Excitation of metastable intermediates in organic photoredox catalysis

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Excitation of metastable intermediates in organic photoredox catalysis: Z-scheme approach decreases catalyst inactivation Alan Aguirre-Soto, Kaja Kaastrup, Seunghyeon Kim, Kasite Ugo-Beke, and Hadley D. Sikes ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00857 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Catalysis

Excitation of metastable intermediates in organic photoredox catalysis: Z-scheme approach decreases catalyst inactivation a

a

a

a

Alan Aguirre-Soto , Kaja Kaastrup , Seunghyeon Kim , Kasite Ugo-Beke , Hadley D. Sikes

a

a,b,

*.

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts

Avenue, Cambridge, MA 02139, USA b

Program in Polymers and Soft Matter, Massachusetts Institute of Technology, 77 Massachusetts

Avenue, Cambridge, MA 02139, USA *Corresponding author

ABSTRACT: Despite the numerous applications of eosin Y as an organic photoredox catalyst, substantial mechanistic aspects of the photoredox process have remained elusive. Through deductive, steady-state kinetic studies, we first propose a mechanism for alkaline, aqueous photoredox catalysis using eosin Y, triethanolamine and oxygen, integrating photo- and nonphotochemical steps. The photoredox cycle begins with a single-electron transfer (SET) induced when eosin Y absorbs green light. This photoinduced SET leads to the formation of a metastable

ACS Paragon Plus Environment

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+ -

+

radical trianion that can be fully reduced to inactivated leuco eosin Y via H /e /H transfer, or regenerated to eosin Y via ground-state SET to oxygen. Since the radical trianion absorbs violet light, we tested the effect of radical trianion photo-excitation on catalyst regeneration. We found that excitation of the metastable radical trianion in the presence of a threshold concentration of oxygen enabled ~100% regeneration of eosin Y. The response to violet light supports the important role of the metastable radical trianion and indicates that the photoredox cycle can be closed via a secondary photoinduced SET event. The idea of photoredox cycles with two consecutive photoinduced electron transfer (PET) steps is not intuitive, and is introduced as a tool to increase photocatalyst turnover by selectively favoring regeneration over “death”. This alludes to the Z-scheme in biological photosynthesis, where multiple PET reactions, often triggered by different frequencies, promote highly selective biochemical transformations by precluding unproductive SET events in plants and bacteria. We expect that the simple Z-scheme model introduced here will enable more efficient use of organic photoredox catalysts in organic and materials chemistry.

KEYWORDS: Photoredox catalysis, reaction kinetics, catalyst inactivation, Z-scheme, reaction mechanism 1. INTRODUCTION Current demands for more sustainable, efficient and benign chemical processes have bolstered the field of photocatalysis, with the goal of providing recyclable, light-absorbing molecules capable of activating chemical reactions under mild irradiation and reaction conditions. Several modes of photocatalysis, distinguished by the mechanism of the recyclable activation, are now readily available, and numerous chemical transformations have been achieved with them.


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In the photoredox mode of photocatalysis, the light-absorbing molecule both activates a reaction and regenerates via single-electron transfer (SET) events. After light absorption by the catalyst, a photoinduced electron transfer (PET) occurs, turning the light-absorbing molecule into an open-shell species with enough thermodynamic driving force to return to the original photocatalyst via a subsequent ground-state SET. While the concept was introduced 40 years 1

ago, it is the most rapidly growing mode of photocatalysis within the last decade.

Organic dyes have been suggested as less toxic and more affordable alternatives to the transition-metal complexes that dominate as photoredox catalysts. Notable examples include 2

3

4

methylene blue, α-sexithiophene and eosin Y. Organic photoredox catalysts have been successfully

employed

trifluoromethylations, desulfurations, 15

catalysis,

11

6,7

for

numerous

reactions,

including

3

hydroxylations,

8

9

2,5

10

dehalogenations, desulfonylations, brominations, cyclizations, 12

13,14

arylations,

C-C and C-P bond-forming reactions, 16-19

and polymerizations,

asymmetric

and in some cases, have even outperformed the efficient 4,20,21

ruthenium and iridium bipyridyl complexes.

