Investigations of the solvent polarity effect on the photophysical properties of coumarin-7 dye.

Photophysical investigations of coumarin-7 (C7) dye in different solvents using absorption, steady-state fluorescence and time-resolved fluorescence measurements reveal the behavioral changes of the dye in nonpolar and other solvents. In moderate to higher polarity solvents, the experimental parameters such as fluorescence quantum yield (Phif), fluorescence lifetime (tauf), radiative rate constant (k(f)), nonradiative rate constant (k(nr)) and Stokes' shift (Deltav) follow almost linear correlations with the Lippert-Mataga solvent polarity parameter Deltaf but show unusual deviations in nonpolar solvents. From the observed results, it is inferred that the dye exists in a planar intramolecular charge transfer structure in moderate to higher polarity solvents, but in nonpolar solvents, the dye exists in a nonplanar structure with its 7-NEt2 group adopting a pyramidal type of configuration.

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-NEt(2) 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-NEt(2) 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.