Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes

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   Gust, Devens; Moore, Thomas A.; Moore, Ana L.

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   A review of general approaches to artificial photosynthetic fuel prodn. that may be useful for eventually overcoming the energy problem. A variety of research groups have prepd. artificial reaction center mols. These systems contain a chromophore, such as a porphyrin, covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. Following the excitation of the chromophore, photoinduced electron transfer generates a primary charge-sepd. state. Electron transfer chains spatially sep. the redox equiv. and reduce electronic coupling, slowing recombination of the charge-sepd. state to the point that catalysts can use the stored energy for fuel prodn. Antenna systems, employing a variety of chromophores that absorb light throughout the visible spectrum, have been coupled to artificial reaction centers and have incorporated control and photoprotective processes borrowed from photosynthesis. Thus far, researchers have not discovered practical solar-driven catalysts for water oxidn. and fuel prodn. that are robust and use earth-abundant elements, but they have developed artificial systems that use sunlight to produce fuel in the lab. For example, artificial reaction centers, where electrons are injected from a dye mol. into the conduction band of nanoparticulate titanium dioxide on a transparent electrode, coupled to catalysts, such as platinum or hydrogenase enzymes, can produce hydrogen gas. Oxidizing equiv. from such reaction centers can be coupled to iridium oxide nanoparticles, which can oxidize water. This system uses sunlight to split water to oxygen and hydrogen fuel, but efficiencies are low and an external elec. potential is required. Although attempts at artificial photosynthesis fall short of the efficiencies necessary for practical application, they illustrate that solar fuel prodn. inspired by natural photosynthesis is achievable in the lab. More research will be needed to identify the most promising artificial photosynthetic systems and realize their potential.
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   Chemical Reviews (Washington, DC, United States) (2010), 110 (11), 6595-6663CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   A review on dye-sensitized solar cells (DSCs). Some brief notes on solar energy in general and DSC in particular are given, followed by a discussion of the operational principles of DSC (energetics and kinetics). Then, the development of material components and some specific exptl. techniques to characterize DSC are described. The current status of module development is also discussed, and finally a brief future outlook is given.
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   Reddy-Marri, A.; Marchini, E.; Cabanes, V. D.; Argazzi, R.; Pastore, M.; Caramori, S.; Gros, P. C. Panchromatic Light Harvesting and Record Power Conversion Efficiency for Carboxylic/Cyanoacrylic Fe(II) NHC Co-Sensitized FeSSCs. Chem. Sci. 2023, 14, 4288– 4301,  DOI: 10.1039/D2SC05971A

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   Panchromatic light harvesting and record power conversion efficiency for carboxylic/cyanoacrylic Fe(II) NHC co-sensitized FeSSCs

   Reddy-Marri, Anil; Marchini, Edoardo; Cabanes, Valentin Diez; Argazzi, Roberto; Pastore, Mariachiara; Caramori, Stefano; Gros, Philippe C.

   Chemical Science (2023), 14 (16), 4288-4301CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

   Fe(II) pyridyl-NHC sensitizers bearing thienylcyanoacrylic (ThCA) anchoring groups have been designed and characterized with the aim of enhancing the metal to surface charge sepn. and the light harvesting window in iron-sensitized DSSCs (FeSSCs). In these new Fe(II) dyes, the introduction of the ThCA moiety remarkably extended the spectral response and the photocurrent, in comparison with their carboxylic analogs. The co-sensitization based on a mixt. of a carboxylic and a ThCA-iron complex produced a panchromatic absorption, up to 800 nm and the best photocurrent and efficiency (Jsc: 9 mA cm-2 and PCE: 2%) ever reported for an FeSSC.
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   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

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   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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   Dalton Transactions (2006), (41), 4869-4883CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)

   A review. Since the mid 1970's interest in the chem. and applications of ruthenium polypyridyl complexes has increased steadily. In this perspective, the development of this area is tracked and discussed taking into account new scientific developments as well as novel applications. The interaction between basic and applied research is of particular importance and selected examples are highlighted.
6. 6

   Happ, B.; Winter, A.; Hager, M. D.; Schubert, U. S. Photogenerated Avenues in Macromolecules Containing Re(I), Ru(II), Os(II), and Ir(III) Metal Complexes of Pyridine-Based Ligands. Chem. Soc. Rev. 2012, 41, 2222– 2255,  DOI: 10.1039/C1CS15154A

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   Photogenerated avenues in macromolecules containing Re(I), Ru(II), Os(II), and Ir(III) metal complexes of pyridine-based ligands

   Happ, Bobby; Winter, Andreas; Hager, Martin D.; Schubert, Ulrich S.

   Chemical Society Reviews (2012), 41 (6), 2222-2255CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)

   A review. Pyridine-based ligands, such as 2,2'-bipyridine and 1,10-phenanthroline, have gained much interest in the fields of supramol. chem. as well as materials science. The appealing optoelectronic properties of their complexes with heavy d6 transition metal ions, such as Ru(II), Os(II), Re(I) and Ir(III), primarily based on the metal-to-ligand charge-transfer (MLCT) nature featuring access to charge-sepd. states, have provided the starting point for many studies in the field of dye-sensitized solar cells (DSSCs), org. light emitting diodes (OLEDs), artificial photosynthesis and photogenerated electron as well as energy transfer processes. This crit. review provides a comprehensive survey over central advances in the field of sol. metal-contg. macromols. in the last few decades. The synthesis and properties of functionalized 2,2'-bipyridyine- and 1,10-phenanthroline-based d6 metal complexes, in particular, their introduction into different prevailing polymeric structures are highlighted. In the most part of the review metal complexes which have been attached as pendant groups on the polymer side chain are covered. Selected applications of the discussed metal-contg. macromols. are addressed, particularly, with respect to photogenerated electron/energy transfer processes. In order to enable a deeper understanding of the properties of the ligands and metal complexes, the fundamentals of selected photophys. processes will be discussed.
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   Liu, Y.; Harlang, T.; Canton, S. E.; Chábera, P.; Suárez-Alcántara, K.; Fleckhaus, A.; Vithanage, D. A.; Göransson, E.; Corani, A.; Lomoth, R.; Sundström, V.; Wärnmark, K. Towards Longer-Lived Metal-to-Ligand Charge Transfer States of Iron(II) Complexes: An N-Heterocyclic Carbene Approach. Chem. Commun. 2013, 49, 6412– 6414,  DOI: 10.1039/c3cc43833c

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   Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach

   Liu, Yizhu; Harlang, Tobias; Canton, Sophie E.; Chabera, Pavel; Suarez-Alcantara, Karina; Fleckhaus, Andre; Vithanage, Dimali A.; Goeransson, Erik; Corani, Alice; Lomoth, Reiner; Sundstroem, Villy; Waernmark, Kenneth

   Chemical Communications (Cambridge, United Kingdom) (2013), 49 (57), 6412-6414CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

   The prepn., structure and excited state dynamics of two new pincer ligand-contg. Fe(II) N-heterocyclic carbene complexes [(L)2Fe]2+(PF6-)2 [L = 2,6-bis(3-R-imidazol-2-ylidene)pyridine; R = Me (1), Me3C (2)] are reported. A 9 ps 3MLCT lifetime was achieved by the Fe(II) complex 1 as obsd. by ultrafast transient absorption spectroscopy. This is the longest known 3MLCT lifetime so far for any kind of complex of this abundant metal, and increased by almost two orders of magnitude compared to the ref. Fe(II) bis-terpyridine complex.
8. 8

   Duchanois, T.; Etienne, T.; Cebrián, C.; Liu, L.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C. An Iron-Based Photosensitizer with Extended Excited-State Lifetime: Photophysical and Photovoltaic Properties. Eur. J. Inorg. Chem. 2015, 2015, 2469– 2477,  DOI: 10.1002/ejic.201500142

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   An iron-based photosensitizer with extended excited-state lifetime: Photophysical and photovoltaic properties

   Duchanois, Thibaut; Etienne, Thibaud; Cebrian, Cristina; Liu, Li; Monari, Antonio; Beley, Marc; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C.

