Study of Two-Photon induced excited state absorption of Three-Branched triphenylamine Derivatives: Cooperative and Anti-Cooperative effect of electron transition in the excited state

For multi-branched molecules, intramolecular cooperative effect can significantly enhance the molecular nonlinear optical absorption. Three triphenylamine-cored compounds (N1, N2 and N3) with three branches are synthesized to study the cooperative and anti-cooperative effect of electron transition in the excited state on two-photon absorption (TPA) and excited state absorption (ESA). Molecular polarization of these multi-branched triphenylamine derivatives is regulated by changing the molecular symmetry and the planarity of peripheral branches, to regulate their charge distribution and electron transition characteristics in the excited state. Here, we show that due to electronic coupling and interaction between certain branches, the asymmetric distribution of electron clouds in the excited states of these multi-branched molecules will lead to an enhancement of their TPA and ESA cross-sections, which is known as the cooperative effect of electron transitions. On the contrary, electronic coupling and interaction among all branches will lead to a highly symmetric distribution of electron clouds in the excited states of these multi-branched molecules, which will cause anti-cooperative effects and result in significant attenuation of TPA and ESA cross-sections.