Electronic excited states of planar vs bowl-shaped polycyclic aromatic hydrocarbons in interaction with water clusters: a TD-DFT study

Abstract

In an astrochemical and environmental context, this work constitutes a step forward in understanding the photo-reactivity of polycyclic aromatic hydrocarbons (PAHs) with water molecules and in water ice under irradiation with low energy photons. The role of charge transfer states PAH +-H 2O - has been proposed, motivating the study of the electronic excited states up to about 6 eV of planar and bowl-shaped PAHs, namely pyrene C 16H 10 and corannulene C 20H 10 , interacting with water clusters of different sizes and orientations, using a time-dependent density functional theory approach. In the case of pyrene, the systematic occurrence of low energy excitations from π orbitals to diffuse orbitals located on some water molecules, mixed with the Rydberg orbitals (R/wat), was found. Such excitations are more numerous and possess larger oscillator strengths when (i) the number of water molecules increases up to representing a first layer of hexagonal water ice and (ii) for the arrangements leading to the lower vertical ionization potential values. In this case, the /wat orbitals are located on the most external H atoms and they may also mix with π⋆ orbitals. This accounts for the efficient reactivity of pyrene with water in water ice. In the case of corannulene, the main result is that, for the C20H10(H2O)3 isomer formed in a noble gas matrix, where (H2O)3 interacts with the concave face of corannulene, no π→ R/wat transition is observed. It is in line with the lack of reactivity of corannulene with water in a noble gas matrix.