Coherent Biexciton Transport in the Presence of Exciton-Exciton Annihilation in Molecular Aggregates

TL;DR

This paper develops a theoretical framework to analyze biexciton dynamics in molecular aggregates, emphasizing the roles of initial state coherence, exciton delocalization, and annihilation in transport and fluorescence.

Contribution

It extends kinetic models beyond weak coupling, explicitly treating populations and coherences, and explores how initial conditions influence biexciton transport and decay behaviors.

Findings

Coherent initial states show early-time ballistic transport.

Incoherent states lead to non-exponential relaxation and diffusion.

Biexciton transport varies with aggregate band structure and initial coherence.

Abstract

We present a theoretical framework for biexciton dynamics in molecular aggregates that explicitly treats populations and coherences across excitation manifolds within a reduced density-matrix formalism. By extending kinetic descriptions beyond the weak-coupling limit, the approach captures the influence of exciton delocalization and exciton-exciton annihilation while remaining computationally tractable within a Markovian description of environmental relaxation. Using this framework, we investigate how the spatial profile and momentum composition of the initial biexciton state govern fluorescence decay and transport. Incoherent initial conditions lead to strongly non-exponential relaxation and time-dependent diffusion driven by nonlinear population kinetics. In contrast, coherently prepared biexciton states exhibit pronounced early-time coherent transport, whose character depends sensitively on whether the initial state is prepared as a standing-wave or traveling-wave superposition of single-exciton modes. Despite nearly identical emission dynamics for J and H aggregate, biexciton transport properties differ markedly due to band structure-dependent interference effect. Our results demonstrate that biexciton dynamics remains strongly influenced by initial-state coherence and momentum composition. Besides initial-state preparation, the coherent-to-incoherent crossover and the diffusive spreading of the exciton density are sensitive to internal conversion processes such as exciton fusion and the decay to the first excited state. The present work establishes initial-state preparation as a key control parameter for many-exciton transport in excitonic systems and provides a general framework for interpreting nonlinear optical experiments beyond population-based descriptions.