Analysis of lowest energy transitions at TD-DFT of pyrene in vacuum and solvent.

Polycyclic aromatic hydrocarbons are present in interstellar medium and trapped in water ice. Among these compounds, pyrene has a controversial theoretical excitation spectrum. We carried out time-dependent density functional theory, including the long-range correction functionals, with the aim to help to understand the inversion of the first two UV bands of lower energies. The pyrene molecule was optimized at TD-DFT functionals with Def2svp basis set. The spectrum of pyrene molecule was calculated using implicit and explicit solvent models. The explicit solvent effect was studied, including a cluster of 51 water molecules. The implicit solvent PCM model was used with water and benzene as solvents. CAM-B3LYP and ωB97XD give correct band positions and the ω parameter was also optimized. NBO and frontier molecular orbitals were used to study the UV band inversion. Graphical abstract Analysis of frontier orbitals involved in the lowest energy transitions at TD-DFT of pyrene.