Solvation dynamics of Coumarin 503 in the liquid-crystal mixture ZLI 1167

The solvation dynamics of the dye Coumarin 503 in the nematic mixture ZLI 1167 has been investigated by means of time-resolved fluorescence spectroscopy, both in the nematic and isotropic phases of the sample. No alignment was imposed on the micro-domains of the nematic phase, so that a complete depolarization of the emitted light was obtained. For the isotropic phase the usual set-up with vertically polarized excitation light and magic-angle detection was used. A time-dependent frequency shift of the maximum of the fluorescence band, caused by solvent reorganization after pulse excitation of the fluorescent probe, was observed. The analysis of the transient Stokes shift correlation function clearly shows biexponential behavior in the nematic phase, the slowest time constant varying from 1670 ps at 311.5 K to 230 ps at 373 K. The decay of the correlation function appears to be largely unaffected by the nematic-isotropic transition, suggesting that the local environment, rather than long-range ordering, determines Stokes shift dynamics. A theoretical model, which takes into account probe reorientations in the presence of a nematic field, and fluctuation of the local solvent polarization, has been developed to interpret the experimental findings.