Photophysical properties of 7-diethylaminocoumarin dyes in dioxane–water mixtures: hydrogen bonding, dielectric enrichment and polarity effects

The effect of water on the photophysical properties of four 7-diethylaminocoumarin dyes (AC 1, BC I, BC II and C 35) in dioxane–water solvent mixtures is studied. Dielectric enrichment of the dyes is not observed in the binary mixtures. Rate constants for bimolecular quenching are calculated. BC I and C 35 are found to undergo both static and dynamic quenching while only static quenching is observed in the case of BC II. The association constant is however found to be very small (Kb=2–5×10-2 M-1). No such dependence is observed in AC 1 where the formation of an intramolecular hydrogen bond is strongly favored. The increase in the non-radiative rate constant with the water concentration observed in the other three dyes (BC I, BC II and C 35) can be correlated to the solvent polarity and hydrogen bonding parameter ET(30). A threshold value of the solvent polarity–polarizability parameter (Pekar factor C) for the non-radiative process is observed for all these three dyes and it is nearly independent of the nature of substitution. The photophysical properties of AC 1 can be adequately explained by considering only one singlet excited state while an additional singlet excited state needs to be included to understand the photophysics of BC I, BC II and C 35 in highly polar environments.