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Using Lifetime and Quenching Rate Constant to Determine Optimal Quencher Concentration
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  • Xena L. Soto
    Xena L. Soto
    Department of Chemistry, State University of New York at Binghamton, 4400 Vestal Parkway East, P.O. Box 6000, Vestal, New York 13850, United States
    Department of Chemistry, Lehman College/City University of New York, 250 Bedford Park Boulevard West, Bronx, New York 10468, United States
    More by Xena L. Soto
  • John R. Swierk*
    John R. Swierk
    Department of Chemistry, State University of New York at Binghamton, 4400 Vestal Parkway East, P.O. Box 6000, Vestal, New York 13850, United States
    *Email: [email protected]
    More by John R. Swierk
    Orcidhttps://orcid.org/0000-0001-5811-7285
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ACS Omega

Cite this: ACS Omega 2022, 7, 29, 25532–25536
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https://doi.org/10.1021/acsomega.2c02638
Published July 12, 2022

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CC-BY-NC-ND 4.0 .

Abstract

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ABSTRACT Excited state quenching is a key step in photochemical reactions that involve energy or electron transfer. High reaction quantum yields require sufficiently high concentrations of a quencher to ensure efficient quenching. The determination of quencher concentrations is typically done through trial and error. Using kinetic modeling, however, a simple relationship was developed that predicts the concentration of quencher necessary to quench 90% of excited states, using only the photosensitizer lifetime and the rate constant for quenching as inputs. Comparison of the predicted quencher concentrations and quencher concentrations used in photoredox reactions featuring acridinium-based photocatalysts reveals that the majority of reactions used quencher concentrations significantly below the predicted concentration. This suggests that these reactions exhibit low quantum yields, requiring long reaction times and/or intense light sources.

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Introduction

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Quenching an excited state is a key step in photochemical reactions that involve electron or energy transfer (Scheme 1). (1) Poor kinetics at the quenching step lead to inefficient harvesting of excited states and limit the overall quantum yield of the reaction. As a second-order process, the rate of quenching depends on the concentration of excited photosensitizers, the concentration of the quencher (Q), and the rate constant for quenching (kq). The quenching step, however, is in direct competition with the unproductive relaxation of the excited state, which is controlled by the lifetime of the photosensitizer (τ). From a reaction design standpoint, the choice of the photosensitizer controls τ, though other factors such as redox potentials and absorption range often take higher priority in the selection of the photosensitizer. (2) The value of kq depends on a host of factors including the choice of the substrate, driving force for electron/energy transfer, and degree of association in solution. (3−5) While kq can also be tuned in a number of ways (e.g., changing the photosensitizer or substrate, changing the solvent, and adding inert salts), these can be impractical from a reaction design standpoint where specific reaction conditions are needed to produce a given product, simplify purification, or solubilize one or more reagents. In principle, the concentration of excited photosensitizers can be varied with light intensity, though as shown below, that has little impact on the quenching yield. Thus, the most practical parameter that can be varied to impact quenching rates and efficiency is the concentration of the quencher. However, instead of quantitatively predicting ideal quencher conditions, trial and error is typically used to determine the optimal concentration in the design of new photochemical reactions.

Scheme 1

Scheme 1. Quenching Pathways for Excited Photosensitizers (PS*)
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We hypothesized that, using only kq and τ, the Stern–Volmer equation could be used to make simple predictions about the quencher concentration needed for efficient excited state quenching. Photosensitizer lifetimes are widely reported for both organic and inorganic photosensitizers. (1,5,6) Determination of kq for a photochemical reaction is simple using the Stern–Volmer relationship and is commonly reported. In the simplest formulation of Stern–Volmer, all that is needed is a fluorimeter, an emissive photosensitizer, and knowledge of τ to determine a value for kq. Inspired by our recent success using kinetic modeling to reproduce reaction quantum yields, we set out to test the hypothesis that with knowledge of kq and τ, the Stern–Volmer equation could be used as a simple, predictive model to determine the quencher concentration needed to ensure high quenching yields. (7,8)

Results and Discussion

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Using Kinetiscope, (9) a freeware stochastic kinetic simulator widely used to study chemical reactions, (10−12) we developed a simple model that involved competition between the relaxation of the excited state and quenching by a quencher, Q. Our specific reaction involved an oxidative quenching reaction that generated a reduced quencher, Q•–, and an oxidized photosensitizer; however, the result would be unchanged for a reductive quenching or energy transfer. Relaxation, krelax, is a first-order process controlled by τ (krelax = 1/τ), while kq was varied from 107 M–1 s–1 to 1010 M–1 s–1. Values of kq greater than 1010 M–1 s–1 were not explored, as these indicate a reaction that is not diffusion-controlled and requires pre-association of a photosensitizer and a quencher or an intramolecular energy/electron transfer. Initially, we modeled continuous illumination and varied the concentration of Q to achieve a quantum yield for quenching, Φquench, of 0.900 ± 0.001 at the end of the simulation. In this case, Φquench is defined as the final concentration of [Q•–] divided by the number of photons introduced to the reaction. Targeting a value of 0.900 ± 0.001 for Φquench represented an optimal balance of harvesting a high concentration of excited states at quencher concentrations that could be practical. We did not regenerate the oxidized photosensitizer in our reaction; however, the concentration of the photosensitizer was relatively high (100 μM), the length of the simulation was kept short (1 s), and the illumination intensity was kept to 1.63 × 10–5 photons/s, which corresponds to a 10 mW intensity of 415 nm light. As a result, only a relatively small concentration of photosensitizer was oxidized during the simulation. In addition, increasing both the simulation time and illumination intensity (Figures S2 and S3) resulted in no change in Φquench, indicating that the buildup of the oxidized photosensitizer has negligible impact on the reaction.
As shown in Figure 1A, varying kq for a given value of τ resulted in 2–3 orders of magnitude variation in the predicted concentration of the quencher. To our delight, for a given photosensitizer lifetime, the predicted values of [Q] could be fit to a simple power law equation of the form
(1)

Figure 1

Figure 1. (A) Predicted concentration of the quencher needed to achieve quenching of 90% of excited states as a function of quenching rate constant (kq) and photosensitizer lifetime. Modeled with continuous illumination of 1.63 × 10–5 photons/s. Solid lines are fit to power law equation of the general form kq/α. (B) Value of α as a function of photosensitizer lifetime (τ). Solid black line is fit to the equation: α = 8.96τ.

