Temperature dependence of the solvation dynamics in alkylcyanobiphenyls

Earlier investigations of the solvation dynamics of the cationic dye rhodamine 700 in the isotropic phase of the nematogenic substance: octylcyanobiphenyl (Rau, J., Ferrante, C., Deeg, F. W., Brauchle, C. J. Phys. Chem. B 1999, 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 polar solvation, while the second one Is not yet fully understood. The latter one is temperature independent from the nematic-isotropic phase transition temperature. up to 20 K above the transition temperature. We present in this paper two different approaches to a better understanding of the solvation dynamics of the alkylcyanobiphenyls. First, it was investigated if a different 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 octylcyanobiphenyl for a second set of measurements. The time-resolved dynamic Stokes shift S(t) was measured above the nematic-isotropic phase transition. Butylcyanobiphenyl with the probe chromophore rhodamine 700 -and octylcyanobiphenyl with coumarin 153 show qualitatively the same contributions as found in the solution of rhodamine 700 in octylcyanobiphenyl. From this, we conclude that the peculiar dynamics observed is characteristic for the whole family of the alkylcyanobiphenyls and not only for a special system of probe molecule and Liquid. In addition to the solvation dynamics the temperature dependence of the rotational relaxation times of the dye molecules was recorded.