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-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore

Nathan T. La Porte, Davis B. Moravec, and Michael D. Hopkins1

Department of , The University of Chicago, Chicago, IL 60637

Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved May 27, 2014 (received for review November 15, 2013)

Energy-storing artificial-photosynthetic systems for CO2 reduction Neumann and coworkers recently reported a photochemical must derive the reducing equivalents from a renewable source system for the reduction of CO2 to CO in which the reducing rather than from sacrificial donors. To this end, a homogeneous, equivalents are derived from the oxidation of H2 by colloidal I + integrated chromophore/two-catalyst system is described that (15). This system, which contains a [Re (phen)(CO)3L] is thermodynamically capable of photochemically driving the (phen is 1,10-phenanthroline) CO2 reduction catalyst linked with -storing reverse –gas shift reaction (CO2 + H2 → CO + a polyoxometalate cluster, drives the reverse water–gas-shift H2O), where the reducing equivalents are provided by renewable reaction (RWGS) (CO2 + H2 → CO + H2O), using light as the H2. The system consists of the chromophore tetraphenylpor- energy source, via the reactions shown in Eqs. 1–3. Unlike R – phyrin (ZnTPP), H2 oxidation catalysts of the form [Cp Cr(CO)3] , photochemical reactions that consume sacrificial donors, this ′ –1 and CO2 reduction catalysts of the type Re(bpy-4,4 -R2)(CO)3Cl. Us- energy-storing system [ΔHf = 41.2 kJ·mol (17)] catalytically ing time-resolved spectroscopic methods, a comprehensive mech- extracts renewable reducing equivalents that can be sourced anistic and kinetic picture of the photoinitiated reactions of to water. mixtures of these compounds has been developed. It has been − + found that absorption of a single by broadly absorbing H2 → 2e + 2H [1] ZnTPP sensitizes intercatalyst to produce the sub- strate-active forms of each. The initial photochemical step is the − + CO2 + 2e + 2H → CO + H2O [2] heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalyti- + → + [3] cally competent state decays with a second-order half-life of CO2 H2 CO H2O ∼15 μs, which is of the right magnitude for substrate trapping The mechanistic, thermodynamic, and kinetic integration of of sensitized catalyst intermediates. two catalytic cycles with a chromophore is a general challenge that cuts across homogeneous molecular approaches to forming artificial | | dual catalysis | photoredox | solar fuels from CO and H O. A photochemical system for the 2 2

