We present results of single and three pulse photoselection studies of the dynamics of a fluorescent probe (Oxazine 4) in the nematic phase of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The combination of these techniques with time resolved fluorescence anisotropy measurements allows the unambiguous determination of the full angular motion of the probe together with the ground and excited state degrees of equilibrium (steady state) alignment. The restricted JHRPHWU\ LPSRVHG E\ WKH QHPDWLF KRVW LV VHHQ WR LPSDUW D PDUNHG DQLVRWURS\ LQ DQG φ GLIIXVLRQ ! ~4 ).
Stimulated emission depletion (STED) population and polarisation dynamics are used to determine 40 α the degree of hexadecapole alignment created in ensembles of rhodamine 6G molecules in solution following two-photon excitation. Hexadecapole molecular alignment is an unavoidable consequence of two-photon excitation but is not observed in spontaneous emission. For a single element diagonal transition tensor measurements of the fluorescence anisotropy R(t) in systems undergoing small step isotropic rotational diffusion can in principle be used to determine 40 α . STED measurements of rhodamine 6G yield a value for 40 α that is larger than that predicted for a single element transition tensor (S XX ). Recent work in our laboratory indicates that whilst S XX is dominant S YY , S XY and S YX are finite, measurements of 40 α appear to be a sensitive probe of the structure of the two-photon transition tensor. STED and fluorescence anisotropy measurements are extended to Rhodamine 6G in the isotropic phase of 5CB a system where small step isotropic rotational relaxation does not take place.Here the values of 40 α are considerably larger. These results are discussed in terms of the initial hexadecapole alignment and 40 α relaxation dynamics in a restricted geometry.
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.
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