Photophysical Properties of Coumarin-500 (C500): Unusual Behavior in Nonpolar Solvents

Photophysical properties of 7-NHEt-4-CF 3 -1,2-benzopyrone dye (coumarin-500, C500), have been investigated in different solvents and solvent mixtures using optical absorption and fluorescence measurements. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 2-methylpentane, 3-methylpentane, and Decalin, the dye displays unusual properties in comparison to those in other solvents. Thus, in the former solvents, the Stokes' shifts Δν̄ and the fluorescence lifetimes (τ f ) for C500 are found to be unusually low. Though in these solvents the fluorescence quantum yields Φ f are seen to be only marginally higher, the radiative decay rate constants k f are found to be unusually higher than those in the other solvents. From comparing the results of C500 with those reported earlier for its lower analogue, C151 (coumarin-151, 7-NH 2 -4-CF 3 -1,2-benzopyrone; Nad, S.; Pal, H. J. Phys. Chem. A. 2001, 105, 1097), it is inferred that just like C151, in nonpolar solvents the dye C500 also exists in nonpolar structures, whereas in all other solvents the dye exists in a polar intramolecular charge transfer (ICT) structure. Unlike C151, the C500, however, does not show any activation-controlled nonradiative deexcitation channel for the S 1 state in nonpolar solvents, which arises due to the flip-flop motion of the 7-amino group. Due to the heavier mass, the flip-flop motion of the 7-NHEt group in C500 is supposed to be much slower than that of the 7-NH 2 group in C151. Thus, such a motion cannot introduce any deexcitation channel, which can compete with the intrinsic fluorescence k f and the internal conversion k IC rates in the excited C500 dye. Thus, unlike C151, there is no activation-controlled nonradiative deexcitation channel for C500 in nonpolar solvents though both the dyes appear to exist in similar structural forms in these solvents, which are different than those existing in other solvents of lower to higher polarities.