Solvent effects on the ESIPT emission of salicylaldehyde Schiff base derivative: A theoretical reconsideration
Journal of Molecular Liquids(2024)
摘要
Salicylaldehyde Schiff base derivatives have potential applications in bioimaging, chemical sensors and organic luminescence materials because of their high fluorescence quantum yield with the excited-state intramolecular proton transfer (ESIPT) process. Recently, Shu et al. studied the luminescent mechanism of the Et2N-substituted salicylaldehyde Schiff base compound (DDHAC) in solvents [Dyes and Pigments 195 (2021) 109708]. It showed double-peaked bands in an n-hexane solvent but a single-peaked band in an acetonitrile solvent. They suggested that the open-enol in the acetonitrile solvent was the dominant species. They considered that open-enol fluorescence and ESIPT fluorescence constituted a dual fluorescent phenomenon in the n-hexane solvent. However, because acetonitrile is a non-protonic solvent, DDHAC molecules in acetonitrile mainly existed as the lower-energy hydrogen-bonded enol form rather than the open-enol form alone. In addition, the open-enol did not undergo the ESIPT process in n-hexane. Therefore, the luminescence mechanism of the DDHAC molecules in n-hexane and acetonitrile solvents needs to be reconsidered. In this study, we investigated the DDHAC molecules in n-hexane and acetonitrile solvents using the density functional theory and time-dependent density functional theory. The potential energy surfaces and optimisation of structures elucidated that the DDHAC molecules in n-hexane and acetonitrile solvents underwent the ESIPT process. The calculated fluorescence peaks demonstrated that the single-peaked broad emission band in the acetonitrile solvent was formed by the combination of the hydrogen-bonded enol* and keto* forms rather than open-enol*. Moreover, the dual fluorescence peaks in the n-hexane solvent were reattributed to the open-enol* form and hydrogen-bonded enol* form. The lack of keto* fluorescence in the n-hexane solvent was attributed to diminished charge coupling in comparison to the enol* form. Our results revise the mechanism of the DDHAC molecules in n-hexane and acetonitrile solvents, providing guidance for designing efficient organic fluorescence probes.
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关键词
Salicylaldehyde Schiff base compound,ESIPT,Luminescence mechanism,Open-enol
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