Excited-State Proton-Coupled Electron Transfer: Different Avenues for

Excited-State Proton-Coupled Electron Transfer: Different Avenues for Promoting Proton/Electron Movement with Solar Photons J. Christian Lennox, Daniel A. Kurtz, Tao Huang, and Jillian L. Dempsey* University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States ABSTRACT: Excited-state proton-coupled electron transfer (ES-PCET) is a promising avenue for solar fuel production and small molecule activation. Although less is known about ES-PCET reactions than related transformations involving exclusively thermal processes, ES-PCET holds promise as a simple and efficient reaction scheme for the formation of solar fuels, and it may provide access to new reactivity not accessible from ground electronic states. This review classifies ES-PCET into six categories based on the identity of the photoexcited reactant: excited-state H+/e− donors, excited-state H+/e− acceptors, photo-oxidants, photoreductants, photoacids, and photobases. A brief overview of each class of ES-PCET is presented. Recent advances and key discoveries within the six classes of ES-PCET are examined, and underexplored reaction systems and promising paths for future research are discussed.

D

Scheme 1. (A) Square and (B) Cube Schemes Describing Thermal and Excited-State (for an ES H+/e− Donor) PCET Reactivity, Respectivelya

riving energy-intensive fuel-producing reactions with sustainable energy inputs such as solar photons presents an exciting opportunity to address energy storage issues. Among the major challenges to realizing light-driven fuel production are (1) the development of catalysts capable of mediating the complex multielectron, multiproton transformations necessary for converting energy-poor molecules to energy-rich fuels and (2) integrating light absorption to this proton-/electron-transfer reactivity in order to couple energy capture and conversion. Proton-coupled electron transfer (PCET) transformations underpin the conversion of feedstock molecules to fuels.1−3 PCET processes can occur via stepwise pathwayselectron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET)which proceed via charged intermediates (Scheme 1A). These discrete charged species can impose an energy penalty on the reaction when these intermediates are high in energy.1,4 Alternatively, the proton and electron can be transferred concertedly in a single kinetic step (concerted proton−electron transfer, CPET) circumventing charged intermediates. Often the concerted process is accompanied by a high activation barrier, though carefully tuned systems such as those found in nature are often optimized such that CPET is favored with a shallower activation barrier.4,5 As such, catalysts capable of choreographing CPET promise access to energy-efficient fuel production.1 Addressing the first challenge of sunlight-to-fuel conversion the development of catalysts which exploit energy-efficient CPET reactions to mediate fuel productionnecessitates a © XXXX American Chemical Society

The * denotes excited-state reagents and reactivity in the cube scheme.

a

deeper understanding of the parameters influencing PCET reaction pathways. Significant research efforts have focused on elucidating PCET reaction mechanisms and identifying the experimental factors that dictate their pathways and kinetics.1 These mechanistic studies, which have been extensively reviewed, generally utilize PCET model substrates such as phenols and quinones, as they provide a convenient and unifying platform with which to explore this complex reactivity.2,5,6 Received: January 20, 2017 Accepted: April 7, 2017

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DOI: 10.1021/acsenergylett.7b00063 ACS Energy Lett. 2017, 2, 1246−1256

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Scheme 3. Six Classes of ES-PCET: (A) ES H+/e− Donors, (B) ES H+/e− Acceptors, (C) ES e− Acceptors, (D) ES e− Donors, (E) ES H+ Donors, and (F) ES H+ Acceptors

Recent fundamental research has also focused on the second challenge facing the development of sunlight-to-fuel schemes: the successful integration of light-harvesting with the aforementioned PCET reactivity. Two limiting approaches have been envisioned for molecular-based systems: (1) a modular approach in which light energy is first converted to chemical energy (by separating charges to form redox equivalents in so-called “flash-quench” schemes) followed by thermally driven fuelproducing PCET reactions7,8 and (2) an integrated approach where PCET reactions are initiated directly from photoexcited states.5,7−9 While the former allows light-harvesting and catalyst components to be individually optimized, the latter offers simplicity and may provide avenues to new solar-to-fuel transformations (Scheme 2).

