We investigated fluorescence from hemicyanine dye molecules in a liquid crystal (4,4′-n-pentylcyanobiphenyl) (5CB) medium at different temperatures. The fluorescence decay lifetime decreased monotonically irrespective of the thermodynamic phases of the host medium as the temperature was increased. This behavior is due to an intramolecular motion of the dye promoted with the decrease in the viscosity of the medium facilitating a nonradiative decay of the excited dye molecules. By contrast, fluorescence intensity from the dyes in the nematic phase was about 3 times stronger than that in the crystalline or isotropic phase. This fluorescence enhancement in the nematic phase was found to be due to an anisotropic alignment of the dye molecules following the anisotropic alignment of the host liquid crystal medium along the pump-beam polarization direction. This light-induced liquid crystal molecular alignment was markedly enhanced by the guest dyes preferentially excited along the pump-beam polarization direction. The orientational order parameter of the dyes in the liquid-crystalline phase deduced from fluorescence anisotropy measurement was similar to the known order parameter of the liquid crystalline 5CB.

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As a reference sample, fluorescence anisotropy was measured from a system in which dyes are completely immobile—hemicyanine molecules incorporated into narrow zeolite pores. This sample gave a fluorescence anisotropy value of 2.9, ensuring that our experimental setup is acceptable. On the contrary, the fluorescence anisotropy from hemicyanine in glycerol was reduced to 2.7. This reduction came from the rotational motion of the dye molecule in glycerol within the fluorescence decay lifetime of the dye. The reduced anisotropy for HC in isotropic phase of 5CB (Imax/Imin ∼ 2.6) is considered to also come from the rotation of the dye molecules. Contrarily, the fluorescence anisotropy from hemicyanine/5CB in the 1-cm thick cell was reduced to 2.2 and 5.5 for the crystalline and nematic phases, respectively. The fluorescence anisotropy in the transparent isotropic phase was exactly the same between the 1-cm and 100 μm cells.
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