   European Journal of Inorganic Chemistry (2015), 2015 (14), 2469-2477CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)

   Herein, we report a homoleptic iron complex bearing tridentate bis-carbene (CNC) ligands designed for sensitization of TiO2 photoanodes. Its excited state has been characterized by ultra-fast transient spectroscopy and time-dependent d. functional theory (TD-DFT) computations, which reveal a record triplet metal-to-ligand charge-transfer (3MLCT) excited-state lifetime (16 ps). The new dye was efficiently chemisorbed on TiO2 and promoted electron injection and photocurrent generation in a dye-sensitized solar cell upon solar irradn.
9. 9

   Harlang, T. C. B.; Liu, Y.; Gordivska, O.; Fredin, L. A.; Ponseca, C. S.; Huang, P.; Chábera, P.; Kjaer, K. S.; Mateos, H.; Uhlig, J. Iron Sensitizer Converts Light to Electrons with 92% Yield. Nat. Chem. 2015, 7, 883– 889,  DOI: 10.1038/nchem.2365

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   Iron sensitizer converts light to electrons with 92% yield

   Harlang, Tobias C. B.; Liu, Yizhu; Gordivska, Olga; Fredin, Lisa A.; Ponseca, Carlito S., Jr.; Huang, Ping; Chabera, Pavel; Kjaer, Kasper S.; Mateos, Helena; Uhlig, Jens; Lomoth, Reiner; Wallenberg, Reine; Styring, Stenbjoern; Persson, Petter; Sundstroem, Villy; Waernmark, Kenneth

   Nature Chemistry (2015), 7 (11), 883-889CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)

   Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogs like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of ESR, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chem. calcns., we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the 3MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.
10. 10

    Liu, L.; Duchanois, T.; Etienne, T.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C. A New Record Excited State 3MLCT Lifetime for Metalorganic Iron(II) Complexes. Phys. Chem. Chem. Phys. 2016, 18, 12550– 12556,  DOI: 10.1039/C6CP01418F

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    A new record excited state 3MLCT lifetime for metalorganic iron(II) complexes

    Liu, Li; Duchanois, Thibaut; Etienne, Thibaud; Monari, Antonio; Beley, Marc; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C.

    Physical Chemistry Chemical Physics (2016), 18 (18), 12550-12556CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    The synthesis and time-resolved spectroscopic characterization are reported of a homoleptic Fe(II) complex exhibiting a record 3MLCT lifetime of 26 ps promoted by benzimidazolylidene-based ligands. Time dependent d. functional mol. modeling of the triplet excited state manifold clearly reveals that, at equil. geometries, the lowest 3MC state lies higher in energy than the lowest 3MLCT one.
11. 11

    Fatur, S. M.; Shepard, S. G.; Higgins, R. F.; Shores, M. P.; Damrauer, N. H. A Synthetically Tunable System to Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines. J. Am. Chem. Soc. 2017, 139, 4493– 4505,  DOI: 10.1021/jacs.7b00700

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    A Synthetically Tunable System To Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines

    Fatur, Steven M.; Shepard, Samuel G.; Higgins, Robert F.; Shores, Matthew P.; Damrauer, Niels H.

    Journal of the American Chemical Society (2017), 139 (12), 4493-4505CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    2,2':6',2''-Terpyridyl (tpy) ligands modified by fluorine (dftpy), chlorine (dctpy), or bromine (dbtpy) substitution at the 6- and 6''-positions are used to synthesize a series of bis-homoleptic Fe(II) complexes. Two of these species, [Fe(dctpy)2]2+ and [Fe(dbtpy)2]2+, which incorporate the larger dctpy and dbtpy ligands, assume a high-spin quintet ground state due to substituent-induced intramol. strain. The smaller fluorine atoms in [Fe(dftpy)2]2+ enable spin crossover with a T1/2 of 220 K and a mixt. of low-spin (singlet) and high-spin (quintet) populations at room temp. Taking advantage of this equil., dynamics originating from either the singlet or quintet manifold can be explored using variable wavelength laser excitation. Pumping at 530 nm leads to ultrafast nonradiative relaxation from the singlet metal-to-ligand charge transfer (1MLCT) excited state into a quintet metal centered state (5MC) as has been obsd. for prototypical low-spin Fe(II) polypyridine complexes such as [Fe(tpy)2]2+. On the other hand, pumping at 400 nm excites the mol. into the quintet manifold (5MLCT ← 5MC) and leads to the observation of a greatly increased MLCT lifetime of 14.0 ps. Importantly, this measurement enables an exploration of how the lifetime of the 5MLCT (or 7MLCT, in the event of intersystem crossing) responds to the structural modifications of the series as a whole. We find that increasing the amt. of steric strain serves to extend the lifetime of the 5,7MLCT from 14.0 ps for [Fe(dftpy)2]2+ to the largest known value at 17.4 ps for [Fe(dbtpy)2]2+. These data support the design hypothesis wherein interligand steric interactions are employed to limit conformational dynamics and/or alter relative state energies, thereby slowing nonradiative loss of charge-transfer energy.
12. 12

    Duchanois, T.; Liu, L.; Pastore, M.; Monari, A.; Cebrián, C.; Trolez, Y.; Darari, M.; Magra, K.; Francés-Monerris, A.; Domenichini, E.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C. NHC-Based Iron Sensitizers for DSSCs. Inorganics 2018, 6, 63,  DOI: 10.3390/inorganics6020063

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    NHC-based iron sensitizers for DSSCs

    Duchanois, Thibaut; Liu, Li; Pastore, Mariachiara; Monari, Antonio; Cebrian, Cristina; Trolez, Yann; Darari, Mohamed; Magra, Kevin; Frances-Monerris, Antonio; Domenichini, Edoardo; Beley, Marc; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C.

    Inorganics (2018), 6 (2), 63/1-63/26CODEN: INORCW; ISSN:2304-6740. (MDPI AG)

    Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal-ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO2. In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophys., electrochem., and computational characterization.
13. 13

    Francés-Monerris, A.; Magra, K.; Darari, M.; Cebrián, C.; Beley, M.; Domenichini, E.; Haacke, S.; Pastore, M.; Assfeld, X.; Gros, P. C.; Monari, A. Synthesis and Computational Study of a Pyridylcarbene Fe(II) Complex: Unexpected Effects of fac/mer Isomerism in Metal-to-Ligand Triplet Potential Energy Surfaces. Inorg. Chem. 2018, 57, 10431– 10441,  DOI: 10.1021/acs.inorgchem.8b01695

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    Synthesis and Computational Study of a Pyridylcarbene Fe(II) Complex: Unexpected Effects of fac/mer Isomerism in Metal-to-Ligand Triplet Potential Energy Surfaces

    Frances-Monerris, Antonio; Magra, Kevin; Darari, Mohamed; Cebrian, Cristina; Beley, Marc; Domenichini, Edoardo; Haacke, Stefan; Pastore, Mariachiara; Assfeld, Xavier; Gros, Philippe C.; Monari, Antonio

    Inorganic Chemistry (2018), 57 (16), 10431-10441CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    The synthesis and the steady-state absorption spectrum of a new pyridine-imidazolylidene Fe(II) complex (Fe-NHC) are presented. A detailed mechanism of the triplet metal-to-ligand charge-transfer states decay is provided on the basis of min. energy path (MEP) calcns. used to connect the lowest-lying singlet, triplet, and quintet state min. The competition between the different decay pathways involved in the photoresponse is assessed by analyzing the shapes of the obtained potential energy surfaces. A qual. difference between facial (fac) and meridional (mer) isomers' potential energy surface (PES) topologies is evidenced for the 1st time in Fe-based complexes. Indeed, the mer complex shows a steeper triplet path toward the corresponding 3MC min., which lies at a lower energy as compared to the fac isomer, thus pointing to a faster triplet decay of the former. Furthermore, while a major role of the metal-centered quintet state population from the triplet 3MC region is excluded, the authors identify the enlargement of Fe-N bonds as the main normal modes driving the excited-state decay.
14. 14

    Chábera, P.; Kjaer, K. S.; Prakash, O.; Honarfar, A.; Liu, Y.; Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundström, V.; Lomoth, R.; Persson, P.; Wärnmark, K. Fe(II) 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.
15. 15

    Magra, K.; Domenichini, E.; Francés-Monerris, A.; Cebrián, C.; Beley, M.; Darari, M.; Pastore, M.; Monari, A.; Assfeld, X.; Haacke, S.; Gros, P. C. Impact of the fac/mer Isomerism on the Excited-State Dynamics of Pyridyl-Carbene Fe(II) Complexes. Inorg. Chem. 2019, 58, 5069– 5081,  DOI: 10.1021/acs.inorgchem.9b00138

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    Impact of the fac/mer Isomerism on the Excited-State Dynamics of Pyridyl-carbene Fe(II) Complexes

    Magra, Kevin; Domenichini, Edoardo; Frances-Monerris, Antonio; Cebrian, Cristina; Beley, Marc; Darari, Mohamed; Pastore, Mariachiara; Monari, Antonio; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C.