An integrated approach where PCET reactions are initiated directly from photoexcited states...offers simplicity and may provide avenues to new solarto-fuel transformations. The work discussed encompasses excited-state PCET reactivity involving thermally equilibrated excited states. Not detailed are a few reports of hydrogen-bound chromophore-amine complexes in which intramolecular charge-transfer excitation directly yields ES-PCET products wherein the ET component is the intramolecular charge transfer.12−15 Highlighted are new understandings of photoexcited-state reactivity involving H+/e− transfer gained from these studies along with perceived opportunities for harnessing these light-driven transformations for fuel production. Excited-State Proton/Electron Donors. For molecules classified as excited-state H+/e− donors (Figure 1), the photoexcited complex acts as both a reducing agent and acid upon light absorption. The first examples of ES H+/e− donors were reported by Nocera and co-workers using a series of donor/acceptor dyads that were noncovalently linked by an amidinium-carboxylate salt bridge.16−24 Many of the dyads explored are composed of either ruthenium tris(bipyridine) (1 and 2)16,17,19 or porphyrin donors (3 and 4),18,20,21,23,24 which form two-point hydrogen bonds to electron deficient benzoate acceptors (A and B) or naphthalenediimide acceptors appended with carboxylate functionality (C). Initial studies investigated how proton movement across the salt bridge affected the rates of ET from the photoexcited donor to the acceptor. Measurement of donor photoluminescence lifetimes, in most cases, indicated that the asymmetric amidiniumcarboxylate salt bridge (Scheme 4A) led to attenuation of ET rates in comparison to symmetric carboxylic acid bridges (Scheme 4B)16 or covalently linked dyads.18,20 The difference in ET rates is attributed to the proton motion accompanying the ET reaction: in the amidinium-carboxylate bridge dyads, proton motion accompanies ET, giving rise to large reorganization energies. Further, the electrostatic field across the asymmetric bridge is thought to affect the PCET reaction energetics. For the symmetric carboxylic acid bridge dyads, any proton displacement is compensated by the second hydrogen bond and balanced symmetrically.16,25 When the orientation of the amidinium-carboxylate bridge dyad is switched (Scheme 4C)17,19 so that PT does not accompany ET across the salt bridge, the ET rate was found to accelerate. This acceleration was attributed to (1) a change in dipole directionality

Scheme 2. Light-Driven PCET Reactions Driven through a Modular Approach (A) in Which Separate Molecules Are Responsible for Light-Harvesting and PCET Reactions and through an Integrated Approach (B) in Which PCET Reactions Are Driven from Electronically Excited States

In contrast to thermal PCET reactions,2,10 excited-state PCET (ES-PCET) processes have been less explored. While Nature provides some examples of ES-PCET in DNA energy dissipation and DNA photolyase,11 the field is faced with many questions.2,7 How does ES-PCET differ from thermal PCET? What parameters dictate ES-PCET reaction pathways? Can ES-PCET be harnessed to drive solar fuel-producing reactions not accessible from ground electronic states?7 As for thermal PCET reactions, molecular model systems offer a versatile platform with which to begin addressing these outstanding questions and help elucidate new pathways by which solar energy can be converted to chemical energy. While many of these reported model systems are based on organic PCET reagents such as phenols and quinones, the fundamental principles elucidated from these studies are expected to readily transfer to systems of consequence to fuel production. In this review, we examine recent advances in ES-PCET, categorizing reactivity into six classes (Scheme 3) based on the role of the excited-state reagent in promoting proton (H+) and electron (e−) transfers: (A) excited-state H+/e− donors (example shown in Scheme 1B), (B) excited-state H+/e− acceptors, (C) excited-state e− acceptors, (D) excited-state e− donors, (E) excited-state H+ donors, and (F) excited-state H+ acceptors. 1247


