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Picosecond polarised fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6- and 7-cyanobiphenyl

Monge, EM; Bryant, J; Obradovic, B; Harsono, A; Bain, AJ; (2003) Picosecond polarised fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6- and 7-cyanobiphenyl. In: Khoo, IC, (ed.) Liquid Crystals VII. Society of Photo-Optical Instrumentation Engineers (SPIE): San Diego, CA, USA. Green open access

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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 polarisation angle beta 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 theta and phi motions yielding two characteristic relaxation times: tau(20) (pure theta-diffusion) and tau(22) (predominantly phi-diffusion). Analysis of the fluorescence intensity decays for excitation polarisation angles of beta=0degrees and beta=54.7degrees 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 theta diffusion shows a characteristic Arrhenius temperature dependence, the rate of diffusion in the phi coordinate is reduced as the system becomes less ordered. In the isotropic phase over a 50degreesC temperature range above T-NI the fluorescence anisotropy is characterised by two correlation times consistent with restricted rotational diffusion (intra-domain relaxation tau(f)) within a slowly relaxing (pseudo-domain) structure (tau(s)). The temperature dependence of tau(f) and tau(s) was in good agreement with recent theoretical models for intra- and inter-domain relaxation.

Type: Proceedings paper
Title: Picosecond polarised fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6- and 7-cyanobiphenyl
Event: Liquid Crystals VII
Location: San Diego, CA
Dates: 05 August 2003 - 07 August 2003
ISBN: 0819450863
Open access status: An open access version is available from UCL Discovery
DOI: 10.1117/12.505794
Publisher version: http://dx.doi.org/10.1117/12.505794
Language: English
Additional information: Copyright © 2003 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
Keywords: picosecond fluorescence polarisation, dye probe, liquid crystal, nematic phase, isotropic phase, orientational relaxation, order parameter, phase transition, oxazine 4, anisotropic fluid media, liquid-crystals, dynamics, depolarization, diffusion, probes, single, decay, time
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/30847
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