Theoretical investigation of the effect of radical substituents on the open-shell character of polycyclic aromatic hydrocarbons

The open-shell character y of a molecule correlates with its physical properties, such as singlet fission and nonlinear optical responses. In this study, we investigated the effect of various radical substituents on the y of polyaromatic hydrocarbons (PAHs) using occupation numbers of natural orbitals calculated at the LC-UBLYP(mu=0.33)/6-311G(d,p) level of theory. It was found that radical substituents significantly altered the y of PAHs compared to non-radical substituents. Specifically, substituting methyl-type radicals such as the dicyanomethyl radical decreased the y value, while substituting allyl-2-yl-type radicals such as nitronyl nitroxide increased it. In addition, we also evaluated the correlation between the y and the HOMO-LUMO gaps and between the y and the spin-state energy gaps. Radical substituents was found to significantly alter the open-shell character y of PAHs compared to non-radical substituents.