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At short values of τ, the fit to eq 1 was excellent with an R2 value of 1. At longer values of τ (e.g., 500 μs), the fit was less accurate at larger kq values (>109 M–1 s–1). This is because at those values, the concentration of quencher needed was essentially independent of kq due to the extremely large values of τ. Figure 1B shows a plot of the value of α as a function of τ and demonstrates that data could also be described by a power law equation of the form
(2)
From the fit in Figure 1B, a value of 8.96 was determined for A. Thus, combining eqs 1 and 2, as well as the value for A, the concentration of Q needed to give Φquench of 0.9 at a given value of τ and kq is given by eq 3
(3)
For values of τ less than or equal to 10 μs, the deviation between [Q] predicted from kinetic modeling and [Q] predicted by eq 3 was 5% or less and typically 1–2% at most. For longer values of τ, the values predicted by eq 3 showed significant deviation at values of kq less than 109 M–1 s–1 and much smaller deviations at smaller values of kq. Percent deviations are shown in Table S1. We note that the majority of photosensitizers used in photochemical reactions have τ on the order of 10 μs or shorter, and in the cases of longer τ values, the kinetic modeling overestimates the concentration of Q needed because of the small quencher concentrations need to achieve 90% quenching. As expected, eq 3 closely matches a rearranged version of the Stern–Volmer equation. If I0/I is set to 10 to account for 90% of excited states being quenched, then the Stern–Volmer equation rearranges to
(4)
This confirms both the validity of using kinetic modeling to study the quenching steps in photochemical reactions and the Stern–Volmer relationship that can be used to predict quencher concentrations needed for efficient harvesting of excited states.
We also performed a similar analysis as above but started with a fixed concentration of excited photosensitizer. This simulates pulsed illumination, like in a laser experiment, where a brief, intense pulse of light excites a significant number of photosensitizers, followed by a dark period. As our benchmark, we set a value of Φquench of 0.9 after 100 ns of reaction time. Again, this represented a balance between a high value of Φquench and reasonable concentrations of Q. We also limited our investigation to photosensitizers with lifetimes of 200 ns to 10 μs. As with continuous illumination, the data could be well fit to eq 1 (Figure S3); however, the plot of α versus τ did not follow the form of eq 2 (Figure S4). As τ gets longer, Figure S3 shows that Φquench has less of a concentration dependence, likely because relaxation becomes a minor unproductive pathway on the timescale of 100 ns.
In order to further validate the results from our simulations, we calculated Φquench using our model for a group of experimental systems described in the literature and compared the predicted Φquench with the measured Φquench. Measurement of Φquench by itself is not common, and accurate measurements can be challenging because of rapid back electron transfer, low cage escape yields, or other unproductive pathways. (13−15) Mindful of this, we carefully selected a set of trial reactions where Φquench was known independently of other unproductive pathways. Figure 2 shows the comparison between predicted and measured Φquench and demonstrates an excellent correlation, suggesting that our kinetic modeling method produces values in good agreement with the experiment.

Figure 2

Figure 2. Predicted quantum yields of quenching (Φquench) compared to experimentally measured quantum yields of quenching (Φquench). Solid blue line shows one-to-one correlation. Details of the measured quantum yields are provided in the Supporting Information.

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We propose that eq 3 can be a useful tool in evaluating the reaction design. Using a series of photoredox reactions that rely on acridinium-based photocatalysts (PCs), we compared the quencher concentrations used in experimental reports to the concentration of quencher predicted by eq 3. Acridinium-based PCs are commonly used in photoredox reactions and typically exhibit short lifetimes on the order of 10 ns. (16)Figure 3 shows that the majority of experimental reports we evaluated used less quencher than what eq 3 predicts, which is needed for efficient quenching. It is also notable that the deviation becomes more pronounced at smaller values of kq. This is largely a function of most reports using a quencher concentration on the order of 0.1–0.2 M. While this concentration range is appropriate for larger values of kq (109 M–1 s–1 or greater), it is too low for smaller values of kq. It is important to note that using a quencher concentration that is too low will impact the quantum yield of the reaction, but not necessarily the overall product yield, as most of the reactions surveyed in Figure 3 achieve product yields of 70% or higher. However, the majority of the reactions ran for more than 24 h and utilized extremely bright light sources. This suggests that the quantum yields of these reactions are indeed low and may prove to be an issue when considering the energy intensity of these photoredox reactions. (17) More generally, this suggests that photoredox reactions that rely on PCs with short lifetimes, that is, those on the nanosecond timescale, will struggle to achieve high quantum yields unless paired with substrates that exhibit a large kq.

Figure 3

Figure 3. Ratio of experimental quencher concentration to predicted quencher concentration from eq 3 as a function of quenching rate constant, kq, for 18 examples of photoredox reactions using acridinium-based PCs. Solid red line indicates a ratio of 1:1 for the experimental-to-predicted quencher concentration. Details for each experimental study are provided in the Supporting Information.

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Conclusions

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In analogy with multistep synthetic reactions, the overall reaction quantum yield for a photochemical reaction is the product of yield for each individual step. Ensuring a high Φquench offers the best chance of achieving a high quantum yield for the overall reaction and is the step that can be most easily impacted via experimental design. Using kinetic modeling, we have shown that the quencher concentration needed for efficient excited state quenching (90% or greater) is simply predicted by the Stern–Volmer equation and relies only on τ and kq, two parameters that are readily accessible. Considering the design of photoredox reactions from a quantum yield perspective, eq 3 predicts that optimal PC lifetimes in the microsecond to tens of microsecond range would be needed to use quencher concentrations on the order of hundreds of millimolar.