RWGS reaction that used a homogeneous H2 oxidation catalyst CHEMISTRY would both provide insights into the fundamental factors that he inexorable growth of global energy consumption and govern this integration and opportunities to exert greater control Tconcern over its attendant environmental consequences has over them than is possible with heterogeneous catalysts. Moti- spurred considerable research into developing means to store vated by these possibilities, we report here the photochemistry solar energy in the form of renewable chemical fuels (1, 2). of a homogeneous system composed of a , CO2- Among the potential feedstocks for these solar fuels, CO2 is reduction catalyst, and H2-oxidation catalyst that, upon excita- a desirable target because it is the end product of the tion with long-wavelength light (λ > 590 nm), yields a product of fossil fuels. Thus, developing solar-driven mechanisms for chemically reducing CO2 to energy-rich products holds the po- Significance tential to recycle conventional fuels and mitigate their – impact (3 7). In natural photosynthesis, the reducing equivalents for the Numerous studies over the past 30 y have investigated ho- reduction of CO2 are derived from photochemical water split- mogeneous artificial-photosynthetic systems for CO2 reduction, ting. In homogeneous artificial-photosynthetic systems for CO2 in which a photoexcited chromophore accomplishes the transfer reduction, the source of the reducing equivalents is generally of from a source of reducing equivalents to a CO2 a nonrenewable sacrificial . These sacrificial reduction catalyst (8–14). With very rare exceptions (15), the reagents aid proof-of-concept experiments by suppressing reducing equivalents consumed in these photochemical CO2 unproductive electron-transfer reactions but negate the solar- reduction reactions have been supplied by sacrificial electron energy-storing potential for the system. Here, we describe an donors. These reagents are used because their prompt decom- integrated homogeneous system in which a zinc porphyrin position following photoinitiated oxidation suppresses un- photosensitizes CO2 reduction and H2 oxidation catalysts, productive back-electron-transfer pathways, which are generally allowing the reducing equivalents for CO2 photoreduction to fast compared with substrate transformation, and because their be derived from renewable H2. This system is thermodynami- decomposition products can provide additional reducing equiv- cally competent to photochemically drive the energy-storing reverse water–gas shift reaction. alents needed for some CO2 reduction reactions, thus circum- venting the one-photon/one-electron limit of molecular photo- Author contributions: N.T.L., D.B.M., and M.D.H. designed research; N.T.L. and D.B.M. sensitizers. Offsetting these practical advantages, however, is the performed research; N.T.L., D.B.M., and M.D.H. analyzed data; and N.T.L., D.B.M., and fact that the stoichiometric consumption of conventional sacri- M.D.H. wrote the paper. ficial donors in these reactions negates their energy-storing The authors declare no conflict of interest. potential. In order for homogeneous systems to drive CO2 re- This article is a PNAS Direct Submission. duction reactions that store energy, these sacrificial reagents 1To whom correspondence should be addressed. E-mail: [email protected]. must be replaced by a second catalytic cycle that extracts the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. reducing equivalents from a renewable source (16). 1073/pnas.1321375111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1321375111 PNAS | July 8, 2014 | vol. 111 | no. 27 | 9745–9750 Downloaded by guest on September 24, 2021 state thermodynamically capable of accomplishing the RWGS Molecular catalysts for H2 oxidation may be divided into two reaction. The system operates via reductive quenching of the mechanistic classes: those with metalloradical active states that low-energy T1 excited state of the common chromophore zinc homolytically cleave H2 via a termolecular process (27), and tetraphenylporphyrin (ZnTPP) by a compound of the form -inspired catalysts that heterolytically activate H2 at a R – – R 5 Cp Cr(CO)3 (1 ;Cp = η -cyclopentadienyl), followed by closed-shell center with assistance of a second-coordination- – thermal electron transfer from the product ZnTPP to sphere Brønsted base (28). Although the hydrogenase-inspired ′ 2 ′ a complex of the type Re(bpy-4,4 -R2)(CO)3Cl ( ; bpy is 2,2 - catalysts exhibit faster rates for H2 activation, their catalytic cycles bipyridyl). The electron-transfer-sensitized radical products of involve three metal oxidation states. In contrast, catalysts with R – these reactions—Cp Cr(CO)3 (1•)and[Re(bpy-4,4′-R2)(CO)3Cl] metalloradical active states cycle through two oxidation states and, – (2 )—can initiate the oxidation of H2 and reduction of CO2, re- thus, can potentially be activated by a single photoinduced elec- spectively. The substituents within each of these classes of tron-transfer reaction. The most extensively studied of these cata- R – compounds allow control over the driving forces and, thus, the lysts are of the form [Cp Cr(CO)3] , which participate in the cycle rates of the productive (and unproductive) electron-transfer of reactions shown in Eqs. 4–6: reactions available to the components. A comprehensive picture  Ã−  à R ð Þ ð1−Þ → R ð Þ ð1•Þ + − [4] of the mechanism and kinetics of this system has been elucidated Cp Cr CO 3 Cp Cr CO 3 e using time-resolved spectroscopic methods.

