Origin of Water-Induced Fluorescence Turn-On from a Schiff Base Compound: AIE or H-Bonding Promoted ESIPT?

A nonfluorescent Schiff base compound (4) in an aprotic solvent (e.g., CH3CN) is found to give blue fluorescence turn-on (lambda(em) approximate to 475 nm) upon addition of H2O. By using a wide range of spectroscopic methods, including H-1 NMR and dynamic light scattering, the fluorescence response is shown to be not originating from the molecular aggregation induced emission (AIE). Spectroscopic studies at low temperatures further reveal a dynamic response of 4 to temperature, showing that the excited state intramolecular proton transfer (ESIPT) can be ON or OFF through interaction with protic solvent. In the binary solvent (with composition CH3CN/H2O = 3:1), the Schiff base gives ESIPT emission (lambda(em) approximate to 524 nm) only at extremely low temperature (below -80 degrees C), which is turned off when being warmed to -60 degrees C, attributing to the increasing photoinduced electron transfer (PET) effect. When the temperature is further raised to -20 degrees C, ESIPT emission is reactivated to give blue emission (lambda(em) approximate to 475 nm) that is observed at room temperature. The observed dynamic fluorescence response reveals that ESIPT could be a predominant mechanism in the fluorescence turn-on of Schiff base compounds, although both AIE and ESIPT mechanisms could operate. The assumption is further verified by examining the response of Schiff base to Al3+ cation.