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The structure of the two-point hydrogen bond has also been found to influence charge transfer across these hydrogenbonded interfaces. The charged amidinium-carboxylate bridge shown in Scheme 4A has been spectroscopically demonstrated to be valid for donor−acceptor supramolecular assemblies only when the acceptor is electron poor; the nonionized amidinecarboxylic acid tautomer dominates with electron-rich acceptors.22,23 In the ionized interface, electronic coupling is enhanced, influencing the relative charge-transfer kinetics.23,24 Together, these systematic studies of salt-bridged complexes have revealed the complex ways in which coupled PT across hydrogenbonded networks can influence ET rates from photoexcited complexes. Similar to the amidinium-carboxylate bridged species, biimidazole complexes form two-point hydrogen bonds with carboxylate anions. While the acid−base reactivity of coordination complexes with imidazole ligands has been demonstrated to couple to the metal oxidation state,26−28 only more recently has this acid−base reactivity been demonstrated to couple to the excited-state redox chemistry of coordination complexes. Notably, Wenger and co-workers demonstrated that when coordinated to a cyclometalated iridium core (5), the acid− base reactivity of the biimidazole ligand couples to the excitedstate redox chemistry generally exhibited by cyclometalated iridium photosensitizers.29,30 In the presence of a dinitrobenzoate H+/e− acceptor (A), which forms two-point hydrogenbonding interactions with the biimidazole ligand, dramatic quenching of the iridium excited state was observed through steady-state luminescence. When the biimidazole nitrogens were methylated, quenching was significantly attenuated, indicating that ET from the metal center of the unmethylated species to the acceptor is accompanied by PT across the biimidazole-dinitrobenzoate salt bridge through an ES-PCET pathway. This work highlights that proton-transfer reactivity can be intimately coupled to photoexcited electronic states when the acidity of the ligand is influenced by electron density at the metal and/or on the ligand. Like imidazole-based ligands, ES-PCET reactivity has also been observed for phenanthrolinediol ligands which similarly contain acidic protons distal to the metal coordination sites. In work by Meyer and co-workers, the ES H+/e− donor reactivity of a ruthenium phenanthrolinediol complex (6) with monoquat (MQ+, D) was studied via transient absorption (TA) spectroscopy.7 This work is noteworthy because ES-PCET reactivity was observed from unique spectroscopic changes in not only the photosensitizer, but also the substrate, providing insight into both ET and PT. Typically, transition metal complexes exhibit optical differences when their oxidation state changes, but the protonation state is often spectroscopically silent in the visible region. However, ES H+/e− transfer from the ruthenium phenanthrolinediol to MQ+ yields HMQ•+, which has a striking blue color similar to reduced methyl viologen. As such, the appearance of the H+/e− transfer product could be spectroscopically monitored in this work, highlighting how suitable reagents can provide enhanced insight into H+/e− reactivity. The aforementioned ES H+/e− donors all incorporate multiple acidic sites on the donor. To simplify the H+/e− donor, Wenger and co-workers synthesized [Ru(CF3-bpy)2(py-imH)]2+ (7) with only a single imidazole proton.31 Upon photoexcitation in the presence of a H+/e acceptor, benzoquinone (E), quenching by ET was observed through TA, despite the incorporation of electron-withdrawing CF3-bpy ligands intended

Figure 1. Schematic of excited-state proton/electron donor PCET along with structures of reactants and substrates described in the main text. The excited-state proton/electron donor consist of both an electron donor and a proton-donating moiety, covalently linked.

Scheme 4. Amidinium-carboxylate and Symmetric Carboxylic Acid Bridges

which affects the energetics of ET, (2) a switch from PCET to pure ET, which reduces the overall reorganization energy, and (3) changes in electron coupling between the donor and acceptor due to differences in hydrogen bonding across the salt bridge.9,17,18 1248


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to minimize the driving force for pure ET vs PCET. However, a very weak but discernible transient signal that resembled that of a reduced and protonated semiquinone radical was observed, indicating that a very small fraction of excited-state deactivation proceeded through an ES-PCET pathway. The competing ET and PCET reactivity was explained by comparing the driving force for ET (0.3 eV) vs PCET; coupling to PT only adds ∼0.2 eV of driving force because the pKa values of the Ru(III) complex and the semiquinone radical are very similar. Studies of ruthenium−pyridine imidazole complexes have been extended to related complexes, such as [Ru(bpy)2(py-imH)]2+ (8).32 The reactivity of this complex with the aforementioned MQ+ H+/e− acceptor was explored, allowing the PCET products to be clearly identified through TA spectroscopy. Interestingly, direct PCET to MQ+ was not observed, but rather a termolecular PCET reaction (either stepwise or concerted) dominates in which MQ+ accepts the electron and the buffer base (in an acetic acid/acetate buffered solution) accepts the proton, followed by PT from acetic acid to MQ• to yield HMQ•+, the appearance of which was readily tracked spectroscopically (eqs 1 and 2). This temporally resolved PCET−PT sequence is consistent with thermochemical analysis.

