Intramolecular Cooperative and Anti-Cooperative Effect on the Two-Photon Absorption Cross Section in Triphenylamine Derivatives.

The intramolecular cooperative effect in branched molecules is a consequence of the interaction and extent of electronic coupling among the different axes of charge transfer. Such an effect is the key to obtain remarkable nonlinear optical response in molecular systems. Here we show that triphenylamine derivative molecules containing only two branches present the strongest electronic interaction between them at the excited state, generating exponential enhancement of the 2PA cross section. The primary factor for such behavior was ascribed to the substantial extent and interaction of the π-electron delocalization promoted by the strong electron-donating and acceptor antisymmetrical groups present in each branch. However, for the three-branch molecules we observed an anticooperative effect, i.e., the 2PA cross section decreases as compared to the one-branch structure as we normalized the signal by the effective π-electron number in each molecule.