Multiproton-Coupled Electron Transfer Across Hydrogen-Bond Networks

E. ODELLA; W.D. GUERRA; S.J. MORA; B.L. WADSWORTH; MARÍA NOEL URRUTIA; M. SECOR; J.J. GOINGS; M.T. HUYNH; Y. YONEDA; J. SHEE; E.A. ARSENAULT; D. HAIT; G. KODIS; M. GERVALDO; L. E. SERENO; G.F. MOORE; M. HEAD-GORDON; G.R. FLEMING; S. HAMMES-SCHIFFER; D. GUST; THOMAS A. MOORE; ANA L. MOORE

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Conferencia; Department of Energy (DOE) Solar Photochemistry - Basic Energy Sciences (BES).; 2021

Constructs consisting of a phenol, a Grotthuss-type hydrogen-bond network based on a polybenzimidazoles (bridge), and a terminal proton acceptor (TPA) demonstrate long-range (~16Å) proton translocation upon oxidation of the phenol.1 The addition of successive benzimidazole moieties to the bridge results in decreasing redox potential of the phenoxyl radical/phenol couple by 60 mV per benzimidazole unit. We interpret this as evidence that benzimidazole-based bridges are non-innocent participants in the PCET processes.2 This is confirmed by structural modification of the bridge, where the drop in redox potential is modulated by selected substituents on the benzimidazoles. The essential role of a well-defined hydrogen-bond network in achieving chemically reversible electrochemical oxidation/reduction was investigated with the models shown in the Figure. With compound 1, experimental and theoretical evidence indicates an overall chemically reversible two-proton coupled electron-transfer process (E2PT). In contrast, with compound 2 electrochemical oxidation of the phenol results in an overall chemically irreversible E2PT process. Insights into the electrochemical behavior of these systems are provided by theoretical calculations of the electrostatic potentials and electric fields at the site of the transferring protons for the forward and reverse processes. To initiate an E1PT process photochemically we prepared a benzimidazole-phenol (BIP) unit covalently attached to a tripentafluorophenyl-porphyrin (BIPPF15). An ultrafast photoinduced process enabled us to observe by 2DEV spectroscopy and long-range-corrected hybrid DFT theoretical techniques the evolution of an initial, partly charge-transferred state to full charge separation on the 120 fs time scale.41. Odella, E., et al., "Proton-Coupled Electron Transfer Drives Long-Range Proton Translocation in Bioinspired Systems." J. Amer. Chem. Soc. 2019, 141, 14057-14061.2. Odella, E., et al., "Proton-Coupled Electron Transfer across Benzimidazole Bridges in Bioinspired Proton Wires." Chem. Science 2020, 11, 3820-3828.3. Guerra, W. D., et al. "Role of Intact Hydrogen-Bond Networks in Multiproton-Coupled Electron Transfer." J. Amer. Chem. Soc. 2020, 142, 21842-21851.4. Yoneda, Y., et al. (2021). "Electron?Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer." J. Amer. Chem. Soc. 2021, 143, 3104-3112.