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

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• 作者：Lucas McDonald ; Junfeng Wang ; Nick Alexander ; Hui Li ; Tianbo Liu ; Yi Pang
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：February 4, 2016
• 年：2016
• 卷：120
• 期：4
• 页码：766-772
• 全文大小：489K
• ISSN：1520-5207

文摘

A nonfluorescent Schiff base compound (4) in an aprotic solvent (e.g., CH₃CN) is found to give blue fluorescence turn-on (λ_em ≈ 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 (λ_em ≈ 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 (λ_em ≈ 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.