From the countless available organic dyes, eosin Y (eosin) has emerged as a preferred organic photoredox catalyst. A wide range of transformations in synthetic organic and materials 4,17

chemistry have been reported, proceed via SET steps.

22

and it is generally theorized that most of these reactions

However, extrapolation of mechanistic insights gained from studies of 23-28

the photochemistry and excited-state kinetics of eosin alone

to more complex and varied


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Page 4 of 28

photoredox catalytic systems involves much uncertainty. In addition, the non-photochemical steps in the photoredox process have been relatively under-studied.

29

Hence, crucial mechanistic

20

aspects remain unclear.

To address this gap, we studied the mechanism of neutral-to-alkaline aqueous photoredox catalysis using eosin, triethanolamine (TEOA) and oxygen in the presence of N-vinylpyrrolidone (NVP) and polyethyleneglycol-diacrylate (PEGDA) under green light. We chose this photoredox system because it is the most widely used for visible-light-induced synthesis of hydrogels via radical chain polymerization of vinyl groups. A deductive investigation of steady-state kinetics was performed with a custom-built high-sensitivity electronic and vibrational spectroscopy 30

apparatus that allows simultaneous tracking of the concentrations of multiple molecules.

This

technique facilitated the elucidation of the direct links between catalyst decay, production of intermediates, and oxygen depletion. Combining our steady-state kinetic results with an analysis of the reported excited-state kinetics, we propose a mechanism that integrates the wellunderstood photochemical steps with the poorly understood non-photochemical steps. Most notably, we highlight the crucial role of a radical trianion intermediate in ensuring efficient catalyst regeneration in the presence of oxygen. The metastability and light-absorbing quality of this radical trianion led us to hypothesize that regeneration of eosin could be enhanced by photo-excitation of the radical trianion using violet light (~405 nm) in addition to the green light absorbed by eosin. We compared exclusive photoexcitation of the catalyst, using a single modern narrow-band light-source, with photo-excitation of both the photoredox catalyst and the visible-light-absorbing metastable intermediates using two different narrow-band light sources. While photo-excitation of intermediate species has most


4

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certainly, albeit inadvertently, occurred since the beginnings of photochemistry, only recently 31-33

has it been discussed in photoredox catalysis,

and it has not been proposed as a method to

reduce catalyst inactivation. To the best of our knowledge, light-absorbing photoredox intermediates from broadly used organic photocatalysts, such as eosin, have not been intentionally and separately photo-excited to improve regeneration in organic or macromolecular synthesis. Lastly, we discuss our expectation that selective photo-excitation of intermediates will enable the design of more efficient photoredox platforms by employing a Z-scheme approach. One of the greatest challenges halting wider adoption of photoredox catalysis lies in ensuring both efficient photocatalyst regeneration and selective radical-mediated processes. Unproductive electron-transfer events, for instance, may lead to low yields. In nature, photosynthetic pathways rely on sequences of photoinduced reactions to increase the selectivity of vital biochemical transformations by precluding unproductive SET events. Our light-assisted photocatalyst regeneration approach alludes to this natural Z-scheme mechanism, which has been used in 34

photocatalytic water splitting,

and extends its use to a versatile class of organophotocatalysts.

