Symmetry breaking and excitonic effects on optical properties of defective nanographenes.

We investigate optical properties of the nanographene family and predict a defect induced effect by utilizing the all-electron first-principles GW+Bethe-Salpeter equation (BSE) method based on the many-body perturbation theory. As an accuracy check of the GW+BSE, photoabsorption spectra are calculated for a grossly warped nanographene (C80H30), which was very recently synthesized [Kawasumi et al., Nat. Chem. 5, 739-744 (2013)]. The calculated spectra are found to faithfully reproduce the shape, height, and position of the measured peaks. Then the method is applied to the flat nanographene without defect (C24H12 and C38H16), the curved ones with single defect (C20H10, C28H14, and C32H16), and fragments of C80H30 with double defect (C36H16 and C42H20). The existence of the defects significantly changes the optical spectra. In particular, the interaction between the defects is found to break the symmetry of the atomic geometries and enhance the excitonic effect, thereby generating the extra peaks at the lower photon energy side of the main peak. The present results might help explain the origin of the first two peaks experimentally observed for C80H30.