Fluorescence enhancement of dye-doped liquid crystal by dye-induced alignment effect

Abstract

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.