 à R ð Þ + 1 = → R ð Þ ð1HÞ [5] Results and Discussion Cp Cr CO 3 2H2 Cp Cr CO 3H Design Criteria. Shown in Fig. 1 is a general scheme for accom-  Ã− R ð Þ + ð :Þ → R ð Þ + +; [6] plishing the RWGS reaction via photochemical electron-transfer Cp Cr CO 3H base B Cp Cr CO 3 BH sensitization of H2-oxidation (CatH2)andCO2-reduction (CatCO2) catalysts by a chromophore (Chr). This scheme imposes where Eq. 5 is a composite of two steps (1• + H2 → 1(H2); 1(H2) + – – a number of fundamental and practical constraints on the com- 1• → 2 1H) (27). The derivatives [Cp*Cr(CO)3] (1a ) and [CpCr – – ponents of the system, which are described here to explain the (CO)3] (1b ) were selected for this study because they possess particular chromophore and catalysts chosen for the present suitable oxidation potentials and pKas for the relevant intermedi- study. These constraints include the obvious thermodynamic ates (29). Further, the deprotonation step that completes the H2 requirements for the pairs of chromophore–catalyst electron- activation cycle (Eq. 6) can be accomplished with alkoxide bases t – transfer reactions, and that the pKas of catalyst intermediates such as BuO (30); the conjugate-acid products of this step must be matched such that there is a Brønsted conjugate acid– are known to be suitable proton sources for the reduction of CO2 2 base pair that can transfer protons from the H2 oxidation cycle to CO by rhenium catalysts of type (31). 6 R – 1– to the CO2 reduction cycle. In addition, the excited chromo- In their d configurations, the [Cp Cr(CO)3] ( )ionsand phore should react by electron transfer with only one of the Re(bpy-4,4′-R2)(CO)3Cl (2) compounds are unreactive with two catalysts (here, CatH2) to ensure unidirectional operation each other and toward their respective H2 and CO2 substrates; of the overall cycle (Fig. 1) and possess lower-energy photo- this constitutes the resting state of the system (Fig. 1). A active excited states than those of the catalysts to prevent chromophore that meets the thermodynamic criteria for pho- – chromophore→catalyst energy transfer. Finally, the catalysts tosensitized electron transfer between these catalysts (1 + 2 → – should be sensitized by one-electron-transfer reactions to be 1• + 2 ), producing their substrate-active forms, is ZnTPP. The – commensurate with the intrinsic one-electron processes of electronic-absorption spectra of ZnTPP, 1a ,and2a (Fig. 2) the chromophore. show that their bands are arrayed in energy such that in mix- Among catalysts that accomplish the two-electron reduction tures of the compounds excitation into the porphyrin Q bands λ = – of CO2 to CO, there is extensive precedent that under photo- ( ex 500 625 nm) will selectively produce the S1 excited state chemical conditions rhenium(I) compounds of the form [Re(bpy-R) of ZnTPP; further, their relative excited-state exclude n+ (CO)3L] (bpy-R is substituted bipyridine or related diimine li- the possibility of energy-transfer processes. The typically used τ ≅ gand; L is anionic (n = 0) or neutral (n = 1) ligand) activate CO2 via S1 excited state of ZnTPP is too short-lived ( 2ns)toallow one-electron-reduced species (11). The initial reduced rhenium efficient bimolecular photochemistry at typical catalyst con- products are ligand-localized radicals of the type [Re(bpy-R•) centrations, but the lower-energy T1 state is very long-lived (n–1)+ = τ ≅ (CO)3L] ; these undergo a series of subsequent steps, be- (E 1.59 eV, 1.5 ms) (32, 33) and is thermodynamically – ginning with dissociation of L, that to the activation of capable of being reductively quenched by 1 but not of being aCO2 by two one-electron-reduced rhenium com- oxidatively quenched by 2 (Table S1). Thus, ZnTPP meets the – pounds through substrate-bridged intermediates (11, 18–22). criterion of photosensitizing the electron-transfer reaction 1 + – The catalysts used in the present study [Re(bpy-4,4′-R2)(CO)3Cl: 2 → 1• + 2 by a single mechanism without short-circuiting or t R = H(2a), Bu (2c), CO2Me (2b), OMe (2d)] (23–26) were energy-transfer reactions. Additionally, the fact that ZnTPP is selected because their one-electron reduction potentials span neutral eliminates pairing of transient species that a range where sensitization by the reduced chromophore will could accelerate unproductive back-electron-transfer reactions. be thermodynamically downhill or uphill, facilitating mechanistic The relative energies of the ZnTPP excited states and the interpretation. chromophore/catalyst redox states are shown in Fig. 3. In this

H2 CO + H2O R

+ – R CatH2 Chr CatCO2 O C N 2x Cr OC Re N R Cat Chr* Chr Cat – C C H2 hν CO2 O C O C Cl O O CO – 2H+ 2 CatH2 (1 ) CatCO2 (2)

Fig. 1. Schematic representation of the photodriven reverse water–gas-shift reaction.

