Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains

TL;DR

This paper models the temperature-dependent fluorescence behavior of weakly localized Frenkel excitons in disordered linear chains, considering various experimental factors, and reveals non-monotonic temperature effects and different relaxation mechanisms.

Contribution

It introduces a kinetic model with microscopically calculated transition rates to analyze exciton dynamics, including localization, vibrational coupling, and non-equilibrium effects, under diverse experimental conditions.

Findings

Non-monotonic temperature dependence of Stokes shift

Fluorescence decay can be vibration-induced rather than radiative

Model applicable to molecular aggregates and conjugated polymers

Abstract

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible non-equilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or non resonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive non-monotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intra-band relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.