Concavity Effects on the Optical Properties of Aromatic Hydrocarbons

We study the modifications on the ground and excited state properties of polycydic aromatic hydrocarbons (PAHs), induced by the variation of concavity and pi-connectivity. Inspired by experimentally feasible systems, we study three series of PAHs, from H-saturated graphene flakes to geodesic buckybowls, related to the formation of fullerene C-60 and C-50-carbon nanotube caps. Working within the framework of quantum chemistry semiempirical methods AM1 and ZINDO/S, we find that the interplay between concavity and pi-connectivity shifts the bright optical lines to higher energies and introduces symmetry-forbidden dark excitations at low energy. A generally good agreement with the available experimental data supports our results, which can be viewed as the basis for designing optical properties of novel curved aromatic molecules.