Excimer states and enhanced two-photon absorption in intramolecular charge-transfer crystals

We explain enhanced two-photon absorption in certain centrosymmetric organic molecular crystals in which intramolecular charge transfer takes place. Specifically we suggest that charge transfer allows the interacting molecules in a unit cell to have non-zero dipole moments in the excited state of the molecules, contributing to a resonance in the two-photon absorption coefficient. Anti-parallel orientation of the molecular dipole moments in the unit cell allows for formation of excimer states and disallows the production of SHG in the same materials. We estimate the ratio of the two-photon to the one-photon absorption coefficient for some typical parameters.