Photophysical studies of triarylamine dyes and an investigation into polyelectrolyte-DNA interactions.

The photophysics and thermal properties of a series of seven novel triarylamine (TAA) dyes are described. Fluorescence characteristics have been studied in solvents of various polarities at room temperature and at 77 K. High molar extinction coefficients of the magnitude of 3.0-4.0(+/-0.50)x104 M-1cm-1 were obtained for most compounds, and relatively short radiative lifetimes were observed. Fluorescence quantum yields of the dyes at room temperature in cyclohexane were found to be between 0.34-0.57 increasing to 0.67-0.95 at 77 K. It has been shown that while at room temperature, solvent shell relaxation around the excited state can occur, and emission is from an equilibrium excited state to a twisted ground state, at 77 K in a rigid matrix environment solvent shell relaxation cannot occur and emission is from a Franck-Condon excited state to a planar ground state. The TAAs studied have excellent thermal properties for possible use in devices with thermal decomposition temperatures of greater than 300 °C, they also do not crystallise readily. Three poly (9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene), fluorescent cationic conjugated polymers (CCP), of average chain lengths- 6, 12 and 100 repeat units, and their interaction with DNA and guanine are reported. Fluorescence microscopy and atomic force microscopy have been utilised to visualise the interaction between the polymers and DNA. Results show both efficient compaction of DNA induced by the polymer and linking and bridging of DNA/polymer aggregates. CCPs are known to aggregate in water, and for the CCPs studied here this is reflected in a decrease in fluorescence. These aggregates can be broken up by mixtures of solvents, e.g. acetonitrile/water. Steady state and ps time resolved techniques have been used to study: (i) aggregation of CCPs in various acetonitrile/water mixtures, and (ii) fluorescence quenching by single and double strand DNA, and guanine. All CCPs are extremely sensitive to quenching by DNA or guanine, with sensitivity increasing with chain length of both the CCP and DNA. Stem-Volmer plots are sigmoidal and have initial quenching rates constants far in excess of the diffusion controlled limit. The results have been analysed in terms of energy migration and trapping within and between polymer chains. Quenching seems best analysed in terms of an equilibrium in which a CCP/DNA aggregate complex is formed which brings polymer chains into close enough proximity to allow interchain excitation energy migration and quenching at aggregate or DNA base traps. We also report preliminary results of modelling time resolved data, of both the aggregation and quenching, using a kinetic model in which energy migration and trapping are represented as a series of energy transfer steps between neighbours