2. RESULTS AND DISCUSSION 2.1 Formation of metastable radical trianion from initial PET event. After absorption of a photon of wavelength in the vicinity of 517 nm, eosin can donate an electron, accept an electron or undergo energy-transfer from its excited states depending on the solvent and other reactants present. It is generally agreed that eosin’s photochemical reactions are mediated by its long-lived triplet excited state, which persists for 24-30 µs in water, aid by eosin’s high molar absorptivity and triplet quantum yield, 0.74 (Figure S1 in the Supporting Information), which makes it


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comparable to the metal bipyridyls. Analysis of the excited-state kinetics from reported values in 7

aqueous neutral-to-alkaline pH (7-10), including quenching rate constants (kqTEOA= 4x10 -1 -1

Lmol s ), quantum yields, and initial concentrations of eosin, TEOA (excess) and oxygen, indicates that photo-oxidation of TEOA is the most kinetically favorable route for triplet quenching under the conditions used here. Electron donation by the amine to eosin’s excited state is so well-accepted that the photoredox community has generally proposed that the photoredox catalysis with eosin proceeds analogous to that with ruthenium and iridium complexes, i.e. via SET, even in air-equilibrated conditions where energy transfer to oxygen can occur. Excited-state oxidation and reduction potentials of ca. -1.11 V and 0.83 V vs SCE, respectively, further support this idea.

4

If SET is the main triplet quenching mechanism, then eosin must be converted into the radical trianion EY˙

3-

from its dianionic ground-state form in aqueous neutral-to-alkaline solution.

3-

Indeed, EY˙ has been characterized by electron paramagnetic resonance (EPR), 23,25,26

transient spectroscopy

27

UV-Vis, and

in neutral-to-alkaline aqueous solutions purged with inert gases.

Here, we first confirmed appearance of a peak in the absorbance spectrum centered around 408 3-

nm, corresponding to EY˙ , in aqueous solution in the absence of oxygen and hydrogen donors (Figure 1A and B, Figure S2 in the Supporting Information). Formation of EY˙ with the decay of the eosin peak. While the EY˙

3-

correlates well

intermediate has been implied in the

26,35-37

photoreduction of eosin, including with amines,

3-

most of the papers using eosin as

photoredox catalyst do not discuss the effects of reaction conditions on EY˙

3-

formation. The


6

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3

2-

photoreduction of the eosin excited-state triplet ( EY *) by triethanolamine (TEOA) most likely occurs via a loosely bound charge-transfer complex (Figure S3 in the Supporting Information). Recent studies suggest that eosin’s singlet excited-state may also participate in PET from amines, which would not modify our observations but only change the pathway for EY˙

3-

38

production.

3-

In slightly acidic aqueous solutions, EY˙ would not be expected as it readily protonates to EY2-

H˙ . In the present study, however, solutions range from neutral to basic. Thus, EY˙ be found unprotonated (Figure 1A, Figure S4 in the Supporting Information).

3-

27,35,36

can then

Based on

our calculations, the majority of the photoreduced eosin appears to be in fact protonated in the 2-

form EY-H˙ (Figure S4 in the Supporting Information). However, both EY˙

3-

and EY-H˙

2-

28,39

sufficiently long-lived in the absence of oxidants and hydrogen donors for detection.

are

This

led us to hypothesize that we could selectively promote the production of either of these intermediates by appropriate control of reaction conditions. 3-

2-

2.2 Metastability of the photoredox intermediates EY˙ / EY-H˙ . Though it is widely known that photoredox systems are pH-sensitive, the acid-base equilibrium of photoredox intermediates has seldom been systematically employed as a control tool to favor the production of SET intermediates. This arguably stems from the generalization that organic photoreductions -

+

are proton-coupled electron (e /H ) transfer (PCET) reactions, where the intermediates formed +

from SET are so basic that a proton (H ) is transferred immediately after electron transfer within 37,40

the solvated ion pair.

As a result, consideration of the role of EY˙

3-

has been precluded.


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However, recent evidence suggests that SET can occur at different distances depending on the nuclear reorganization energy imposed by the solvent and the electron coupling of the system. 3- 25,26

This could explain the detection of species such as EY˙ . 2-

dominant, the EY-H˙ species could be readily deprotonated to EY˙

41

And, even if PCET was

3-

after PCET at appropriate

conditions. This led us to focus our attention on the production and consumption of EY˙

3-

under

different conditions, including the presence of oxygen and NVP.