Method

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Reactions were simulated using Kinetiscope (https://hinsberg.net/kinetiscope/). Scheme 2 shows a typical reaction setup. A simple reaction scheme was used that involved the excitation of a PC and then a competition between relaxation and quenching. The excitation was handled in two steps following our previous work. (7) In the first step, photon generation, photons were generated via a zeroth-order reaction at a rate of 1.63 × 10–5 photons/s. This simulates 10 mW illumination at 415 nm. In the next step, excitation, the photons were captured by the PC to generate the excited photocatalyst (PC*) at a rate of 1 × 1014 M–1 s–1. An arbitrarily large rate constant was selected to ensure that every time a photon interacted with a PC, excitation occurred. This also ensured efficient capture of all photons, with the simulation modeling continuous, uniform excitation across the reaction volume. While in a real system absorption of photons would be governed by the Beer–Lambert–Bouguer law, we are not specifying a specific PC and, by extension, molar extinction coefficient. Even in a real system with high PC concentrations and a large molar extinction coefficient at the excitation wavelength, the instantaneous concentration of excited PC generated by constant illumination would be 13 orders of magnitude smaller than the quencher concentrations used in this study.

Scheme 2

Scheme 2. Steps Used in Kinetic Modeling of Quantum Yields
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After excitation, the excited PC could then relax back to the ground state (PC* → PC), relaxation, or be quenched by a quencher in the pathway labeled “quenching”. Though the quenching step was written as an oxidative quenching reaction, the results would be identical for a reductive quenching or energy transfer. In order to simplify the analysis, no back electron transfer between PC+ and Q-was allowed to occur nor was PC+ regenerated. In this study, we investigated the quantum yield of quenching, Φquench, which is simply defined as the number of PCs that undergo quenching divided by the number of excited PCs. This parameter is unaffected by other reaction pathways (e.g., back electron transfer), and so to simplify the model, no other pathways were included. The simulation length was 1 s, which decreased the simulation running time and ensured that only a small concentration of PC was converted into PC+ by the end of the simulation. Φquench results were unchanged when running the simulation for longer timescales or when adding in a PC regeneration step.
For steady-state illumination simulation, a PC concentration of 100 μM was used, while for simulations involving a fixed initial concentration of PC*, a concentration of 1 μM was used. In Kinetiscope, individual particles are used to track ensembles of molecules. Variation of the number of moles represented by a given particle showed no effect on Φquench for steady-state illumination but some deviation in Φquench for a fixed concentration of PC* (Figure S1). As a result, the total number of particles was adjusted so that there was 10–13 mol/particle. For constant illumination, the simulation ran for 1 s, and the concentration of Q varied until a Φquench value of 0.900 ± 0.001 was achieved at the end of the reaction. For a fixed starting concentration of PC*, the concentration of Q was varied until Φquench at 100 ns equaled 0.900 ± 0.001.
Simulation of published reactions followed an identical approach as above with minor modifications. The concentration of Q used in the simulation was the same as that used in the published report and was not varied. Where possible, the concentration of PC used matched that in the published report. If the concentration of PC was not reported, a value of 100 μM was utilized. The concentration of PC was found to have no impact on the quantum yield of the reaction unless the concentration of PC was less than 20 μM.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c02638.

  • Details of kinetic modeling; impact of illumination time and intensity on quantum yield; predicted quencher concentrations for starting with the fixed concentration of excited PC; comparison of quencher concentrations predicted by eq 3 with those derived from kinetic modeling; and details of experimental studies included in Figures 2 and 3 (PDF)

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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 Author
  • Author
    • Xena L. Soto - Department of Chemistry, State University of New York at Binghamton, 4400 Vestal Parkway East, P.O. Box 6000, Vestal, New York 13850, United States;  Department of Chemistry, Lehman College/City University of New York, 250 Bedford Park Boulevard West, Bronx, New York 10468, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the National Science Foundation (NSF CHE-2047492). X.L.S. thanks the State University of New York Louis Stokes Alliance for Minority Participation (NSF 1619619) for a summer research fellowship. Finally, the authors wish to acknowledge ChemRxiv for publishing an earlier version of this manuscript as a preprint. (18)