Excited-State Proton/Electron Acceptors. For molecules classified as excited-state H+/e− acceptors (Figure 2), a single excited species acts as both an electron acceptor and proton acceptor, oxidizing and deprotonating a donor molecule or moiety. In contrast to other forms of ES-PCET, reactions involving ES H+/e− acceptors have been observed solely with bimolecular and intramolecular systems, with no examples of termolecular ES-PCET reported. These reactions offer intriguing possibilities for new reactivity that contrast with the more extensively explored ES e− acceptor PCET scheme. Although there is a long history of ES H+/e− acceptor PCET with small organic molecules (e.g., flavins, quinones, benzophenones, and pyridines), many require ultraviolet light excitation and are therefore less applicable toward solar energy capture and conversion.33−39 Most recent and relevant examples within this category of ES-PCET involve a small class of acceptor and donor molecules. The majority of these ES H+/e− acceptors are characterized by a polypyridyl Ru(II) or Re(I) complex which acts as a powerful oxidant in the excited state, and ligands containing a pyrazine or imidazolate ring as a proton acceptor. The H+/e− donors used in reactions with ES H+/e− acceptors are similarly limited in scope, consisting almost entirely of hydroquinone and phenol derivatives (F−J). These donors are well-suited for the reaction, as they are known to undergo 1 H+/1 e− reactivity (to form semiquinone and phenoxyl radicals, respectively), and can be substituted with electron-donating and -withdrawing groups to vary the oxidation potential and/or acidity to afford a suitable thermodynamic match for the acceptor. The first case of ES-PCET involving an inorganic molecule as an ES H+/e− acceptor was demonstrated in 2005 by Kramer and co-workers, where photoexcited [Ru(bpy)2(2-(2-pyridyl)benzimidazolate)]+ (9) was shown to oxidize and deprotonate

Figure 2. Schematic of excited-state proton/electron acceptor PCET along with structures of reactants and substrates described in the main text. Excited-state proton/electron acceptors consist of both an electron acceptor and proton-accepting moiety, covalently linked.

a substituted hydroquinone (H).40 These results were backed by the detection of the neutral semiquinone radical via timeresolved EPR and the presence of a large kinetic isotope effect (KIE) in the quenching of the Ru(II)* excited state. Since then, five other studies have been performed on ruthenium polypyridine scaffolds incorporating bipyrazine (bpz) and imidazole ligand motifs.41−45 In 2013, Wenger and co-workers demonstrated the importance of substrate selection on PCET reaction pathways.41 Using a [Ru(bpz)3]2+ complex (10) and a series of substituted thiophenols (I), Wenger showed that the primary PCET pathway could be modulated between ET-PT, CPET, and PT-ET by switching from electron-donating to electron-withdrawing substituents on the thiophenol substrate. This has been the only definitive example of an operative PT-ET reaction pathway in ES H+/e− acceptor PCET. ES PT-ET reactivity must inherently be driven by changes in the pKa of the acceptor upon photoexcitation, frequently through an MLCT excited state in which the excited electron localized on the ligand increases the basicity of any protonatable site. However, Wenger has also shown this localization is irrelevant in cases dominated by ET.46 In ET-PT and CPET reaction schemes, the PT component can instead 1249


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Figure 3. Schematic of excited-state electron acceptor PCET along with structures of reactants and substrates described in the main text.

its protonated and redox states, which allows for straightforward identification of the operative PCET mechanism via timeresolved spectroscopy. Also of note is a recent report by Kennis and co-workers in which a modified fullerene with an attached amine moiety accepts a proton and electron intramolecularly from a covalently connected phenol (14).48 These schemes present opportunities for research on ES H+/e− acceptor PCET systems. Excited-State Electron Acceptors. ES-PCET can also occur with discrete molecules acting as H+ and e− acceptors in what is often referred to as a “bidirectional” reaction. Light-absorption by excited-state e− acceptors (Figure 3) initiates oxidation of a H+/e− donor substrate coupled to thermal deprotonation of the substrate to a separate proton-accepting moiety or molecule. Early examples of ES e− acceptor PCET were reported by Linschitz and co-workers,49−51 in which the luminescence quenching of triplet 3C60* (15) and singlet tetracene (16) by phenol (L) and naphthol (M) substrates was found to be accelerated by the addition of pyridine bases. Identification of C60•−, protonated bases, and phenoxyl radicals by TA spectroscopy and observation of kinetic isotope effects (kH/kD = 1.2−1.7) were used to assign reactivity of 3C60* with hydrogen-bonded phenol−pyridine pairs as a ES-CPET pathway.

