Performance of density functionals for excited-state properties of isolated chromophores and exciplexes: Emission spectra, solvatochromic shifts, and charge-transfer character

This study assesses the performance of various meta-generalized gradient approximation (meta-GGA), global hybrid, and range-separated hybrid (RSH) density functionals in capturing excited-state properties of organic chromophores and their excited-state complexes (exciplexes). Motivated by their uses in solar energy harvesting and photoredox CO2 reduction, we use oligo-(p-phenylenes) and their excited-state complexes with triethylamine as model systems. We focus on the fluorescence properties of these systems, specifically emission energies, solvatochromic shifts, and wavefunction characteristics. The latter is described using reduced quantities such as natural transition orbitals (NTOs) and exciton descriptors. The functionals are benchmarked against the experimental fluorescence spectra and the equation-of-motion coupled-cluster method with single and double excitations (EOM-CCSD). The results show that, both in isolated chromophores and in exciplexes, meta-GGA functionals drastically underestimate the emission energies, and exhibit significant exciton delocalization and anti-correlation between electron and hole pair. The performance of global hybrid functionals depends strongly on the percentage of exact exchange. RSH GGAs are the best-performing functionals identified in our study, with ωPBE demonstrating the best agreement with experimental results. RSH meta-GGAs often overestimate emission energies in exciplexes and yield larger hole NTOs. Their performance can be improved by optimally tuning the range-separation parameter.