References

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    Liu, M. J.; Wiegel, A. A.; Wilson, K. R.; Houle, F. A. Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals. J. Phys. Chem. A 2017, 121, 5856– 5870,  DOI: 10.1021/acs.jpca.7b04892
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    Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals
    Liu, Matthew J.; Wiegel, Aaron A.; Wilson, Kevin R.; Houle, Frances A.
    Journal of Physical Chemistry A (2017), 121 (31), 5856-5870CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    A key uncertainty in the heterogeneous oxidn. of carboxylic acids by hydroxyl radicals (OH) in aq.-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chem. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimol. decompn. of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of org. carbon between the gas and aq. phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidn. of citric acid aerosol at high relative humidity. The reaction scheme, contg. both free-radical and acid-base elementary reaction steps with phys. validated rate coeffs., accurately predicts the exptl. obsd. mol. compn., particle size, and av. elemental compn. of the aerosol upon oxidn. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimol. fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equil. activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low mol. wt., highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and vol. growth that accompany oxidn. These results frame the oxidative lifecycle of atm. aerosol: it is governed by feedbacks between reactions that first increase the particle oxidn. state, then eventually promote water uptake and acid-base chem. When coupled to free-radical reactions, acid-base channels lead to formation of low mol. wt. gas-phase reaction products and decreasing particle size.
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    Bolshchikov, B. D.; Tsvetkov, V. B.; Alikhanova, O. L.; Serbin, A. V. How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether-alt-maleic anhydride). Macromol. Chem. Phys. 2019, 220, 1900389,  DOI: 10.1002/macp.201900389
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    How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether-alt-maleic anhydride)
    Bolshchikov, Boris D.; Tsvetkov, Vladimir B.; Alikhanova, Olga L.; Serbin, Alexander V.
    Macromolecular Chemistry and Physics (2019), 220 (23), 1900389CODEN: MCHPES; ISSN:1022-1352. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Products of free-radical polymn. (FRP) are usually not regulated on the mol. scale, consisting of blocks obtained through the fastest kinetic scheme pathways. The side or kinetically restricted products can be a source of impurities in a complex FRP case, or possess new properties if isolated solely. FRP synthesis of poly(divinyl ether-alt-maleic anhydride), known as "DIVEMA", serves as a polymn. example with such kinetic and thermodn. complexities. Uncertainty in factors regulating polymer structure is a challenge in advancement "DIVEMA" derivs. toward medical practice. In-depth investigation via quantum-chem. and mol. mechanics methods unveils mechanistic aspects of polymer stereoisomerism and confirms possible isolation of thermodynamically or kinetically controlled products on a large data set. Strategies toward regulation of 5-exo/6-endo cycloisomerism are theorized and then studied via microkinetic modeling. Thermodynamically controlled products can be isolated utilizing lower monomer concns., in range of 10-3 to 10-1 M, and/or application of a complexing agent that is better to realize via solvents, capable of formation π- and σ-radical complexes. Change of electrophilic monomer is proposed as an approach for designing more molecularscale adjustable copolymn. processes. Methodol., obtained results, and conclusions for "DIVEMA" can be valuable to control other FRP processes on the mol. scale, unlocking polymers with improved or new functionalities.
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    Bonaldo, F.; Mattivi, F.; Catorci, D.; Arapitsas, P.; Guella, G. D Exchange Processes in Flavonoids: Kinetics and Mechanistic Investigations. Molecules 2021, 26, 3544,  DOI: 10.3390/molecules26123544
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    H/D exchange processes in flavonoids: kinetics and mechanistic investigations
    Bonaldo, Federico; Mattivi, Fulvio; Catorci, Daniele; Arapitsas, Panagiotis; Guella, Graziano
    Molecules (2021), 26 (12), 3544CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
    Several classes of flavonoids, such as anthocyanins, flavonols, flavanols, and flavones, undergo a slow H/D exchange on arom. ring A, leading to full deuteration at positions C(6) and C(8). Within the flavanol class, H-C(6) and H-C(8) of catechin and epicatechin are slowly exchanged in D2O to the corresponding deuterated analogs. Even quercetin, a relevant flavonol representative, shows the same behavior in a D2O/DMSOd6 1:1 soln. Detailed kinetic measurements of these H/D exchange processes are here reported by exploiting the time-dependent changes of their peak areas in the 1H-NMR spectra taken at different temps. A unifying reaction mechanism is also proposed based on our detailed kinetic observations, even taking into account pH and solvent effects. Mol. modeling and QM calcns. were also carried out to shed more light on several mol. details of the proposed mechanism.
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    The factors influencing the escape of donor-acceptor charge-transfer pairs from their solvent cage were examd., using methylviologen (MV2+) as the acceptor and several different excited metal complexes and arom. org. mols. as donors. The ratios of cage escape yields were detd. by measuring the amt. of transient absorption due to MV•+ radical cation in the absence and in the presence of an anthracene deriv. as an energy shuttle, under conditions where no irreversible photochem. takes place. For a series of different Ru and Os complexes, the cage escape yield of the [M3+,MV•+] pair varied only slightly, ranging from 0.14 to 0.27. When the donor was an org. triplet excited state (9-methylanthracene or acridine yellow), the cage escape yield was near unity. Perturbation by heavy atoms reduced the yield to 0.3 (9-bromoanthracene in soln. contg. CH3I). The cage escape yield was strongly affected by the rate of triplet-singlet interconversion of the triplet charge pair generated in the quenching event. When no mechanism for triplet-singlet mixing was present (org. donors), back-electron-transfer, which requires a spin change, was slow compared to diffusion out of the cage. When spin-orbit coupling was substantial (heavy-atom perturbation, transition-metal complexes), back-electron-transfer was competitive with diffusional cage escape.
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    Sun, H.; Hoffman, M. Z. Reductive Quenching of the Excited State of Ruthenium(II) Complexes Containing 2,2’-Bipyridine, 2,2’-Bipyrazine, and 2,2’-Bipyramidine. J. Phys. Chem. 1994, 98, 11719– 11726,  DOI: 10.1021/j100096a015
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    Reductive Quenching of the Excited States of Ruthenium(II) Complexes Containing 2,2'-Bipyridine, 2,2'-Bipyrazine, and 2,2'-Bipyrimidine Ligands
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    Journal of Physical Chemistry (1994), 98 (45), 11719-26CODEN: JPCHAX; ISSN:0022-3654.
