CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The ruthenium complexes of dppz (dipyrido[3,2-a:2',3'-c]phenazine) have found wide interest due to their environment-sensitive luminescence and are used, for example, as spectroscopic probes for DNA. The deactivation process for the excited state of the "light-switch" complex [Ru(phen)2dppz]2+ (phen = 1,10-phenanthroline) has been studied in water, glycerol, ethylene glycol, and 1,2- and 1,3-propanediol by using fluorescence spectroscopy and single photon counting. In all solvents anomalous temp. dependence is found (increasing quantum yield and excited-state lifetime with increasing temp.). Model-independent anal. shows that only two emissive species, with solvent- and temp.-invariant emission spectral profiles, are sufficient to account for all the data in the polyol solvents. Van't Hoff plots of the ratio of the two species are linear at higher temps. in all solvents, indicating rapid thermal equilibration of the two species, except for lower temps. in the most viscous solvent glycerol. Kinetic modeling of the system with microscopic rate consts. with pos. Arrhenius activation energies requires a third nonemissive species, which is assigned to an excited state with two hydrogen bonds from the solvent, whereas the first two species are assigned to the mono-hydrogen-bonded and non-hydrogen-bonded excited-state species. This assignment is supported by the observation of a growing luminescence intensity as temp. is increased, but no wavelength shift, of high-purity [Ru(phen)2dppz]2+ in water soln.
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    46. 46
      Kuhnt, C.; Karnahl, M.; Tschierlei, S.; Griebenow, K.; Schmitt, M.; Schäfer, B.; Krieck, S.; Görls, H.; Rau, S.; Dietzek, B. Substitution-controlled ultrafast excited-state processes in Ru-dppz-derivatives. Phys. Chem. Chem. Phys. 2010, 12, 1357– 1368,  DOI: 10.1039/B915770K
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      Substitution-controlled ultrafast excited-state processes in Ru-dppz-derivatives
      Kuhnt, Christian; Karnahl, Michael; Tschierlei, Stefanie; Griebenow, Kristin; Schmitt, Michael; Schaefer, Bernhard; Krieck, Sven; Goerls, Helmar; Rau, Sven; Dietzek, Benjamin; Popp, Juergen
      Physical Chemistry Chemical Physics (2010), 12 (6), 1357-1368CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Ru-dppz (dppz = dipyrido[3,2-a:2',3,3'-c]phenazine) complexes play an important role as environmentally sensitive luminescence sensors and building blocks for larger supramol. compds. Their photophys. properties are known to be highly sensitive to intermol. solvent-solute interactions and solvent bulk-properties. The synthesis and characterization of a novel Ru-dppz deriv. is reported. The potential of drastically tuning the photophys. properties of such complexes is exemplified, by introducing very simple structural modifications, namely bromine, into the dppz-ligand scaffold. The photophysics i.e. nature of excited states and the excited-state relaxation pathway of the various complexes has been investigated by means of electrochem. measurements, steady-state emission expts. and femtosecond time-resolved spectroscopy. It could be shown that the location of bromine substitution influences the relative energy between a luminescent and a non-luminescent metal-to-ligand charge-transfer state and therefore quenches or facilitates transitions between both. Hence it is illustrated that the luminescent properties and the underlying ultrafast excited-state dynamics of the complexes can be controlled by structural variations, i.e. by intramol. interactions as opposed to changes in the intermol. interactions.
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    47. 47
      Pourtois, G.; Beljonne, D.; Moucheron, C.; Schumm, S.; Kirsch-De Mesmaeker, A.; Lazzaroni, R.; Brédas, J.-L. Photophysical Properties of Ruthenium(II) Polyazaaromatic Compounds: A Theoretical Insight. J. Am. Chem. Soc. 2004, 126, 683– 692,  DOI: 10.1021/ja034444h
      47
      Photophysical Properties of Ruthenium(II) Polyazaaromatic Compounds: A Theoretical Insight
      Pourtois, Geoffrey; Beljonne, David; Moucheron, Cecile; Schumm, Stephan; Kirsch-De Mesmaeker, Andree; Lazzaroni, Roberto; Bredas, Jean-Luc
      Journal of the American Chemical Society (2004), 126 (2), 683-692CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Quantum-chem. methods are applied to study the nature of the excited states relevant in the photophys. processes (absorption and emission) of a series of polyazaarom.-ligand-based ruthenium(II) complexes. The electronic and optical properties of the free polyazaarom. ligands and their corresponding ruthenium(II) complexes are detd. on the basis of correlated Hartree-Fock semiempirical approaches. While the emission of complexes contg. small-size ligands, such as 1,10-phenanthroline or 2,2'-bipyridine, arises from a manifold of metal-to-ligand charge-transfer triplet states (3MLCTs), an addnl. ligand-centered triplet state (3L) is identified in the triplet manifold of complexes contg. a π-extended ligand such as dipyrido[3,2-a:2',3'-c]phenazine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene. Recent exptl. data are interpreted in light of these theor. results; namely, the origin for the abnormal solvent- and temp.-dependent emission measured in π-extended Ru complexes is revisited.
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    48. 48
      Olofsson, J.; Wilhelmsson, L. M.; Lincoln, P. Effects of Methyl Substitution on Radiative and Solvent Quenching Rate Constants of [Ru(phen) 2 dppz] 2+ in Polyol Solvents and Bound to DNA. J. Am. Chem. Soc. 2004, 126, 15458– 15465,  DOI: 10.1021/ja047166a
      48
      Effects of Methyl Substitution on Radiative and Solvent Quenching Rate Constants of [Ru(phen)2dppz]2+ in Polyol Solvents and Bound to DNA
      Olofsson, Johan; Wilhelmsson, L. Marcus; Lincoln, Per
      Journal of the American Chemical Society (2004), 126 (47), 15458-15465CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Me substituents on the distant benzene ring of the dppz ligand in the "light switch" complex [Ru(phen)2dppz]2+ have profound effects on the photophysics of the complexes in water as well as in the polyol solvents ethylene glycol, glycerol, and 1,2- and 1,3-propanediol. Whereas 11,12-dimethyl substitution decreases the rate of quenching by diminishing hydrogen bonding by solvent, the 10-Me substituent in addn. also decreases both the radiative and the nonradiative rate const. for decay to the ground state of the non-hydrogen-bonded excited state species. For both the 10-Me and the 11,12-dimethyl derivs., the effect of Me substitution on the equil. of solvent hydrogen bonding to the excited state is due to changes in the entropy terms, rather than in the enthalpy, indicating that the effect is a steric perturbation of the solvent cage around the mol. When intercalated into DNA, the effects of Me substitution is smaller than those in polyol solvent or water, suggesting that the water mols. that quench the excited state by hydrogen bonding to the phenazine aza nitrogens mainly access them from the same groove as in which the Ru(II) ion resides. Since the Δ-enantiomer of [Ru(phen)210-methyl-dppz]2+ has an abs. quantum yield of up to 0.23 when bound to DNA, a value 7000 times higher than in pure water soln., it is promising as a new luminescent DNA probe.
