Benchmark Study on the Triplet Excited-State Geometries and Phosphorescence Energies of Heterocyclic Compounds: Comparison Between TD-PBEO and SAC-CI

In this work, we investigated the properties of the triplet excited states of heterocyclic compounds including their geometries, electronic properties, and phosphorescence energies by using both the direct symmetry-adapted cluster-configuration interaction (SAC-CI) method and the TD-DFT approach with the PBE0 exchange-correlation functional (TD-PBE0). The target states are the pi pi* and n pi* triplet states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the Rydberg states. The present benchmark demonstrates that these two methods provide reasonably accurate geometries for the excited states of these heterocyclic compounds. The calculated Strokes shifts, which reflect geometry changes, were consistent for both these methods. The trends of agreement with experimental or reference values obtained for a panel of exchange-correlation functionals used to compute the absolute emission energies from the triplet states, differ from those found for the singlet excited states. Some of the low-lying triplet excited states were examined in detail for the first time, including vibrational analysis.