Theoretical Study on Stacking Distance Dependence of One- and Two-Photon Absorption Properties of Phenalenyl π-Dimer Models

Abstract Stacking distance (d) dependence of the peak positions and intensities of one-photon absorption (OPA) and two-photon absorption (TPA) spectra of phenalenyl π-dimer models was investigated theoretically. Excitation energies and transition dipole moments required to simulate the OPA and TPA spectra were evaluated at the extended multi-configurational quasi-degenerate second-order perturbation theory (XMC-QDPT2) level. The first TPA peak intensity for the model with d = 3.0 Å, related to the electronic transition along the stacking direction, was enhanced by a two-order magnitude compared with that for the model with d = 6.0 Å. We also obtained diradical character (y)-dependent behavior of the TPA property for the first TPA peak predicted from the valence-bond configuration interaction method based on the two-site model.