Strengthened near-IR two-photon absorption induced emission of ESIPT chromophores by molecular aggregation

In this study, a range of new free open-chain conjugated organic chromophores including salicylidene methylamine part were synthesized and characterized. The steady state and transient state absorption and emission properties of the target dyes in DMF were investigated. The results showed that the target dyes processed the excited state intramolecular proton transfer (ESIPT) in DMF under one-photon excitation. Furthermore, the ESIPT was more competitive under the exposure of pulsed pumping near-IR femotosecond laser in DMF. The further survey showed that the target dyes yielded gradually the regular molecular aggregates in the mixed DMF/ H2O (the water volume fractions, 60-80%) with evolving time. In particular, the target molecular aggregates exhibited the strong enol emission but the quenched keto emission under one-photon exposure and near-IR twophoton irradiation, respectively. In addition, the near-IR two-photon absorption (TPA) induced enol emission of the target dyes showed an enhancement in the aggregation state. This study demonstrates that it is feasible to strengthen TPA induced emission of enol phototautomers of some ESIPT compounds by the molecular aggregation under the irradiation of near-IR femotosecond laser.