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    49. 49
      Önfelt, B.; Olofsson, J.; Lincoln, P.; Nordén, B. Picosecond and Steady-State Emission of [Ru(phen) 2 dppz] 2+ in Glycerol: Anomalous Temperature Dependence. J. Phys. Chem. A 2003, 107, 1000– 1009,  DOI: 10.1021/jp0269266
      There is no corresponding record for this reference.
    50. 50
      Reback, J.; jbrockmendel; McKinney, W.; den Bossche, J. V.; Augspurger, T.; Roeschke, M.; Hawkins, S.; Cloud, P.; gfyoung; Sinhrks pandas-dev/pandas: Pandas, version 1.4.. Zenodo 2020,  DOI: 10.5281/zenodo.3509134
      There is no corresponding record for this reference.
    51. 51
      Uhlig, J. KiMoPack v.6.6.2 Documentation on “ReadTheDocs.io”, Section Opening ; 2022; https://kimopack.readthedocs.io/en/latest/Opening.html.
      There is no corresponding record for this reference.
    52. 52
      Dietzek, B.; Pascher, T.; Sundström, V.; Yartsev, A. Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design. Laser Physics Letters 2007, 4, 38– 43,  DOI: 10.1002/lapl.200610070
      There is no corresponding record for this reference.
    53. 53
      Dobryakov, A. L.; Kovalenko, S. A.; Ernsting, N. P. Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution. J. Chem. Phys. 2005, 123, 044502,  DOI: 10.1063/1.1948383
      53
      Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution
      Dobryakov, A. L.; Kovalenko, S. A.; Ernsting, N. P.
      Journal of Chemical Physics (2005), 123 (4), 044502/1-044502/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      A unified description is presented of sequential and coherent contributions to optical transient absorption measured by femtosecond pump-supercontinuum probe spectroscopy. All inherent transient terms are taken into account. The "coherence spike" seen during pump-probe overlap is thereby decompd. into its components. The method is demonstrated with rhodamine 110 in methanol. Pure homogeneous dephasing times are obtained from a simultaneous fit of all pertinent measurements. Vibronic structure in the coherence spectrum is assigned to stimulated Raman scattering between vibrational levels in the first excited electronic state. The time-zero spectrum for stimulated emission and the solvation relaxation function are also obtained.
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    54. 54
      Uhlig, J. KiMoPack v.6.6.2 Documentation on “ReadTheDocs.io”, Section Shaping ; 2022; https://kimopack.readthedocs.io/en/latest/Shaping.html.
      There is no corresponding record for this reference.
    55. 55
      Uhlig, J. KiMoPack v.6.6.2 Documentation on “ReadTheDocs.io”, Section Fitting ; 2022; https://kimopack.readthedocs.io/en/latest/Fitting.html.
      There is no corresponding record for this reference.
    56. 56
      Nelder, J. A.; Mead, R. A Simplex Method for Function Minimization. Computer Journal 1965, 7, 308– 313,  DOI: 10.1093/comjnl/7.4.308
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    57. 57
      Virtanen, P.; Gommers, R.; Oliphant, T. E.; Haberland, M.; Reddy, T.; Cournapeau, D.; Burovski, E.; Peterson, P.; Weckesser, W.; Bright, J. SciPy 1.0: Fundamental Algorithms for Scientific Computing in Python. Nat. Methods 2020, 17, 261– 272,  DOI: 10.1038/s41592-019-0686-2
      57
      SciPy 1.0: fundamental algorithms for scientific computing in Python
      Virtanen, Pauli; Gommers, Ralf; Oliphant, Travis E.; Haberland, Matt; Reddy, Tyler; Cournapeau, David; Burovski, Evgeni; Peterson, Pearu; Weckesser, Warren; Bright, Jonathan; van der Walt, Stefan J.; Brett, Matthew; Wilson, Joshua; Millman, K. Jarrod; Mayorov, Nikolay; Nelson, Andrew R. J.; Jones, Eric; Kern, Robert; Larson, Eric; Carey, C. J.; Polat, Ilhan; Feng, Yu; Moore, Eric W.; Vander Plas, Jake; Laxalde, Denis; Perktold, Josef; Cimrman, Robert; Henriksen, Ian; Quintero, E. A.; Harris, Charles R.; Archibald, Anne M.; Ribeiro, Antonio H.; Pedregosa, Fabian; van Mulbregt, Paul
      Nature Methods (2020), 17 (3), 261-272CODEN: NMAEA3; ISSN:1548-7091. (Nature Research)
      Abstr.: SciPy is an open-source scientific computing library for the Python programming language. Since its initial release in 2001, SciPy has become a de facto std. for leveraging scientific algorithms in Python, with over 600 unique code contributors, thousands of dependent packages, over 100,000 dependent repositories and millions of downloads per yr. In this work, we provide an overview of the capabilities and development practices of SciPy 1.0 and highlight some recent tech. developments.
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    58. 58
      Gavana, A. Webpage: Python implementation of Ampgo, online; http://infinity77.net/global_optimization/index.html, accessed May 17, 2022.
      There is no corresponding record for this reference.