9746 | www.pnas.org/cgi/doi/10.1073/pnas.1321375111 La Porte et al. Downloaded by guest on September 24, 2021 not detected at concentrations of the latter up to 3.5 mM, which is the concentration used in termolecular photoreactions de- – – scribed later in this paper. In the presence of 1a and 1b (0–8 mM in THF or DMF), the ZnTPP fluorescence lifetime and in- tensity were observed to decrease only slightly (e.g., from 1.90 ns – to 1.64 ns for 1a at 8 mM). Stern–Volmer analyses of the data for – the 1 ions (Fig. S1) provide reductive quenching rate constants 10 –1 –1 – 9 [kq(S1)] of 1.05 ± 0.03 × 10 M ·s for 1a and 9.2 ± 0.2 × 10 – – – – M 1·s 1 for 1b , with the faster rate for 1a being in accord with the 0.22-V larger driving force (Table S1). The products of these – reductive quenching reactions are ZnTPP and 1•. Based on these observations and the kinetics of the intrinsic excited-state decay processes of ZnTPP (fluorescence, kr = 1.4 × 7 –1 7 –1 10 s ; internal conversion, kic = 6.3 × 10 s ; intersystem 8 –1 crossing, kisc = 4.0 × 10 s ) (34), the ZnTPP S1 state will decay Fig. 2. Electronic-absorption spectra of [NEt4]1a, 2a, and ZnTPP in DMF solution at room temperature. principally via intersystem crossing to form the T1 excited state (ϕisc = 0.78, Fig. 3, pathway c) when in the presence of any – combination of compounds of types 1 and 2 at the highest figure, the possible reaction pathways following excitation of the concentrations used in the photochemical experiments described – 1a• S1 excited state are labeled a through g. These processes are below; the quantum yield for formation of ZnTPP and via discussed in order below. reductive quenching of the S1 state at these concentrations is – ϕ (S ) = 0.08 (Fig. 3, pathway b). The ZnTPP and 1• products – q 1 Photochemistry of the ZnTPP S1 Excited State with 1 and 2. Excita- formed from reductive quenching of the S1 state are also pro- tion of solutions of ZnTPP containing compounds of types duced via reductive quenching of the T1 state (discussed below); 1– 2 λ = – and/or in the porphyrin Q band region ( ex 500 625 nm) thus, their presence does not complicate subsequent analyses. selectively produces the ZnTPP S1 excited state (Fig. 2). The – photochemical processes of the S1 state in these mixtures were Reductive Quenching of the ZnTPP T1 State by 1 . In mixtures of – probed by measuring the effect on the ZnTPP fluorescence ZnTPP, 1 , and 2, the only thermodynamically downhill bi- lifetime of added 2b, the only rhenium compound of this study for molecular process for the ZnTPP T1 excited state is reductive 1– which oxidative quenching of the S1 state is thermodynamically quenching by (Fig. 3, pathway d). The decay of the T1 state in – – – – downhill (ΔG = –0.32 eV; Fig. 3, pathway a), and of 1a and 1b , the presence of 1a and 1b in DMF solution was probed using which are capable of reductively quenching the S1 state (Fig. 3, nanosecond timescale transient-absorption (TA) spectroscopy, λ ≥ pathway b). Owing to the short lifetime of the S1 state [τ = 1.9 ns with monitoring in the 670 nm wavelength region to avoid – in THF and dimethylformamide (DMF)], even fast (diffusion- excitation of transient 1•. In the absence of 1 , the TA spectrum CHEMISTRY limited) bimolecular quenching processes will be characterized by observed following ZnTPP S1 excitation (Fig. 4) exhibits the – small quantum yields at millimolar concentrations of 1 and 2, characteristic features of the T1 state: a band at 710 nm and an such as those used throughout this study. In accord with this absorption edge at ∼815 nm associated with a band at 840 nm expectation, quenching of the ZnTPP S1 state by 2b in DMF was (33, 35). (The full 840-nm band is not observed owing to the loss

eV + – – R R 2.5 ZnTPP / 1 / 2 2d 2c Cr O C N 2a C CO O C OC Re N R O – C Cl ZnTPP S1 / 1 / 2 O a 2 1a–: CpR = Cp* 2a: R = H – R 1b : Cp = Cp 2b: R = CO2Me 2b t c 2c: R = Bu 2d: R = OMe ZnTPP T / 1– / 2 1 b 1.5

d

– 1b ZnTPP / 1• / 2 2d 1 hν 1a 2c ZnTPP– / 1• / 2 f 2a

e 0.5 2b g Fig. 3. Jablonski diagram showing intermolecular – electron-transfer processes among ZnTPP, 1 ,and

2 initiated by excitation of the S1 state of ZnTPP in DMF. Relative energies of redox states are determined from electrochemical data shown in 0 – ZnTPP / 1 / 2 Table S1.

