Simulating Excited-State Complex Ensembles: Fluorescence and Solvatochromism in Amine-Arene Exciplexes

Exciplexes are excited-state complexes formed as a result of partial charge-transfer from the donor to the acceptor species. They are formed when one moiety of the donor-acceptor pair is electronically excited. The arene-amine exciplex formed between oligo-(p-phenylene) (OPP) and triethylamine (TEA) is of interest in photoredox catalysis for CO2 reduction as it can compete with complete quenching and electron transfer to OPP. Formation of the exciplex can therefore hinder the generation of a radical anion OPP·− necessary for subsequent CO2 reduction. We report an implementation of a workflow automating quantum-chemistry calculations that generate and characterize an ensemble of structures to represent this exciplex state. We use FireWorks, Pymatgen, and Custodian python packages for high-throughput ensemble generation via TDDFT optimization, verification of excited-state minima, and exciplex characterization with natural transition orbitals, exciton analysis, excited-state Mulliken charges, and energy decomposition analysis. Fluorescence spectra computed for these ensembles using Boltzmann-weighted contributions from each structure agree better with experiment than our previous work that employed a single representative exciplex structure (Kron, K. J. et al. J. Phys. Chem. A, 2022, 126, 2319–2329). The ensemble description of the exciplex state also reproduces an experimentally observed red shift of the emission spectrum of [OPP-4−TEA]∗ relative to [OPP-3−TEA]∗. The workflow developed here streamlines otherwise labor-intensive calculations that would require significant user intervention.