    59. 59
      Henrich, J. D.; Zhang, H.; Dutta, P. K.; Kohler, B. Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand. J. Phys. Chem. B 2010, 114, 14679– 14688,  DOI: 10.1021/jp102776r
      59
      Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand
      Henrich, Joseph D.; Zhang, Haoyu; Dutta, Prabir K.; Kohler, Bern
      Journal of Physical Chemistry B (2010), 114 (45), 14679-14688CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The excited-state dynamics of two mixed-ligand mononuclear Ru(II) complexes, [(bpy)2RuLDQ]4+ (bpy = 2,2'-bipyridine, LDQ = 1-[4-(4'-methyl)-2,2'-bipyridyl]-2-[4-(4'-N,N'-tetramethylene-2,2'-bipyridinium)]), and [(bpy)2RuL]2+ (L = 1, 2-bis[4-(4'-methyl)-2,2'-bipyridyl]ethene), were investigated by femtosecond transient absorption spectroscopy. Photoexcitation of the [(bpy)2RuLDQ]4+ complex at three sep. pump wavelengths leads to a common charge-sepd. state consisting of Ru3+ and an excited electron delocalized over the extended π-system centered on the ethenyl moiety of the LDQ ligand. In [(bpy)2RuL]2+, the excited electron is unable to delocalize throughout the π system and remains on the bipyridyl end of ligand L closest to the ruthenium atom. Vibrational cooling in the charge-sepd. state of [(bpy)2RuLDQ]4+ indicates that this state is formed faster than excess energy can be dispersed to the solvent and orders of magnitude more rapidly than in previously studied ruthenium-diquat or Ru-viologen dyads with nonconjugated linkers.
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    60. 60
      Greenough, S. E.; Roberts, G. M.; Smith, N. A.; Horbury, M. D.; McKinlay, R. G.; Żurek, J. M.; Paterson, M. J.; Sadler, P. J.; Stavros, V. G. Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine) 2 (nicotinamide) 2 ] 2+: experimental and theoretical insight into its photoactivation mechanism. Phys. Chem. Chem. Phys. 2014, 16, 19141– 19155,  DOI: 10.1039/C4CP02359E
      60
      Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine)2(nicotinamide)2]2+: experimental and theoretical insight into its photoactivation mechanism
      Greenough, Simon E.; Roberts, Gareth M.; Smith, Nichola A.; Horbury, Michael D.; McKinlay, Russell G.; Zurek, Justyna M.; Paterson, Martin J.; Sadler, Peter J.; Stavros, Vasilios G.
      Physical Chemistry Chemical Physics (2014), 16 (36), 19141-19155CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Mechanistic insight into the photo-induced solvent substitution reaction of cis-[Ru(bipyridine)2(nicotinamide)2]2+ (1) is presented. Complex 1 is a photoactive species, designed to display high cytotoxicity following irradn., for potential use in photodynamic therapy (photochemotherapy). In Ru(II) complexes of this type, efficient population of a dissociative triplet metal-centered (3MC) state is key to generating high quantum yields of a penta-coordinate intermediate (PCI) species, which in turn may form the target species: a mono-aqua photoproduct [Ru(bipyridine)2(nicotinamide)(H2O)]2+ (2). Following irradn. of 1, a thorough kinetic picture is derived from ultrafast UV/visible transient absorption spectroscopy measurements, using a 'target anal.' approach, and provides both timescales and quantum yields for the key processes involved. We show that photoactivation of 1 to 2 occurs with a quantum yield ≥0.36, all within a time-frame of ∼400 ps. Characterization of the excited states involved, particularly the nature of the PCI and how it undergoes a geometry relaxation to accommodate the water ligand, which is a keystone in the efficiency of the photoactivation of 1, is accomplished through state-of-the-art computation including complete active space SCF methods and time-dependent d. functional theory. Importantly, the conclusions here provide a detailed understanding of the initial stages involved in this photoactivation and the foundation required for designing more efficacious photochemotherapy drugs of this type.
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    61. 61
      Oviedo, P. S.; Baraldo, L. M.; Cadranel, A. Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry. Proc. Natl. Acad. Sci. U. S. A. 2021, 118, e2018521118  DOI: 10.1073/pnas.2018521118
      61
      Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry
      Oviedo, Paola S.; Baraldo, Luis M.; Cadranel, Alejandro
      Proceedings of the National Academy of Sciences of the United States of America (2021), 118 (4), e2018521118CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The concept that differential wave function overlap between excited states can be engineered within a mol. chromophore is explored. The aim is to control excited state wave function symmetries, so that symmetry matches or mismatches result in differential orbital overlap and define low-energy trajectories or kinetic barriers within the excited state surface, that drive excited state population toward different reaction pathways. Two donor-acceptor assemblies were explored, where visible light absorption preps. excited states of different wave function symmetry. These states could be resolved using transient absorption spectroscopy, thanks to wave function symmetry-specific photoinduced optical transitions. One of these excited states undergoes energy transfer to the acceptor, while another undertakes a back-electron transfer to restate the ground state. This differential behavior is possible thanks to the presence of kinetic barriers that prevent excited state equilibration. This strategy can be exploited to avoid energy dissipation in energy conversion or photoredox catalytic schemes.
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    62. 62
      Berera, R.; van Stokkum, I. H. M.; Kodis, G.; Keirstead, A. E.; Pillai, S.; Herrero, C.; Palacios, R. E.; Vengris, M.; van Grondelle, R.; Gust, D. Energy Transfer, Excited-State Deactivation, and Exciplex Formation in Artificial Caroteno-Phthalocyanine Light-Harvesting Antennas. J. Phys. Chem. B 2007, 111, 6868– 6877,  DOI: 10.1021/jp071010q
      62
      Energy transfer, excited-state deactivation and exciplex formation in artificial carotene-phthalocyanine light-harvesting antennas
      Berera, Rudi; van Stokkum, Ivo H. M.; Kodis, Gerdenis; Keirstead, Amy E.; Pillai, Smitha; Herrero, Christian; Palacios, Rodrigo E.; Vengris, Mikas; van Grondelle, Rienk; Gust, Devens; Moore, Thomas A.; Moore, Ana L.; Kennis, John T. M.
      Journal of Physical Chemistry B (2007), 111 (24), 6868-6877CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Results from transient absorption spectroscopy on artificial light-harvesting dyads, made up of a Zn phthalocyanine (Pc) covalently linked to carotenoids with 9, 10 or 11 conjugated double bonds, referred to as dyads 1, 2 and 3, resp., are presented. Energy transfer and excited-state deactivation pathways, following excitation of the strongly allowed carotenoid S2 state as a function of the conjugation length, were studied. The S2 state rapidly relaxes to the S* and S1 states. In all systems, there was a new pathway of energy deactivation in the carotenoid manifold in which the S* state acts as an intermediate state in the S2 → S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state was obsd. that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only obsd. upon direct carotenoid excitation and is not fluorescent. In dyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet-singlet energy transfer to Pc, making the process efficient (> 90%), but for dyads 2 and 3, the S1 energy transfer channel is prohibited and only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a quencher of the phthalocyanine excited-state.
