UC San Diego

An ongoing goal in chemistry is to develop cheaper and greener methods of catalysis. Recent experiments show modified reaction kinetics when placing molecules inside an optical cavity and achieving vibrational strong coupling (VSC). VSC occurs when N - 1 molecular vibrations strongly interact with a cavity photon mode to form two hybrid light-matter modes called polaritons, as well as N - 1 optically dark vibrational modes. Since the aforementioned changes in reactivity occur without external pumping, VSC holds promise as a future tool in industrial catalysis. However, VSC-modified chemistry is not well understood. Initial theoretical efforts demonstrate that transition-state theory (TST), the most commonly used reaction-rate theory, predicts negligible changes in reaction rate due to VSC for typical experimental conditions. Subsequent works have begun to consider how VSC influences reactions for which the assumptions of TST break down.

In this dissertation, we theoretically explore how VSC affects reactions where thermalization occurs on a similar or longer time scale compared to reactive events. Such reactions, which include photochemical processes, can violate the TST assumption that internal thermal equilibrium is maintained throughout the reaction.

First, we study thermally activated electron transfer. For two molecules under VSC, we find that thermalization can be accelerated by cavity decay, a dissipative channel available to polaritons but not the uncoupled molecular vibrations. As a result, nonequilibrium effects that impact the reaction rate are suppressed. For a disordered ensemble of many molecules, VSC yields dark modes that are delocalized across several molecules. We reveal an unconventional mechanism by which this delocalization suppresses reactive events but not vibrational decay, thus speeding up the reaction.

We then investigate photochemistry under VSC. Specifically, we propose a “remote control” of chemistry, where photoexcitation of molecules in one optical cavity enhances the photoisomerization of molecules in another optical cavity. This idea challenges the standard paradigm that a catalyst must bind its substrate to change reactivity.

Finally, we develop a comprehensive theory for a related phenomenon: polariton-assisted energy transfer between spatially separated molecules. This theory not only sheds light on experimental observations but even predicts a number of intriguing effects, including the role reversal of donor and acceptor chromophores.