La Porte et al. PNAS | July 8, 2014 | vol. 111 | no. 27 | 9747 Downloaded by guest on September 24, 2021 absence of other reaction pathways. The only competing pathway under the conditions of the preceding experiments is the di- merization of 1• to form the Cr–Cr bonded dimers [Cp*Cr(CO)3]2 6 –1 –1 (kdimer(1a) ≅ 7 × 10 M ·s ) and [CpCr(CO)3]2 [kdimer(1b) ≅ 3 × – – 108 M 1·s 1] (38). Under conditions where dimerization of 1• is – negligible, the concentrations of ZnTPP and 1• are identical and – the kinetics for the decay of ZnTPP , which is the observable species in the TA spectra, are described by the second-order ex- – – 2 pression −d[ZnTPP ]/dt = kBET[ZnTPP ] ,wherekBET is the back- – electron-transfer rate. The concentration of ZnTPP as a function of time was calculated from the absorbance of the TA band at 705 λ = –1 –1 Fig. 4. Temporally integrated TA spectra of ZnTPP (0.07 mM; ex 558 nm) nm (Fig. 4; e = 12,000 M ·cm ) (36) because it does not overlap Δ = – μ – Δ = – μ in the absence ( t 1 100 s) and presence of 1a (3.5 mM, t 1 200 s) in with other transient features. The temporal decay of the 705-nm DMF solution. The smooth curve is the ground-state absorption spectrum of – –1 – band was found to be linear with [ZnTPP ] ,asshowninFig.6 ZnTPP (adapted from ref. 36). for 1a•. This is consistent with the expected second-order kinetics – for electron transfer between ZnTPP and 1• without kinetically >> of photomultiplier tube sensitivity and probe-beam intensity in competitive dimerization (i.e., kBET kdimer). Linear fits to these = ± × 9 –1· –1 1a• this region.) Although the intrinsic T lifetime is 1.5 ms, under data (Fig. 6 and Fig. S3)providekBET 7 1 10 M s for 1 Δ = – = ± × 9 –1· –1 1b• Δ = the experimental conditions the TA signals of this state are de- ( G 0.97 eV) and kBET 7 1 10 M s for ( G tectable for only ∼200 μs and decay via non-first-order kinetics –1.12 eV); these rates are larger than those for dimerization of 1b• owing to triplet–triplet annihilation (SI Text) (33). Addition of and 1a• by one and three orders of magnitude, respectively. – – a 50-fold excess of 1a ([1a ] = 3.5 mM, [ZnTPP] = 0.07 mM) Under the experimental conditions, the initial concentra- – 1a• ∼ results in marked changes to the appearance and kinetics of the tions of photogenerated ZnTPP and are each 0.008 mM Δ = Δ = TA spectrum (Fig. 4 and Fig. S2): The T features at 710 and 815 [ A(705 nm) 0.1 at t 0, Fig. 6]. Thus, the second-order half- 1 – ∼ μ nm are not detected and, instead, a sharper band at 705 nm with life of ZnTPP is 15 s, and the ion is present in spectroscop- ically detectable concentrations for ∼200 μs. This is sufficient to a pronounced shoulder at 720 nm is observed. The position and – shape of this band are identical to those of a prominent elec- allow the strongly reducing ZnTPP radical to be oxidized by – 2 tronic-absorption band of ZnTPP , as measured by spectroelec- CO2 reduction catalysts of type (Fig. 3, pathway f). For this reaction to occur its electron-transfer kinetics must out-compete trochemistry (36). This demonstrates that the T1 state of ZnTPP is – – 1• reductively quenched by 1a (Fig. 3, pathway d). A similar result is back electron transfer between ZnTPP and , as could be the – 2 obtained when ZnTPP is excited in the presence of 1b .Byin- case under diffusion control when is present in greater con- 1• ference, the oxidation products 1a• and 1b• are also present in centration than transient . the samples, but they are not detected because the TA probe – Electron-Transfer Sensitization of 2 by Photogenerated ZnTPP . The beam is blocked in the wavelength region in which they absorb – oxidation of photogenerated ZnTPP by catalysts of type 2 (λ ≤ 650 nm) owing to their photosensitivity (37). To our knowl- – produces ZnTPP and the ligand-centered radical 2 (Fig. 3, edge, this is the first example of reductive quenching of the T1 1• state of ZnTPP. pathway f); also present is from the initial quenching reaction. The rate constants for photoinitiated electron transfer be- This product state is thermodynamically competent to accom- – – plish the substrate-activation steps of the reverse water–gas-shift tween ZnTPP and 1a and 1b were determined by measuring – reaction: 2 undergoes subsequent reactions that, under CO2 the dependence of the ZnTPP T1 lifetime on the concentration 1– 1a– = – 1b– = – atmosphere, lead ultimately to the reduction of CO2 to CO (11), of the ions ([ ] 0.056 0.46 mM, [ ] 0.065 0.54 mM). 1• 1– ∼ – whereas reacts with and cleaves H2 (27). To understand and These measurements used concentrations of that are 5 50 2 – times lower than those used in the preceding spectroscopic develop the conditions under which sensitization of by ZnTPP is favored over back electron transfer to 1•, the photochemistry experiments (e.g., Fig. 4) so that the T1 bands were still ob- 1a– 2a–2d servable in the TA spectra. Under these conditions, the yield for of mixtures of ZnTPP, ,and was probed using – formation of ZnTPP and 1• by the reductive quenching of the TA spectroscopy. ϕ < The nanosecond timescale TA spectra of a mixture of ZnTPP S1 state is q(S1) 0.01. A representative kinetic trace from 1a– 2a these measurements (Fig. 5) illustrates the dramatic acceleration (0.070 mM), (3.5 mM), and (2.4 mM) observed following 1a– ZnTPP S1 excitation are markedly different from those described of the T1 decay by submillimolar concentrations of . Owing 1a– 2a to the greatly reduced lifetime of the T state, the contribution to above for a mixture of ZnTPP and without . In the 670- to 1 820-nm region, the spectra in the presence and absence of 2a are its decay from triplet–triplet annihilation is negligible and the profiles exhibit first-order kinetics. Stern–Volmer analyses of these data (Fig. 5) provide rate constants for reductive quench- – 9 –1 –1 ing of the T1 state by 1a of kq(T1) = 4.4 × 10 M ·s (ΔG = – – – –0.61 eV) and by 1b of 2.8 × 109 M 1·s 1 (ΔG = –0.46 eV). The ordering of these rates and those for reductive quenching of the – – – 10 –1 –1 S1 state by 1a and 1b (1a :kq(S1) = 1.05 × 10 M ·s , ΔG = – 9 –1 –1 –1.14 eV; 1b :kq(S1) = 9.2 × 10 M ·s , ΔG = –0.92 eV) are entirely consistent with their relative driving forces. Based on these data and the intramolecular decay kinetics of the T1 state – of ZnTPP, the quantum yields for formation of ZnTPP and – – 1• from the T1 state exceed 99% for [1a ] ≥ 20 μM and [1b ] ≥ 30 μM.

