Elucidating the Synergetic Interplay between Average Intermolecular Coupling and Coupling Disorder in Short-Time Exciton Transfer

TL;DR

This paper develops an analytical framework for understanding short-time exciton dynamics in disordered molecular aggregates, revealing the key roles of intermolecular coupling and disorder in ultrafast energy transfer.

Contribution

It introduces a reciprocal-space analytical approach to quantify short-time exciton transport considering both diagonal and off-diagonal disorder, highlighting their synergistic effects.

Findings

Short-time ballistic expansion is mainly governed by off-diagonal disorder.

A synergistic interplay exists between average intermolecular coupling and coupling disorder.

The model integrates with quantum electrodynamics for realistic molecular systems.

Abstract

Exciton transport in molecular aggregates is a fundamental process governing the performance of organic optoelectronics and light-harvesting systems. While most theoretical studies have emphasized long-time transport behavior, recent advances in ultrafast spectroscopy have brought into focus the short-time regime, in which exciton motion remains ballistic on femtosecond-to-picosecond timescales. In this work, we develop an analytical framework for short-time exciton dynamics in a one-dimensional lattice subject to both on-site energetic (diagonal) disorder and intermolecular coupling (off-diagonal) fluctuations. Utilizing the reciprocal-space analysis, we derive closed-form expressions for the first and second spatial moments considering both localized excitation and moving Gaussian initial conditions. Our analytical and numerical results show that, while the long-time dynamics are influenced by diagonal disorder, the short-time ballistic expansion is governed primarily by off-diagonal disorder. Crucially, we reveal a synergistic interplay between the average intermolecular coupling and the off-diagonal coupling disorder strength, demonstrating that they contribute equivalently to short-time exciton transport. Moreover, we integrate this generic disorder model with a realistic molecular system within the framework of macroscopic quantum electrodynamics, thereby providing a theoretical foundation for characterizing and optimizing ultrafast energy flow of disordered molecular aggregates in complex dielectric media.