    The reductive quenching of the luminescent excited states of Ru(II) complexes of the general formula Ru(bpy)3-m-z(bpm)m(bpz)z2+ (bpy = 2,2'-bipyridine, bpm = 2,2'- bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0,1,2,3 and m + z ≤ 3) by arom. amines and methoxybenzenes as nonsacrificial electron donors and by C2O42-, EDTA, and TEOA as sacrificial donors in aq., acetonitrile, and propylene carbonate soln. was examd. by continuous and pulsed laser flash photolysis techniques. From kq, a value of E° for the irreversible oxidn. of TEOA (-0.84 ± 0.12 V vs. NHE) in acetonitrile was obtained. Values of the cage escape yield of redox products (ηce) showed weak or no dependencies on the driving forces of back electron transfer within the geminate redox pair in the solvent cage (ΔGbt°), suggesting that the simple model of competition between cage escape and back electron transfer may be inadequate to describe the results. A modification of the simple model, in which is introduced a kinetically important reorientation of the geminate redox pair, is proposed.
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    Aydogan, A.; Bangle, R. E.; Cadranel, A.; Turlington, M. D.; Conroy, D. T.; Cauët, E.; Singleton, M. L.; Meyer, G. J.; Sampaio, R. N.; Elias, B.; Troian-Gautier, L. Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light. J. Am. Chem. Soc. 2021, 143, 15661– 15673,  DOI: 10.1021/jacs.1c06081
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    Accessing photoredox transformations with an iron(III) photosensitizer and green light
    Aydogan, Akin; Bangle, Rachel E.; Cadranel, Alejandro; Turlington, Michael D.; Conroy, Daniel T.; Cauet, Emilie; Singleton, Michael L.; Meyer, Gerald J.; Sampaio, Renato N.; Elias, Benjamin; Troian-Gautier, Ludovic
    Journal of the American Chemical Society (2021), 143 (38), 15661-15673CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Efficient excited-state electron transfer between an iron(III) photosensitizer and org. electron donors was realized with green light irradn. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)2]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}), that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the UV to the IR regions, provided a quant. description of the catalytic mechanism with assocd. rate consts. and yields.
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    Joshi-Pangu, A.; Lévesque, F.; Roth, H. G.; Oliver, S. F.; Campeau, L.-C.; Nicewicz, D.; DiRocco, D. A. Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis. J. Org. Chem. 2016, 81, 7244– 7249,  DOI: 10.1021/acs.joc.6b01240
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    Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis
    Joshi-Pangu, Amruta; Levesque, Francois; Roth, Hudson G.; Oliver, Steven F.; Campeau, Louis-Charles; Nicewicz, David; DiRocco, Daniel A.
    Journal of Organic Chemistry (2016), 81 (16), 7244-7249CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    The emergence of visible light photoredox catalysis has enabled the productive use of lower energy radiation, leading to highly selective reaction platforms. Polypyridyl complexes of iridium and ruthenium have served as popular photocatalysts in recent years due to their long excited state lifetimes and useful redox windows, leading to the development of diverse photoredox-catalyzed transformations. The low abundances of Ir and Ru in the earth's crust and, hence, cost make these catalysts nonsustainable and have limited their application in industrial-scale manufg. Herein, we report a series of novel acridinium salts as alternatives to iridium photoredox catalysts and show their comparability to the ubiquitous [Ir(dF-CF3-ppy)2(dtbpy)](PF6).
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    Ruccolo, S.; Qin, Y.; Schnedermann, C.; Nocera, D. G. General Strategy for Improving the Quantum Efficiency of Photoredox Hydroamidation Catalysis. J. Am. Chem. Soc. 2018, 140, 14926– 14937,  DOI: 10.1021/jacs.8b09109
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    General Strategy for Improving the Quantum Efficiency of Photoredox Hydroamidation Catalysis
    Ruccolo, Serge; Qin, Yangzhong; Schnedermann, Christoph; Nocera, Daniel G.
    Journal of the American Chemical Society (2018), 140 (44), 14926-14937CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The quantum efficiency in photoredox catalysis is the crucial determinant of energy intensity and, thus, is intrinsically tied to the sustainability of the overall process. Here, we track the formation of different transient species of a catalytic photoredox hydroamidation reaction initiated by the reaction of an Ir(III) photoexcited complex with 2-cyclohexen-1-yl(4-bromophenyl)carbamate. We find that the back reaction between the amidyl radical and Ir(II) photoproducts generated from the quenching reaction leads to a low quantum efficiency of the system. Using transient absorption spectroscopy, all of the rate consts. for productive and nonproductive pathways of the catalytic cycle have been detd., enabling us to establish a kinetically competent equil. involving the crucial amidyl radical intermediate that minimizes its back reaction with the Ir(II) photoproduct. This strategy of using an off-pathway equil. allows us to improve the overall quantum efficiency of the reaction by a factor of 4. Our results highlight the benefits from targeting the back-electron transfer reactions of photoredox catalytic cycles to lead to improved energy efficiency and accordingly improved sustainability and cost benefits of photoredox synthetic methods.
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    Soto, X.; Swierk, J. Using Lifetime and Quenching Rate Constant to Determine Quencher Concentration. ChemRxiv, 26, 2022. ver. 1. DOI: 10.26434/chemrxiv-2022-q6klm .
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  1. Bernard G. Stevenson, Cameron Gironda, Eric Talbott, Amanda Prascsak, Nora L. Burnett, Victoria Kompanijec, Roman Nakhamiyayev, Lisa A. Fredin, John R. Swierk. Photoredox Product Selectivity Controlled by Persistent Radical Stability. The Journal of Organic Chemistry 2024, 89 (19) , 13818-13825. https://doi.org/10.1021/acs.joc.3c00490
  2. Felicity Draper, Stephen DiLuzio, Hannah J. Sayre, Le Nhan Pham, Michelle L. Coote, Egan H. Doeven, Paul S. Francis, Timothy U. Connell. Maximizing Photon-to-Electron Conversion for Atom Efficient Photoredox Catalysis. Journal of the American Chemical Society 2024, 146 (39) , 26830-26843. https://doi.org/10.1021/jacs.4c07396
  3. John R. Swierk. The Cost of Quantum Yield. Organic Process Research & Development 2023, 27 (7) , 1411-1419. https://doi.org/10.1021/acs.oprd.3c00167
  4. Xuezhen Song, Yixuan Mao, Shusheng Wang, Jiayong Luo, Shitao Wang, Yun Qian, Dapeng Cao. 1,2,3,4,5,6-Hexakis(4-bromophenyl)benzene-based covalent organic polymers as specific luminescent probes for the selective sensing of nitro-explosives. New Journal of Chemistry 2025, 49 (3) , 687-691. https://doi.org/10.1039/D4NJ04596C
  5. Zhanpeng Wang, Quanxiao Liu, Jigang Wang, Yuansheng Qi, Zhenjun Li, Junming Li, Zhanwei Zhang, Xinfeng Wang, Cuijuan Li, Rong Wang. Study on the Luminescence Performance and Anti-Counterfeiting Application of Eu2+, Nd3+ Co-Doped SrAl2O4 Phosphor. Nanomaterials 2024, 14 (15) , 1265. https://doi.org/10.3390/nano14151265
  6. Eric D. Talbott, Nora L. Burnett, John R. Swierk. Mechanistic and kinetic studies of visible light photoredox reactions. Chemical Physics Reviews 2023, 4 (3) https://doi.org/10.1063/5.0156850
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  • Abstract