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    63. 63
      Venkatesh, Y.; Venkatesan, M.; Ramakrishna, B.; Bangal, P. R. Ultrafast Time-Resolved Emission and Absorption Spectra of meso -Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy. J. Phys. Chem. B 2016, 120, 9410– 9421,  DOI: 10.1021/acs.jpcb.6b05767
      63
      Ultrafast Time-Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy
      Venkatesh, Yeduru; Venkatesan, M.; Ramakrishna, B.; Bangal, Prakriti Ranjan
      Journal of Physical Chemistry B (2016), 120 (35), 9410-9421CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      A comprehensive study of ultrafast mol. relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and THF solvent mixt. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate consts. assocd. with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is obsd. at low-frequency mode, ca. 26 cm-1 region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful anal. of time profiles of up-converted signals at different emission wavelengths helps to reveal detail mol. dynamics. The obsd. lifetimes are as indicated: A very fast decay component with 80 ± 20 fs obsd. at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time consts. are due to intramol. vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, resp. The weighted av. of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An addnl. ∼20 ± 2 ps rise component is obsd. in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model anal. of TA data exclusively generate the true spectra of each excited species/state with their resp. lifetimes along with microscopic rate consts. assocd. with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are obsd. which are assocd. with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, resp., when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), resp., as expected. The TA results parallel the fluorescence up-conversion studies, and both the results not only compliment each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx, and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.
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    64. 64
      Müller, C.; Schwab, A.; Randell, N. M.; Kupfer, S.; Dietzek-Ivansic, B.; Chavarot-Kerlidou, M. A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents. Chem. Eur. J. 2022, 28, e202103882  DOI: 10.1002/chem.202103882
      64
      A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents
      Mueller, Carolin; Schwab, Alexander; Randell, Nicholas M.; Kupfer, Stephan; Dietzek-Ivansic, Benjamin; Chavarot-Kerlidou, Murielle
      Chemistry - A European Journal (2022), 28 (18), e202103882CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophys. properties of the ruthenium photosensitizer [(bpy)2Ru(oxim-dppqp)]2+ (Ru), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent d. functional theory; the results are discussed in comparison to the parent [(bpy)2Ru(dppz)]2+ and [(bpy)2Ru(oxo-dppqp)]2+ complexes. In addn., this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equiv. on the reactive excited states.
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    65. 65
      Kaufmann, M.; Müller, C.; Cullen, A. A.; Brandon, M. P.; Dietzek, B.; Pryce, M. T. Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands. Inorg. Chem. 2021, 60, 760– 773,  DOI: 10.1021/acs.inorgchem.0c02765
      65
      Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands
      Kaufmann, Martin; Muller, Carolin; Cullen, Aoibhin A.; Brandon, Michael P.; Dietzek, Benjamin; Pryce, Mary T.
      Inorganic Chemistry (2021), 60 (2), 760-773CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradn. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophys. characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent d. functional theory calcns. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (1MLCT) and thiazole (tz)-induced intraligand charge-transfer (1ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered 3MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived 3ILCT state (τ ≈ 40-300 ns) with excess charge d. localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τEm,1 ≈ 40-300 ns and τEm,2 ≈ 100-200 ns, which are attributed to emission from the 3ILCT and 3MLCT manifolds, resp. A single-bond extension of the dipyrido[3,2-a:2',3'-c]phenazine ligand with thiazole units leads to changes in the electrochem. and photophys. properties of the corresponding ruthenium complexes. Dual emission is obsd., which is based on two possible pathways following photoexcitation; the first leads to a metal-based 3MLCT emission, and the second relaxes through a ligand-based 3ILCT into the ground state.
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    66. 66
      Müller, C.; Isakov, D.; Rau, S.; Dietzek, B. Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands. J. Phys. Chem. A 2021, 125, 5911– 5921,  DOI: 10.1021/acs.jpca.1c03856
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      Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands
      Mueller, Carolin; Isakov, Dajana; Rau, Sven; Dietzek, Benjamin
      Journal of Physical Chemistry A (2021), 125 (27), 5911-5921CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Ruthenium(II) complexes, like [(tbbpy)2Ru(dppz)]2+ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there was ongoing interest in the design of π-extended Ru-dppz systems with red shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, the authors explore the photophys. properties of protonation isomers of the linearly π-extended [(tbbpy)2Ru(L)]2+-type complexes bearing a dppz ligand with directly fused imidazole (Im) and methyl-imidazole units (mim) as L. Steady-state UV-visible absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-Im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the 3ILCT and 3MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-) imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.
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    67. 67
      Chábera, P.; Lindh, L.; Rosemann, N. W.; Prakash, O.; Uhlig, J.; Yartsev, A.; Wärnmark, K.; Sundström, V.; Persson, P. Photofunctionality of iron(III) N-heterocyclic carbenes and related d transition metal complexes. Coord. Chem. Rev. 2021, 426, 213517,  DOI: 10.1016/j.ccr.2020.213517
      67
      Photofunctionality of iron(III) N-heterocyclic carbenes and related d5 transition metal complexes
      Chabera, Pavel; Lindh, Linnea; Rosemann, Nils W.; Prakash, Om; Uhlig, Jens; Yartsev, Arkady; Waernmark, Kenneth; Sundstroem, Villy; Persson, Petter
      Coordination Chemistry Reviews (2021), 426 (), 213517CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A Review. Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d5 electronic configuration, the photophysics and photochem. of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophys. and photochem. properties assocd. with low-spin (doublet) ligand-to-metal charge transfer (2LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidn. and photo-redn. reactions. These encouraging results are at first sight surprising in light of the general scarcity of known photofunctional complexes of any transition metal complexes with a d5 electronic configuration, including 1st, 2nd and 3rd row transition metal complexes of Mn(II), Tc(II), Re(II), Fe(III), Ru(III) and Os(III). Here, we review the photophys. and photochem. properties of the new Fe(III) NHC complexes together with related d5 transition metal complexes as a basis for a broader understanding of the unorthodox photophys. and photochem. properties assocd. with this open-shell electronic configuration. This includes considerations of the role of charge and spin effects on the ground state electronic structure, as well as discussions of charge transfer (CT) and metal centered (MC) excited state properties.
