Unraveling Abnormal Collective Effects via the Non-Monotonic Number Dependence of Electron Transfer in Confined Electromagnetic Fields

TL;DR

This paper develops an analytical framework to understand how collective light-matter interactions in optical cavities non-monotonically influence electron transfer rates in molecular systems, revealing optimal conditions for cavity-controlled chemistry.

Contribution

It introduces a new analytical model for electron transfer in cavity systems that captures non-monotonic effects and predicts optimal molecule numbers for reactivity enhancement.

Findings

Electron transfer rate shows non-monotonic dependence on molecule number N.

Cavity-induced quantum yield increases linearly for small N, then suppresses at large N.

Analytical formulas relate thermal bath frequency, N, and electron transfer dynamics.

Abstract

Strong light-matter coupling within an optical cavity leverages the collective interactions of molecules and confined electromagnetic fields, giving rise to the possibilities of modifying chemical reactivity and molecular properties. While collective optical responses, such as enhanced Rabi splitting, are often observed, the overall effect of the cavity on molecular systems remains ambiguous for a large number of molecules. In this paper, we investigate the non-adiabatic electron transfer (ET) process in electron donor-acceptor pairs influenced by collective excitation and local molecular dynamics. Using the timescale difference between reorganization and thermal fluctuations, we derive analytical formulas for the electron transfer rate constant and the polariton relaxation rate. These formulas apply to any number of molecules ($N$) and account for the collective effect as induced by cavity photon coupling. Our findings reveal a non-monotonic dependence of the rate constant on $N$, which can be understood by the interplay between electron transfer and polariton relaxation. As a result, the cavity-induced quantum yield increases linearly with $N$ for small $N$ (as predicted by a simple Dicke model), but shows a turnover and suppression for large $N$ (consistent with the large $N$ problem of polariton chemistry). We also interrelate the thermal bath frequency and the number of molecules, suggesting the optimal number for maximizing enhancement. The analysis provides an analytical insight for understanding the collective excitation of light and electron transfer, helping to predict the optimal condition for effective cavity-controlled chemical reactivity.