Third-Order Nonlinear Optical Properties of Asymmetric Non-Alternant Open-Shell Condensed-Ring Hydrocarbons: Effects of Diradical Character, Asymmetricity, and Exchange Interaction

Abstract

Using spin-unrestricted density functional theory, the longitudinal static second hyperpolarizability γ has been investigated for an asymmetric azulene-like open-shell condensed-ring molecule (1) as well as for its symmetric almost closed-shell and open-shell analogues (pentacene (2) and its dicyclopenta- (3) and dicyclohepta-fused (4) analogues). It is found that the symmetric systems, 3 and 4, having intermediate diradical characters show a 6–9 times enhancement of γ as compared to 2, but that this enhancement is even larger (by a factor of 30) for the asymmetric diradicaloid, 1. Geometrical, electronic, and magnetic descriptors have shown that in 1 the terminal seven- and five-membered rings display a cooperative push–pull effect, leading to a large dipole moment, that seems to reinforce its aromaticity as well as its ionic character, whereas 3 and 4 are mostly neutral diradicaloids and are less aromatic. These features have been rationalized by resorting to an asymmetric two-site model within the vale...