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1.4 CHEMICAL SCIENCES
Much is known about the electronic, vibrational and steric effects resulting from the addition of differing organic functional groups as side chains, to the parent conjugated polymer. However the same cannot be said about the substitution of constituents of the polymer backbone itself. To this end a study is presented here which systematically studies the effects of naphthalene and anthracene substitution of the properties of poly phenylene vinylene (PPV) derivative conjugated polymers. The synthesis by the Witting-Horner reaction of derivatives is presented whereby the phenylene units of the PPV structure are methodically substituted by naphthylene and anthrylene units to form a homologous series of structures. Alkyloxy substituents are added to all aromatic systems to aid solubility. The 2,6-attachment of the vinylene linkage on the naphthylene and anthrylene ring provides novel structures whose properties may be compared to the soluble PPV derivative POPV. All intermediates, polymerisation starting materials and polymers are fully structurally characterised by nuclear magnetic resonance (NMR). Fourier transform infrared (FTIR), and Raman spectroscopies. The molecular weights of the polymer materials are characterised by gel permeation chromatography (GPC), and these results are supported by end group analysis of the proton NMR spectra. The electronic properties if the polymers are investigated using ultra violet-visible (UV-vis) and fluorescence soectroscopies. The introduction of naphthylene units results in a hypsochromatic shift in the absorption and emission spectra, while the introduction of anthrylene units lead to a bathochromatic shift relative to the naphthalene structures. The observed structural variation of the spectroscopic properties is explained in terms of a combination of the increased conjugation of the substituent acene units and the decreased electronic contribution across the vinylene linkage. The electronic properties of the series are shown to follow a well defined trend using a relationship derived from each polymer constituent’s electron affinity to estimate the total energetics of each system. The substitution of different aromatic units into the polymer backbone will result in the introduction of differing characteristics frequencies which may result in a damping of the electron-phonon coupling which results in the non-radiative decay of the excited state. This damping is anticipated to increase radiative decay efficiency. To investigate this, the fluorescence quantum yield is measured for each polymer and the results confirm that the co-polymers have the highest quantum yield in line with the above hypothesis. As a major restriction in the commercialisation of organic display technologies is the stability of the active material, the kinetics of the photooxidative degradation of each of the polymers is investigated. It is found that all substitutions improve the stability of the p[olymers and that the materials containing naphthylene units are particularly stable due an alactron deficient vinylene bond.
Lynch, Patrick.(2008). Systematic synthesis and study of the structure-property relationships of arylene vinylene polymers. Dublin Institute of Technology. doi:10.21427/D7J599