5 January 2004 Picosecond polarized fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6-, and 7-cyanobiphenyl
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Picosecond fluorescence anisotropy and lifetime measurements are used to investigate the orientational dynamics and steady state order of the fluorescent probe oxazine 4 in the nematic and isotropic phases of 5, 6 and 7 cyanobiphenyl. Variation of the excitation polarization angle β with respect to the nematic director allows the preparation of both cylindrically symmetric and asymmetrically aligned probe distributions whose relaxation dynamics are sensitive to both θ and φ motions yielding two characteristic relaxation times: τ20 (pure θ-diffusion) and τ22 (predominantly φ-diffusion). Analysis of the fluorescence intensity decays for excitation polarisation angles of β=0° and β=54.7° allows a determination of the effect of local field and differential reflection losses without the measurement of sample refractive indices. A striking feature of oxazine 4 dynamics in the approach to the nematic-isotropic phase transition temperature (TNI) is that whilst θ diffusion shows a characteristic Arrhenius temperature dependence, the rate of diffusion in the φ coordinate is reduced as the system becomes less ordered. In the isotropic phase over a 50°C temperature range above TNI the fluorescence anisotropy is characterised by two correlation times consistent with restricted rotational diffusion (intra-domain relaxation τf) within a slowly relaxing (pseudo-domain) structure (τs). The temperature dependence of τf and τs was in good agreement with recent theoretical models for intra- and inter-domain relaxation.
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Eugenio M. Monge, Jason Bryant, Bojan Obradovic, Andrian Harsono, Angus J. Bain, "Picosecond polarized fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6-, and 7-cyanobiphenyl", Proc. SPIE 5213, Liquid Crystals VII, (5 January 2004); doi: 10.1117/12.505794; https://doi.org/10.1117/12.505794

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