PCET in Artificial Photosynthesis

ANA L. MOORE; THOMAS A MOORE; GARY F. MOORE; EMMANUEL ODELLA; WALTER D. GUERRA; MARÍA NOEL URRUTIA

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Conferencia; 38th Eastern Regional Photosynthesis Conference: Photosynthesis: Following the Energy; 2021

Inspired by the proton-coupled electron transfer (PCET) process in photosystem II involving Tyrz-His190, a benzimidazole-phenol (BIP) system was used to illustrate an E1PT process, in which a one-electron oxidation of the phenol is accompanied by the transfer of one proton to the benzimidazole.1 With amino- or imino-substituted BIPs, we showed that a one-electron, two-proton PCET reaction takes place, described as an E2PT process. Aiming at long-range proton translocation, we designed and synthesized constructs consisting of a phenol, a Grotthuss-type hydrogen-bond network based on a polybenzimidazole framework (bridge), and a terminal proton acceptor (TPA). Translocation of protons up to ∼16 Å was observed to occur by an E4PT process.2 In all cases, infrared spectroelectrochemistry (IRSEC) was used to demonstrate that upon oxidation of these complexes, protons translocate across a well-defined hydrogen-bond network from the phenol to the TPA. As is usually the case in energy coupling systems in nature, the process is fully reversible. That is, upon reduction of the oxidized phenol, protons translocate from the TPA across the hydrogen-bond network back to the phenol. For the same TPA, the addition of successive benzimidazole moieties to the bridge results in a decrease of the redox potential of the phenoxyl radical/phenol couple by 60 mV per benzimidazole unit. We interpret this experimental finding as evidence that the benzimidazole-based bridge is a non-innocent participant in the PCET process.3 This is confirmed by structural modification of the bridge, where the redox potential drop by the successive addition of substituted benzimidazole units can be modulated by the substitutions. To initiate an E1PT process photochemically we studied a benzimidazole unit covalently attached to a tripentafluorophenylporphyrin (BIPPF15) where the porphyrin mimics P680. Proton-coupled electron transfer in BIPPF15 takes place on two-time scales, an ultrafast process from the initially prepared, unrelaxed excited level and a slower, process on the time scale of a few hundred picoseconds arising from the relaxed lowest excited singlet state of the PF15 moiety. The ultrafast process, though not the dominant pathway, enables us to observe via the developing dipole moment the evolution of the initial, partly charge transferred state, to full charge separation as molecular movement consisting of a dihedral twist between the BIP and the macrocycle occurs on the 120 fs time scale.4 In the long term, we envision the construction of molecular proton wires where proton transport across lipid bilayers (~30Å) would generate proton-motive force (PMF) in conjunction with photochemically induced PCET. These constructs provide a path towards artificial photosynthesis in which PCET-based proton management plays a role in efficient catalysis in both oxidative and reductive processes.1.Mora, S. J.; Odella, E.; Moore, G. F.; Gust, D.; Moore, T. A.; Moore, A. L., Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems. Acc. Chem. Res. 2018, 51 (2), 445-453.2.Odella, E.; Wadsworth, B. L.; Mora, S. J.; Goings, J. J.; Huynh, M. T.; Gust, D.; Moore, T. A.; Moore, G. F.; Hammes-Schiffer, S.; Moore, A. L., Proton-Coupled Electron Transfer Drives Long-Range Proton Translocation in Bioinspired Systems. J. Am. Chem. Soc. 2019, 141 (36), 14057-14061.3.Odella, E.; Mora, S. J.; Wadsworth, B. L.; Goings, J. J.; Gervaldo, M. A.; Sereno, L. E.; Groy, T. L.; Gust, D.; Moore, T. A.; Moore, G. F.; Hammes-Schiffer, S.; Moore, A. L., Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires. Chemical Science 2020, 11 (15), 3820-3828.4.Yoneda, Y.; Mora, S. J.; Shee, J.; Wadsworth, B. L.; Arsenault, E. A.; Hait, D.; Kodis, G.; Gust, D.; Moore, G. F.; Moore, A. L.; Head-Gordon, M.; Moore, T. A.; Fleming, G. R., Electron?Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2021, 143 (8), 3104-3112.