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 are necessarily equal. However, in 5CB, this was seen to correlate strongly with equilibrium order parameters, indicating the need to quantify fully the probe orientational distribution function. In Chapter 3, single and two-photon time resolved fluorescence anisotropy measurements are used to determine the hitherto unmeasured (ground state) K=6 moment of P. The final chapters concern the development of the next generation of photoselection techniques where an excited state population is engineered by stimulated emission depletion (STED). In Chapter 4 STED following two-photon excitation (PUMP) has been demonstrated in a range of well established single photon probes and two recently developed quadrupolar chromophores. Saturation of the DUMP transition due to excited state re-pumping was investigated, and from a two level rate equation model stimulated emission cross-sections and ground state vibrational relaxation times were determined. In Chapter 5 the polarisation properties of STED are investigated; orientation-selective depletion was shown to yield highly polarised excited state arrays. Polarisation resolved STED measurements were shown to circumvent spontaneous emission selection rules (which forbid the measurement of excited state moments of rank K > 2) allowing the first measurements of hexadecapole (K=4) orientational relaxation in two fluorescent probes.