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    68. 68
      Rein, C.; Uhlig, J.; Carrasco-Busturia, D.; Khalili, K.; Gertsen, A. S.; Moltke, A.; Zhang, X.; Katayama, T.; Lastra, J. M. G.; Nielsen, M. M. Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution. Structural Dynamics 2021, 8, 024501,  DOI: 10.1063/4.0000055
      68
      Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution
      Rein, Christian; Uhlig, Jens; Carrasco-Busturia, David; Khalili, Khadijeh; Gertsen, Anders S.; Moltke, Asbjoern; Zhang, Xiaoyi; Katayama, Tetsuo; Lastra, Juan Maria Garcia; Nielsen, Martin Meedom; Adachi, Shin-Ichi; Haldrup, Kristoffer; Andreasen, Jens Wenzel
      Structural Dynamics (2021), 8 (2), 024501CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
      Ultrafast, light-induced dynamics in copper-zinc-tin-sulfide (CZTS) photovoltaic nanoparticles are investigated through a combination of optical and x-ray transient absorption spectroscopy. Laser-pump, x-ray-probe spectroscopy on a colloidal CZTS nanoparticle ink yields element-specificity, which reveals a rapid photo-induced shift of electron d. away from Cu-sites, affecting the MO occupation and structure of CZTS. We observe the formation of a stable charge-sepd. and thermally excited structure, which persists for nanoseconds and involves an increased charge d. at the Zn sites. Combined with d. functional theory calcns., the results provide new insight into the structural and electronic dynamics of CZTS absorbers for solar cells. (c) 2021 American Institute of Physics.
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    69. 69
      Kaufhold, S.; Rosemann, N. W.; Chábera, P.; Lindh, L.; Bolaño Losada, I. B.; Uhlig, J.; Pascher, T.; Strand, D.; Wärnmark, K.; Yartsev, A. Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene–Co(III) Complex. J. Am. Chem. Soc. 2021, 143, 1307– 1312,  DOI: 10.1021/jacs.0c12151
      69
      Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex
      Kaufhold, Simon; Rosemann, Nils W.; Chabera, Pavel; Lindh, Linnea; Bolano Losada, Iria; Uhlig, Jens; Pascher, Torbjoern; Strand, Daniel; Waernmark, Kenneth; Yartsev, Arkady; Persson, Petter
      Journal of the American Chemical Society (2021), 143 (3), 1307-1312CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The photofunctionality of the cobalt-hexacarbene complex [Co(III)(PhB(MeIm)3)2]+ (PhB(MeIm)3 = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ~ 10-4) but remarkably long-lived (τ ~ 1μs) orange photoluminescence at 690 nm in soln. at room temp. following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chem. calcns. as a rare case of luminescence from a metal-centered state in a 3d6 complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.
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    70. 70
      Lindh, L.; Gordivska, O.; Persson, S.; Michaels, H.; Fan, H.; Chábera, P.; Rosemann, N. W.; Gupta, A. K.; Benesperi, I.; Uhlig, J. Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality. Chemical Science 2021, 12, 16035– 16053,  DOI: 10.1039/D1SC02963K
      70
      Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality
      Lindh, Linnea; Gordivska, Olga; Persson, Samuel; Michaels, Hannes; Fan, Hao; Chabera, Pavel; Rosemann, Nils W.; Gupta, Arvind Kumar; Benesperi, Iacopo; Uhlig, Jens; Prakash, Om; Sheibani, Esmaeil; Kjaer, Kasper S.; Boschloo, Gerrit; Yartsev, Arkady; Freitag, Marina; Lomoth, Reiner; Persson, Petter; Waernmark, Kenneth
      Chemical Science (2021), 12 (48), 16035-16053CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A new generation of octahedral iron(II)-N-heterocyclic carbene (NHC) complexes, employing different tridentate Ĉ N̂ C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(II) center, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochem., and quantum chem. calcns. demonstrate the improved mol. excited state properties in terms of a broader absorption spectrum compared to the ref. complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I-/I3- redox mediator and std. operating conditions, outcompeting the corresponding DSSC based on the homoleptic ref. complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.
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    71. 71
      Lindh, L.; Chábera, P.; Rosemann, N. W.; Uhlig, J.; Wärnmark, K.; Yartsev, A.; Sundström, V.; Persson, P. Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis. Catalysts 2020, 10, 315,  DOI: 10.3390/catal10030315
      71
      Photophysics and photochemistry of iron carbene complexes for solar energy conversion and photocatalysis
      Lindh, Linnea; Chabera, Pavel; Rosemann, Nils W.; Uhlig, Jens; Warnmark, Kenneth; Yartsev, Arkady; Sundstrom, Villy; Persson, Petter
      Catalysts (2020), 10 (3), 315CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
      A review. Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compds. based on iron are esp. interesting, as iron is the most common transition metal element in the Earth's crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compds. with significantly improved photophys. and photochem. properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochem. of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochem. properties of iron carbenes and related complexes for photovoltaic, photoelectrochem. and photocatalytic applications.
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    72. 72
      Kjær, K. S.; VanDriel, T. B.; Harlang, T. C. B.; Kunnus, K.; Biasin, E.; Ledbetter, K.; Hartsock, R. W.; Reinhard, M. E.; Koroidov, S.; Li, L. Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy. Chemical Science 2019, 10, 5749– 5760,  DOI: 10.1039/C8SC04023K
      72
      Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy
      Kjaer, Kasper S.; Van Driel, Tim B.; Harlang, Tobias C. B.; Kunnus, Kristjan; Biasin, Elisa; Ledbetter, Kathryn; Hartsock, Robert W.; Reinhard, Marco E.; Koroidov, Sergey; Li, Lin; Laursen, Mads G.; Hansen, Frederik B.; Vester, Peter; Christensen, Morten; Haldrup, Kristoffer; Nielsen, Martin M.; Dohn, Asmus O.; Papai, Matyas I.; Moeller, Klaus B.; Chabera, Pavel; Liu, Yizhu; Tatsuno, Hideyuki; Timm, Cornelia; Jarenmark, Martin; Uhlig, Jens; Sundstom, Villy; Warnmark, Kenneth; Persson, Petter; Nemeth, Zoltan; Szemes, Dorottya Sarosine; Bajnoczi, Eva; Vanko, Gyorgy; Alonso-Mori, Roberto; Glownia, James M.; Nelson, Silke; Sikorski, Marcin; Sokaras, Dimosthenis; Canton, Sophie E.; Lemke, Henrik T.; Gaffney, Kelly J.