be driven by changes to the substrate pKa upon oxidation. A comparison was made between two Re(I) complexes (11 and 12) in the oxidation of 4-cyanophenol (G). In 11, the excited electron is localized on the bipyrazine ligand that acts as the proton acceptor, while in 12 the electron is localized on the bipyridine ligand and a separate pyrazine ligand acts as the proton acceptor. Interestingly, the rates for the ES-CPET oxidation of 4-cyanophenol for these complexes remain similar, showing the insignificance of MLCT localization in ES-CPET reactivity. Furthermore, the thermodynamic principles that allow for the above CPET reactivity also hold for ET-PT reactions, although a direct comparison of two analogous complexes has not yet been made for the corresponding ET-PT process. For an ET-PT reaction, it is not necessary that the electronically excited H+/e− acceptor be capable of deprotonating the substrate. Provided that it is thermodynamically favorable for the reduced acceptor to deprotonate the oxidized substrate, ES-PCET can occur. Some recent examples of ES H+/e− acceptor reactivity extend beyond the framework of polypyridyl Ru(II) and Re(I) complexes. Notably, our group has shown that photoexcited acridine orange (13) reacts with both tri-tert-butylphenol (J) and TEMPOH (K) through a CPET mechanism.47 Acridine orange exhibits unique spectral profiles for each of 1250


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quenching of an excited photo-oxidant to initiate the reaction.68 Notably, the reaction of a Re(I) photo-oxidant (19b) with pyridylphenol dyads (R) was compared to that with simple phenols. While efficient oxidation of all untethered phenols by the Re(I) photo-oxidant via pure ET was observed in photoluminescence quenching experiments, the covalently bound phenol−base moieties show significantly faster quenching rates and evidence for CPET in both TA and KIE studies. While the phenol-only experiments indicate that an ET-PT reaction pathway is thermodynamically accessible for the pyridylphenol substrates, the observed CPET reactivity suggests the importance of a covalently bound base at promoting oxidation by coupling the proton release in an intramolecular, hydrogen-bound system. Free base porphyrins have also been found to function as ES e− acceptors in PCET reactions involving phenolbenzimidazole dyads.69−71 For example, a porphyrin covalently bound to phenol-benzimidazole (22-S) has been studied as a unimolecular mimic of the chlorophyll-TyrZ-His190 complex in photosystem II.69 Emission quenching studies revealed that the singlet porphyrin excited state acts as an ES e− acceptor, triggering the intramolecular phenolic PT to benzimidazole. Relatedly, Bangal and co-workers have shown nondiffusive ultrafast PCET coupled with fluorescence up-conversion via time-correlated single photon counting, in which 4-methoxyphenol is oxidized by the excited singlet state of meso-tetrakis5,10,15,20-pentafluorophenyl porphyrin (23) with concerted motion of bound protons to associated pyridine on the scale of tens of picoseconds.71 In addition to phenol-base dyads, ES-PCET reactions involving nucleotides have also been examined. The excited state of [Ru(TAP)2(dppz)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene, dppz = dipyrido[3,2-a:2′,3′-c]phenazine, 24) has been demonstrated to oxidize guanosine monophosphate and guanine-containing polynucleotides (T).72 In these reactions, ET from guanine to the Ru(II)* complex is coupled to deprotonation of guanine to a solvent molecule, or in the case of doubly stranded DNA, the proton is transferred to hydrogenbound cytosine. Application of ES e− acceptors to promote PCET reactions for chemical transformations has also been recently explored in the field of photoredox catalysis. Among several examples recently reviewed by Knowles and co-workers,61,73 olefin hydroamidation catalysis60 and remote C−H bond alkylation62 are two reactions that have been promoted through ES e− acceptors. In these experiments, it is postulated that oxidation of the N−H bond (e.g., in N-aryl amide derivatives, U) by [Ir(dF(CF3)ppy)2(bpy)]+* (25, dF(CF3)ppy = 2-(2,4difluorophenyl)-5-(trifluoromethyl)pyridine) is coupled to PT to dibutyl phosphate. Together with the extensive literature describing fundamental studies of ES-PCET with excited-state e− acceptors discussed above, these application-based studies reveal the extensive opportunities to utilize excited-state e− acceptors to drive H+/e− chemistry. Excited-State Electron Donors. For molecules classified as excited-state e− donors (Figure 4), a single excited species acts as an electron donor, reducing a proton-accepting molecule or moiety. In comparison to reports involving ES e− acceptors, less work has focused on ES-PCET reactions of this type. Among the studies reported, photoreductants such as modified [Ru(bpy)3]2+ complexes74 (26), Ir(ppy)3 (27),75 and pyrene (28)76 have been used as ES e− donors. For example, ultrafast dynamics of both ET and PT have been studied in pyrene-modified pyrimidine

