Photophysical investigations of coumarin-7 (C7) dye in different solvents using absorption, steady-state fluorescence and time-resolved fluorescence measurements reveal the behavioral changes of the dye in nonpolar and other solvents. In moderate to higher polarity solvents, the experimental parameters such as fluorescence quantum yield (Φf), fluorescence lifetime (τf), radiative rate constant (kf), nonradiative rate constant (knr) and Stokes' shift (Δν̄) follow almost linear correlations with the Lippert–Mataga solvent polarity parameter Δf but show unusual deviations in nonpolar solvents. From the observed results, it is inferred that the dye exists in a planar intramolecular charge transfer structure in moderate to higher polarity solvents, but in nonpolar solvents, the dye exists in a nonplanar structure with its 7-NEt2 group adopting a pyramidal type of configuration. Unlike some of the other coumarin dyes, namely coumarin-120 (C120) (4-CH3-7-NH2-1,2-benzopyrone) and coumarin-151 (C151) (4-CF3-7-NH2-1,2-benzopyrone), which also show similar structural changes in nonpolar and other solvents, the C7 dye does not show any activation-controlled deexcitation process in nonpolar solvents. This is attributed to the very slow flip–flop motion of the 7-NEt2 group of the C7 dye in comparison with the very fast flip–flop motion of the 7-NH2 group in the C120 and C151 dyes. Qualitative potential energy diagrams are presented to rationalize the observed results of C7 dye and to compare these with those of the other dyes such as C120 and C151. A support for the observed results and interpretation has also been obtained from quantum chemical calculations on the structures of the C7 dye.
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Vol. 81 • No. 2