H-bond sensing with 3-hydroxyflavones: steady-state and time-resolved fluorescence studies

The 4'-dialkylamino derivatives of 3-hydroxyflavone find many applications as molecular probes, since their two-band fluorescence spectra produce a strong response to different intermolecular interactions including H-bonding. The results of our steady-state and time-resolved studies in neat and mixed solvents reveal an important and probably unique property of these dyes: their ground-state equilibrium between H-bonded and non H-bonded forms is not changed significantly on excitation to the normal (N*) excited state. In the 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. These dyes do not change significantly the distribution between H-bonded and non H-bonded species in their environment and therefore they can provide a measure of the H-bonding potential of their environment. Due to this feature, they can serve as unique sensors of the H-bonding potential in unknown media. This sensing can be provided by the dramatic change of the relative intensities of their two separated emission bands.

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