Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays

Abstract

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the TyrZ-His190 redox relay in photosystem II. These compounds provide a platform to theoretically and experimentally explore and expand proton-coupled electron transfer (PCET) processes. The models feature H-bonds between the phenol and the nitrogen at the 3-position of the benzimidazole and between the 1H-benzimidazole proton and the imine nitrogen. Protonation of the benzimidazole and the imine can be unambiguously detected by infrared spectroelectrochemistry (IRSEC) upon oxidation of the phenol. DFT calculations and IRSEC results demonstrate that with sufficiently strong electron-donating groups at the para-position of the N-phenylimine group (e.g.,-OCH3 substitution), proton transfer to the imine is exergonic upon phenol oxidation, leading to a oneelectron, two-proton (E2PT) product with the imidazole acting as a proton relay. When transfer of the second proton is not sufficiently exergonic (e.g.,-CN substitution), a one-electron, one-proton transfer (EPT) product is dominant. Thus, the extent of proton translocation along the H-bond network, either ~1.6 Å or ~6.4 Å, can be controlled through imine substitution. Moreover, the H-bond strength between the benzimidazole NH and the imine nitrogen, which is a function of their relative pKa values, and the redox potential of the phenoxyl radical/phenol couple are linearly correlated with the Hammett constants of the substituents. In all cases, a high potential (~1 V vs SCE) is observed for the phenoxyl radical/phenol couple. Designing and tuning redox-coupled proton wires is important for understanding bioenergetics and developing novel artificial photosynthetic systems.

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires

“…Model compounds 1-3 were obtained by reaction of a derivative of BIP containing a formyl group at the 7-position (BIP-CHO) with the corresponding aromatic or aliphatic primary amino derivatives in the presence of a catalytic amount of pyrrolidine. 26,27 Compound 4 was obtained by reduction of the tertiary amide group in the 7-position of BIP (BIP-CONEt 2 ) following a procedure previously described. 22 The second benzimidazole moiety of the BI 2 P family was introduced by condensation of BIP-CHO with methyl 2,3-diaminobenzoate followed by conversion of the ester group to the aldehyde (BI 2 P-CHO), the precursor of compounds 1 0 -4 0 .…”

“…22 Indeed, BIP compounds substituted with N-phenylimines as a TPA group did have midpoint potentials near 1.00 V vs. SCE. 26 Moreover, the ratio of E2PT to E1PT products following oxidation of the phenol could be modulated by the nature of the substituents at the paraposition of the N-phenylimine moiety. Density functional theory (DFT) calculations and infrared spectroelectrochemistry (IRSEC) results showed that with stronger electron-donating substituents the proton of the benzimidazole is transferred to the imine group upon phenol oxidation, leading to a net proton movement of $6.4Å along the hydrogen-bond network.…”

“…Density functional theory (DFT) calculations and infrared spectroelectrochemistry (IRSEC) results showed that with stronger electron-donating substituents the proton of the benzimidazole is transferred to the imine group upon phenol oxidation, leading to a net proton movement of $6.4Å along the hydrogen-bond network. 26 To extend the proton translocation, new constructs bearing an additional benzimidazole moiety, which generates a dibenzimidazole bridge and increases the spatial separation between the oxidation center (the phenol moiety) and the TPA group, have been synthesized (see Fig. 1).…”

“…1). These molecules were designed to investigate the control of phenol redox potentials in amino and imine-substituted BIP systems, 22,26 and to further explore the thermodynamics of proton transfer across a dibenzimidazole bridge unit (see Fig. 2).…”

Highly Polarized Coumarin Derivatives Revisited: Solvent‐Controlled Competition Between Proton‐Coupled Electron Transfer and Twisted Intramolecular Charge Transfer

“…A possible explanation for this effect is that the proton‐donating (amide NH) and proton‐accepting (coumarin C=O) moieties are not mutually electronically conjugated. In other words, a more proper classification of this reaction is electron‐driven proton transfer (EDPT), more commonly abbreviated as proton‐coupled electron transfer (PCET) …”