    Inorganic Chemistry (2019), 58 (8), 5069-5081CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    The control of photophys. properties of iron complexes and esp. of their excited states decay is a great challenge in the search for sustainable alternatives to noble metals in photochem. applications. Herein we report the synthesis and investigations of the photophysics of mer and fac iron complexes bearing bidentate pyridyl-NHC ligands, coordinating the iron with three ligand-field-enhancing carbene bonds. Ultrafast transient absorption spectroscopy reveals two distinct excited state populations for both mer and fac forms, ascribed to the populations of the T1 and the T2 states, resp., which decay to the ground state via parallel pathways. We find 3-4 ps and 15-20 ps excited-state lifetimes, with resp. amplitudes depending on the isomer. The longer lifetime exceeds the one reported for iron complexes with tridentate ligands analogs involving four iron-carbene bonds. By combining exptl. and computational results, a mechanism based on the differential trapping of the triplet states in spin-crossover regions is proposed for the first time to explain the impact of the fac/mer isomerism on the overall excited-state lifetimes. Our results clearly highlight the impact of bidentate pyridyl-NHC ligands on the photophysics of iron complexes, esp. the paramount role of fac/mer isomerism in modulating the overall decay process, which can be potentially exploited in the design of new Fe(II)-based photoactive compds.
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    Steube, J.; Burkhardt, L.; Päpcke, A.; Moll, J.; Zimmer, P.; Schoch, R.; Wölper, C.; Heinze, K.; Lochbrunner, S.; Bauer, M. Excited-State Kinetics of an Air-Stable Cyclometalated Iron(II) Complex. Chem.─Eur. J. 2019, 25, 11826– 11830,  DOI: 10.1002/chem.201902488

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    Excited-State Kinetics of an Air-Stable Cyclometalated Iron(II) Complex

    Steube, Jakob; Burkhardt, Lukas; Paepcke, Ayla; Moll, Johannes; Zimmer, Peter; Schoch, Roland; Woelper, Christoph; Heinze, Katja; Lochbrunner, Stefan; Bauer, Matthias

    Chemistry - A European Journal (2019), 25 (51), 11826-11830CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The title complex class with an Earth-abundant metal ion has been repeatedly suggested as a chromophore and potential photosensitizer on the basis of quantum chem. calcns. Synthesis and photophys. properties of the parent complex [Fe(pbpy)(tpy)]+ (Hpbpy=6-phenyl-2,2'-bipyridine and tpy=2,2':6',2''-terpyridine) of this new chromophore class are now reported. Ground-state characterization by X-ray diffraction, electrochem., spectroelectrochem., UV/Vis, and X-ray spectroscopy in combination with DFT calcns. proves the high impact of the cyclometalating ligand on the electronic structure. The photophys. properties are significantly improved compared to the prototypical [Fe(tpy)2]2+ complex. In particular, the metal-to-ligand absorption extends into the near-IR and the 3MLCT lifetime increases by 5.5, whereas the metal-centered excited triplet state is very short-lived.
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    Braun, J.; Lozada, I.; Kolodziej, C.; Burda, C.; Newman, K.; Van Lierop, J.; Davis, R.; Herbert, D. Iron(II) Coordination Complexes with Panchromatic Absorption and Nanosecond Charge-Transfer Excited State Lifetimes. Nat. Chem. 2019, 11, 1144– 1150,  DOI: 10.1038/s41557-019-0357-z

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    Iron(II) coordination complexes with panchromatic absorption and nanosecond charge-transfer excited state lifetimes

    Braun, Jason D.; Lozada, Issiah B.; Kolodziej, Charles; Burda, Clemens; Newman, Kelly M. E.; van Lierop, Johan; Davis, Rebecca L.; Herbert, David E.

    Nature Chemistry (2019), 11 (12), 1144-1150CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)

    Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is crit. Here, we present a sensitizer design in which iron(II) centers are supported by frameworks contg. benzannulated phenanthridine and quinoline heterocycles paired with amido donors. These complexes exhibit panchromatic absorption and nanosecond charge-transfer excited state lifetimes, enabled by the combination of vacant, energetically accessible heterocycle-based acceptor orbitals and occupied MOs destabilized by strong mixing between amido nitrogen atoms and iron. This finding shows how ligand design can extend metal-to-ligand charge-transfer-type excited state lifetimes of iron(II) complexes into the nanosecond regime and expand the range of potential applications for iron-based photosensitizers.
18. 18

    Tang, Z.; Chang, X.-Y.; Wan, Q.; Wang, J.; Ma, C.; Law, K.-C.; Liu, Y.; Che, C.-M. Bis(tridentate) Iron(II) Complexes with a Cyclometalating Unit: Photophysical Property Enhancement with Combinatorial Strong Ligand Field Effect. Organometallics 2020, 39, 2791– 2802,  DOI: 10.1021/acs.organomet.0c00149

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    Bis(tridentate) Iron(II) Complexes with a Cyclometalating Unit: Photophysical Property Enhancement with Combinatorial Strong Ligand Field Effect

    Tang, Zhou; Chang, Xiao-Yong; Wan, Qingyun; Wang, Jian; Ma, Chensheng; Law, Kwok-Chung; Liu, Yungen; Che, Chi-Ming

    Organometallics (2020), 39 (15), 2791-2802CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    A series of bis(tridentate) cyclometalated oligopyridine complexes [FeII(C N N)(L)]+ have been synthesized via the reactions of [FeII(C Npy Npy)Br(PMe3)2] (HC Npy Npy = 6-(phenyl)-2,2'-bipyridine) and [FeII(C Nppy Nisoquin)Br(PMe3)2] (HC Nppy Nisoquin = 3-(4,6-diphenylpyridin-2-yl)isoquinoline) with tridentate ligands L, affording [FeII(C Npy Npy)(4'-R-tpy)]+ (R = H, Cl, p-ClC6H4, p-MeOC6H4, p-tol, resp.; tpy = 2,2':6',2''-terpyridine), [FeII(C Nppy Nisoquin)(tpy)]+, [FeII(C Npy Npy)(6-(Ph2PCH2)-bpy)]+ (bpy = 2,2'-bipyridine), and [FeII(C Npy Npy)(6-NHC-bpy)]+ (NHC = N-heterocyclic carbene). Some of the complexes exhibit bathochromically shifted absorption bands, markedly cathodically shifted E1/2(FeIII/FeII) values, and narrowed electrochem. band gaps in comparison to those of [FeII(tpy)2]2+, as revealed by UV-vis absorption spectroscopy and cyclic voltammetry. DFT and TDDFT calcn. studies revealed strong mixing of MLCT with LLCT transitions as origins to the major absorption bands. Complex [FeII(C Npy Npy)(6-NHC-bpy)]+ incorporating a cyclometalating unit and an NHC donor exhibits panchromic absorption, and [FeII(C Npy Npy)(6-(Ph2PCH2)-bpy)]+ shows markedly enhanced molar absorptivity in the visible region. A femtosecond transient absorption spectroscopic study of [FeII(C Npy Npy)(6-NHC-bpy)]+, coupled with DFT and TDDFT calcns. and spectroelectrochem. measurements, gave evidence for the possible involvement of a 3MLCT state with a lifetime of 21.4 ps.
19. 19

    Darari, M.; Francés-Monerris, A.; Marekha, B.; Doudouh, A.; Wenger, E.; Monari, A.; Haacke, S.; Gros, P. C. Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties. Molecules 2020, 25, 5991,  DOI: 10.3390/molecules25245991

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    19

    Towards iron(II) complexes with octahedral geometry: synthesis, structure and photophysical properties

    Darari, Mohamed; Frances-Monerris, Antonio; Marekha, Bogdan; Doudouh, Abdelatif; Wenger, Emmanuel; Monari, Antonio; Haacke, Stefan; Gros, Philippe C.

    Molecules (2020), 25 (24), 5991CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)

    The control of ligand-field splitting in iron (II) complexes is crit. to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepd. from pyridylNHC and pyridylquinoline type ligands, resp., have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that-despite the geometrical improvement-the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent deactivation channel into the metal-centered quintet state, as for other Fe-polypyridine complexes.
20. 20

    Magra, K.; Darari, M.; Domenichini, E.; Francés-Monerris, A.; Cebrián, C.; Beley, M.; Pastore, M.; Monari, A.; Assfeld, X.; Haacke, S.; Gros, P. C. Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands. J. Phys. Chem. C 2020, 124, 18379– 18389,  DOI: 10.1021/acs.jpcc.0c03638

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    20

    Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

    Magra, Kevin; Darari, Mohamed; Domenichini, Edoardo; Frances-Monerris, Antonio; Cebrian, Cristina; Beley, Marc; Pastore, Mariachiara; Monari, Antonio; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C.

