| Autor: |
Angus J. Bain, Andrian Harsono, Eugenio M. Monge, Bojan Obradovic, Jason Bryant |
| Rok vydání: |
2003 |
| Předmět: |
|
| Zdroj: |
Liquid Crystals VII. |
| ISSN: |
0277-786X |
| Popis: |
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: τ 20 (pure θ-diffusion) and τ 22 (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 (T NI ) 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 T NI the fluorescence anisotropy is characterised by two correlation times consistent with restricted rotational diffusion (intra-domain relaxation τ f ) within a slowly relaxing (pseudo-domain) structure (τ s ). The temperature dependence of τ f and τ s was in good agreement with recent theoretical models for intra- and inter-domain relaxation. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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