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    Scheme 1

    Scheme 1. Quenching Pathways for Excited Photosensitizers (PS*)
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    Figure 1

    Figure 1. (A) Predicted concentration of the quencher needed to achieve quenching of 90% of excited states as a function of quenching rate constant (kq) and photosensitizer lifetime. Modeled with continuous illumination of 1.63 × 10–5 photons/s. Solid lines are fit to power law equation of the general form kq/α. (B) Value of α as a function of photosensitizer lifetime (τ). Solid black line is fit to the equation: α = 8.96τ.

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    Figure 2

    Figure 2. Predicted quantum yields of quenching (Φquench) compared to experimentally measured quantum yields of quenching (Φquench). Solid blue line shows one-to-one correlation. Details of the measured quantum yields are provided in the Supporting Information.

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    Figure 3

    Figure 3. Ratio of experimental quencher concentration to predicted quencher concentration from eq 3 as a function of quenching rate constant, kq, for 18 examples of photoredox reactions using acridinium-based PCs. Solid red line indicates a ratio of 1:1 for the experimental-to-predicted quencher concentration. Details for each experimental study are provided in the Supporting Information.

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    Scheme 2

    Scheme 2. Steps Used in Kinetic Modeling of Quantum Yields
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  • References

    This article references 18 other publications.