– Thermal Back Electron Transfer Between ZnTPP and 1•. Photo- – – Fig. 5. Quenching of the ZnTPP T1 state by 1a in DMF. (Left) TA kinetic 1• – – generated ZnTPP and are expected to undergo back electron profiles of the ZnTPP T1 state (λ = 818 nm) with [1a ] = 0 and [1a ] = 0.46 transfer to fully reform the ground state (Fig. 3, pathway e), in the mM. (Right) Stern–Volmer analysis.

9748 | www.pnas.org/cgi/doi/10.1073/pnas.1321375111 La Porte et al. Downloaded by guest on September 24, 2021 order kinetics (Fig. S6) and provided a rate constant of kBET = 8 ± – – 1 × 109 M 1·s 1. This rate is within experimental error of that – measured for electron transfer from ZnTPP to 1a• (kBET = 7 ± – – 1 × 109 M 1·s 1), for which the driving force is approximately the same (ΔG = –0.97 eV). Conclusions Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of R – ZnTPP, [Cp Cr(CO)3] , and Re(bpy-4,4′-R2)(CO)3Cl has been – Fig. 6. (Left) TA kinetic profile showing decay of photo-generated ZnTPP developed. These results demonstrate the feasibility of sensitizing an H oxidation catalyst and a CO reduction catalyst through (λ = 705 nm) via back electron transfer to 1a• in DMF (λex = 558 nm). (Right) 2 2 Second-order rate plot for these data. sequential electron-transfer steps initiated by the absorption of a single photon by a broadly absorbing chromophore. Although the rates of competing productive (pathway f, Fig. 3) and un- – similar in that they exhibit the band at 705 nm owing to ZnTPP productive pathways (e) are of similar magnitude and the pre- – (Fig. S4); this indicates, as expected, that the presence of 2a does catalysts 1 and 2 are present in similar ground-state concentrations, not interfere with reductive quenching of the ZnTPP T1 state by the pseudo first-order conditions of the photochemical experiment – – 1a (Fig. 3, pathway d). However, the ZnTPP is consumed in bias the system to produce the desired state in which both catalysts <5 μs in the presence of 2a (Fig. S4), whereas in the absence of are activated. It is noteworthy that photosensitization of this system 2a it remains detectable for ∼200 μs. Examination of the spec- can be accomplished with the low-energy T1 state of ZnTPP, which trum in the 450- to 550-nm region in the time window after the participates via heretofore unobserved reductive quenching; this – ZnTPP has fully decayed (Fig. 7) shows the presence of an raises the prospect of capturing light across the full visible spectrum unsymmetrical band maximizing at 505 nm that closely matches to drive these reactions. Under the experimental conditions, the the position and shape of an absorption band of electrochemi- catalytically competent state decays with a second-order half-life of – cally prepared 2a (39). This demonstrates that the accelerated ∼15 μs, which is of the right magnitude for substrate trapping of – – decay of ZnTPP is due to oxidation by 2a to produce 2a and sensitized catalyst intermediates. ZnTPP (Fig. 3, pathway f); presumably, this occurs with con- servation of the initial 1a• photoproduct. Materials and Methods The kinetics of the thermal electron-transfer reactions be- General Procedures. Procedures for purification, chemical syntheses, – tween photogenerated ZnTPP and 2a–2d were determined by and electrochemical and spectroscopic measurements are provided in SI Text. – All experiments were performed at room temperature under a measuring the dependence of the ZnTPP lifetime on the con- – atmosphere using standard Schlenk and glovebox techniques. Solution centration of 2 (SI Text). The concentrations of ZnTPP and 1a – samples for optical experiments were prepared on a vacuum line