Time Resolved Excited State Photo-Engineering

This thesis concerns the development and application of time resolved fluorescence which yield new and detailed information on molecular order and motion in both isotropic and ordered systems. Polarised fluorescence studies involve the photoselection of a non-equilibrium distribution of excited state probe orientations whose relaxation is monitored by time resolved fluorescence anisotropy. Chapter 1 introduces the basic concepts of probe photophysics and orientational photoselection. Chapter 2 combines photoselection with single and multiple laser pulses to provide a full description of probe alignment dynamics in the nematic phase of 5-cyanobiphenyl (5CB). In isotropic media the cylindrically symmetric and asymmetric alignment relaxation times (T20 and T22) are necessarily equal. However, in 5CB, T20 > T22; this was seen to correlate strongly with equilibrium order parameters, indicating the need to quantify fully the probe orientational distribution function P(0,2) allowing the first measurements of hexadecapole (K=4) orientational relaxation in two fluorescent probes.