      Chemical Science (2019), 10 (22), 5749-5760CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Light-driven mol. reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, exptl. approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 fs time-resoln. provide a solid exptl. foundation for detg. the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such expts., it becomes possible to identify the dominant trajectory followed during the excited state cascade and to det. the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2'-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered mol. systems involving 3d transition metals.
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    73. 73
      Skov, A. B.; Ree, N.; Gertsen, A. S.; Chabera, P.; Uhlig, J.; Lissau, J. S.; Nucci, L.; Pullerits, T.; Mikkelsen, K. V.; Brøndsted Nielsen, M. Excited-State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity. ChemPhotoChem. 2019, 3, 619– 629,  DOI: 10.1002/cptc.201900088
      73
      Excited-State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity
      Skov, Anders B.; Ree, Nicolai; Gertsen, Anders S.; Chabera, Pavel; Uhlig, Jens; Lissau, Jonas S.; Nucci, Luigi; Pullerits, Tonu; Mikkelsen, Kurt V.; Brondsted Nielsen, Mogens; Solling, Theis I.; Hansen, Thorsten
      ChemPhotoChem (2019), 3 (8), 619-629CODEN: CHEMYH ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The gain and loss of aromaticity plays a key role in org. chem. and in the prediction of rate-detg. steps. Herein, we explore the concept of aromaticity in photoisomerization reactions. Benzannulated derivs. of the dihydroazulene-vinylheptafulvene (DHA-VHF) photoswitch were investigated using transient absorption spectroscopy and time-dependent d. functional theory to elucidate the effect of built-in aromaticity on the switching properties. We found that benzannulation hampered the switching ability by enhancing an already existing barrier on the excited state surface. This enhancement was found to arise from a significant loss of aromaticity in the DHA-to-VHF transition state on the excited state potential energy surface. The VHF was found to be highly arom. on the excited state surface, showing a reversal of aromaticity compared to the ground state. The barrier was found to be dependent on the position of benzannulation, since one deriv. was found to switch as fast as the non-benzannulated mol. although with lower efficiency, whereas another deriv. completely lost the ability to undergo reversible photoswitching. The findings herein provide novel principles for the design of mol. photoswitches, shedding new light on excited state aromaticity, as previous discussions have mainly considered excited state aromaticity to be beneficial to switching. Our findings show that this view must be reconsidered.
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    74. 74
      Chábera, P.; Kjær, K. S.; Prakash, O.; Honarfar, A.; Liu, Y.; Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundström, V. FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime. J. Phys. Chem. Lett. 2018, 9, 459– 463,  DOI: 10.1021/acs.jpclett.7b02962
      74
      FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime
      Chabera, Pavel; Kjaer, Kasper S.; Prakash, Om; Honarfar, Alireza; Liu, Yizhu; Fredin, Lisa A.; Harlang, Tobias C. B.; Lidin, Sven; Uhlig, Jens; Sundstroem, Villy; Lomoth, Reiner; Persson, Petter; Waernmark, Kenneth
      Journal of Physical Chemistry Letters (2018), 9 (3), 459-463CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN soln. that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidn. states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, resp., demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.
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    75. 75
      Kjær, K. S.; Kunnus, K.; Harlang, T. C. B.; Van Driel, T. B. V.; Ledbetter, K.; Hartsock, R. W.; Reinhard, M. E.; Koroidov, S.; Li, L.; Laursen, M. G. Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2’-bipyridine)(CN)4]2–. Phys. Chem. Chem. Phys. 2018, 20, 4238– 4249,  DOI: 10.1039/C7CP07838B
      75
      Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2'-bipyridine)(CN)4]2-
      Kjaer, Kasper S.; Kunnus, Kristjan; Harlang, Tobias C. B.; Van Driel, Tim B.; Ledbetter, Kathryn; Hartsock, Robert W.; Reinhard, Marco E.; Koroidov, Sergey; Li, Lin; Laursen, Mads G.; Biasin, Elisa; Hansen, Frederik B.; Vester, Peter; Christensen, Morten; Haldrup, Kristoffer; Nielsen, Martin M.; Chabera, Pavel; Liu, Yizhu; Tatsuno, Hideyuki; Timm, Cornelia; Uhlig, Jens; Sundstom, Villy; Nemeth, Zoltan; Szemes, Dorottya Sarosine; Bajnoczi, Eva; Vanko, Gyorgy; Alonso-Mori, Roberto; Glownia, James M.; Nelson, Silke; Sikorski, Marcin; Sokaras, Dimosthenis; Lemke, Henrik T.; Canton, Sophie E.; Warnmark, Kenneth; Persson, Petter; Cordones, Amy A.; Gaffney, Kelly J.
      Physical Chemistry Chemical Physics (2018), 20 (6), 4238-4249CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and x-ray emission transient spectroscopies, the authors have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 ps lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as DMSO and MeCN. The authors use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in H2O, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in <100 fs, forming a quasi-stable metal centered excited state with a 13 ps lifetime. This MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(II)-centered coordination compds., which form MC quintet (5MC) states. The solvent dependent changes in excited state nonradiative relaxation for [Fe(bpy)(CN)4]2- allows the authors to infer the influence of the solvent on the electronic structure of the complex. Also, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.
