A new approach to the photochemistry of organic semi-conductors

In this work a theoretical description of isomerisation is provided using stilbene as a reference material, together with an investigation of the influence of nitrile and methoxy substitution on the thermal and photochemical isomensation of 1,2-bis(2-thienyl)ethenes via a more fundamental approach. In this new approach, the use of computational chemistry proved its relevance.; The synthetic pathways towards the compounds are discussed theoretically and experimentally. The thermal isomerisation is studied by means of the thermodynamic equilibrium, which is easily catalytically achieved and analysed. The obtained results could be rationalised by ground-state energy calculations of the most stable geometries which are determined by a theoretical conformational study. The E → Z isomerisation breaks down the efficiency of OLEDs already, but a consecutive cyclisation reaction followed by an irreversible oxidation step irreversibly destroys the active compound. We performed a theoretical energetic study of the thermal cycloreversion process.; In order to investigate the photochemical isomerisations, we performed quantum yield determinations. The photon dose is determined by the potassium ferrioxalate actinometer and the amount of E → Z conversion is analysed by fitting the UV/Vis absorption spectra of the E/Z mixtures to a linear combination of the spectra of the pure E- and Z-isomers. The results of the quantum yield determinations could also be rationalised by the performed CASSCF calculations of the potential energy surfaces of first and second excited states. The quantum yields fit the calculated activation energies nicely. The latter are inextricably associated with the energy barriers present in the excited state potential energy surfaces. This can be utilised as a guiding principle for future molecular design.; In another part of this thesis we synthesised the oligomers 2,5-bis(2,2-diphenylethenyl)-thiophene and 4,4'-bis[2,2-bis(4-methoxyphenyl)ethenyl]biphenyl in order to solve the stability problems of this class of materials by an empirical approach. Their syntheses are described and their structural properties discussed; the gas-phase geometries are calculated, the solid-phase geometries obtained from single crystal XRD. Both methods clearly show the non-planarity of these compounds. Deviation from planarity is an improvement because it prevents excimer formation to a large extent. In order to correctly assign the NMR spectra, the 1D and the 2D HETCOR spectra, with both long- and short-range couplings, were recorded and the results of high level theoretical calculations of NMR related properties are applied.; Finally, the photochemistry and photophysics of the oligomers are investigated. We quantified the fluorescence by its quantum yield and detected no improvement in terms of the derivatives. The influence of solvent polarity on excited state lifetimes is also studied. We concluded that polarity enhances a non-radiative decay channel, i.e. isomerisation. (Abstract shortened by UMI.)...