in sealable in these samples were chosen to be sufficient to produce ZnTPP cuvettes; those for photophysical measurements were degassed by at least in quantitative yield from the T1 state ([ZnTPP] = 0.070 mM, five freeze–pump–thaw cycles and sealed under purified nitrogen. Tran- CHEMISTRY – [1a ] = 3.7 mM (2a–c) or 3.5 mM (2d)); the concentration ranges sient-absorption spectroscopic experiments were performed using λ ≥ 670- for 2a–2d differed for each derivative, based on the driving force nm probe light to ensure that transient 1a• and 1b• were not irradiated, for the electron-transfer reaction. In the presence of 2a–2c, because they are reported to be photosensitive (37). – photogenerated ZnTPP decays with first-order kinetics, rather ZnTPP S1 Quenching Measurements. The quenching of the S1 state of ZnTPP by than the second-order kinetics observed for the back-electron- – – – 1a (in THF) and 1b (in DMF) was studied by steady-state emission spec- transfer reaction between ZnTPP and 1• when 2 is absent, – troscopy and fluorescence-lifetime measurements (Fig. S1). Samples con- 2 1 – – because even though the ground-state concentrations of and taining systematically varied concentrations of 1a (0–8 mM) or 1b (0–6.15 are similar the concentration of 2 is two to three orders of mM) were prepared from a stock solution of ZnTPP ([ZnTPP] = 0.007 mM). – • – magnitude greater than those of transient ZnTPP and 1a . The The samples were excited in the ZnTPP Q band region, at which the 1 ions – measured lifetime of ZnTPP varies linearly with the concen- do not absorb (Fig. 2). tration of 2 (Fig. S5). These data provide rate constants for – 2a–2c ZnTPP T1 Quenching Measurements. The products and kinetics of the reductive electron transfer from ZnTPP to that vary according to – quenching of the T1 state of ZnTPP by 1 , and of back electron transfer the driving force for the reaction: The fastest rate is observed for – – – • 2b (k = 4.0 × 109 M 1·s 1, ΔG = –0.46 eV) and slower rates between ZnTPP and 1 , in DMF solution were examined by TA spectroscopy ET [λ = 558 nm, ZnTPP Q(1,0)] in the wavelength range 670–820 nm (see SI Text are found for the approximately thermoneutral reactions with 2a ex 9 –1 –1 for details). Kinetic profiles for T1 quenching were measured at 818 nm, (kET = 1.8 × 10 M ·s , ΔG = –0.05 eV) and 2c (kET = 1.7 × 8 –1 –1 7 –1 –1 10 M ·s , ΔG = 0.05 eV). For 2d,(kET = 4.8 × 10 M ·s , ΔG = 0.09 eV), the electron-transfer rate is sufficiently slow – that the decay of the ZnTPP anion cannot be fit by a single- exponential function, indicating that back electron transfer – between ZnTPP and 1a• is kinetically competitive with re- duction of 2d. The electron-transfer rates for these processes are depicted in the Rehm–Weller plot shown in Fig. 8. In the absence of reactive substrates, photochemically gener- – ated 1• and 2 will react by electron transfer to reform the ground- – state mixture of ZnTPP, 1a ,and2 (Fig. 3, pathway g). The rate of – this reaction was measured for 2a and 1a• (ΔG = –0.92 eV) in a TA spectroscopic experiment that broadened the probe wave- length window to 450–550 nm, so that the kinetic profile of the TA – Fig. 7. Temporally integrated (Δt = 6–400 μs) TA spectrum of a mixture of band of 2a at 505 nm (Fig. 7) could be measured. Although these – ZnTPP (0.070 mM; λex = 558 nm), 1a (3.5 mM), and 2a (2.4 mM) in DMF. The – conditions have the potential to trigger photochemical reactions of TA bands of photogenerated ZnTPP have completely decayed before this – 1a• (37), the decay of 2a was found to obey the expected second- time window. The spectrum of 2a– (adapted from ref. 39) is overlaid.