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    76. 76
      Kjær, K. S.; Kaul, N.; Prakash, O.; Chábera, P.; Rosemann, N. W.; Honarfar, A.; Gordivska, O.; Fredin, L. A.; Bergquist, K.-E.; Häggström, L. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 2019, 363, 249– 253,  DOI: 10.1126/science.aau7160
      76
      Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime
      Kjaer, Kasper Skov; Kaul, Nidhi; Prakash, Om; Chabera, Pavel; Rosemann, Nils W.; Honarfar, Alireza; Gordivska, Olga; Fredin, Lisa A.; Bergquist, Karl-Erik; Haeggstroem, Lennart; Ericsson, Tore; Lindh, Linnea; Yartsev, Arkady; Styring, Stenbjoern; Huang, Ping; Uhlig, Jens; Bendix, Jesper; Strand, Daniel; Sundstroem, Villy; Persson, Petter; Lomoth, Reiner; Waernmark, Kenneth
      Science (Washington, DC, United States) (2019), 363 (6424), 249-253CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Iron's abundance and rich coordination chem. are potentially appealing features for photochem. applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is phenyl[tris(3-methylimidazol-1-ylidene)]borate-, exhibits strong, visible, room temp. photoluminescence with a 2.0-ns lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were obsd. for the 2LMCT state of [Fe(phtmeimb)2]+ in bimol. quenching studies with methylviologen and diphenylamine.
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    77. 77
      ElNahhas, A.; Shameem, M. A.; Chabera, P.; Uhlig, J.; Orthaber, A. Synthesis and characterization of cyclopentadithiophene heterofulvenes - Design tools for light activated processes. Chem. Eur. J. 2017, 23, 5673– 5677,  DOI: 10.1002/chem.201700917
      There is no corresponding record for this reference.
    78. 78
      Pascher, T.; Chesick, J. P.; Winkler, J. R.; Gray, H. B. Protein Folding Triggered by Electron Transfer. Science 1996, 271, 1558– 1560,  DOI: 10.1126/science.271.5255.1558
      78
      Protein folding triggered by electron transfer
      Pascher, Torbjorn; Chesick, John P.; Winkler, Jay R.; Gray, Harry B.
      Science (Washington, D. C.) (1996), 271 (5255), 1558-60CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Rapid photochem. electron injection into unfolded ferricytochrome c titrated with 2.3 to 4.6 M guanidine hydrochloride (GuHCl) at pH 7 and 40° produced unfolded ferrocytochrome, which then converted to the folded protein. Two folding phases were obsd.: a fast process with a time const. of 40 μs (4.6 M GuHCl), and a slower phase with a rate const. of 90 per s (2.3 M GuHCl). The activation free energy for the slow step varied linearly with GuHCl concn.; the rate const., extrapolated to aq. soln., was 7600 per s. Electron-transfer methods can bridge the nanosecond to millisecond measurement time gap for protein folding.
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    79. 79
      De, S.; Pascher, T.; Maiti, M.; Jespersen, K. G.; Kesti, T.; Zhang, F.; Inganäs, O.; Yartsev, A.; Sundström, V. Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends. J. Am. Chem. Soc. 2007, 129, 8466– 8472,  DOI: 10.1021/ja068909q
      79
      Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends
      De, Swati; Pascher, Torbjoern; Maiti, Manisankar; Jespersen, Kim G.; Kesti, Tero; Zhang, Fengling; Inganaes, Olle; Yartsev, Arkady; Sundstroem, Villy
      Journal of the American Chemical Society (2007), 129 (27), 8466-8472CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      By measuring excited state and charge dynamics in blends of an alternating polyfluorene copolymer and fullerene deriv. over nine orders in time and two orders in light intensity, the light-induced processes were monitored from ultrafast charge photogeneration to much slower decay of charges by recombination. At low light intensities relevant to solar cell operation, relatively fast (∼30 ns) geminate recombination is the dominating charge decay process, while nongeminate recombination has a negligible contribution. Under solar illumination conditions, geminate recombination of charges may be directly competing with efficient charge collection in polymer/fullerene solar cells.
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    80. 80
      Newville, M.; Stensitzki, T.; Allen, D. B.; Ingargiola, A. LMFIT: Non-Linear Least-Square Minimization and Curve-Fitting for Python (1.0.3). Zenodo 2014,  DOI: 10.5281/zenodo.598352
      There is no corresponding record for this reference.
    81. 81
      Lasdon, L.; Duarte, A.; Glover, F.; Laguna, M.; Martí, R. Adaptive memory programming for constrained global optimization. Computers. & Operations Research 2010, 37, 1500– 1509,  DOI: 10.1016/j.cor.2009.11.006
      There is no corresponding record for this reference.
    82. 82
      Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io”, Section Plotting ; 2022; https://kimopack.readthedocs.io/en/latest/Plotting.html.
      There is no corresponding record for this reference.
    83. 83
      Uhlig, J. KiMoPack v.6.6.2 Documentation on ”ReadTheDocs.io”, Section Saving ; 2022; https://kimopack.readthedocs.io/en/latest/Saving.html.
      There is no corresponding record for this reference.
