Theoretical study on two-photon absorption for symmetric molecular systems composed of charged groups linked with a π-conjugated bridge

Two-photon absorption (TPA) properties of symmetric molecular systems composed of charged groups linked with a π-conjugated bridge are investigated using the time-dependent perturbation theory with the single and double excitation CI method in the INDO/S approximation in order to clarify the effect of introducing charges on the TPA property. It turns out that the introduction of charges causes the red shift and significant enhancement of TPA cross section in the first peak position as compared to that in the neutral molecule with a similar framework. This is primarily ascribed to be the increase of transition moment between the one-photon-allowed, first excited state and the two-photon allowed, higher excited state for the charged molecules because of the localization of contributing MOs in the both-end regions.