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

A. Patra, G. Pippim, A.I. Krylov, and S.M. Sharada
J. Chem. Theo. Comp.  20, 2520 – 2537 (2024)

This study assesses the performance of \textcolor{black}{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 wavefunc- tion 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.

Download this paper (PDF)

Related Research

Interface between electronic structure, spectroscopy, and dynamics