Photophysical properties of coumarin-30 dye in aprotic and protic solvents of varying polarities

Experimental results on various photophysical properties of coumarin-30 (C30) dye, namely, Stokes' shift (Deltanu), fluorescence quantum yield (Phi(f)), fluorescence lifetime (tau(f)), radiative rate constant (k(f)) and nonradiative rate constant (k(nr)) as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Deltaf = {(epsilon - 1)/(2epsilon + 1) - (n(2) - 1)/ (2n(2) + 1)}, they show unusual deviations in nonpolar solvents at one end and in high-polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: (1) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7-NEt2 substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone pi cloud; (2) in medium to higher polarity solvents, the dye exists in a polar intramolecular charge transfer structure, where the 7-NEt2 group and the 1,2-benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone pi cloud; (3) in protic solvents, the dye-solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and (4) in high-polarity protic solvents, the excited C30 undergoes a new activation-controlled nonradiative deexcitation process because of the involvement of a twisted intramolecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity-dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.