Excited State Structure and Decay Rates for Aggregates

Abstract

Electronic excited state in molecular aggregate or exciton states continue to attract great attention due to the increasing demands for applications of molecular optoelectronics and sensing technology. The working principle behind the application is closely related to the excited state structure and dynamic processes in molecular aggregate. In our previous review article (Aggregate 2021; 2: e91), we focused more on the molecular mechanism for aggregation-induced emission process. Here, we are going to summarize our recent progress on theoretical investigations on the effects of excitonic coupling (J) and the intermolecular charge transfer (CT) on the excited state structure and dynamic processes. These are in general missing for molecular quantum chemistry studies. We will first present a novel definition of exciton coherence length which can present a bijective relation with the radiative decay rate and obviously we have clarified the confusion appeared in literature. Then, we will look at the CT effect for aggregate starting from a simple three-state model coupled with quantum chemical calculation for molecular dimer and we focus on the intensity borrowing, which can turn H-aggregate into emissive when the electron transfer and hole transfer integrals possessing the same sign and being large enough. We are able to propose a molecular descriptor to design molecular materials possibly possessing both high photoluminescence quantum yield and carrier mobility. Finally, we introduce our work on the modified energy gap law for non-radiative decay rate in aggregates. We found there exist optimal J to minimize the non-radiative decay loss.