    Journal of Physical Chemistry C (2020), 124 (34), 18379-18389CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    The possibility of achieving luminescent and photophys. active metal-org. compds. relies on the stabilization of charge transfer states and kinetically and thermodynamically blocking non-radiative dissipative channels. In this contribution, we explore the behavior of bidentate iron complexes bearing N-heterocyclic carbene ligands with extended conjugation systems by a multidisciplinary approach combining chem. synthesis, ultrafast time-resolved spectroscopy, and mol. modeling. Lifetimes of the metal-to-ligand charge transfer and metal-centered states reaching up to ≈20 ps are evidenced, while complex decay mechanisms are pointed out, together with a possible influence of the facial and meridional isomerism. The structural degrees of freedom driving the non-radiative processes are highlighted, and their rigidification is suggested as an effective way to further increase the lifetimes.
21. 21

    Vittardi, S. B.; Magar, R. T.; Schrage, B. R.; Ziegler, C. J.; Jakubikova, E.; Rack, J. J. Evidence for a Lowest Energy 3MLCT Excited State in [Fe(tpy)(CN)₃]⁻. Chem. Commun. 2021, 57, 4658– 4661,  DOI: 10.1039/D1CC01090E

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    21

    Evidence for a lowest energy 3MLCT excited state in [Fe(tpy)(CN)3]-

    Vittardi, Sebastian B.; Magar, Rajani Thapa; Schrage, Briana R.; Ziegler, Christopher J.; Jakubikova, Elena; Rack, Jeffrey J.

    Chemical Communications (Cambridge, United Kingdom) (2021), 57 (38), 4658-4661CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

    Transient absorption data of [FeII(tpy)(CN)3]- reveals spectroscopic signatures indicative of 3MLCT with a ~ 10 ps kinetic component. These data are supported by DFT and TD-DFT calcns., which show that excited state ordering is responsive to the no. of cyanide ligands on the complex.
22. 22

    Leis, W.; Argüello Cordero, M. A.; Lochbrunner, S.; Schubert, H.; Berkefeld, A. A Photoreactive Iron(II) Complex Luminophore. J. Am. Chem. Soc. 2022, 144, 1169– 1173,  DOI: 10.1021/jacs.1c13083

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    22

    A Photoreactive Iron(II) Complex Luminophore

    Leis, Wolfgang; Argueello Cordero, Miguel A.; Lochbrunner, Stefan; Schubert, Hartmut; Berkefeld, Andreas

    Journal of the American Chemical Society (2022), 144 (3), 1169-1173CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Controlling the order and lifetimes of electronically excited states is essential to effective light-to-potential energy conversion by mol. chromophores. This work reports a luminescent and photoreactive iron(II) complex, the first performant group homolog of prototypical sensitizers of ruthenium. Double cyclometalation of a phenylphenanthroline framework at iron(II) favors the population of a triplet metal-to-ligand charge transfer (3MLCT) state as the lowest energy excited state. Near-IR (NIR) luminescence exhibits a monoexponential decay with τ = 2.4 ns in the solid state and 1 ns in liq. phase. Lifetimes of 14 ns at 77 K are in line with a narrowing of the NIR emission band at λem,max = 1170-1230 nm. Featuring a 3MLCT excited-state redox potential of -2 V vs the ferrocene/ferrocenium couple, the use of the Fe(II) chromophore as a sensitizer for light-driven synthesis is exemplified by the radical cross-coupling of 4-chlorobromobenzene and benzene.
23. 23

    Moll, J.; Förster, C.; König, A.; Carrella, L. M.; Wagner, M.; Panthöfer, M.; Möller, A.; Rentschler, E.; Heinze, K. Panchromatic Absorption and Oxidation of an Iron(II) Spin Crossover Complex. Inorg. Chem. 2022, 61, 1659– 1671,  DOI: 10.1021/acs.inorgchem.1c03511

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    23

    Panchromatic Absorption and Oxidation of an Iron(II) Spin Crossover Complex

    Moll, Johannes; Foerster, Christoph; Koenig, Alexandra; Carrella, Luca M.; Wagner, Manfred; Panthoefer, Martin; Moeller, Angela; Rentschler, Eva; Heinze, Katja

    Inorganic Chemistry (2022), 61 (3), 1659-1671CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    In order to expand and exploit the useful properties of d6-iron(II) and d5-iron(III) complexes in potential magnetic, photophys., or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push-pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal-ligand orbital overlap. The electronic ground states of the pseudo-octahedral d6- and d5-complexes mer-[Fe(dgpy)2]2+, cis-fac-[Fe(dgpy)2]2+, and mer-[Fe(dgpy)2]3+ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temp. UV/visible absorption, NMR, EPR, and Mossbauer spectroscopy, magnetic susceptibility measurements at variable temp. as well as quantum chem. calcns.
24. 24

    Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N. Lifetimes, Spectra, and Quenching of the Excited States of Polypyridine Complexes of Iron(II), Ruthenium(II), and Osmium(II). J. Am. Chem. Soc. 1980, 102, 1309– 1319,  DOI: 10.1021/ja00524a014

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    24

    Lifetimes, spectra, and quenching of the excited states of polypyridine complexes of iron(II), ruthenium(II), and osmium(II)

    Creutz, Carol; Chou, Mei; Netzel, Thomas L.; Okumura, Mitchio; Sutin, Norman

    Journal of the American Chemical Society (1980), 102 (4), 1309-19CODEN: JACSAT; ISSN:0002-7863.

    The lifetimes of the excited states formed by 530-nm excitation of polypyridine complexes of Fe(II) (FeL32+) and Os(II) (OsL32+) were detd. by laser flash-photolysis techniques. The FeL32+ lifetimes, measured in H2O at room temp. using ps absorption spectrometry, are as follows (L, τ ± σ (ns)): 2,2',2''-terpyridine (Z) (2.54 ± 0.13); 2,2'-bipyridine (bpy) (0.81 ± 0.07); 4,4'-dimethyl(2,2'-bipyridine) (0.76 ± 0.04); 1,10-phenanthroline (0.80 ± 0.07); 4,7-(di-Ph sulfonate)-1,10-phenanthroline (0.43 ± 0.03). Lifetimes for the analogous complexes of Os(II) are 10-100 ns under the same conditions. Unlike the excited states of Ru(bpy)32+ and Os(bpy)32+ (λmax 430-460 nm, ε ∼5 × 103 M-1 cm-1), the excited state of Fe(bpy)32+ does not luminesce or absorb significantly in the visible (ε <103 M-1 cm-1 at λ ≥350 nm) but does exhibit intense absorption <∼325 nm. Rate consts. for the quenching of the excited states polypyridine complexes of Os(II) and Ru(II) by ground-state polypyridine complexes of Fe(II), Ru(II), and Os(II) are reported and are ascribed to either electron-transfer or energy-transfer processes. The excited states of Fe(bpy)32+ and of FeZ22+ undergo reaction with Feaq3+ (0.5 M H2SO4, 25°) with a rate const. ≤ 1 × 107 M-1 s-1. Based on a comparison of its properties with those of the luminescent charge-transfer excited states of Ru(II) and Os(II) polypyridine complexes the excited state of FeL32+ is identified as a ligand-field state. The potential of the excited-state couple Fe(bpy)33+ + e .dblharw. *Fe(bpy)32+ is ∼+0.1 V.
25. 25

    Gawelda, W.; Cannizzo, A.; Pham, V.-T.; van Mourik, F.; Bressler, C.; Chergui, M. Ultrafast Nonadiabatic Dynamics of [Fe^II(bpy)3]²⁺ in Solution. J. Am. Chem. Soc. 2007, 129, 8199– 8206,  DOI: 10.1021/ja070454x

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    25

    Ultrafast Nonadiabatic Dynamics of [FeII(bpy)3]2+ in Solution

    Gawelda, Wojciech; Cannizzo, Andrea; Pham, Van-Thai; van Mourik, Frank; Bressler, Christian; Chergui, Majed

