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Synthesis, characterisation and spectroscopic analysis of fluorescent dyes for applications-based chemistry
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posted on 21.02.2017by Higginbotham, Heather Fay
This research project investigates the synthesis and spectroscopic analysis of organic and inorganic fluorescent and luminescent materials for future use in applications-based chemistry. This work specifically focuses upon fluorescent dyes with tunable emission properties designed for the investigation of applications less commonly explored in literature. The use of advanced spectroscopic and single particle fluorescence techniques is also an underlying theme of each chapter, used to elucidate the mechanism of emissive output from synthesised materials.
Initial research focused upon the development water-soluble naphthalene diimides for use in biophysical applications (Chapter 3). Quenching ratios in water were first probed using steady state and time-resolved fluorescence techniques revealing that both static and dynamic quenching mechanisms play a role in visible fluorescence emission. Encapsulation of fluorescent naphthalene diimides into reverse and liposomal micelles (both used as biological membrane mimics) highlighted their potential for biophysical applications. It was shown using single particle techniques that the cavity size of dye loaded reverse micelles and diffusional parameters and hydrodynamic radii of dye loaded liposomal micelles could be accurately determined.
Further research using naphthalene diimides led to the development of a series of supramolecular host-guest materials used to investigate non-bonding interactions (Chapter 4). Small naphthalene diimides were synthesised and the change in their photophysical properties probed upon incorporation into a cavity molecule, cucurbituril, which served as host. Results showed that the non-bonding interactions between naphthalene diimides and cucurbituril could be probed using numerous steady state and time-resolved fluorescence techniques, with the most interesting of the complexes forming an emissive ‘excimer like’ species with a fluorescence quantum yield of 7% in water, and a Stokes shift of more than 100 nm.
Core-substituted naphthalene diimides were further developed as fluorescent sensing materials for small analytes such as protons as well as redox active compounds (Chapter 5). Two proton sensing naphthalene diimides and a series of redox sensing derivatives were synthesised and the mechanism of the sensing event spectroscopically probed using both steady state and time-resolved spectroscopic techniques. The mechanism of proton sensing was found to be photoinduced electron transfer from an electron donating tertiary nitrogen attached to the naphthalene diimide, which is subsequently suppressed upon protonation leading to an increase in emission. The emission output from one of the proton sensing materials was also found to be highly dependent upon solvent polarity and time-resolved methods were used to elucidate that the closeness in energy between the singly excited and the charge separated state leads to delayed fluorescence. Redox sensitive naphthalene diimides were analysed using steady state, cyclic voltammetry and computational modeling revealing the presence of highly conjugated small molecule systems that absorb and emit in the red/deep red region of the electromagnetic spectrum. This emission was modulated by the addition of oxidants and reductants and switched the fluorescence output ‘on’ and ‘off’ many times.
Studies at the single molecule level were also completed using CdSe quantum nanocrystals (Chapter 6). Taking advantage of the inherent photostability of these semiconducting materials, the emission dipoles of quantum nanocrystals of different sizes and shapes were probed using defocused wide-field microscopy. Comparison of emission patterns to theoretically determined patterns showed that unlike CdSe spheres, which have a no discreet emission dipole, CdSe rods contain a single dipole moment down the c-axis of the crystal. This discreet dipole emission however, appeared to breakdown in rods with very small aspect ratios behaving more like spheres again.