Theoretical study on stacking distance dependence of one- and two-photon absorption properties of phenalenyl π-dimer models

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 multiconfigurational 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.Graphical AbstractTheoretical calculations predicted the appearance of a new two-photon absorption peak related to the intermolecular electronic polarization in phenalenyl π-dimer models when the stacking distance becomes sufficiently small.