Solvation dynamics of Rhodamine 700 in the nematogenic liquid octylcyanobiphenyl

An investigation of the solvation dynamics of rhodamine 700 in the isotropic phase of the nematogenic substance octylcyanobiphenyl is presented. The time-resolved dynamic Stokes shift S(t) was measured above the nematic-isotropic phase transition and at two temperatures below it. At all temperatures S(t) is characterized by a fast and a slow component on a 10 and 100 ps time scale, respectively. The slow component shows an exponentially activated temperature dependence analogous to the one observed in polar liquids. The fast component exhibits a distinctly different temperature dependence. It is temperature independent from the nematic-isotropic phase transition temperature to 20 K above the transition temperature. Only at higher temperatures does this decay becomes faster. These fast dynamics are associated with the specific anisotropic potential in the locally ordered nematogenic liquid. Additional measurements of the reorientational dynamics of the chromophore are compared with the solvation dynamics investigations.