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      A review. This review will highlight the early work on the use of transition metal complexes as photoredox catalysts to promote reactions of org. compds. (prior to 2008), as well as cover the surge of work that has appeared since 2008. We have for the most part grouped reactions according to whether the org. substrate undergoes redn., oxidn., or a redox neutral reaction and throughout have sought to highlight the variety of reactive intermediates that may be accessed via this general reaction manifold.
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      A review. The use of visible light sensitization as a means to initiate org. reactions is attractive due to the lack of visible light absorbance by org. compds., reducing side reactions often assocd. with photochem. reactions conducted with high energy UV light. This tutorial review provides a historical overview of visible light photoredox catalysis in org. synthesis along with recent examples which underscore its vast potential to initiate org. transformations.
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      A review. Use of org. photoredox catalysts in a myriad of synthetic transformations with a range of applications was reviewed. This overview was arranged by catalyst class where the photophysics and electrochem. characteristics of each was discussed to underscore the differences and advantages to each type of single electron redox agent. Net reductive and oxidative as well as redox neutral transformations that could be accomplished using purely org. photoredox-active catalysts was highlighted. An overview of the basic photophysics and electron transfer theory was presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.
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      Photophysical Properties and Redox Potentials of Photosensitizers for Organic Photoredox Transformations
      Wu, Yanyu; Kim, Dooyoung; Teets, Thomas S.
      Synlett (2022), 33 (12), 1154-1179CODEN: SYNLES; ISSN:0936-5214. (Georg Thieme Verlag)
      A review. Photoredox catalysis has proven to be a powerful tool in synthetic org. chem. The rational design of photosensitizers with improved photocatalytic performance constitutes a major advancement in photoredox org. transformations. This review summarizes the fundamental ground-state and excited-state photophys. and electrochem. attributes of mol. photosensitizers, which are important determinants of their photocatalytic reactivity.
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    7. 7
      Stevenson, B. G.; Spielvogel, E. H.; Loiaconi, E. A.; Wambua, V. M.; Nakhamiyayev, R. V.; Swierk, J. R. Mechanistic Investigations of an α-Aminoarylation Photoredox Reaction. J. Am. Chem. Soc. 2021, 143, 8878– 8885,  DOI: 10.1021/jacs.1c03693
      7
      Mechanistic Investigations of an α-Aminoarylation Photoredox Reaction
      Stevenson, Bernard G.; Spielvogel, Ethan H.; Loiaconi, Emily A.; Wambua, Victor Mulwa; Nakhamiyayev, Roman V.; Swierk, John R.
      Journal of the American Chemical Society (2021), 143 (23), 8878-8885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      While photoredox catalysis continues to transform modern synthetic chem., detailed mechanistic studies involving direct observation of reaction intermediates and rate consts. are rare. By use of a combination of steady state photochem. measurements, transient laser spectroscopy, and electrochem. methods, an α-aminoarylation mechanism that is the inspiration for a large no. of photoredox reactions was rigorously characterized. Despite high product yields, the external quantum yield (QY) of the reaction remained low (15-30%). By use of transient absorption spectroscopy, productive and unproductive reaction pathways were identified and rate consts. assigned to develop a comprehensive mechanistic picture of the reaction. The role of the cyanoarene, 1,4-dicyanobenzne, was found to be unexpectedly complex, functioning both as initial proton acceptor in the reaction and as a neutral stabilizer for the 1,4-dicyanobenzene radical anion. Finally, kinetic modeling was utilized to analyze the reaction at an unprecedented level of understanding. This modeling demonstrated that the reaction is limited not by the kinetics of the individual steps but instead by scattering losses and parasitic absorption by a photochem. inactive donor-acceptor complex.
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    8. 8
      Spielvogel, E. H.; Stevenson, B. G.; Stringer, M. J.; Hu, Y.; Fredin, L. A.; Swierk, J. R. Insights into the Mechanism of An Allylic Arylation Reaction via Photoredox Coupled Hydrogen Atom Transfer. J. Org. Chem. 2022, 87, 223– 230,  DOI: 10.1021/acs.joc.1c02235
      8
      Insights into the Mechanism of an Allylic Arylation Reaction via Photoredox-Coupled Hydrogen Atom Transfer
      Spielvogel, Ethan H.; Stevenson, Bernard G.; Stringer, Michael J.; Hu, Yue; Fredin, Lisa A.; Swierk, John R.
      Journal of Organic Chemistry (2022), 87 (1), 223-230CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Despite widespread use as a synthetic method, the precise mechanism and kinetics of photoredox coupled hydrogen atom transfer (HAT) reactions remain poorly understood. This results from a lack of detailed kinetic information as well as the identification of side reactions and products. In this report, a mechanistic study of a prototypical tandem photoredox/HAT reaction coupling cyclohexene and 1,4-dicyanobenzene (DCB) using an Ir(ppy)3 photocatalyst and thiol HAT catalyst is reported. Through a combination of electrochem., photochem., and spectroscopic measurements, key unproductive pathways and side products are identified and rate consts. for the main chem. steps are extd. The reaction quantum yield was found to decline rapidly over the course of the reaction. An unreported cyanohydrin side product was identified and thought to play a key role as a proton acceptor in the reaction. Transient absorption spectroscopy (TAS) and quantum chem. calcns. suggested a reaction mechanism that involves radical addn. of the nucleophilic DCB radical anion to cyclohexene, with cooperative HAT occurring as the final step to regenerate the alkene. Kinetic modeling of the reaction, using rate consts. derived from TAS, demonstrates that the efficiency of the reaction is limited by parasitic absorption and unproductive quenching between excited Ir(ppy)3 and the cyanohydrin photoproduct.
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    9. 9
      Hinsberg, W. D.; Houle, F. A. Kinetiscope. http://www.hinsberg.net/kinetiscope (accessed May 12, 2019).
      There is no corresponding record for this reference.
    10. 10
      Liu, M. J.; Wiegel, A. A.; Wilson, K. R.; Houle, F. A. Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals. J. Phys. Chem. A 2017, 121, 5856– 5870,  DOI: 10.1021/acs.jpca.7b04892
      10
      Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals
      Liu, Matthew J.; Wiegel, Aaron A.; Wilson, Kevin R.; Houle, Frances A.
      Journal of Physical Chemistry A (2017), 121 (31), 5856-5870CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      A key uncertainty in the heterogeneous oxidn. of carboxylic acids by hydroxyl radicals (OH) in aq.-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chem. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimol. decompn. of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of org. carbon between the gas and aq. phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidn. of citric acid aerosol at high relative humidity. The reaction scheme, contg. both free-radical and acid-base elementary reaction steps with phys. validated rate coeffs., accurately predicts the exptl. obsd. mol. compn., particle size, and av. elemental compn. of the aerosol upon oxidn. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimol. fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equil. activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low mol. wt., highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and vol. growth that accompany oxidn. These results frame the oxidative lifecycle of atm. aerosol: it is governed by feedbacks between reactions that first increase the particle oxidn. state, then eventually promote water uptake and acid-base chem. When coupled to free-radical reactions, acid-base channels lead to formation of low mol. wt. gas-phase reaction products and decreasing particle size.
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    11. 11
      Bolshchikov, B. D.; Tsvetkov, V. B.; Alikhanova, O. L.; Serbin, A. V. How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether-alt-maleic anhydride). Macromol. Chem. Phys. 2019, 220, 1900389,  DOI: 10.1002/macp.201900389
      11
      How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether-alt-maleic anhydride)
      Bolshchikov, Boris D.; Tsvetkov, Vladimir B.; Alikhanova, Olga L.; Serbin, Alexander V.
      Macromolecular Chemistry and Physics (2019), 220 (23), 1900389CODEN: MCHPES; ISSN:1022-1352. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Products of free-radical polymn. (FRP) are usually not regulated on the mol. scale, consisting of blocks obtained through the fastest kinetic scheme pathways. The side or kinetically restricted products can be a source of impurities in a complex FRP case, or possess new properties if isolated solely. FRP synthesis of poly(divinyl ether-alt-maleic anhydride), known as "DIVEMA", serves as a polymn. example with such kinetic and thermodn. complexities. Uncertainty in factors regulating polymer structure is a challenge in advancement "DIVEMA" derivs. toward medical practice. In-depth investigation via quantum-chem. and mol. mechanics methods unveils mechanistic aspects of polymer stereoisomerism and confirms possible isolation of thermodynamically or kinetically controlled products on a large data set. Strategies toward regulation of 5-exo/6-endo cycloisomerism are theorized and then studied via microkinetic modeling. Thermodynamically controlled products can be isolated utilizing lower monomer concns., in range of 10-3 to 10-1 M, and/or application of a complexing agent that is better to realize via solvents, capable of formation π- and σ-radical complexes. Change of electrophilic monomer is proposed as an approach for designing more molecularscale adjustable copolymn. processes. Methodol., obtained results, and conclusions for "DIVEMA" can be valuable to control other FRP processes on the mol. scale, unlocking polymers with improved or new functionalities.
