Photoinduced Charge Transfer in Push/Pull Systems of Two-Photon Absorption

A series of stilbene derivatives have been constructed by modifying the stilbene systems with different H, CN, NH₂, NMe₂, and NO₂ groups. In a vacuum, it was found that a redshift in the ultraviolet–visible spectrum occurred because of the enhancement of the donor/acceptor capabilities of this group, with the order of redshift being NO₂ > NMe₂ > CN > NH₂ > H. For stilbene molecular systems, the peak of two-photon absorption (TPA) observed in the simulated spectra should be attributed to two transitions that are contributed by two excited states with similar energy. In the case of derivatives, such a transition is separated by energy, and two TPA peaks can be clearly observed (derivatives containing NO₂ and NMe₂ groups have two TPA peaks), where the magnitude of the separation is directly related to the intensity of the peripheral group. In addition, the S₁ state is the intermediate state in the TPA transitions to both of the final excited states.