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

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journal contribution
posted on 04.02.2016, 14:53 by Lucas McDonald, Junfeng Wang, Nick Alexander, Hui Li, Tianbo Liu, Yi Pang
A nonfluorescent Schiff base compound (4) in an aprotic solvent (e.g., CH3CN) is found to give blue fluorescence turn-on (λem ≈ 475 nm) upon addition of H2O. By using a wide range of spectroscopic methods, including 1H 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 (λem ≈ 524 nm) only at extremely low temperature (below −80 °C), which is turned off when being warmed to −60 °C, attributing to the increasing photoinduced electron transfer (PET) effect. When the temperature is further raised to −20 °C, ESIPT emission is reactivated to give blue emission (λem ≈ 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.