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 excita tion light and magic-angle detection was used. A time-dependent freque ncy shift of the maximum of the fluorescence band, caused by solvent r eorganization after pulse excitation of the fluorescent probe, was obs erved. The analysis of the transient Stokes shift correlation function clearly shows biexponential behavior in the nematic phase, the slowes t time constant varying from 1670 ps at 311.5 K to 230 ps at 373 K. Th e decay of the correlation function appears to be largely unaffected b y the nematic-isotropic transition, suggesting that the local environm ent, rather than long-range ordering, determines Stokes shift dynamics . A theoretical model, which takes into account probe reorientations i n the presence of a nematic held, and fluctuation of the local solvent polarization, has been developed to interpret the experimental findin gs.