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 M-w = 10.2 kDa. The thermal behavior and the X-ray diffraction (XRD) patterns of our native PBT samples allow a crystal polymorph (form I') 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 I' 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. Using such a combined approach we are able to show that the studied polymorph, in analogy with poly-3-(S)-2-methylbutylthiophene (PMBT), is well described by a limit-ordered orthorhombic model in space group C222(1) with refined lattice parameters a = 7.64(1) angstrom, b = 7.75(1) angstrom (chain axis), and c = 24.97(8) angstrom, yielding a calculated density of 1.24 g/cm(3) in good agreement with the experimental value. The refined structure, which presents stacking and layering similar to form 1, is shown by MM to be a local potential energy minimum. We also find lower energy structures with looser stacking periodicity of the polythiophene chains, consistently with data on PBT polymorphism. Both the form I' PBT and the PMBT crystal structures, the only two poly(3-alkylthiophenes) (P3ATs) structures for which detailed models are presently available, confirm features of the inverse comb model of P3ATs by Prosa et al. (Macromolecules, 1992, 25, 4364). Our models also suggest important new aspects: isodirectional arrangements are preferred over antiparallel ones within tightly packed polythiophene stacks. This feature is incompatible with chain-folding occurring between chains within individual stacks. On the contrary crystal growth with chain-folding between chains in adjacent layers is compatible with the refined structures and in principle possible, although plausibly slow, as it requires disruption of preexistent pi-stacking interactions. The molecular and crystal models we devised allow a more detailed understanding of the reasons causing crystallization to be slow and limited in extent, especially for high molecular weight P3ATs.