Electronic–Vibrational Coupling and Electron Transfer

Abstract

This Feature Article describes our recent work on implementing pump-degenerate four-wave mixing for studying electronic vibrational coupling in solid-state systems and in particular in interfacial electron transfer systems. Interfacial electron transfer (ET) reactions constitute key physical phenomena central to a variety of light-induced energy transport and conversion processes such as vision, photocatalysis, photovoltaics, energy storage, molecular electronics, etc. Heterogeneous material systems like organic/inorganic interfaces are of particular interest due to the variety of possible modifications to their properties. Although ET dynamics has been studied intensively in heterogeneous systems, many questions remain unanswered. The significance of electronic–vibrational coupling and coherence is one aspect that has yet to be fully explored. It is well-known that ET between a molecule and a semiconductor can be accompanied by vibrational excitation due to Franck–Condon overlap of the excited state and cation. The energy necessary for exciting these high-energy vibrational modes is taken from the excess energy of the transferred electron and leads to an ET spectrum. This spectrum carries information that is associated with the vibrational wavepacket that is involved in the ET process, including the coherent beating that results from periodic attempts to overcome the transition state. The dynamics of the wavepacket, and in particular its coherence time, provides insight into the strength of the coupling between electron donor and acceptor states and the adiabaticity of the process. This information is important but not readily available for interfacial ET systems comprising a large number of possible electron acceptor states because neither the rate of the reaction nor the electronic spectrum allows unambiguous assignment. These recent studies give access to vibrational wave packet dynamics triggered by ET reaction and have the potential to allow direct measurement of Huang–Rys parameters and study fundamental properties of interfacial ET like the adiabaticity and the amount of coherence—electronic and vibrational—in the system.