Effect of solvent polarity on the photophysical properties of coumarin-1 dye

Effects of solvent polarity on the photophysical properties of coumarin-1 (C1; $\text{7-}{\mathrm{NEt}}_2\text{-4-}{\mathrm{CH}}_3\text{-1,2-}\mathrm{benzopyrone})$ dye have been investigated using steady-state and time-resolved fluorescence measurements. In moderate to higher polarity solvents the properties like Stokes’ shifts $\mathrm{(\Delta \nu ̄),}$ fluorescence quantum yields ${\mathrm{(\Phi }}_f\mathrm{),}$ fluorescence lifetimes ${\mathrm{(\tau }}_f\mathrm{),}$ radiative rate constants ${\mathrm{(k}}_f\mathrm{),}$ and nonradiative rate constants ${\mathrm{(k}}_{\mathrm{nr}})$ follow more or less linear correlation with the solvent polarity function ${\text{Δf\hspace{0.17em}[={(D−1)/(2D+1)}−{(n}}^2{\text{−1)/(2n}}^2\text{+1)}].}$ In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 3-methylpentane, and decalin, however, all the above-mentioned properties show unusual deviation in comparison to the trend observed in moderate to higher polarity solvents. Thus, the $\mathrm{\Delta \nu ̄}$ and $k_{\mathrm{nr}}$ values are unusually lower and the ${\Phi }_f,$ ${\tau }_f,$ and $k_f$ values are unusually higher in nonpolar solvents. Comparing the results of C1 with those of its lower analogue, namely, coumarin-120 (C120; $\text{7-}{\mathrm{NH}}_2\text{-4-}{\mathrm{CH}}_3\text{-1,2-}\mathrm{benzopyrone}\mathrm{),}$ it has been inferred that like C120, the dye C1 also exists in different structural form in nonpolar solvents than in other solvents of moderate to higher polarities. It is proposed that in nonpolar solvents the dye exists in a nonpolar structure, where the $\text{7-}{\mathrm{NEt}}_2$ group of the dye adopts a pyramidal configuration and consequently out of plane of the 1,2-benzopyrone moiety. In moderate to higher polarity solvents, the dye is supposed to exist in a polar intramolecular charge transfer structure with its $\text{7-}{\mathrm{NEt}}_2$ group in resonance with the 1,2-benzopyrone moiety. The observed differences in some of the properties of C1 and C120 dyes in nonpolar solvents have been rationalized based on the relative rates of the flip-flop motions of the 7-amino groups of the two dyes. It is understood that where a very fast flip-flop motion of the $\text{7-}{\mathrm{NH}}_2$ group in C120 introduces an activation-controlled nonradiative deexcitation channel for the dye excited state in nonpolar solvents, the very slow flip-flop motion of the $\text{7-}{\mathrm{NEt}}_2$ group in C1 cannot introduce any such nonradiative deexcitation channel for the dye.