Zwitterion Mediator/Quenchers. Coulombic Minimization of the Back-Reaction in Photocatalysis

Article abstract of DOI:10.1021/j100395a021

Zwitterionic analogues of methyl viologen, N,N'-bis(carboxyethyl)-4,4'-bipyridyl (CEB), and N,N'-bis(sulfonatopropyl)-4,4'-bipyridyl (SPB) have been studied as electrontransfer quenchers of excited-state photocatalysts.The molecules are formally neutral when oxidized but become negatively charged when reduced.This charge buildup minimizes undesirable back-comnination of the photogenerated redox products.This Coulombic effect is demonstrated both by direct flash photolysis measurements of back-recombination and by catalytic studies of water reduction.Results are compared for the zwitterion quenchers and methyl viologen, reacting with soluble anionic porphyrines and with micelle-associated reactants.For the anionic porphyrin system zinc tetrakis(sulfonatophenyl)porphyrin(4-) the rates of back-recombination are as follows: ZnTSPP^3- + MV¹⁺ -> ZnTSPP^4- + MV²⁺ (k), k = 2E9 M^-1s^-1; ZnTSPP^3- + CEB^1- -> ZnTSPP^4- + CEB^+/- (k), k = 6E8 M^-1s^-1.In a corresponding catalytic system incorporating ZnTSPP as the photoacceptor, CEB or methyl viologen as the quencher, and N-phenylgycine as the electron donor, a >2-fold enhancement of catalysis is observed by replacing MV²⁺ with CEB.These events can be greatly accentuated by increasing the charge on the component, via micellar localization.Thus, by using an amphiphilic derivetive of Ru(bpy)3²⁺ as photoactive donor solubilized in anionic micelles and SPB as electron acceptor, one can achieve 200-fold reduction in the rate of back-reaction.