AIMD-Based Protocols for Modeling Exciplex Fluorescence Spectra and Inter-System Crossing in Photocatalytic Chromophores

This study introduces a computational protocol for modeling the emission spectra of exciplexes using excited-state ab initio molecular dynamics (AIMD) simulations. The protocol is applied to a model exciplex formed by oligo-p-phenylenes (OPPs) and triethylamine (TEA), which is of interest in the context of photocatalytic reduction of ${\text{CO}}_2$. AIMD facilitates efficient sampling of the conformational space of OPP3 and OPP4 exciplexes with TEA, offering a dynamic alternative to previously employed static methods. The AIMD-based protocol successfully reproduces experimental emission spectra for OPP-TEA exciplexes, agreeing with previous computational and experimental findings. The results show that AIMD simulations provide an efficient means of sampling the conformational space of these exciplexes, requiring less user input and, in some instances, fewer computational resources than multiple excited-state optimizations initiated from user-specified initial structures. The study also evaluates the yield of intersystem crossing (ISC) using AIMD and Landau-Zener probability. The results suggest that ISC is a minor decay channel for OPP3 and OPP4. This work provides new insights into the structural flexibility and emission characteristics of OPP-TEA photoredox catalyst systems, potentially contributing to improved design strategies for organic chromophores in ${\text{CO}}_2$ reduction applications.