Spectroscopy, relaxation, and transport of molecular excitons in noisy and disordered environments

Abstract

In this thesis contribution we theoretically investigate the spectroscopy, relaxation, and transport properties of Frenkel excitons in molecular aggregates, with extensive comparison to or prediction of experimental observables. Particular emphasis is devoted to the effects of thermal noise, static disorder, and system dimensionality. Our key contributions are summarized as the following. We study the spectroscopic signatures of excitonic molecular aggregates of dimensionality larger than unity as functions of temperature and disorder strength. These findings are applied to the determination of essential system characteristics and quantitatively explain the spectroscopic traits seen in experiments where either the temperature or disorder strength is altered. A classification scheme generalized from Kasha's seminal work on J- and H-aggregates is proposed that is compatible with experimental observations previously unexplained. We recognize the importance of long-wavelength approximations in understanding the density of states in two-dimensional excitonic aggregates. And for tubular aggregates this leads to a simple expression for the energy gap between the parallel- and the perpendicular-polarized peaks useful in inferring key system parameters. This long-wavelength approach is then extended to the analysis of 2D excitonic molecular aggregates in general. A universal scaling relation concerning the steady-state diffusive transport of excitons in molecular tubes is predicted and analyzed, where the key order parameter is identified as the ratio between the localization length of the exciton wavefunctions and the tube circumference. A unified theoretical framework is proposed to explain the relaxation of hot excitons generated in emissive conjugated polymers across three orders of magnitude in timescale, with quantitative agreements with experiments.