Ordered stacking of regioregular head-to-tail polyalkylthiophenes: insights from the crystal structure of form I' poly(3-n-butylthiophene)

We report the synthesis, structural characterization, and features of the crystallization behavior of a highly regioregular head-to-tail poly(3-n-butylthiophene) (PBT) with average molecular weight Mw ) 10.2 kDa. The thermal behavior and the X-ray diffraction (XRD) patterns of our native PBT samples allow a crystal polymorph (form I2) to be identified, never previously discussed in the literature but closely related to the more common but more disordered form I. The crystal structure of form I2 PBT has been determined and refined by Rietveld analysis of XRD patterns from polycrystalline samples and has been confirmed by molecular mechanics (MM) calculations adopting a thiophene-specific force field developed in our group.