An organized combination of x-ray diffraction, infrared spectroscopy, and computer simulation in a study of crystal structural change in thermochromic phase transition of poly(3-alkyl thiophene)s

Summary form only given. In a series of papers we investigated the molecular and crystal structural changes occurring in the thermochromic phase transition of poly(3-alkylthiophene)s. At low temperature the polythiophine takes a planar conformation with all-trans alkyl side chains. Above the transition temperature [ca. 80/spl deg/C for poly(3-dodecylthiophene)] the alkyl chains begin to disorder through trans-gauche conformational change, which induces a twisting of polythiophene skeletal chain, resulting in a shortening of /spl pi//spl pi/-conjugation length. In the present paper we will report the detailed analysis of crystal structure of poly(3-alkylthiophene) made by an organized combination of X-ray diffraction, infrared spectroscopy, and computer simulation. Molecular mechanics calculation gave several energetically stable crystal structure models. Among them the two models have been found to give a good agreement with the observed X-ray diffraction and infrared spectral data. The crystals are consisted of a stacking of layers in which the essentially planar (slightly bent) molecular chains are arranged side by side with the trans-zigzag-like side chains packed as closely as possible. There are two possible ways of stacking of layers: (a) two molecular layers are packed in a unit cell in an anti-parallel mode. This model corresponds to that reported by Winokur et al. (b) one layer is included in a unit cell of half size (parallel single layer). A choice of the best model is now in progress.