These termolecular reactions have laid important groundwork for subsequent ES-PCET studies involving ES e− acceptors. More recently, MLCT excited states of d6 transition metal complexes (e.g., Re(I), Ru(II), Ir(III)) have been extensively used as ES e− acceptors in PCET reactions.52−63 Among these, Re(I) polypyridyl complexes (17−19a) with covalently linked phenol substrates (N−Q ) have been the most heavily studied.52−57,64,65 Nocera and co-workers reported the formation of the neutral tyrosyl radical following photoexcitation of the Re(I) moiety in several covalently linked Re−tyrosine (and fluorinated tyrosine) dyads (17-N, 17-O, 18-N, 19a-N) with deprotonation by either solvent or an external base primarily through a ES ET-PT mechanism.52−55 Notably, that a clear relationship between PCET rates and the strength of bases was revealed by 19a-N with pyridine and imidazole as external bases,55 suggesting parallels between the effects of varied PT driving force on ES and thermal e− acceptor PCET reactions.66 In another study, the influence of the linking ligand between the Re core and the tyrosine moiety (17-N vs 18-N) was compared, and it was found that the lifetime of the tyrosyl radical could be extended through differing ligand binding group identity and location.52 In a study by Wenger and co-workers, ES-PCET involving a series of Re−phenol dyads (19a-P) linked through a variable number of p-xylene spacers via a coordinated pyridine moiety was explored. PCET reactivity was found to differ substantially as the number of xylene spacer units was varied, with shorter dyads exhibiting apparent photoacid behavior of the phenolic proton upon excitation and phenol oxidation proceeding via an ET-PT mechanism for the dyads with longer linker ligands.56 This example highlights how the coupling of the electron acceptor and the phenol donor can dramatically influence reactivity in such donor−bridge−acceptor molecules. ES-PCET reactivity of Ru(II) polypyridyl complexes with covalently bound phenols (20-P, 20-Q ) has also been well studied, and from these works new understanding about ES-PCET reactivity has been gained.58,63,67 One example by Wenger and co-workers analyzed [Ru(bpz)3]2+ complexes with a covalently connected phenol (20-Q, n = 1) through a xylene spacer, and showed that PCET mechanisms can be altered from CPET to stepwise ET-PT by switching from electron-withdrawing to electron-donating phenol substituents (Q , R = CN vs H).58 In the study of a related [Ru(bpy)3]2+-based system with a covalently linked phenol (21-P, R = H, n = 1−3) and pyridine as a proton acceptor, PCET reactivity differed between dyads without (n = 0) and with (n = 1) a xylyl spacer.63 The former reacted simply as a photoacid, while the latter exhibited apparent photoacid behavior that was diagnosed to proceed via a PCETET (net PT) pathway. Major differences in activation energies were found, and were suggested to drive the differing reactivities. In subsequent studies of this dyad structure incorporating 1−3 xylyl spacers (21-P, R = H, n = 1−3) with pyrrolidine as a proton acceptor, CPET reactivity was observed.59 Quantification of the CPET rate constants vs xylyl spacer units allowed the distance dependence of bidirectional PCET to be explored, revealing that the distance decay constant βCPET = 0.67 ± 0.23 Å−1 for CPET was similar to that measured for pure ET in related systems (βET = 0.52−0.77 Å−1). This work provides a clear example of how the distance decay model for electron transfer can also be extended to PCET reactions. Although most bimolecular schemes reported for ES e− acceptor PCET employ a photo-oxidant−substrate dyad as discussed above, ES-PCET reactions have also been explored with covalently bound substrate−base dyads, relying on diffusional 1251


