[摘要]：Many squaraines exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions. The electronic and vibronic contributions to these low-energy two-photon absorptions were elucidated by performing correlated quantum-chem. calcns. on model chromophores that differ in terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)-phenylpolyenyl donors, the calcd. energy of the lowest two-photon active state approaches that of the lowest energy one-photon active (1Bu) state. This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these chromophores. For all squaraines contg. indolenylidenemethyl donors, the calcns. indicate that there are no low-lying electronic states of appropriate symmetry for TPA. The lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1Bu state with bu vibrational modes. Through implementation of Herzberg-Teller theory, the vibrational modes responsible for the low-energy TPA peak were identified and exptl. TPA spectra of several squaraines were reproduced, at least qual.
