Dynamics of Intermolecular Hydrogen Bonds in the Excited States of 4‘-Dialkylamino-3-hydroxyflavones. On the Pathway to an Ideal Fluorescent Hydrogen Bonding Sensor

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 108; no. 40; pp. 8151 - 8159
• Main Authors: Shynkar, Vasyl V, Klymchenko, Andrey S, Piémont, Etienne, Demchenko, Alexander P, Mély, Yves
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07.10.2004
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp047990l

The spectroscopic behavior of the 4‘-dialkylamino-3-hydroxyflavones in protic environments is very unusual. Previous studies showed that in contrast to other solvatochromic dyes containing carbonyl group (coumarins, Nile Red, PRODAN, etc.), their Stokes shift does not increase on the formation of intermolecular H-bonds with protic solvents. The present steady-state and time-resolved studies show that the ground-state equilibrium between the H-bonded and non-H-bonded forms of this derivative in mixed solvents is not changed significantly when the dye is excited to the normal (N*) excited state. New H-bonds do not form, but those already existing in the ground state can disrupt on a slow time scale. This last process is probably coupled with the slow excited-state intramolecular proton transfer (ESIPT) reaction of the H-bonded form of the dye. Therefore, the fluorescence spectra of the dye provide a measure of the ground state distribution between its H-bonded and non-H-bonded forms, which in turn reflects the H-bonding potential of the environment. Due to this feature, this dye can serve not only as a calibrator of solvent properties but also as a unique sensor of H-bonding potential in unknown media. This sensing can be provided by the relative intensities of the two separated emission bands in the fluorescence spectra.