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acidity in its electronic excited state (i.e., a photoacid)77−82 to a substrate, coupled to thermal reduction of the substrate by a separate reductant. There are very few examples of this form of ES-PCET, partially due to a relative scarcity of well-behaved photoacids. Many organic photoacids suffer from short excited-state lifetimes, irreversible photochemistry, and rapid photodegredation, and many require UV light excitation, hindering kinetic analysis.83 One instance of ES H+ donor PCET was reported by Gray and co-workers in the study of a cobaloxime complex known to catalyze H2 production (Scheme 5).84 Excitation of the organic Scheme 5. Mechanism for Co(II)-Hydride Formation

photoacid 6-bromo-2-naphthol with ultraviolet light in the presence of the Co(I) species yielded the Co(II) hydride. TA spectroscopy was used to diagnose the reactivity as a stepwise PT-ET reaction, whereby ES proton transfer from the photoacid to the Co(I) species yielded a Co(III) hydride species that was subsequently reduced through comproportionation with unreacted Co(I) to form Co(II) hydride and 1 equiv of Co(II). This work is noteworthy for two reasons: it is a rare example of ES-PCET involving an excited-state H+ donor and it involves ES-PCET reactivity of a substrate of relevance to fuel production. While inorganic molecules displaying photoacid behavior have been reported, and inorganic photosensitizers are ubiquitous in other types of ES-PCET, they may not be wellsuited for use as pure ES proton donors in PCET reactions (or as purely ES proton acceptors; see below).85−87 The MLCT excited states that give rise to most photoacid and photobase behavior in transition metal complexes also dramatically alter the reduction potentials of the complexes.85−88 These changes in reduction potentials are useful for designing reactions involving ES H+/e− donors and acceptors, but they can complicate reactions of ES H+ donor PCET by introducing unwanted ET reactions involving the photoexcited complex.56,63,89 For this reason, organic photoacids, which are generally harder to oxidize in their excited state, may be preferred when designing schemes for ES H+ donor PCET. However, these inorganic photoacids may be useful as ES H+/e− donors and are underexplored for such applications. Although ES-PCET reactions involving ES H+ donors are scarce, this approach to photoactivating PCET reactions may be valuable for the development of solar fuel-forming reactions. For instance, photoacids can promote PT-ET reaction pathways less commonly favored with other ES-PCET reactants, thus allowing access to new, desirable substrates

Figure 4. Schematic of excited-state electron donor PCET along with structures of reactants and substrates described in the main text.

nucleosides (28-V and 28-W) via steady-state fluorescence and femtosecond TA spectroscopy.76 The ET occurs in several picoseconds, which is promoted in a more acidic environment, suggesting PCET reactivity. In other reports, quenching of the 3 MLCT excited states of Ir(ppy)3 or [Ru(bpy)3]2+ chromophores (26 and 27) by acetophenone (X), yielding the ketyl radical, was explored in acidic conditions.73−75 Sequential ET-PT and PT-ET pathways were determined to contain thermodynamically unfavorable steps, and a first order dependence on both acid and ketone was observed, supporting a proposed CPET mechanism for these reactions.61,73,75 Furthermore, the rate of reductive PCET was found to increase by appending electrondonating methoxy groups to the backbone of the bpy ligands, which increases the reductive power of the photoexcited [Ru(bpy)3]2+.74 These limited examples of excited-state e− donors employed in ES-PCET reactions come as somewhat of a surprise, considering the breadth of literature surrounding excited-state e− acceptor PCET. ES e− donor PCET is likely rich for discovery of new reactions that can be built off the extensive number of photoreductants studied for ET reactivity. Excited-State Proton Donors. ES-PCET reactions involving excited-state H+ donors (Figure 5) are characterized by proton transfer from a photoexcited molecule that exhibits enhanced

Photoacids can promote PT-ET reaction pathways less commonly favored with other ES-PCET reactants, thus allowing access to new, desirable substrates for fuel production.

Figure 5. General schematic describing excited-state proton donor PCET described in the main text. 1252


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for fuel production. Recently reported photoacids with visible absorption features and reversible protonation states may provide new avenues for convenient study in this field.83 Excited-State Proton Acceptors. ES-PCET reactions involving excited-state H+ acceptors (Figure 6) are characterized by proton

Recognizing and harnessing excitedstate PCET reactivity that cannot be accessed from electronic ground states is crucial. engender H+/e− reactivity in their excited states. Further, work must extend beyond model systems based on organic reagents such as phenols and quinones to substrates of consequence to fuel production, such as transition metal hydride complexes (putative intermediates in fuel-producing catalytic cycles) and energy-poor fuel feedstocks such as CO2 and H2O. By pushing the frontiers of ES-PCET through fundamental research, opportunities to harness excited-state proton-coupled electron transfer reactivity for fuel production will be revealed.