    84. 84
      Schindler, J.; Zhang, Y.; Traber, P.; Lefebvre, J.-F.; Kupfer, S.; Demeunynck, M.; Gräfe, S.; Chavarot-Kerlidou, M.; Dietzek, B. A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex. J. Phys. Chem. C 2018, 122, 83– 95,  DOI: 10.1021/acs.jpcc.7b08989
      84
      A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex
      Schindler, Julian; Zhang, Ying; Traber, Philipp; Lefebvre, Jean-Francois; Kupfer, Stephan; Demeunynck, Martine; Graefe, Stefanie; Chavarot-Kerlidou, Murielle; Dietzek, Benjamin
      Journal of Physical Chemistry C (2018), 122 (1), 83-95CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The π-extended dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand of the Ru(II) complex [Ru(bpy)2(oxo-dppqp)](PF6)2 (oxo-dppqp = dipyrido[3,2-a:2',3'-c]pyrido[2'',3''-4,5,6]quinolino[2,3-h]phenazin-15-one, bpy = 2,2'-bipyridine) enables the mononuclear complex for visible-light-driven accumulation of two electrons on a single ligand structure. Although this has been shown before, the excited-state physics underlying this promising feature are exploited in this work. The photophysics of the complex was investigated by excitation-wavelength-dependent resonance Raman and transient absorption spectroscopy in combination with time-dependent d. functional theory. The results show that excitation with visible light leads to the population of the two excited-state branches: (i) the population of a short-lived 3MLCT state in which the excess electronic d. is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and (ii) the population of a more long-lived 3ππ* state (τ = 9 ns). Notably, the long-lived 3ππ* state rather than a 3MLCT state is prone to reductive quenching by the sacrificial electron donor and, hence, presents the crit. excited-state intermediate in the photochem. charge accumulation expts.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvV2mtrzF&md5=32b5950cb024497dc521b5ce9962a1e0
    85. 85
      Mengele, A. K.; Müller, C.; Nauroozi, D.; Kupfer, S.; Dietzek, B.; Rau, S. Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes. Inorg. Chem. 2020, 59, 12097– 12110,  DOI: 10.1021/acs.inorgchem.0c01022
      85
      Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes
      Mengele, Alexander K.; Mueller, Carolin; Nauroozi, Djawed; Kupfer, Stephan; Dietzek, Benjamin; Rau, Sven
      Inorganic Chemistry (2020), 59 (17), 12097-12110CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Bi(benz)imidazoles (b(b)i.m.) acting as N,N-chelates in ruthenium complexes represent a unique class of ligands. They do not harbor MLCT excited states in Ru polypyridyl complexes upon visible light excitation provided that no substitution is introduced at the N atoms. Hence, they can be used to steer light-driven electron transfer pathways into a desired direction. Nonetheless, the free N atoms are susceptible to protonation and hence introduce highly pH-dependent properties into the complexes. Previous results for ruthenium complexes contg. R2bbim ligands with alkylic or arylic N,N'-substitution indicated that although pH insensitivity was accomplished unexpected loss of spectator ligand features incurred simultaneously. Here, the authors report the synthesis and photophys. characterization of a series of differently N,N'-alkylated b(b)i.m. ligands along with their corresponding [(tbbpy)2Ru(R2b(b)i.m.)](PF6)2 complexes (tbbpy = 4,4'-tert-butyl-2,2'-bipyridine). The data reveal that elongation of a rigid ethylene bridge by just one methylene group drastically increases the emission quantum yield, emission lifetime and photostability of the resultant complexes. Quantum chem. calcns. support these findings and allow the authors to rationalize the obsd. effects based on the energetic positions of the resp. excited states. The authors suggest that N,N'-propylene protected 1H,1'H-2,2'-biimidazole (prbim) is a suitable spectator ligand as it stabilizes sufficiently long-lived MLCT excited states exclusively localized at auxiliary bipyridine ligands. This ligand represents therefore a vital building block for next generation photochem. mol. devices in artificial photosynthesis. A spectator ligand appears on the horizon: Detailed spectroscopic techniques on a series of bi(benz)imidazole-based Ru polypyridine model compds. reveal a propylene protected biimidazole as suitable spectator ligand which allows to evade the dilemma of intertwined pH-sensitive photophysics and excited state localization.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhs1Kjt73M&md5=283727c26c987abf0e35d9b1bdd4530f
    86. 86
      Zhang, Y.; Traber, P.; Zedler, L.; Kupfer, S.; Gräfe, S.; Schulz, M.; Frey, W.; Karnahl, M.; Dietzek, B. Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system. Phys. Chem. Chem. Phys. 2018, 20, 24843– 24857,  DOI: 10.1039/C8CP04595J
      86
      Cu(I) vs. Ru(II) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system
      Zhang, Ying; Traber, Philipp; Zedler, Linda; Kupfer, Stephan; Graefe, Stefanie; Schulz, Martin; Frey, Wolfgang; Karnahl, Michael; Dietzek, Benjamin
      Physical Chemistry Chemical Physics (2018), 20 (38), 24843-24857CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Heteroleptic Cu(I) complexes are a promising alternative towards traditional Ru(II) photosensitizers. In particular, Cu(I) complexes of the type [Cu(P̂ P)(N̂ N)]+, where N̂ N represents a diimine and P̂ P a bulky diphosphine ligand, are already successfully applied for photocatalysis, org. light-emitting diodes or dye-sensitized solar cells. Therefore, this study aims for the systematic comparison of three novel heteroleptic Cu(I) compds., composed of xantphos (xant) as P̂ P ligand and different diimine ligands with an extended π-system in the backbone, with their structurally related Ru(II) analogs. In these Ru(II) photosensitizers [Ru(bpy)2(N̂ N)]2+ (bpy = 2,2'-bipyridine) the same N̂ N ligands were used, namely, dipyrido[3,2-f:2',3'-h]quinoxaline (dpq) and dipyrido[3,2-a:2',3'-c]phenazine (dppz). To gain an in-depth understanding of the photoinduced charge transfer processes, the photophys. features of these complexes and their electrochem. oxidized/reduced species were studied by a combination of UV-visible absorption, resonance Raman and spectroelectrochem. (TD)DFT calcns. were applied to qual. analyze these measurements. As a result, the heteroleptic Cu(I) complexes exhibit comparable charge transfer properties to their Ru(II) analogs, i.e. upon visible light excitation they undergo a metal-to-ligand charge transfer to the diimine ligand(s). In contrast, the reduced Cu(I)- and Ru(II)-dppz complexes show considerably different electronic transitions. The singly reduced Cu(I)-dppz complexes are able to accumulate an addnl. electron at the phenanthroline moiety upon blue-light excitation, which is beneficial for multi-electron-transfer reactions. Upon low-energy light irradn. electronic transitions from the dppz- anion to the xant ligand are excited, which could shorten the lifetime of the photosensitizer intermediates in an unwanted way.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.2c00907.

    • Detailed explanation of the model construction, error analysis, data export, and a summary of the tutorials (PDF)

    • File containing Jupyter notebook demonstrating the general workflow of KiMoPack to analyze TA data of Ru-dppz collected in DCM, ACN and H2O employing two built-in kinetic models (parallel and sequential model), Jupyter notebook showing how to define and implement a user-defined kinetic model in the global fit using Ru-dppz in acetonitrile as an example, Jupyter notebook using the example of Ru-dppz to show how TA data recorded in different solvents, namely DCM, ACN, and H2O, can be (visually) compared to each other and external spectra, i.e., steady-state and spectroelectrochemical absorption data, Jupyter notebook showing how individual TA scans of Ru-dppz (in ACN, DCM, or H2O) can be selected and averaged using an interactive plot, a python module file that contains the documented definitions of three function for the generation of a consecutative model, a non linear power dependent model and a model with distributed rate, folder containing the raw data used for the tutorials, where the data are subdivided into a specific folder per tutorial, and a folder containing the images rendered in the tutorial notebooks (ZIP)


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