    Journal of the American Chemical Society (2007), 129 (26), 8199-8206CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The ultrafast relaxation of aq. Fe(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) was characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence expt. shows a very short-lived broad 1MLCT emission band at ∼600 nm, which decays in ≤20 fs, and a weak emission at ∼660 nm, which the authors attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) <400 nm, and a longer-lived 1 >550 nm, along with the ground-state bleach (GSB). The authors identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decompn. and a global anal. of the TA data, based on a sequential relaxation model, reveal 3 characteristic time scales:120 fs, 960 fs, and 665 ps. The 1st is the decay of the 3MLCT, the 2nd is identified as the population time of the 5T2 state, while the 3rd is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit const. of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, the authors propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate ∼2000 cm-1/100 fs. This rate is discussed, and it nevertheless can be described by the Fermi golden rule, despite its high value.
26. 26

    Auböck, G.; Chergui, M. Sub-50-fs Photoinduced Spin Crossover in [Fe(bpy)3]²⁺. Nat. Chem. 2015, 7, 629– 633,  DOI: 10.1038/nchem.2305

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    26

    Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+

    Aubock, Gerald; Chergui, Majed

    Nature Chemistry (2015), 7 (8), 629-633CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)

    Excitation by visible light of the singlet metal-to-ligand charge-transfer (1MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state (5T) with unity quantum yield. This so-called spin crossover (SCO) transition in aq. tris(bipyridine)iron(2+) was studied. The authors use pump-probe transient absorption spectroscopy with a high time resoln. of <60 fs in the UV probe range, in which the 5T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the 5T state absorb. The 5T state is impulsively populated in <50 fs, which is the time the authors measured for the depopulation of the MCLT manifold. Probably nontotally-sym. modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. Even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) 2-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.
27. 27

    Abrahamsson, M.; Jäger, M.; Österman, T.; Eriksson, L.; Persson, P.; Becker, H.-C.; Johansson, O.; Hammarström, L. A 3.0 micros Room Temperature Excited State Lifetime of a Bistridentate RuII-Polypyridine Complex for Rod-like Molecular Arrays. J. Am. Chem. Soc. 2006, 128, 12616– 12617,  DOI: 10.1021/ja064262y

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    27

    A 3.0 μs Room Temperature Excited State Lifetime of a Bistridentate RuII-Polypyridine Complex for Rod-like Molecular Arrays

    Abrahamsson, Maria; Jaeger, Michael; Oesterman, Tomas; Eriksson, Lars; Persson, Petter; Becker, Hans-Christian; Johansson, Olof; Hammarstroem, Leif

    Journal of the American Chemical Society (2006), 128 (39), 12616-12617CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    A bistridentate RuII-polypyridine complex [Ru(bqp)2]2+ (bqp = 2,6-bis(8'-quinolinyl)pyridine) has been prepd., which has a coordination geometry much closer to a perfect octahedron than the typical Ru(terpyridine)2-type complex. Thus, the complex displays a 3.0 μs lifetime of the lowest excited metal-to-ligand charge transfer (3MLCT) state at room temp. This is, to the best of the authors knowledge, the longest MLCT state lifetime reported for a RuII-polypyridyl complex at room temp. The structure allows for the future construction of rod-like, isomer-free mol. arrays by substitution of donor and acceptor moieties on the central pyridine units. This makes it a promising photosensitizer for applications in mol. devices for artificial photosynthesis and mol. electronics.
28. 28

    Mengel, A. K. C.; Förster, C.; Breivogel, A.; Mack, K.; Ochsmann, J. R.; Laquai, F.; Ksenofontov, V.; Heinze, K. A Heteroleptic Push–Pull Substituted Iron(II) Bis(tridentate) Complex with Low-Energy Charge-Transfer States. Chem.─Eur. J. 2015, 21, 704– 714,  DOI: 10.1002/chem.201404955

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    28

    A Heteroleptic Push-Pull Substituted Iron(II) Bis(tridentate) Complex with Low-Energy Charge-Transfer States

    Mengel, Andreas K. C.; Foerster, Christoph; Breivogel, Aaron; Mack, Katharina; Ochsmann, Julian R.; Laquai, Frederic; Ksenofontov, Vadim; Heinze, Katja

    Chemistry - A European Journal (2015), 21 (2), 704-714CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]2+ with a strongly electron-withdrawing ligand (dcpp, 2,6-bis(2-carboxypyridyl)pyridine) and a strongly electron-donating tridentate tripyridine ligand (ddpd, N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine) is reported. Both ligands form six-membered chelate rings with the Fe center, inducing a strong ligand field. This results in a high-energy, high-spin state (5T2, (t2g)4(eg*)2) and a low-spin ground state (1A1, (t2g)6(eg*)0). The intermediate triplet spin state (3T1, (t2g)5(eg*)1) probably is between these states from the rapid dynamics after photoexcitation. The low-energy π* orbitals of dcpp allow low-energy MLCT absorption plus addnl. low-energy LL'CT absorptions from ddpd to dcpp. The directional charge-transfer character is probed by electrochem. and optical analyses, Mossbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]3+ and [Fe(dcpp)(ddpd)]+, augmented by d. functional calcns. The combined effect of push-pull substitution and the strong ligand field paves the way for long-lived charge-transfer states in Fe(II) complexes.
29. 29

    Britz, A.; Gawelda, W.; Assefa, T. A.; Jamula, L. L.; Yarranton, J. T.; Galler, A.; Khakhulin, D.; Diez, M.; Harder, M.; Doumy, G. Using Ultrafast X-ray Spectroscopy to Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]²⁺. Inorg. Chem. 2019, 58, 9341– 9350,  DOI: 10.1021/acs.inorgchem.9b01063

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    29

    Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]2+

    Britz, Alexander; Gawelda, Wojciech; Assefa, Tadesse A.; Jamula, Lindsey L.; Yarranton, Jonathan T.; Galler, Andreas; Khakhulin, Dmitry; Diez, Michael; Harder, Manuel; Doumy, Gilles; March, Anne Marie; Bajnoczi, Eva; Nemeth, Zoltan; Papai, Matyas; Rozsalyi, Emese; Sarosine Szemes, Dorottya; Cho, Hana; Mukherjee, Sriparna; Liu, Chang; Kim, Tae Kyu; Schoenlein, Robert W.; Southworth, Stephen H.; Young, Linda; Jakubikova, Elena; Huse, Nils; Vanko, Gyoergy; Bressler, Christian; McCusker, James K.

    Inorganic Chemistry (2019), 58 (14), 9341-9350CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    A range of ultrafast x-ray spectroscopies were used in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (dcpp is 2,6-(dicarboxypyridyl)pyridine). This compd. exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temp. fluid soln., raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ x-ray emission spectroscopies were used to unambiguously det. the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compd. Geometric changes assocd. with the photoinduced ligand-field state conversion also were monitored with extended x-ray absorption fine structure. The data show the typical av. Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved x-ray techniques can provide insights into the electronic structures of mols.-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.
30. 30

    Smeigh, A. L.; Creelman, M.; Mathies, R. A.; McCusker, J. K. Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching. J. Am. Chem. Soc. 2008, 130, 14105– 14107,  DOI: 10.1021/ja805949s

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    Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching

    Smeigh, Amanda L.; Creelman, Mark; Mathies, Richard A.; McCusker, James K.

    Journal of the American Chemical Society (2008), 130 (43), 14105-14107CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    A combination of femtosecond electronic absorption and stimulated Raman spectroscopies was employed to det. the kinetics assocd. with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in soln. A time const. of τ = 190 ± 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the UV in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; addnl. dynamics obsd. in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time const. of ca. 10 Ps. The results set an important precedent for extremely rapid, formally forbidden (ΔS = 2) nonradiative relaxation as well as defining the time scale for intramol. optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.
31. 31

    Consani, C.; Prémont-Schwarz, M.; ElNahhas, A.; Bressler, C.; van-Mourik, F.; Cannizzo, A.; Chergui, M. Vibrational Coherences and Relaxation in the High-Spin State of Aqueous [Fe^II(bpy)3]²⁺. Angew. Chem. 2009, 48, 7184– 7187,  DOI: 10.1002/ange.200902728

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    Zhang, W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.; Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.; Kroll, T. Tracking Excited-State Charge and Spin Dynamics in Iron Coordination Complexes. Nature 2014, 509, 345– 348,  DOI: 10.1038/nature13252

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    Tracking excited-state charge and spin dynamics in iron coordination complexes

    Zhang, Wenkai; Alonso-Mori, Roberto; Bergmann, Uwe; Bressler, Christian; Chollet, Matthieu; Galler, Andreas; Gawelda, Wojciech; Hadt, Ryan G.; Hartsock, Robert W.; Kroll, Thomas; Kjaer, Kasper S.; Kubicek, Katharina; Lemke, Henrik T.; Liang, Huiyang W.; Meyer, Drew A.; Nielsen, Martin M.; Purser, Carola; Robinson, Joseph S.; Solomon, Edward I.; Sun, Zheng; Sokaras, Dimosthenis; van Driel, Tim B.; Vanko, Gyorgy; Weng, Tsu-Chien; Zhu, Diling; Gaffney, Kelly J.