Figure 1. Chemical structure of eosin Y used as photoredox catalyst, one-electron reduction to its radical trianion, two hydrogen atom reduction to leuco-eosin, steady-state kinetic studies of eosin, radical trianion, and leuco eosin concentrations, and colorimetric assay for hydrogen peroxide detection using superoxide dismutase (SOD). (A) The metastable radical trianion is the partially reduced intermediate EY˙

3-

(λmax = 408 nm) produced during irradiation of an aqueous


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solution of eosin Y (λmax =517 nm) and triethanolamine (TEOA) with ~520 nm light in 3-

anaerobic conditions. (B) Radical trianion, EY˙ , yield as a function of irradiation time in the absence and presence of oxygen or N-vinylpyrrolidone (NVP). (C) Formation of the leuco-eosin, 2-

fully reduced “dead” photocatalyst EY-H2 , during irradiation with green light in the presence of NVP. (D) Steady-state kinetics of eosin Y decay during irradiation time in the presence and absence of oxygen or NVP. (E) Effect of SOD in H2O2 production in solutions irradiated with 2

500 nm (FWHM 20 nm) light at ~ 30 mW/cm as a function of exposure time. The intensity of the blue color is proportional to the H2O2 concentration and was quantified using the CIELAB color space. ∆CIE values were calculated by subtracting the values for control samples before irradiation. Data points indicate the average of three replicates, and error bars represent standard deviation. 2-

2.3 Photocatalyst inactivation by full reduction of EY-H˙

competes with regeneration

3-

by oxidation of EY˙ . We examined the role of NVP as a hydrogen donor. Hydrogen abstraction from TEOA is also possible but appears to be significantly slower. While SET from 3-

EY˙ to O2, shown in Figure 1D, regenerates eosin (reaction 1), the transfer of a hydrogen atom from NVP to EY-H˙

2-

-

+

(reaction 2) leads to photocatalyst “death” via the 2e /2H transfer

process (Figure 1C and D). On one hand, the hydrogen donor, NVP, precluded detection of EY˙

3-

and dramatically increased the rate of eosin decay (Figure 1B-D and Figure S5 in the 3-

Supporting Information). In contrast, O2 also consumed EY˙ , but regenerated eosin instead


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(Figure 1B-D and Figure S6 in the Supporting Information). While the fully bent, “dead” leuco eosin EY-H2

2-

(Figure 1C and Figure S7 in the Supporting Information) is difficult to oxidize

back to eosin (Figure S8 in Supporting Information),

35,36

EY˙

3-

can be reasonably expected to

readily oxidize back to eosin in the presence of O2. We confirm that regeneration appears to be faster than full reduction in presence of NVP, TEOA and oxygen (Figure S9 in the Supporting 3-

Information). These results support the metastability of the EY˙ /EY-H˙

2-

intermediates (Figure

S5 in the Supporting Information), and of the importance of ground-state SET from the radical trianion to oxygen in precluding irreversible inactivation of the photoredox catalyst. 3-

2-

EY˙ + O2 EY + O2˙ 3-

+

-

(1)

2-

2-

EY˙ + H EY-H˙ + NVP-H EY-H2 + NVP˙

(2)

3-

2.4 Production of superoxide as evidence of ground-state SET from EY˙

to oxygen. To

further test our hypothesized mechanism, we explored the ground-state reactions in which EY˙

3-

participates. A proposed pathway is SET to oxygen to form superoxide (reaction 1). Superoxide is known to readily decay by disproportionation to oxygen and hydrogen peroxide H2O2.

