| In the present thesis, the influence of temperature on the structural, electronic and optical properties of model conjugated oligomers of PPV, PPP and PA has been investigated. In order to compute these effects at various temperatures in the gas phase, the validated MM3 classical force field and the semi-empirical INDO/S-CIS scheme have been used in a combined INDO/S-CIS//MD(MM3) procedure to generate absorption spectra.; The thermal broadening simulated for the first absorption band seems to indicate that the smallest conjugated molecule required to ensure the convergence of thermal broadening effects to about 24 nm at 300 K, should consist of a chain of 8 conjugated carbon atoms. The position of the first absorption band maximum λmax of PPV oligomers is found to undergo blue shifting with increasing temperature, whereas it is found to get red shifted in the case of PPP and PA oligomers. These shifts can be related to correlations between λmax and specific geometrical parameters, such as the dihedral angles describing the rotations of the phenyl rings in PPV.; Finally, the influence of intermolecular steric hindrances and crystalline packing on the thermal broadening and λmax of trans-stilbene, the smallest PPV oligomer, is evaluated. In a crystal, room temperature is found to result in a thermal broadening of 18 nm. In addition, the synchronous rotation of both phenyl rings around the central double bond of trans-stilbene, known as the pedal motion, has been investigated. |