Figure 6. General schematic describing excited-state proton acceptor PCET described in the main text.

transfer from a substrate to a photoexcited molecule that exhibits enhanced basicity in its electronic excited state (i.e., a photobase),47,90−95 coupled to thermal oxidation of the substrate by a separate oxidant. Many parallels can be drawn between these reactions of ES H+ acceptors and those of ES H+ donors, including the relative incompatibility of metal complex photosensitizers and a dearth of well-behaved photobases that absorb visible light.85 Furthermore, among visible light-absorbing organic photobases, some also readily undergo redox chemistry in their excited states (e.g., acridines), which limits their viability as discrete ES H+ acceptors.47,96 To our knowledge, there have been no reports of photobase-initiated ES-PCET, but this underexplored field holds promise as another approach capable of bypassing the restrictions of more conventional ET-dominated ES-PCET reaction schemes. Outlook. Decades of work understanding excited-state electron transfer and excited-state proton-transfer reactions have laid the foundation for application of excited-state reagents in PCET reactions. Many complexes that undergo ES-ET and ES-PT reactions, especially excited-state electron acceptors, have been utilized to promote excited-state PCET reactivity across a variety of substrates. Studies incorporating these reactants have provided new understanding of general PCET reactivity; for example, the ability to modulate PCET reaction pathways through the systematic variation of reaction parameters and the role of ET distance on PCET reaction rates have been elucidated through the use of excited-state reactants. While these excited-state reagents have proven valuable in enhancing our understanding of PCET reactivity in general, in many cases their reactivity does not differ from that observed for the ground-state reagents that promote thermal PCET reactions. For example, recent work exploring CPET reactivity involving noncovalently linked Ru(II) photooxidants and substituted phenols has shown similar relationships between driving force and reaction rates for both termolecular systems involving ES e− acceptors and bimolecular systems involving ES H+/e− acceptors,42 underscoring the thermodynamic equivalence between these two ES-PCET reagents.45 However, there are many reagentsespecially among those classified in this work as excited-state proton/electron acceptors and excited-state proton/electron donorsthat react via pathways by which there are no analogues for thermal reagents. Opportunities to exploit this reactivity to drive fuel-producing reactions are promising. As such, recognizing and harnessing excited-state PCET reactivity that cannot be accessed from electronic ground states is crucial. Toward this, it is essential that contemporary research efforts pursue a deeper understanding of the fundamental science behind excited-state PCET reactivity and seek new ways that molecules can be designed to

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jillian L. Dempsey: 0000-0002-9459-4166 Notes

The authors declare no competing financial interest. Biographies J. Christian Lennox was born in Charlotte, North Carolina. He received his B.S. in chemistry from the College of William & Mary in 2013, where he performed undergraduate research with William McNamara. He is currently a graduate student at UNC, where he is investigating the factors underlying PCET pathway selection. Daniel A. Kurtz was born in Clinton Township, Michigan, and received his B.S. in Chemistry from Oakland University in 2013. Daniel worked under the supervision of Greg A. N. Felton while at Oakland University, and he spent a summer at Brookhaven National Laboratory with David C. Grills. He is currently a graduate student at UNC. Tao Huang was born in Quanzhou, China. He completed his B.S. in Chemistry at Wuhan University in 2011, working with Bin Hu. He earned his Ph.D. at North Carolina State University in 2016, where he worked with Walter Weare. He is now a postdoctoral associate at UNC. Jillian L. Dempsey was born in Summit, New Jersey. She received her S.B. from MIT in 2005, where she worked with Daniel Nocera, and her Ph.D. from Caltech in 2011, where she worked with Harry Gray and Jay Winkler. After a postdoc with Daniel Gamelin at University of Washington, she joined the faculty at the University of North Carolina in 2012. Website link: http://dempsey.web.unc.edu/

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation through a CAREER Award (CHE-1452615). J.L.D. acknowledges support from a Packard Fellowship in Science and Engineering and a Sloan Research Fellowship.

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