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 (?f ~ 0.18) than in aprotic solvents (?f~ 0.2). The interesting
and unusual observation that in lower polarity solvents (?f< ~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.