Electron-correlated theory of excited states and optical absorption of low-symmetry Polycyclic Aromatic Hydrocarbons

In the field of optoelectronics, polycyclic aromatic hydrocarbons (PAHs) have received remarkable attention. However, for the realization of their full potential, understanding of their electronic structures, low-lying excited states, and optical properties is essential. With this aim in mind, we performed a rigorous study of the low-lying excited states (both electronic and spin) of three PAHs, both using a first-principles time-dependent density-functional theory (TDDFT) based approach, as well as an effective π -electron approach based on the Pariser-Parr-Pople model Hamiltonian, coupled with the configuration interaction (PPP-CI) method. Our very large-scale PPP-CI calculations account for the electron-correlation effects quite well. The considered PAHs are coronene derivatives having relatively low point group symmetries: two have C_2v symmetry, and one has C_2h symmetry, among the three PAHs, two are isomers of each other. We have compared calculated optical absorption spectra with the available experimental data and find very good agreement. We have also calculated the singlet-triplet gaps (spin gaps) of the PAHs using both DFT and PPP-CI approach, in order to obtain further understanding of the nature of electron-correlation effects and spin excitations in these systems.