TEMPERATURE-DEPENDENT INFRARED SPECTROSCOPIC AND CRYSTALLOGRAPHIC STUDY OF ALPHA,ALPHA'-DIMETHYL OLIGOTHIOPHENES IN NEUTRAL FORM

We have studied structural and spectroscopic characteristics on a seri es of alpha,alpha'-dimethyl end-capped oligothiophenes (from two to si x thiophene units) by varying temperature. Powder x-ray diffraction an d infrared spectra at different temperatures between - 170 and 150 deg rees C were recorded. The diffractometric study shows that the thermal expansion coefficients of these materials are highly anisotropic. The expansion in the ab plane, which is perpendicular to the chain direct ion, is an order of magnitude bigger than that along the chain directi on. Corresponding to these structural features, the infrared spectra s how interesting temperature dependence. For instance, the separation b etween adjacent components of the multiplets near 800 cm(-1), that are assigned to the C-II out-of-plane bending vibrations, increases with decreasing temperatures. This behavior reflects the change in the inte rmolecular forces caused by the thermal expansion. Furthermore, the sh ifts in the C=C stretching vibrations indicate that the extent of the pi-electron delocalization increases with increasing temperatures. The origin for this is briefly discussed. (C) 1998 American Institute of Physics.