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.

Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuĊ6H6) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

“…Thus, partly in response to the questions raised above as well as to address the likely possibility mentioned earlier that the unpaired electron on the MuĊ 6 H 6 radical is not in contact with the metal surface in this study, an alternate mechanism to () is proposed in (), in which the intermediate MuĊ 6 state from (, ) on the AuNP surface undergoes an electron transfer from the radical prior to forming stabilized MuĊ 6 H 6 , as in where f has the same meaning as in (), here in competition with electron transfer from MuĊ 6 to the metal surface, with rate constant k E . Such a mechanism would imply that electron transfer is facilitated by thermal excitation populating low-lying vibrational states in MuĊ 6 H 6 , also addressed in a different context in ref , but here giving the diamagnetic final state previewed above. This process might also be influenced by plasmon excitation on these metal NPs, arising via this energy transfer process on the gold NP surfaces, also thought to play a role in macro and mesoporous silica supports for silver NP catalysts …”

Synthesis and Properties of Perylene-Bridge-Anchor Chromophoric Compounds

“…In this context, perylene-bridge-anchor sensitizers, Figure a, are of special significance because of their role in pioneering electron transfer studies on TiO 2 nanoparticles conducted by Willig and co-workers. ,− Recent publications − point to a renewed interest in perylene sensitizers as excellent model compounds to probe electronic vibrational coupling . This is because photoexcitation of perylene in the visible (λ max = 440 nm) has almost pure S 0 → S 1 character with a long (∼5 ns) lifetime, and the absorption spectra of the ground, excited, and charge-separated states do not overlap, Figure b.…”

“…11,23−27 Recent publications 28−33 point to a renewed interest in perylene sensitizers as excellent model compounds to probe electronic vibrational coupling. 34 This is because photoexcitation of perylene in the visible (λ max = 440 nm) has almost pure S 0 → S 1 character with a long (∼5 ns) lifetime, and the absorption spectra of the ground, excited, and charge-separated states do not overlap, Figure 1b. Finally, perylene is a good photoreductant for TiO 2 as the energy of the perylene first excited singlet state (Pe*) lies ∼0.5 eV above the conduction band edge, as shown in Figure 1c.…”

Triple sum frequency pump-probe spectroscopy of transition metal dichalcogenides

“…By preceding a CMDS pulse sequence by a pump, the selectivity of CMDS can be leveraged as a probe in a "pump-CMDS-probe" measurement. [2,3,6,18,19] In this paper we introduce triple sum-frequency (TSF) spectroscopy as a new probe for material systems by measuring the pump-induced TSF response of model semiconductor systems: transition metal dichalcogenides (TMDCs).…”