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

ABSTRACT: A nonfluorescent Schiff base compound (4) in an aprotic solvent (e.g., CH3CN) is found to give blue fluorescence turn-on (?em ? 475 nm) upon addition of H2O. By using a wide range of spectroscopic methods, including (1)H 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 (?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 (?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 Al(3+) cation.

SUBMITTER: McDonald L

PROVIDER: S-EPMC4844177 | biostudies-literature | 2016 Feb

REPOSITORIES: biostudies-literature

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

McDonald Lucas L Wang Junfeng J Alexander Nick N Li Hui H Liu Tianbo T Pang Yi Y

The journal of physical chemistry. B 20160126 4

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 (1)H 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 intra ...[more]

PMID: 26761400

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Project description:In our previous work, we discussed the emergence of the dual fluorescence phenomenon in selected compounds from the group of 1,3,4-thiadiazoles. The results obtained in a number of experimental studies, supported by [TD]DFT calculations, clearly indicated that the phenomenon of dual fluorescence stemmed from an overlap of several factors, including the correct conformation of the analyzed molecule and, very significantly in this context, aggregation effects. Where those two conditions were met, we could observe the phenomenon of intermolecular charge transfer (CT) and the emergence of electronic states responsible for long wave emissions. However, in light of the new studies presented in this paper, we were able, for the first time, to provide a specific theory for the effect of dual fluorescence observed in the analyzed group of 1,3,4-thiadiazoles. We present the results of spectroscopic measurements conducted for two selected analogues from the 1,3,4-thiadiazole group, both in polar and non-polar solvents, which clearly evidence (as we have already suspected in the past, albeit have not shown in publications to date) the possibility of processes related to emission from the tautomer formed in the process of excited state intramolecular proton transfer, which is responsible for the long-wavelength emissions observed in the selected analogues. The presented results obtained with the use of UV-Vis, fluorescence (stationary and time-resolved), FTIR, and Raman spectroscopy, as well as from calculations of dipole moment changes between the ground and excited state with the use of two derivatives with different structures of the resorcylic system, corroborated our standing hypothesis. At the same time, they excluded the presence of ground state keto forms of the analyzed analogues unless necessitated by the structure of the molecule itself. In this case, aggregation factors enhance the observed effects related to the dual fluorescence of the analyzed compounds (by way of AIE-aggregated induced emissions).

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

Publications

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

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