Photophysical properties of coumarin-152 and coumarin-481 dyes: Unusual behavior in nonpolar and in higher polarity solvents

Solvent polarity and temperature effect on the photophysical properties of two 1,2-benzopyrone dyes, namely, coumarin-152 (C152) and coumarin-481 (C481), have been investigated using steady-state and time-resolved fluorescence measurements. In nonpolar (NP) solvents (e.g., hexane, cyclohexane, methylcyclohexane, 2-methylpentane, and 3-methylpentane), the Stokes' shifts (Delta(v) over bar) and fluorescence lifetimes (tau(f)) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Delta(v) over bar correlates linearly with the solvent polarity function, Deltaf = {(is an element of - 1)/(2is an element of + 1) - (n(2) - 1)/(2n(2) + 1)}, and the slopes of these plots indicate that the fluorescent states for both the dyes in these solvents are of intramolecular charge transfer (ICT) character. The unusually lower Delta(v) over bar and tf values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields ((Df) and tau(f) values of the two dyes also show an unusual reduction in higher polarity solvents (Deltaf > similar to0.2). Furthermore, unlike in all other solvents, in solvents with Deltaf > similar to0.2, the Phi(f) and tau(f) values are strongly temperature-dependent. These results indicated the involvement of a new activation-controlled nonradiative deexcitation channel in higher polarity solvents, assigned to the involvement of the nonfluorescent twisted intramolecular charge transfer (TICT) state. Though the activation barrier (DeltaE(a)) for such processes usually decreases with solvent polarity, for the present systems the DeltaE(a) is seen to increase with Deltaf. This unusual behavior has been rationalized assuming that the DeltaE(a) arises because of the potential energy (PE) crossing of the TICT and ground states rather than that of the ICT and TICT states as is usually the case for most TICT molecules. On increasing Deltaf, since the highly polar TICT state not only gets better stabilization but also its PE surface becomes more steeper than the ICT state, the DeltaE(a) effectively increases with solvent polarity.