    Nature (London, United Kingdom) (2014), 509 (7500), 345-348CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)

    Femtosecond resoln. x-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. The charge and spin dynamics were tracked, and the crit. role of intermediate spin states in the crossover mechanism was established. Possibly these capabilities will make the method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered mol. phenomena involving 3d transition metal complexes.
33. 33

    Haldrup, K.; Gawelda, W.; Abela, R.; Alonso-Mori, R.; Bergmann, U.; Bordage, A.; Cammarata, M.; Canton, S. E.; Dohn, A. O.; van Driel, T. B. Observing Solvation Dynamics with Simultaneous Femtosecond X-ray Emission Spectroscopy and X-ray Scattering. J. Phys. Chem. B 2016, 120, 1158– 1168,  DOI: 10.1021/acs.jpcb.5b12471

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    33

    Observing Solvation Dynamics with Simultaneous Femtosecond X-ray Emission Spectroscopy and X-ray Scattering

    Haldrup, Kristoffer; Gawelda, Wojciech; Abela, Rafael; Alonso-Mori, Roberto; Bergmann, Uwe; Bordage, Amelie; Cammarata, Marco; Canton, Sophie E.; Dohn, Asmus O.; van Driel, Tim Brandt; Fritz, David M.; Galler, Andreas; Glatzel, Pieter; Harlang, Tobias; Kjaer, Kasper S.; Lemke, Henrik T.; Moeller, Klaus B.; Nemeth, Zoltan; Papai, Matyas; Sas, Norbert; Uhlig, Jens; Zhu, Diling; Vanko, Gyorgy; Sundstrom, Villy; Nielsen, Martin M.; Bressler, Christian

    Journal of Physical Chemistry B (2016), 120 (6), 1158-1168CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)

    In liq. phase chem. dynamic solute-solvent interactions often govern the path, ultimate outcome, and efficiency of chem. reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramol. electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermol. dynamics of a model chromophore, aq. [Fe(bpy)3]2+, with complementary X-ray tools in a single expt. exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering anal. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramol. structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can ext. new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent d. increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio mol. dynamics simulations. Structural changes in the aq. solvent assocd. with d. and temp. changes occur with ∼1 ps time consts., characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute mols. well before the solvent contributions become dominant.
34. 34

    Lemke, H.; Kjaer, K.; Hartsock, R.; Van Driel, T.; Chollet, M.; Glownia, J.; Song, S.; Zhu, D.; Pace, E.; Matar, S.; Nielsen, M.; Benfatto, M.; Gaffney, K.; Collet, E.; Cammarata, M. Coherent Structural Trapping Through Wave Packet Dispersion during Photoinduced Spin State Switching. Nat. Commun. 2017, 8, 15342,  DOI: 10.1038/ncomms15342

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    Coherent structural trapping through wave packet dispersion during photoinduced spin state switching

    Lemke, Henrik T.; Kjaer, Kasper S.; Hartsock, Robert; van Driel, Tim B.; Chollet, Matthieu; Glownia, James M.; Song, Sanghoon; Zhu, Diling; Pace, Elisabetta; Matar, Samir F.; Nielsen, Martin M.; Benfatto, Maurizio; Gaffney, Kelly J.; Collet, Eric; Cammarata, Marco

    Nature Communications (2017), 8 (), 15342CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)

    The description of ultrafast nonadiabatic chem. dynamics during mol. photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here, we gain exptl. insights, beyond the Born-Oppenheimer approxn., into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy)3]2+ compd. by time-resolved X-ray absorption spectroscopy at sub-30-fs resoln. and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as mol. breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramol. vibronic coupling before a slower vibrational energy dissipation to the soln. environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional mols.
35. 35

    Carey, M. C.; Adelman, S.; McCusker, J. K. Insights into the Excited State Dynamics of Fe(II) Polypyridyl Complexes from Variable-Temperature Ultrafast Spectroscopy. Chem. Sci. 2019, 10, 134– 144,  DOI: 10.1039/C8SC04025G

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    Insights into the excited state dynamics of Fe(II) polypyridyl complexes from variable-temperature ultrafast spectroscopy

    Carey, Monica C.; Adelman, Sara L.; McCusker, James K.

    Chemical Science (2019), 10 (1), 134-144CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

    In an effort to better define the nature of the nuclear coordinate assocd. with excited state dynamics in first-row transition metal-based chromophores, variable-temp. ultrafast time-resolved absorption spectroscopy has been used to det. activation parameters assocd. with ground state recovery dynamics in a series of low-spin Fe(II) polypyridyl complexes. Our results establish that high-spin (5T2) to low-spin (1A1) conversion in complexes of the form [Fe(4,4'-di-R-2,2'-bpy')3]2+ (R = H, CH3, or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300-350 cm-1 in fluid CH3CN soln., a value that more than doubles to ∼750 cm-1 for [Fe(terpy)2]2+ (terpy = 2,2':6',2''-terpyridine). The data were analyzed in the context of semi-classical Marcus theory. Changes in the ratio of the electronic coupling to reorganization energy (specifically, Hab4/λ) reveal an approx. two-fold difference between the [Fe(bpy')3]2+ complexes (∼1/30) and [Fe(terpy)2]2+ (∼1/14), suggesting a change in the nature of the nuclear coordinate assocd. with ground state recovery between these two types of complexes. These exptl.-detd. ratios, along with ests. for the 5T2/1A1 energy gap, yield electronic coupling values between these two states for the [Fe(bpy')3]2+ series and [Fe(terpy)2]2+ of 4.3 ± 0.3 cm-1 and 6 ± 1 cm-1, resp., values that are qual. consistent with the second-order nature of high-spin/low-spin coupling in a d6 ion. In addn. to providing useful quant. information on these prototypical Fe(II) complexes, these results underscore the utility of variable-temp. spectroscopic measurements for characterizing ultrafast excited state dynamics in this class of compds.
36. 36

    Miller, J. N.; McCusker, J. K. Outer-Sphere Effects on Ligand-Field Excited-State Dynamics: Solvent Dependence of High-Spin to Low-Spin Conversion in [Fe(bpy)3]²⁺. Chem. Sci. 2020, 11, 5191– 5204,  DOI: 10.1039/D0SC01506G

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    Outer-sphere effects on ligand-field excited-state dynamics: solvent dependence of high-spin to low-spin conversion in [Fe(bpy)3]2+

    Miller, Jennifer N.; McCusker, James K.

    Chemical Science (2020), 11 (20), 5191-5204CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

    In condensed phase chem., the solvent can have a significant impact on everything from yield to product distribution to mechanism. With regard to photo-induced processes, solvent effects have been well-documented for charge-transfer states wherein the redistribution of charge subsequent to light absorption couples intramol. dynamics to the local environment of the chromophore. Ligand-field excited states are expected to be largely insensitive to such perturbations given that their electronic rearrangements are localized on the metal center and are therefore insulated from so-called outer-sphere effects by the ligands themselves. In contrast to this expectation, we document herein a nearly two-fold variation in the time const. assocd. with the 5T2 → 1A1 high-spin to low-spin relaxation process of tris(2,2'-bipyridine)iron(II) ([Fe(bpy)3]2+) across a range of different solvents. Likely origins for this solvent dependence, including relevant solvent properties, ion pairing, and changes in solvation energy, were considered and assessed by studying [Fe(bpy)3]2+ and related derivs. via ultrafast time-resolved absorption spectroscopy and computational analyses. It was concluded that the effect is most likely assocd. with the vol. change of the chromophore arising from the interconfigurational nature of the 5T2 → 1A1 relaxation process, resulting in changes to the solvent-solvent and/or solvent-solute interactions of the primary solvation shell sufficient to alter the overall reorganization energy of the system and influencing the kinetics of ground-state recovery.
37. 37

    Jiang, Y.; Liu, L.; Sarracini, A.; Krawczyk, K.; Wentzell, J.; Lu, C.; Field, R.; Matar, S.; Gawelda, W.; Müller-Werkmeister, H. M.; Miller, R. Direct Observation of Nuclear Reorganization Driven by Ultrafast Spin Transitions. Nat. Commun. 2020, 11, 1530,  DOI: 10.1038/s41467-020-15187-y

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    37

    Direct observation of nuclear reorganization driven by ultrafast spin transitions

