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1.3 PHYSICAL SCIENCES
This project sets out to investigate the variations of the optical properties of organic conjugated polymers with a view towards understanding the transition from isolated molecule (solution) to the solid state. The polymer poly9p-phenylene vinylene-co-2,5-dioctyloxy-m-phenylene vinylene) (PmPV) is chosen as it is of interest for both light emitting and nanotube composite applications. As a model compound, 2,5-dioctoxy-p-distrylbenzene, termed the trimer, is employed for comparison. The objective are to characterize the effect of the environment, using solvent and concentration dependent studies, on the optical properties of the polymer and to compare the results for the more complex polymeric system to the more definable molecular trimer. Low concentration UV/vis absorption and fluorescence studies indicate that the trimer well represents the electronic properties of the polymer. Solvent dependent studies of the trimer indicate that although the absorption and fluorescence spectra are relatively solvent independent, the relative fluorescence yield is strongly dependent on the solvent environment of the trimer. The variation does not correlate with traditional solvatochromic parameters, but rather can be correlated with the integrated overlap of the Raman spectra of the trimer and the solvent. The variation of the fluorescence lifetime with solvent is similarly related to this vibrational overlap parameter. This indicates that the primary effect of the changing environment is on the vibrational coupling and thus the nonradiative decay mechanisms. No such clear correlation is seen for they polymer. However, it is seen that the fluorescence lifetime of the trimer and to a greater extent the polymer is strongly dependant on concentration, implying aggregation effects. The absorption and fluorescence spectra of the polymer are seen to be strongly concentration dependant, and the results indicate that the polymer has a strong tendency to aggregate. Moreover, the degree of concentration dependence is seen to be different for polymers made by different routes. Raman studies of the trimer made via the Wittig condensation and the Horner Emmons condensation routes in both toluene and DMF, indicated that different reaction schemes produce different degrees of isomerisation along the conjugated backbone. The cis/trans ration was quantified by comparison to known mixture of cis/trans stilbene characterized by H1-NMR. The variation was seen to yield a 5:1 ratio of Trans to Cis ratio for the trimer. The variation of the cis/trans ratio was also investigated utilizing Raman spectroscopy, for the trimer the synthetic route proved massively important in the degree of isomerisation. Cis/trans ratio ranging from 17 -51%. Similar variations in degree of isomerisation in the polymer were observed and quantified. The % Cis in the polymer varied between 8-55%, depending on synthetic route. As the Wittig DMF polymer is that in which the strongest aggregation is seen, the degree of aggregation can be correlated with the cis content of the backbone. The effect of the cis isomeriation on the backbone conformation can be visualized using Hyper Chem. Whereas the all trans backbone forms a regular helix due to the meta linkage, the introduction of cis linkages disrupts this regular coiling. IN the tightly coiled trans form the п electrons of the successive coils can interact giving a well defined intramolecular. Disruption of the coiling opens up the п backbone to intermolecular interactions which can have considerable effect on the optical properties. It is concluded that engineering the intramolecular interactions in this fashion may be a route towards controlling and limiting the detrimental effect of the less definable intermolecular interactions.
Henderson, K. (2004). Optimisation of organic materials for laser applications. Masters dissertation. Technological University Dublin. doi:10.21427/D73K7N