Aggregation Enhancement on Two-Photon Optical Properties of AIE-Active D-TPE-A Molecules

We present an aggregation enhancement in two-photon-excited fluorescence (TPEF) of about 2 orders of magnitude in a series of novel noncentrosymmetric D-π-A molecules. Detailed analysis shows that their TPEF is enhanced concomitantly with both fluorescence quantum efficiency and two-photon absorptivity. These D-π-A molecules use tetraphenylethylene (TPE) as the π-bridge, giving them aggregation-induced emission characteristics. These branched or butterfly-configured molecules demonstrate two-photon absorption (TPA) transition bands similar to those in their linear absorption. The molecular TPA cross sections in the aggregation environment reach around 50–130 GM, and peak within the available wavelength range of a Ti:Sapphire femtosecond oscillator, which makes the molecules quite favorable for bioimaging applications. We observe that two-photon absorptivity increases progressively with the addition of more donor and acceptor moieties on the TPE backbone, which is presumably attributed to the improved conjugation length and enhanced intramolecular charge transfer, and hence better delocalization of π-electrons.