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    12. 12
      Bonaldo, F.; Mattivi, F.; Catorci, D.; Arapitsas, P.; Guella, G. D Exchange Processes in Flavonoids: Kinetics and Mechanistic Investigations. Molecules 2021, 26, 3544,  DOI: 10.3390/molecules26123544
      12
      H/D exchange processes in flavonoids: kinetics and mechanistic investigations
      Bonaldo, Federico; Mattivi, Fulvio; Catorci, Daniele; Arapitsas, Panagiotis; Guella, Graziano
      Molecules (2021), 26 (12), 3544CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
      Several classes of flavonoids, such as anthocyanins, flavonols, flavanols, and flavones, undergo a slow H/D exchange on arom. ring A, leading to full deuteration at positions C(6) and C(8). Within the flavanol class, H-C(6) and H-C(8) of catechin and epicatechin are slowly exchanged in D2O to the corresponding deuterated analogs. Even quercetin, a relevant flavonol representative, shows the same behavior in a D2O/DMSOd6 1:1 soln. Detailed kinetic measurements of these H/D exchange processes are here reported by exploiting the time-dependent changes of their peak areas in the 1H-NMR spectra taken at different temps. A unifying reaction mechanism is also proposed based on our detailed kinetic observations, even taking into account pH and solvent effects. Mol. modeling and QM calcns. were also carried out to shed more light on several mol. details of the proposed mechanism.
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    13. 13
      Olmsted, J.; Meyer, T. J. Factors Affecting Cage Escape Yields Following Electron-Transfer Quenching. J. Phys. Chem. 1987, 91, 1649– 1655,  DOI: 10.1021/j100290a071
      13
      Factors affecting cage escape yields following electron-transfer quenching
      Olmsted, John, III; Meyer, Thomas J.
      Journal of Physical Chemistry (1987), 91 (6), 1649-55CODEN: JPCHAX; ISSN:0022-3654.
      The factors influencing the escape of donor-acceptor charge-transfer pairs from their solvent cage were examd., using methylviologen (MV2+) as the acceptor and several different excited metal complexes and arom. org. mols. as donors. The ratios of cage escape yields were detd. by measuring the amt. of transient absorption due to MV•+ radical cation in the absence and in the presence of an anthracene deriv. as an energy shuttle, under conditions where no irreversible photochem. takes place. For a series of different Ru and Os complexes, the cage escape yield of the [M3+,MV•+] pair varied only slightly, ranging from 0.14 to 0.27. When the donor was an org. triplet excited state (9-methylanthracene or acridine yellow), the cage escape yield was near unity. Perturbation by heavy atoms reduced the yield to 0.3 (9-bromoanthracene in soln. contg. CH3I). The cage escape yield was strongly affected by the rate of triplet-singlet interconversion of the triplet charge pair generated in the quenching event. When no mechanism for triplet-singlet mixing was present (org. donors), back-electron-transfer, which requires a spin change, was slow compared to diffusion out of the cage. When spin-orbit coupling was substantial (heavy-atom perturbation, transition-metal complexes), back-electron-transfer was competitive with diffusional cage escape.
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    14. 14
      Sun, H.; Hoffman, M. Z. Reductive Quenching of the Excited State of Ruthenium(II) Complexes Containing 2,2’-Bipyridine, 2,2’-Bipyrazine, and 2,2’-Bipyramidine. J. Phys. Chem. 1994, 98, 11719– 11726,  DOI: 10.1021/j100096a015
      14
      Reductive Quenching of the Excited States of Ruthenium(II) Complexes Containing 2,2'-Bipyridine, 2,2'-Bipyrazine, and 2,2'-Bipyrimidine Ligands
      Sun, Hai; Hoffman, Morton Z.
      Journal of Physical Chemistry (1994), 98 (45), 11719-26CODEN: JPCHAX; ISSN:0022-3654.
      The reductive quenching of the luminescent excited states of Ru(II) complexes of the general formula Ru(bpy)3-m-z(bpm)m(bpz)z2+ (bpy = 2,2'-bipyridine, bpm = 2,2'- bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0,1,2,3 and m + z ≤ 3) by arom. amines and methoxybenzenes as nonsacrificial electron donors and by C2O42-, EDTA, and TEOA as sacrificial donors in aq., acetonitrile, and propylene carbonate soln. was examd. by continuous and pulsed laser flash photolysis techniques. From kq, a value of E° for the irreversible oxidn. of TEOA (-0.84 ± 0.12 V vs. NHE) in acetonitrile was obtained. Values of the cage escape yield of redox products (ηce) showed weak or no dependencies on the driving forces of back electron transfer within the geminate redox pair in the solvent cage (ΔGbt°), suggesting that the simple model of competition between cage escape and back electron transfer may be inadequate to describe the results. A modification of the simple model, in which is introduced a kinetically important reorientation of the geminate redox pair, is proposed.
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    15. 15
      Aydogan, A.; Bangle, R. E.; Cadranel, A.; Turlington, M. D.; Conroy, D. T.; Cauët, E.; Singleton, M. L.; Meyer, G. J.; Sampaio, R. N.; Elias, B.; Troian-Gautier, L. Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light. J. Am. Chem. Soc. 2021, 143, 15661– 15673,  DOI: 10.1021/jacs.1c06081
      15
      Accessing photoredox transformations with an iron(III) photosensitizer and green light
      Aydogan, Akin; Bangle, Rachel E.; Cadranel, Alejandro; Turlington, Michael D.; Conroy, Daniel T.; Cauet, Emilie; Singleton, Michael L.; Meyer, Gerald J.; Sampaio, Renato N.; Elias, Benjamin; Troian-Gautier, Ludovic
      Journal of the American Chemical Society (2021), 143 (38), 15661-15673CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Efficient excited-state electron transfer between an iron(III) photosensitizer and org. electron donors was realized with green light irradn. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)2]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}), that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the UV to the IR regions, provided a quant. description of the catalytic mechanism with assocd. rate consts. and yields.
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    16. 16
      Joshi-Pangu, A.; Lévesque, F.; Roth, H. G.; Oliver, S. F.; Campeau, L.-C.; Nicewicz, D.; DiRocco, D. A. Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis. J. Org. Chem. 2016, 81, 7244– 7249,  DOI: 10.1021/acs.joc.6b01240
      16
      Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis
      Joshi-Pangu, Amruta; Levesque, Francois; Roth, Hudson G.; Oliver, Steven F.; Campeau, Louis-Charles; Nicewicz, David; DiRocco, Daniel A.
      Journal of Organic Chemistry (2016), 81 (16), 7244-7249CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      The emergence of visible light photoredox catalysis has enabled the productive use of lower energy radiation, leading to highly selective reaction platforms. Polypyridyl complexes of iridium and ruthenium have served as popular photocatalysts in recent years due to their long excited state lifetimes and useful redox windows, leading to the development of diverse photoredox-catalyzed transformations. The low abundances of Ir and Ru in the earth's crust and, hence, cost make these catalysts nonsustainable and have limited their application in industrial-scale manufg. Herein, we report a series of novel acridinium salts as alternatives to iridium photoredox catalysts and show their comparability to the ubiquitous [Ir(dF-CF3-ppy)2(dtbpy)](PF6).
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    17. 17
      Ruccolo, S.; Qin, Y.; Schnedermann, C.; Nocera, D. G. General Strategy for Improving the Quantum Efficiency of Photoredox Hydroamidation Catalysis. J. Am. Chem. Soc. 2018, 140, 14926– 14937,  DOI: 10.1021/jacs.8b09109
      17
      General Strategy for Improving the Quantum Efficiency of Photoredox Hydroamidation Catalysis
      Ruccolo, Serge; Qin, Yangzhong; Schnedermann, Christoph; Nocera, Daniel G.
      Journal of the American Chemical Society (2018), 140 (44), 14926-14937CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The quantum efficiency in photoredox catalysis is the crucial determinant of energy intensity and, thus, is intrinsically tied to the sustainability of the overall process. Here, we track the formation of different transient species of a catalytic photoredox hydroamidation reaction initiated by the reaction of an Ir(III) photoexcited complex with 2-cyclohexen-1-yl(4-bromophenyl)carbamate. We find that the back reaction between the amidyl radical and Ir(II) photoproducts generated from the quenching reaction leads to a low quantum efficiency of the system. Using transient absorption spectroscopy, all of the rate consts. for productive and nonproductive pathways of the catalytic cycle have been detd., enabling us to establish a kinetically competent equil. involving the crucial amidyl radical intermediate that minimizes its back reaction with the Ir(II) photoproduct. This strategy of using an off-pathway equil. allows us to improve the overall quantum efficiency of the reaction by a factor of 4. Our results highlight the benefits from targeting the back-electron transfer reactions of photoredox catalytic cycles to lead to improved energy efficiency and accordingly improved sustainability and cost benefits of photoredox synthetic methods.
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    18. 18
      Soto, X.; Swierk, J. Using Lifetime and Quenching Rate Constant to Determine Quencher Concentration. ChemRxiv, 26, 2022. ver. 1. DOI: 10.26434/chemrxiv-2022-q6klm .
      There is no corresponding record for this reference.

  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c02638.

    • Details of kinetic modeling; impact of illumination time and intensity on quantum yield; predicted quencher concentrations for starting with the fixed concentration of excited PC; comparison of quencher concentrations predicted by eq 3 with those derived from kinetic modeling; and details of experimental studies included in Figures 2 and 3 (PDF)


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