42

Several reports were found of photoredox cycles where peroxides are formed from the 4

superoxide produced from the SET cycle. Here, we utilized superoxide dismutase (SOD) for the detection of superoxide via a colorimetric peroxidase assay in solution. Results clearly show a significant increase in the color associated with the production of H2O2 when SOD is present


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(Figure 1E). Quantification of the colorimetric results clearly shows that H2O2 was produced in proportion to irradiation dose as SOD catalyzes H2O2 disproportionation. These observations support the production of superoxide under aerobic irradiation, which most likely results from the ground-state SET from EY˙

3-

to O2 at the concentrations of oxygen, TEOA, and NVP used

here (Figure S1 in the Supporting Information). We confirmed that eosin regeneration by ground-state SET from EY˙

3-

to O2 is exergonic (Figures S6 & S10 in the Supporting

Information). The electrostatically corrected standard free energy change ∆G°’ET is estimated between – (13.8-9.2 kcal/mol), using Marcus Theory (Figure S9 in the Supporting Information). A kinetic constant for electron transfer cannot be estimated from our experiments, but our kinetic data indicate the reaction rate is close to diffusion-controlled. Others have reported the used of H2O2 decomposition to extend the radical initiation process beyond the irradiation time by 43

reaction with ascorbic acid.

Here, we confirmed that H2O2 forms directly from the

regeneration step with oxygen. 2.5 Excitation of metastable radical trianion intermediate accelerates regeneration. With an understanding of how to control the formation of EY˙

3-

and EY-H˙

2-

through the kinetics of

eosin photoreduction, ground-state oxidation and hydrogen-abstraction reactions, we examined whether photocatalyst regeneration could be selectively accelerated by photo-excitation of EY˙ , as shown in Figure 2. Kimura et al. observed that visible light irradiation of EY˙

3-

3-

induces

debromination in deoxygenated basic methanolic solutions via a hypothesized SET to ground44-46

state eosin.

However, the implications and potential of photo-exciting EY˙

3-

were not


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examined. This motivated us to explore the possibility of accelerating the ground-state electron transfer from EY˙

3-

to O2 by photo-activating EY˙

3-

with violet light, which would be mostly

3-

absorbed by EY˙ due to eosin’s lesser molar absorptivity at this frequency.

We confirmed that the rate of eosin regeneration is faster when EY˙

3-

absorbs a violet (405

nm) photon in the presence of a threshold concentration of oxygen under green light irradiation (Figure 2). While eosin

Figure 2. Steady-state kinetics of eosin decay during irradiation with green light and/or violet light. Exposing air-equilibrated aqueous eosin/TEOA solutions to violet light (405 nm) improves the regeneration of eosin during continuous irradiation with a green LED (500 nm). When violet and green lights are used simultaneously, the rate of single electron transfer from EY˙ increases, as compared to exposure to green light alone. More EY˙

3-

3-

to O2

is converted back into

eosin, thus reducing irreversible catalyst inactivation.


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has a non-zero absorption in the violet, this result is opposite of what would be expected if the additional photons were being absorbed by ground state eosin, i.e. increase in the eosin consumption rate. These initial observations are impressive considering that the irradiance at 405 nm, ~ 200-700 µW/cm2, is at approximately two orders of magnitude weaker than the irradiance at frequencies greater than 500 nm (Figure 2), absorbed by eosin. Irradiance levels above a certain threshold, especially in the absence of oxygen, appear to have an opposite effect, expected from the absorption of excess 405 nm photons by eosin. Only 15% eosin was consumed after 800 s of irradiation, which constitutes over 20% improvement in regeneration by photoinduced electron transfer as compared to ground-state electron transfer to oxygen. As expected, PET to O2 occurs at a higher rate than full reduction of EY-H˙

2-

by hydrogen

abstraction from NVP (Figure S10 in the Supporting Information). This light-driven 3-

photocatalyst regeneration provides additional evidence of the formation of dissociated EY˙ , as well as of the utility of its metastability to prevent irreversible catalyst inactivation. Photoexcitation of EY˙

3-

3-

accelerates regeneration by SET to O2, shifts the EY˙ /EY-H˙

2-

acid-

3-

base equilibrium towards EY˙ , and reduces photocatalyst inactivation.

