Long-range proton-coupled electron transfer in phenol–Ru(2,2′-bipyrazine)32+ dyads

Abstract

Two dyads in which either 4-cyanophenol or un-substituted phenol is connected via a p-xylene spacer to a Ru(bpz)3(2+) (bpz = 2,2'-bipyrazine) complex were synthesized and investigated. Selective photo-excitation of Ru(bpz)3(2+) at 532 nm in a CH3CN-H2O mixture leads to the formation of 4-cyanophenolate or phenolate along with Ru(bpz)3(2+) in its electronic ground state. This apparent photoacid behavior can be understood on the basis of a reaction sequence comprised of an initial photoinduced proton-coupled electron transfer (PCET) during which 4-cyanophenol or phenol is oxidized and deprotonated, followed by a thermal electron transfer event in the course of which 4-cyanophenoxyl or phenoxyl is reduced by Ru(bpz)3(+) to 4-cyanophenolate or phenolate. Conceptually, this reaction sequence is identical to a sequence of photoinduced charge-separation and thermal charge-recombination events as observed previously for many electron transfer dyads, with the important difference that the initial photoinduced electron transfer process is proton-coupled. The dyad containing 4-cyanophenol reacts via concerted-proton electron transfer (CPET) whereas the dyad containing un-substituted phenol appears to react predominantly via a stepwise PCET mechanism. Long-range PCET is a key reaction in photosystem II. Understanding the factors that govern the kinetics of long-range PCET is desirable in the broader context of light-to-energy conversion by means of proton-electron separation across natural or artificial membranes.

Electron Accumulation on Naphthalene Diimide Photosensitized by [Ru(2,2′-Bipyridine)3]2+

“…Our study demonstrates that for applications aiming at multi-electron (photo-)redox reactions, the covalent linkage of photosensitizers and catalytic reaction centers introduces significant challenges with regard to avoiding unproductive (but energy-consuming) electron transfer reactions upon sequential absorption of two (or more) photons. These unproductive electron transfers could presumably be decelerated significantly with suitable PCET photochemistry, [70][71][72][73][74] leading ultimately to the accumulation of redox equivalents rather than the accumulation of charge, similar to what is observed in the oxygen-evolving complex of photosystem II. 75,76 ASSOCIATED CONTENT Supporting Information.…”

An improved procedure for the preparation of Ru(bpz)3(PF6)2 via a high-yielding synthesis of 2,2’-bipyrazine

“…Unfortunately, the synthesis of 2,2’-bipyrazyl (bpz) is quite challenging compared to the syntheses of other, structurally similar bidentate bisazene ligands. Various methods for the preparation of bpz have been reported [ 26 – 28 ], the most common of which involve transition metal-catalyzed reductive homocouplings of halopyrazine electrophiles [ 29 – 30 ]. However, we found these procedures to be capricious in our hands, and after a survey of known reductive dimerization protocols, the highest yields of bpz we were able to obtain resulted from a Pd-catalyzed procedure reported by Plé ( Scheme 1 ) [ 31 ].…”

Proton switching as a driving force for the metal–organic complex-mediated growth of gold colloids

“…21,22 The mechanistic details of the electrochemical oxidation of phenol has been well discussed through the Pourbaix diagram by Jean-Michel et al 23 They identified the reaction pathway, where a concerted proton-electron transfer mechanism operated with water acting as a proton acceptor. 21,22 The mechanistic details of the electrochemical oxidation of phenol has been well discussed through the Pourbaix diagram by Jean-Michel et al 23 They identified the reaction pathway, where a concerted proton-electron transfer mechanism operated with water acting as a proton acceptor.…”