    Jiang, Yifeng; Liu, Lai Chung; Sarracini, Antoine; Krawczyk, Kamil M.; Wentzell, Jordan S.; Lu, Cheng; Field, Ryan L.; Matar, Samir F.; Gawelda, Wojciech; Mueller-Werkmeister, Henrike M.; Miller, R. J. Dwayne

    Nature Communications (2020), 11 (1), 1530CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)

    One of the most basic mol. photophys. processes is that of spin transitions and intersystem crossing between excited states surfaces. The change in spin states affects the spatial distribution of electron d. through the spin orbit coupling interaction. The subsequent nuclear reorganization reports on the full extent of the spin induced change in electron distribution, which can be treated similarly to intramol. charge transfer with effective reaction coordinates depicting the spin transition. Here, single-crystal [FeII(bpy)3](PF6)2, a prototypical system for spin crossover (SCO) dynamics, is studied using ultrafast electron diffraction in the single-photon excitation regime. The photoinduced SCO dynamics are resolved, revealing two distinct processes with a (450 ± 20)-fs fast component and a (2.4 ± 0.4)-ps slow component. Using principal component anal., we uncover the key structural modes, ultrafast Fe-N bond elongations coupled with ligand motions, that define the effective reaction coordinate to fully capture the relevant mol. reorganization.
38. 38

    Oppermann, M.; Zinna, F.; Lacour, J.; Chergui, M. Chiral Control of Spin-Crossover Dynamics in Fe(II) Complexes. Nat. Chem. 2022, 14, 739– 745,  DOI: 10.1038/s41557-022-00933-0

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    38

    Chiral control of spin-crossover dynamics in Fe(II) complexes

    Oppermann, Malte; Zinna, Francesco; Lacour, Jerome; Chergui, Majed

    Nature Chemistry (2022), 14 (7), 739-745CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)

    Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in mol. devices. However, real-world technol. applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temps. Here the authors demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in soln., assocd. for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ2O1,O2)phosphorus(V) (P(O2C6Cl4)3- or TRISPHAT) anions. The authors characterized the high-spin-state relaxation using broadband ultrafast CD spectroscopy in the deep UV in combination with transient absorption and anisotropy measurements. The authors find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, the authors' results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.
39. 39

    Wolf, M. M. N.; Groß, R.; Schumann, C.; Wolny, J. A.; Schünemann, V.; Døssing, A.; Paulsen, H.; McGarvey, J. J.; Diller, R. Sub-Picosecond Time Resolved Infrared Spectroscopy of High-Spin State Formation in Fe(II) Spin Crossover Complexes. Phys. Chem. Chem. Phys. 2008, 10, 4264– 4273,  DOI: 10.1039/b802607f

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    39

    Sub-picosecond time resolved infrared spectroscopy of high-spin state formation in Fe(II) spin crossover complexes

    Wolf, Matthias M. N.; Gross, Ruth; Schumann, Christian; Wolny, Juliusz A.; Schuenemann, Volker; Dossing, Anders; Paulsen, Hauke; McGarvey, John J.; Diller, Rolf

    Physical Chemistry Chemical Physics (2008), 10 (29), 4264-4273CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    The photoinduced low-spin (S = 0) to high-spin (S = 2) transition of the Fe(II) spin-crossover systems [Fe(btpa)](PF6)2 and [Fe(b(bdpa))](PF6)2 in soln. were studied for the 1st time by ultrafast transient IR spectroscopy at room temp. Neg. and pos. IR difference bands between 1000 and 1065 cm-1 that appear within the instrumental system response time of 350 fs after excitation at 387 nm display the formation of the vibrationally unrelaxed and hot high-spin 5T2 state. Vibrational relaxation is obsd. and characterized by the time consts. 9.4 ± 0.7 ps for [Fe(btpa)](PF6)2/acetone and 12.7 ± 0.7 ps for both [Fe(btpa)](PF6)2/MeCN and [Fe(b(bdpa))](PF6)2/MeCN. Vibrational anal. was performed via DFT calcns. of the low-spin and high-spin state normal modes of both compds. as well as their resp. IR absorption cross sections. The simulated IR difference spectra are dominated by an increase of the absorption cross section upon high-spin state formation in accordance with the exptl. IR spectra.
40. 40

    Mukuta, T.; Tanaka, S.; Inagaki, A.; Koshihara, S. y.; Onda, K. Direct Observation of the Triplet Metal-Centered State in [Ru(bpy)3]²⁺ Using Time-Resolved Infrared Spectroscopy. ChemistrySelect 2016, 1, 2802– 2807,  DOI: 10.1002/slct.201600747

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    Direct Observation of the Triplet Metal-Centered State in [Ru(bpy)3]2+ Using Time-Resolved Infrared Spectroscopy

    Mukuta, Tatsuhiko; Tanaka, Sei'ichi; Inagaki, Akiko; Koshihara, Shin-ya; Onda, Ken

    ChemistrySelect (2016), 1 (11), 2802-2807CODEN: CHEMUD; ISSN:2365-6549. (Wiley-VCH Verlag GmbH & Co. KGaA)

    [Ru(bpy)3]2+ is well-known as a prototype for the Ru(II) complexes used in a wide variety of photofunctional materials. The triplet metal-centered (3MC) state is important in this complex, since it dominates the phosphorescence lifetime and photoreaction processes. Despite this, the 3MC state has not yet been obsd. by spectroscopic methods. In the present study, we demonstrated that time-resolved IR vibrational spectroscopy enables observations of the 3MC state. A vibrational band at 1599 cm-1 was found to exhibit unique temporal behavior that differed from that of other bands assignable to the triplet metal-to-ligand charge-transfer (3MLCT) state. This unique behavior was assessed under various exptl. conditions and it was concluded that the band arises from the short-term population (∼23 ps) of the 3MC state during relaxation to the bottom of the 3MLCT state. These results agree with [Fe(bpy)3]2+ spectra, which show that the 5MC state is the most stable excited state.
41. 41

    Sun, Q.; Dereka, B.; Vauthey, E.; Lawson Daku, L. M.; Hauser, A. Ultrafast Transient IR Spectroscopy and DFT Calculations of Ruthenium(II) Polypyridyl Complexes. Chem. Sci. 2017, 8, 223– 230,  DOI: 10.1039/C6SC01220E

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    Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(II) polypyridyl complexes

    Sun, Qinchao; Dereka, Bogdan; Vauthey, Eric; Lawson Daku, Latevi M.; Hauser, Andreas

    Chemical Science (2017), 8 (1), 223-230CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

    Ultrafast time-resolved IR spectroscopy of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine), [Ru(mbpy)3]2+ (mbpy = 6-methyl-2,2'-bipyridine) and [Ru(mphen)3]2+ (mphen = 2-methyl-1,10'-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the 1MLCT band at 400 nm. While for [Ru(bpy)3]2+ no intermediate state can be evidenced for the relaxation of the corresponding 3MLCT state back to the ground state, for [Ru(mbpy)3]2+ and [Ru(mphen)3]2+ an intermediate state with a lifetime of about 400 ps is obsd. The species assocd. IR difference spectra of this state are in good agreement with the calcd. difference spectra of the lowest energy 3dd state using DFT. The calcd. potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)3]2+ the 3dd state is at a higher energy than the 3MLCT state and that there is a substantial barrier between the two min. For [Ru(mbpy)3]2+ and [Ru(mphen)3]2+, the 3dd state is at a lower energy than the 3MLCT state.
42. 42

    Bouzaid, J.; Schultz, M.; Lao, Z.; Bartley, J.; Bostrom, T.; McMurtrie, J. Supramolecular Selection in Molecular Alloys. Cryst. Growth Des. 2012, 12, 3906– 3916,  DOI: 10.1021/cg300320r

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    Supramolecular Selection in Molecular Alloys

    Bouzaid, Jocelyne; Schultz, Madeleine; Lao, Zane; Bartley, John; Bostrom, Thor; McMurtrie, John

    Crystal Growth & Design (2012), 12 (8), 3906-3916CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)