La Porte et al. PNAS | July 8, 2014 | vol. 111 | no. 27 | 9749 Downloaded by guest on September 24, 2021 – Rate Constants for ZnTPP Oxidation by 2a–2d. The products and kinetics of the reaction of ZnTPP– and 2a–2d in DMF solution were studied using TA spec- troscopy in the λ > 670-nm region (see SI Text for details). Samples contained – constant concentrations of ZnTPP and 1a ([ZnTPP] = 0.070 mM (2a, 2c)or 0.140 mM (2b, 2d); [1a–] = 3.7 mM (2a–2c)or3.5mM(2d)) and varied in concentration of 2. The samples were excited at 558 nm (2a, 2c) or 598 nm – (2b, 2d). The ZnTPP ion, formed via reductive quenching of the ZnTPP T1 state by 1a–, exhibits a characteristic band at 705 nm, which was moni- tored to obtain kinetic information. Electron-transfer kinetics between 2a– and 1a• were monitored in a separate experiment using the same samples and excitation conditions, but in which the 670-nm cutoff filter on the probe beam was removed to allow monitoring of the 505-nm Fig. 8. Plot of log kET vs. ΔG for electron transfer from photogenerated – – ZnTPP to 2a–2d in DMF. band of 2a .

ACKNOWLEDGMENTS. We thank Prof. Richard Dallinger and Dylan Lynch for their assistance with constructing the transient-absorption spectrometer. which is the blue edge of an intense ZnTPP T1 band at 840 nm. The lifetime of – – This research was supported by the US Department of Energy, Office of Basic the T1 state was determined as a function of the concentration of 1 ([1a ] = – Energy Sciences, Solar Photochemistry Program, under Grant DE-FG02-07- 0–0.56 mM; [1b ] = 0–0.65 mM) of samples prepared from a stock solution of ER15910. Fluorescence lifetime measurements were performed in the Institute ZnTPP in DMF ([ZnTPP] = 0.070 mM). The kinetics of back electron transfer – for Biophysical Dynamics NanoBiology Facility, which is supported in part by between ZnTPP and 1a• were determined for a sample containing 0.070 mM – – National Institutes of Health Grant 1S10RR026988-01. N.T.L. was supported ZnTPP and 3.7 mM 1a ; the decay of the 705-nm TA band assigned to ZnTPP by National Science Foundation Graduate Research Fellowship Program was monitored. Grant DGE-0638477.

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