We tested these expectations when the photoredox catalytic system was used to convert PEGDA and NVP to a hydrogel. During this reaction, aerobic exposure to green and violet light resulted in ~100% regeneration of eosin in the presence of oxygen, i.e. eosin concentration remained constant as long as a threshold concentration of oxygen was retained (Figure 3A-C). The rate of eosin consumption and polymerization were the same with or without violet light when oxygen is absent (Figures 2 and 3). As expected, the rate of anaerobic eosin decay under both irradiation regimes was the same as the rate obtained after O2 depletion under exclusive


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exposure to green light (Figure 3). Increase in eosin recyclability has been reported through other methods, including the grafting of eosin to nanoparticles.

47

However, this is the first report, to

our knowledge, of increase recyclability by photoexcitation of metastable intermediates.


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Figure 3. Steady-state kinetics of eosin decay during radical chain polymerization of NVP/PEGDA (A-C) Residual eosin Y and extent of polymerization during aerobic and anaerobic continuous irradiation of aqueous eosin Y/triethanolamine/N-vinylpyrrolidone/PEGDA solutions for A) anaerobic exposure to green light (500 nm), B) aerobic exposure to green light alone, and C) aerobic exposure to green and violet light (405 nm). Conservation of 100 % of the initial eosin Y is achieved only with oxygen and violet light, as the regeneration of eosin Y becomes highly sensitive to the presence of oxygen. 2.6 The second photoinduced electron transfer (PET) closes the photoredox cycle more 3-

rapidly than a single PET event. The excited state of the metastable radical trianion (EY˙ *) is expected to have a higher ionization potential than its ground state, thus making electron transfer more thermodynamically feasible (Scheme 1). The standard free energy change for photoinduced electron transfer (∆G°’PET) will be lower than ∆GET by the energy of the excited state E0,0 (Figure S3 in the Supporting Information), as classically conveyed by the semi-empirical RehmWeller equation. While little information is available on the excited states of photoredox radical 3-

ion intermediates (e.g. EY˙ ), the rate of photoinduced electron transfer can surely be expected 3-

to be faster than the rate of the ground-state electron transfer from EY˙ to O2.

2.7 Organophotocatalysis cycle with two sequential PETs alludes to Z-scheme in biological photosynthesis. We propose the photoredox cycle shown in Scheme 1, where green 3-

light drives photoreduction of eosin to EY˙ , while violet light accelerates conversion of EY˙ -

+

back to eosin to prevent “death” by full 2e /2H

3-

reduction. No reports were found of a

photocatalysis mechanism where two consecutive PETs are employed to selectively enhance


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Page 16 of 28

photocatalyst regeneration. Our mechanism alludes to the photosynthetic mechanisms observed in Nature,

48,49

e.g. Z-scheme, where multiple reaction centers, such as photosystems I and II,

participate in sequential photoinduced electron transfer reactions. In plants and

Scheme 1. Organophotocatalysis mechanism with two photoinduced electron transfers 2-

selectively promotes photocatalyst regeneration over inactivation via Z-scheme. Eosin Y (EY ) is photoreduced by TEOA upon absorption of green light (500-560 nm). Dissociated EY˙ found unprotonated, which allows it to regenerate EY oxygen. Full reduction of the protonated EY-H˙ 2-

2-

2-

3-

is

via direct single-electron transfer to

intermediate, via intramolecular hydrogen -

+

abstraction (EY-H˙ ) and proton-coupled electron (e /H ) transfer, competes with regeneration


16

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ACS Catalysis

2-

of eosin Y. Fully reduced EY-H

(gray box) is difficult to oxidize back to eosin Y, and is

therefore considered inactivated or “dead” catalyst (colorless solution). Eosin Y regeneration (colored solution) is accelerated by excitation of EY˙

3-

with violet light (400-420 nm) due to a

more thermodynamically feasible electron transfer to oxygen. bacteria, protein-bound electron donors and acceptors participate in sequences of PET reactions that aid in controlling the rates of charge separation and recombination to favor productive SET steps, while kinetically hindering unproductive reactions. While photosynthesis is more complex, our mechanism alludes to the strategy of using multiple sequential PETs to harness enough energy to favor a desired reaction pathway, catalyst regeneration in this case, with readily available organic photoredox catalysts.

