ESIPT and AIE characteristics of three Schiff base derivatives and the relevant photophysical mechanism analyses

Recently, three novel Schiff base derivatives (L1-3) were experimentally found to show water-induced aggregation-induced emission (AIE) and ultrafast excited state intramolecular proton transfer (ESIPT) characteristics, which have potential applications in fields of both optical material and fluoride ion detection. However, the corresponding optical properties have not been recognized completely and the relevant theoretical mechanism is lacking in analysis. In this work, based on the constructed relaxed potential energy curves, we investigated the stable configurations and clarified the proton transfer reaction mechanism for L1-3 with different substituents. The results show that the proton transfer associated with the interconversion from enol to keto isomer in S0 state is increasingly activated as substituted from electron-withdrawing group to electron-donating group, while the reverse trend is observed in S1 state. Above phenomenon can be ascribed to the change in strength of intramolecular hydrogen bond contributed to the reaction, which is demonstrated by hydrogen-bond parameters and interaction region indicator (IRI) analyses. Based on the optimized geometries, the single-fluorescence emission is found following the dual-absorption. The calculated fluorescence peaks (545 nm for L1, 536 nm for L2, 704 nm for L3) show little difference from those of experiment, which are reassigned to the ESIPT induced formation of keto isomer instead of enol form. In addition, the mechanism of water-induced AIE is clarified by increasing radiative rate and orbital overlap. To essentially comprehend the photophysical properties of organic luminogens with AIE and ESIPT characteristics is of fundamental interests in molecular electronics and photonics, and is also helpful to design new optical materials.