Hydrogen bonding effects on the fluorescence properties of 4′-diethylamino-3-hydroxyflavone in water and water-acetone mixtures

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 203; pp. 96 - 105
• Main Authors: Hessz, Dóra, Bojtár, Márton, Mester, Dávid, Szakács, Zoltán, Bitter, István, Kállay, Mihály, Kubinyi, Miklós
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05-10-2018
• Subjects: 3-Hydroxychromone dye; Excited state intramolecular proton transfer; Fluorescence decay; Implicit-explicit solvent model; Solute-solvent hydrogen bond; TD-DFT; Solute-solvent hydrogen bond; Implicit-explicit solvent model; 3-Hydroxychromone dye; Excited state intramolecular proton transfer; Fluorescence decay; TD-DFT
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2018.05.104

The fluorescence properties of 4′-diethylamino-3-hydroxyflavone (FET), a dye probe sensitive to the polarity as well as the hydrogen bonding ability of its environment, have been studied in acetone-water mixtures by measuring spectra and decay curves over the whole composition range and analyzing the results on the basis of theoretical calculations. In acetone, like in most of organic solvents, the dye showed dual fluorescence, due to an excited state intramolecular proton transfer (ESIPT), in which a quasi-equilibrium between the two excited species, N* and T*, was reached. In acetone-water mixtures with lower molar fractions of water, where the water molecules are largely dispersed, only one type of hydrate could be detected, a complex with 1:1 composition, showing only N* emission, but with a high (0.45) fluorescence quantum yield. At higher water concentrations, the interaction of FET with the hydrogen-bonded water clusters resulted in fluorescence quenching. In neat water the fluorescence quantum yield fell to ~0.001. Theoretical calculations on a FET-acetone complex, a FET-water complex and a FET-water-acetone triple complex (the latter as model for the samples with low water concentrations) concluded that ESIPT was energetically favored in all the models, but the E(N*)-E(T*) energy difference for the water complexes was much lower. The kinetic barrier of ESIPT was found greatly higher in the FET-water complex than in the isolated solute. The intermolecular hydrogen bonds in the water complexes became significantly stronger following the excitation, stabilizing the N* form of the hydrated dye.
[Display omitted]
•The 3-hydroxyflavone dye FET has two emissive forms in acetone, N* and T*, due to excited state proton transfer (ESIPT).•FET and water form a hydrogen bonded 1:1 complex in acetone, which shows only N* emission.•Theoretical calculations confirmed that ESIPT is less favored in the FET-water complex than in the free dye.•The interaction of FET with water clusters results in a strong fluorescence quenching.•In neat water the fluorescence quantum yield of FET falls to ~0.001.