3. CONCLUSIONS Our mechanism introduces a set of tools to favor photocatalyst regeneration by controlling the chemical kinetic rates of reactions of the photocatalysis intermediates. For instance, selection of 3-

the oxidant and the hydrogen donor ensures that the EY˙ /EY-H˙

2-

open-shell intermediates are

sufficiently long-lived to participate in subsequent reactions. Then, the photoexcitation of one of the intermediates is used to increase the kinetic rate of one of the two reactions allowing us to selectively promote a particular pathway, e.g. photocatalyst regeneration over inactivation. In this case, ground-state SET from EY˙ 3-

photoexcitation of EY˙

3-

to oxygen is so fast that the rate improvement with

is not as dramatic as can be reasonably be expected for other organic


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photoredox catalysts. However, it is reasonable to expect that other intermediates could be stabilized long enough to absorb a photon, where perhaps the oxidant has a lower electron affinity to the metastable intermediate than the case introduced here. The result would be that the preclusion of catalyst inactivation can be greater in other systems, which is significant for many chemical transformations. This work serves as proof-of-concept of how sequences of photoinduced electron transfers can be designed to selectively favor catalyst regeneration. AUTHOR INFORMATION Corresponding Author *Correspondence to: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Materials and methods, photochemical and photophysical properties of eosin Y, detection of visible light absorbing metastable radical trianion, Eosin Y/triethanolamine complexation study, Estimation of relative yields, metastable intermediates from the photoreduction of eosin Y, evidence of SET from radical trianion to oxygen, bent structure of leuco eosin Y, colorless


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solution after irreversible catalyst inactivation, analysis of Gibbs free energy, steady-state kinetics of eosin inactivation, oxygen inhibition kinetics, oxygen permeation through hydrogels. (PDF)

ACKNOWLEDGMENT A CONACYT Postdoctoral Research Award (263622 to A.A.) an NSF GRFP Award (to K.K.) and the Department of Defense (W81XWH-13-1-0272) supported this work. We thank Prof. Jeffrey W. Stansbury for support with the use of the coupled UV-Vis/FT-NIR spectroscopy apparatus. We thank Jose Enrique Arizpe Femat and Felice Frankel for help with visuals. REFERENCES 1. Hedstrand, D. M.; Kruizinga, W. H.; Kellogg, R. M. Light induced and dye accelerated reductions of phenacyl onium salts by 1,4-dihydropyridines. Tetrahedron Lett. 1978 19, 1255– 1258. 2. Pitre, S. P.; McTiernan, C. D.; Ismaili, H.; Scaiano, J. C. Mechanistic Insights and Kinetic Analysis for the Oxidative Hydroxylation of Arylboronic Acids by Visible Light Photoredox Catalysis: A Metal-Free Alternative. J. Am. Chem. Soc. 2013 135,13286–13289. 3. McTiernan, C. D.; Pitre S. P.; Scaiano, J. C. Photocatalytic Dehalogenation of Vicinal Dibromo Compounds Utilizing Sexithiophene and Visible-Light Irradiation. ACS Catal. 2014, 4, 4034–4039. 4. Hari, D. P.; König, B. Synthetic applications of eosin Y in photoredox catalysis. Chem. Commun. 2014, 50, 6688–6699.


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TOC Graphic


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Excitation of metastable intermediates in organic photoredox catalysis

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