Hydrogen Bonds in Excited State Proton Transfer

TL;DR

This study uses time-resolved photoelectron imaging to reveal how hydrogen bonds influence excited state proton transfer, significantly affecting photochemical reaction pathways in biological chromophores.

Contribution

It uncovers the mechanism of excited state proton transfer at hydrogen bonds, demonstrating how a single hydrogen bond alters reaction outcomes and stabilizes the system.

Findings

Hydrogen bonding dramatically changes photochemical pathways.

Excited state proton transfer involves sequential hydrogen and charge transfer.

Hydrogen bonds promote ground state recovery instead of dissociation.

Abstract

Hydrogen bonding interactions between biological chromophores and their surrounding protein and solvent environment significantly affect the photochemical pathways of the chromophore and its biological function. A common first step in the dynamics of these systems is excited state proton transfer between the non-covalently bound molecules, which stabilises the system against dissociation and principally alters relaxation pathways. Despite such fundamental importance, studying excited state proton transfer across a hydrogen bond has proven difficult, leaving uncertainties about the mechanism. Through time-resolved photoelectron imaging measurements we demonstrate how the addition of a single hydrogen bond and the opening of an excited state proton transfer channel dramatically changes the outcome of a photochemical reaction, from rapid dissociation in the isolated chromophore, to efficient stabilisation and ground state recovery in the hydrogen bonded case, and uncover the mechanism of excited state proton transfer at a hydrogen bond, which follows sequential hydrogen and charge transfer processes.