Temperature dependence of solvation dynamics in alkylcyanobiphenyls

Earlier investigations of the solvation dynamics of the cationic dye rhodam ine 700 in the isotropic phase of the nematogenic substance: octylcyanobiph enyl (Rau, J., Ferrante, C., Deeg, F. W., Brauchle, C. J. Phys. Chem. B 199 9, 103, 931) have shown that in this liquid two different contributions to the solvation dynamics exist. The slower one can-be explained in terms of p olar solvation, while the second one Is not yet fully understood. The latte r one is temperature independent from the nematic-isotropic phase transitio n temperature. up to 20 K above the transition temperature. We present in t his paper two different approaches to a better understanding of the solvati on dynamics of the alkylcyanobiphenyls. First, it was investigated if a dif ferent member of the family of the alkylcyanobiphenyls, butylcyanobiphenyl, exhibits similar dynamic behavior. To check the possible influence of the probe chromophore, the neutral dye coumarin 153 was dissolved in octylcyano biphenyl for a second set of measurements. The time-resolved dynamic Stokes shift S(t) was measured above the nematic-isotropic phase transition. Buty lcyanobiphenyl with the probe chromophore rhodamine 700 -and octylcyanobiph enyl with coumarin 153 show qualitatively the same contributions as found i n the solution of rhodamine 700 in octylcyanobiphenyl. From this, we conclu de that the peculiar dynamics observed is characteristic for the whole fami ly of the alkylcyanobiphenyls and not only for a special system of probe mo lecule and Liquid. In addition to the solvation dynamics the temperature de pendence of the rotational relaxation times of the dye molecules was record ed.