Single- and two-photon time-resolved polarized fluorescence studies of probe molecule dynamics in nematic liquid crystals

Abstract

We present the results of combined single and two photon linearly polarised time resolved fluorescence anisotropy measurements of the order and motion of a fluorescent probe (rhodamine 6G) in the nematic phase of 4-n-pentyl4'-cyanobiphenyl (5CB). Variation of the excitation polarisation angle (β) with respect to the nematic director yield a set of initial single and two photon anisotropies R(0,β). Single photon R(0,β) measurements yield the and moments of the ground state orientational distribution function. For rhodamine 6G in 5CB these indicate that the inclusion of higher moments ( and above) are necessary to describe the probe ordering within the nematic host.. Two photon R(0,β) measurements however allow the direct measurement of , for rhodamine 6G these yield a value close to theoretical predictions. Two and single photon initial anisotropy measurements are wholly consistent with an approximately Gaussian probe distribution at an angle of 38° to the nematic director with a full width half maximum of c.a. 26°. Variation of β affords the photoselection of both cylindrically symmetric and asymmetric degrees of probe alignment that are sensitive respectively to θ and θ plus φ diffusion in the laboratory (nematic director) frame. Cylindrically symmetric and asymmetric alignment relaxation are observed to be linear but with distinctly different relaxation rates, indicating highly restricted probe motion within the nematic environment.