Validation of Local Hybrid Functionals for Excited States: Structures, Fluorescence, Phosphorescence, and Vibronic Spectra.

Abstract

Local hybrid functionals are evaluated in linear-response TDDFT computations for a broad range of excited-state properties including excited-state structures, fluorescence, and phosphorescence energies and the vibronic shape of absorption and phosphorescence spectra. Computation of such properties requires the optimization of excited states, which is facilitated by the recent implementation of excited-state gradients for local hybrid functionals in the TURBOMOLE program (Grotjahn, R.; Furche, F.; Kaupp, M. J. Chem. Theory Comput. 2019, 15, 5508). Comparison with coupled-cluster reference values reveals competitive performance of local hybrids for excited-state bond lengths with particular advantages for carbon-halogen, carbon-carbon, and carbon-nitrogen bonds. As with most global and range-separated hybrid functionals, carbonyl and thionyl bonds in n → π* excited states are found to be too compact. For the emission energies, results depend on the multiplicity of the excited state. While the local hybrid functionals tested perform moderately well, comparable to global hybrids, for singlet states (fluorescence energies), they provide outstanding accuracy for triplet states (phosphorescence energies), only matched by those from the highly empirical M06-2X hybrid functional. The assessment of the shape of vibronic spectra reveals rather small differences between local hybrid functionals and conventional hybrid functionals with comparable exact-exchange admixture. The advantages for phosphorescence energies and the robust performance for the shape of vibronic spectra are combined to showcase the potential of local hybrid functionals for the prediction of phosphorescence spectra.