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 (Δν̄) and fluorescence lifetimes (τ f) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Δν̄ correlates linearly with the solvent polarity function, Δ f = (ε − 1) / (2ε + 1) − (n 2 − 1) / (2 n 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 Δν̄ and τ f values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields (Φ f) and τ f values of the two dyes also show an unusual reduction in higher polarity solvents (Δ f > ∼0. 2). Furthermore, unlike in all other solvents, in solvents with Δ f > ∼0. 2, the Φ f and τ 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 (Δ E a) for such processes usually decreases with solvent polarity, for the present systems the Δ E a is seen to increase with Δ f. This unusual behavior has been rationalized assuming that the Δ E 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 Δ f, since the highly polar TICT state not only gets better stabilization but also its PE surface becomes more steeper than the ICT state, the Δ E a effectively increases with solvent polarity.