    [M(phen)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and phen = 1,10-phenanthroline) co-crystallize to form mol. alloys (solid solns. of mols.) [MAxMB1-x(phen)3](PF6)2·0.5H2O in which the relative concns. of the metal complexes in the crystals closely match those in the crystg. soln. Consequently, the compn. of the co-crystals can be accurately predicted and controlled by modulating the relative concns. of the metal complexes in the crystg. soln. Although they are chem. and structurally similar, complexes [M(bipy)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and bipy = 2,2'-bipyridine) display markedly different behavior upon co-crystn. In this case, the resulting co-crystals [MAxMB1-x(bipy)3](PF6)2 have relative concns. of the constituent complexes that are markedly different from the relative concns. of the complexes initially present in the crystg. soln. For example, when the Ni and Fe complexes are co-crystd. from a soln. contg. a 50:50 ratio of each, the result is the formation of some crystals with a higher proportion of Fe and others with a higher proportion of Ni. The relative concns. of the metal complexes in the crystals can vary from those in the crystg. solns. by ≤15%. This result was obsd. for a range of combinations of metal complexes (Ni/Fe, Ni/Ru, and Fe/Ru) and a range of starting concns. in the crystg. solns. (90:10 through to 10:90 in 10% increments). To explain this remarkable result, the authors introduce the concept of supramol. selection, which is a process driven by mol. recognition that leads to the partially selective aggregation of like mols. during crystn.
43. 43

    Kaindl, R. A.; Wurm, M.; Reimann, K.; Hamm, P.; Weiner, A. M.; Woerner, M. Generation, Shaping, and Characterization of Intense Femtosecond Pulses Tunable from 3 to 20 μm. J. Opt. Soc. Am. 2000, 17, 2086– 2094,  DOI: 10.1364/JOSAB.17.002086

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    Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 μm

    Kaindl, Robert A.; Wurm, Matthias; Reimann, Klaus; Hamm, Peter; Weiner, Andrew M.; Woerner, Michael

    Journal of the Optical Society of America B: Optical Physics (2000), 17 (12), 2086-2094CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)

    We report on an intense mid-IR light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3-20-μm wavelength range with pulse durations as short as 50 fs at 5 μm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-IR signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-IR pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-IR pulses.
44. 44

    Stensitzki, T. Femtosecond Pump-Probe Spectroscopy on Corroles, Phytochromes, Channelrhodopsins and Ground-state Reactions. Ph.D. Thesis, Department of Physics, Freie Universität Berlin, 2019.

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    Stensitzki, T. Skultrafast─A Python Package for Time-Resolved Spectroscopy , 2021. DOI: 10.5281/zenodo.5713589 .

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46. 46

    Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344

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    A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu

    Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge

    Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
47. 47

    Frisch, M. J.; Gaussian 16. Revision C.01; Gaussian Inc.: Wallingford CT, 2016.

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    Wallin, S.; Davidsson, J.; Modin, J.; Hammarstrom, L. Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]²⁺ and [Ru(bpy)(py)4]²⁺. Ultrafast Interligand Randomization of the MLCT State. J. Phys. Chem. A 2005, 109, 4697– 4704,  DOI: 10.1021/jp0509212

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    Femtosecond transient absorption anisotropy study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. ultrafast interligand randomization of the MLCT state

    Wallin, Staffan; Davidsson, Jan; Modin, Judit; Hammarstroem, Leif

    Journal of Physical Chemistry A (2005), 109 (21), 4697-4704CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)

    It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron "hops" from ligand to ligand in the MLCT state with a time const. of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond, the mol. has no memory of which bipyridine was initially photo-selected, which suggests an ultrafast interligand randomization of the MLCT state.
49. 49

    Stensitzki, T.; Yang, Y.; Berg, A.; Mahammed, A.; Gross, Z.; Heyne, K. Ultrafast Electronic and Vibrational Dynamics in Brominated Aluminum Corroles: Energy Relaxation and Triplet Formation. Struct. Dynam. 2016, 3, 043210,  DOI: 10.1063/1.4949363

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50. 50

    Zahn, C.; Stensitzki, T.; Heyne, K. Femtosecond Anisotropy Excitation Spectroscopy to Disentangle the Q_x and Q_y Absorption in Chlorophyll a. Chem. Sci. 2022, 13, 12426– 12432,  DOI: 10.1039/D2SC03538C

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    Hamm, P.; Ohline, S. M.; Zinth, W. Vibrational Cooling After Ultrafast Photoisomerization of Azobenzene Measured by Femtosecond Infrared Spectroscopy. J. Chem. Phys. 1997, 106, 519– 529,  DOI: 10.1063/1.473392

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    Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy

    Hamm, P.; Ohline, S. M.; Zinth, W.

    Journal of Chemical Physics (1997), 106 (2), 519-529CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resoln. Transient difference spectra were obtained in a frequency range where Ph ring modes and the central N:N-stretching mode absorbs. The exptl. data are discussed in terms of a simple theor. model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyat. mol. It is shown that these off-diagonal anharmonic consts. dominate the obsd. transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene mol. can be described as follows: After an initial ultrafast intramol. energy redistribution process, the decay of the related intramol. temp. occurs via intermol. energy transfer to the solvent on a time scale of ca. 20 ps.
52. 52

    Kovalenko, S. A.; Schanz, R.; Hennig, H.; Ernsting, N. P. Cooling Dynamics of an Optically Excited Molecular Probe in Solution from Femtosecond Broadband Transient Absorption Spectroscopy. J. Chem. Phys. 2001, 115, 3256– 3273,  DOI: 10.1063/1.1380696

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    Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

    Kovalenko, S. A.; Schanz, R.; Hennig, H.; Ernsting, N. P.

    Journal of Chemical Physics (2001), 115 (7), 3256-3273CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in soln. is studied exptl. and theor. Using the pump-supercontinuum probe (PSCP) technique the authors obsd. the complete spectral evolution of hot absorption induced by fs optical pumping. In t-stilbene the hot S1 state results from Sn → S1 internal conversion with 50 fs characteristic time. The time const. of intramol. thermalization or intramol. vibrational redistribution (IVR) in S1 is estd. as τIVR«100 fs. In PNA and DPNA the hot ground state is prepd. by S1 → S0 relaxation with characteristic time 0.3-1.0 ps. The initial mol. temp. is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency νm and width square Γ2 on mol. temp. T. Within this framework analytic expressions are derived for the differential absorption signal ΔOD(T(t),ν). After calibration with stationary absorption spectra in a low temp. range, the solute temp. T(t) may be evaluated from a transient absorption expt. For highly polar PNA and DPNA, T(t) is well described by a biexponential decay which reflects local heating effects, while for nonpolar t-stilbene the local heating is negligible and the cooling proceeds monoexponentially. To rationalize this behavior, an analytic model is developed, which considers energy flow from the hot solute to a 1st solvent shell and then to the bulk solvent. Fastest cooling is found for PNA in H2O: a time const. of 0.64 ps (68%) corresponds to solute-solvent energy transfer while 2.0 ps (32%) characterizes the cooling of the 1st shell. In aprotic solvents cooling is slower than in alcs. and slows down further with decreasing solvent polarity. This contrasts with nonpolar t-stilbene which cools down with 8.5 ps both in MeCN and cyclohexane. Comparison of the cooling kinetics for PNA in H2O with those for DPNA in H2O-MeCN mixts. suggests that the solute-solvent energy transfer proceeds mainly through H bonds.
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    Marciniak, H.; Lochbrunner, S. On the Interpretation of Decay Associated Spectra in the Presence of Time Dependent Spectral Shifts. Chem. Phys. Lett. 2014, 609, 184– 188,  DOI: 10.1016/j.cplett.2014.05.006

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    On the interpretation of decay associated spectra in the presence of time dependent spectral shifts

    Marciniak, Henning; Lochbrunner, Stefan

    Chemical Physics Letters (2014), 609 (), 184-188CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)

    Time dependent spectral shifts caused by energetic relaxation processes frequently contribute to time resolved spectral data. They make a global anal. of transient spectra challenging, since the basic assumption of separability of time and frequency dependence does not hold. We compare the global anal. results of synthetic transient spectra to a Taylor expansion of their spectral shape at infinite time. Thereby we can elucidate the correlation of time consts. and decay assocd. spectra resulting from global anal. with the underlying shift dynamics. The obtained insights are applied to transient absorption spectra of Coumarin 152 and its solvation dynamics.
54. 54

    Zhang, K.; Ash, R.; Girolami, G. S.; Vura-Weis, J. Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)³²⁺ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy. J. Am. Chem. Soc. 2019, 141, 17180– 17188,  DOI: 10.1021/jacs.9b07332

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    Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy

    Zhang, Kaili; Ash, Ryan; Girolami, Gregory S.; Vura-Weis, Josh

    Journal of the American Chemical Society (2019), 141 (43), 17180-17188CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chem. and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a long-lived quintet state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme UV (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)32+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 ps is obsd. on the 5T2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N sym. stretch. The results show that the shape of the M2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resoln., and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.