Effect of protic solvents on twisted intramolecular charge transfer state formation in coumarin-152 and coumarin-481 dyes

Photophysical properties of coumarin-152 and coumarin-481 dyes in different protic solvents indicate that solute–solvent H-bonding interaction stabilizes the twisted intramolecular charge transfer (TICT) states of these dyes. Accordingly, the onset of TICT mediated nonradiative deexcitation channel in these dyes occurs at lower solvent polarity in protic solvents (Δf ∼ 0.18) than in aprotic solvents (Δf ∼ 0.2). The interesting and unusual observation that in lower polarity solvents (Δf < ∼0.17) the fluorescence lifetime of these dyes increases with temperature is interpreted considering an equilibrium between the free and the solute–solvent H-bonding complexes in protic solvents.