Monge, Eugenio Martin; (2003) Single and two photon fluorescence studies of ordered molecular systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Time resolved polarised fluorescence of molecular probes is used to study motion and order in the nematic and isotropic phases of cyanobiphenyl liquid crystals. Linearly polarised pulses with a variable polarisation angle create both cylindrically symmetric and asymmetric degrees of excited state alignment sensitive to θ and ϕ diffusion. Three beam and single photon photoselection techniques are combined for the first time in a study of highly aligned (oxazine) probe dynamics in 5CB, confirming previous symmetry-based predictions for alignment relaxation in aligned media to be correct. Single beam photoselection techniques are substantially refined and new measurements involving the analysis of intensity as well as anisotropy decays are developed. This approach yields information on optical correction factors present in highly anisotropic systems without recourse to either approximation or separate experimental determination. In addition a separate path to the determination of the cylindrically asymmetric alignment relaxation time was also developed. These developments permitted an extensive and precise study of probe dynamics in 6 and 7-cyanobiphenyl to be undertaken. A combination of single and two photon photoselection techniques was used to determine previously 'hidden' (K=6) moments of the ground state probe distribution function. These experiments provided the first rigorous validation of well established theory by which 'hidden' moments of the ground state distribution and hence the full distribution function is calculated. Single and two-photon fluorescence anisotropy measurements were also undertaken in the isotropic phase of 5,6 and 7CB confirming earlier observations that a domain structure persists up to 50°C above the nematic-isotropic phase transition. The temperature dependence of both the inter and intra-domain probe relaxation times were in good agreement with the predictions of recent theory developed for the dynamics of pure liquid crystals. Finally a combination of polarised photoselection techniques with surface wave excitation was developed and applied to the study of nematic probe dynamics in the vicinity of the cell surface.

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