Effects of hydrogen bonding on intramolecular/intermolecular proton-coupled electron transfer using a Ruthenium-anthraquinone dyad in ultrafast time domain

Abstract

A ruthenium-anthraquinone dyad consisting of the bi-imidazole spacer in which one imidazole proton acts as a hydrogen bonding agent is employed to investigate the effects of H-bonding on both photo-induced electron transfer (PET) and proton-coupled electron transfer process (PCET). The H-bonding of the anthraquinone radical anion (AQ•-) with a bi-imidazole spacer in complex 1 generated by the electron transfer reduction of complex 1 is indicated by the symbolic anodic shift of the reduction potential of anthraquinone. The present investigation has established that the arrangement of an intramolecular H-bond between the attached AQ and the imidazole proton consequences in a notable increase in the rate of intramolecular electron transfer from Ru(II) to AQ. Further, we have also demonstrated the ultrafast electron transfer and PCET dynamics in presence of water. We have successfully detected the transient absorption photoproduct Ru(III) by which we could determine the time constants of each step which is rare in literature. Most importantly, our study demonstrates electron transfer and intra-molecular PCET dynamics in the femtosecond time domain. The results of the steady-state and transient absorption spectroscopic studies suggest that bi-imidazole spacer and water molecules have a significant role in both PET and PCET dynamics as a proton source in the excited state. We have demonstrated PCET reaction in ultrafast time scale using Ru-polypyridyl complex, with significant kinetic isotope effect.