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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Proceedings Volume 5213, Liquid Crystals VII; (2004); doi: 10.1117/12.505794
Event: Optical Science and Technology, SPIE's 48th Annual Meeting, 2003, San Diego, California, United States
Abstract
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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KEYWORDS
Diffusion

Fluorescence anisotropy

Luminescence

Anisotropy

Polarization

Liquid crystals

Temperature metrology

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