Single and two-photon fluorescence studies of linear and non-linear optical chromophores.

The subject matter presented in this thesis concerns the structural and dynamic studies of new fluorescent probe molecules and the application of polarised fluorescence techniques and analysis to molecular motion, order and solvation in a highly ordered environment. Chapter 1 reviews recent group research providing a context to the work in this thesis, whilst Chapters 2 and 3 concern the study of fluorescent probe dynamics. Time resolved photoselection techniques were used to probe the order and full angular motion of Coumarin 6 and Coumarin 153 in the nematic and isotropic phases of the liquid crystal 5CB. The uptake of coumarin molecules into this host differs from previously studied (Xanthene) probes - in particular, Coumarin 6 is seen to adopt a disruptive position within the alkyl tails due to its size and hydrophobic nature this is discussed in Chapter 2. Furthermore, Coumarin 153 undergoes a substantial increase in dipole moment upon electronic excitation this led to a unique study of time dependent solvation dynamics in both a globally and locally structured environment. The presence of strong solvent- solute interactions necessitated the development of a new approach to the analysis of time resolved polarised fluorescence in ordered systems. This approach and the study of time dependent solvation dynamics in the isotropic and nematic phases of 5CB is presented in Chapter 3. Structural studies of new two-photon fluorescent probes in collaboration with CNRS Rennes and Los Alamos are described in the final two chapters. Large two-photon resonances in the green-visible were observed together with a fuller characterisation of those in the near IR. Polarised two-photon absorption and anisotropy measurements were used to examine the structure of the two-photon resonances. Finally, the stimulated emission depletion dynamics of a branched two-photon fluorophore were investigated and found to differ markedly from conventional (non-degenerate) fluorophores