Extensive Studies on π-Stacking of Poly(3-alkylthiophene-2,5-diyl)s and Poly(4-alkylthiazole-2,5-diyl)s by Optical Spectroscopy, NMR Analysis, Light Scattering Analysis, and X-ray Crystallography

Abstract

Stacking of poly(3-alkylthiophene)s P3RThs and poly(4-alkylthiazole)s P4RTzs has been studied. Light scattering analysis indicates that head-to-tail (HT) type HT-P3HexTh (R = n-C6H13) gives a degree of depolarization (ρv) of 0.26 in CHCl3, which reveals that HT-P3HexTh takes a stiff structure even in the good solvent. Addition of CH3OH to CHCl3 solutions of HT-P3HexTh and head-to-head (HH) type HH-P4HepTz (R = n-C7H15) leads to π-stacking of the polymer molecules to form stable colloidal particles. The light scattering analysis of the colloidal solution of HT-P3HexTh in a 2:1 solution of CHCl3 and CH3OH reveals that HT-P3HexTh is aggregated in a parallel style. Results of filtration experiments using membranes with 0.20 and 0.02 μm pores agree with the degree of the aggregation. P3HexThs with irregular structures (P3HexTh (Fe) and P3HexTh (Ni) with HT/HH ratios of about 7/3 and 1/2, respectively) show a weaker trend to aggregate; however, P3HexTh (Fe) is considered to stack in a surface region of a stretched poly(ethylene terephthalate) PET film. A dichroism observed with the stretched PET film indicates that the π−π* absorption as well as photoluminescence of the stacked P3HexTh molecules have a transition moment along the direction of the polymer main chain. X-ray diffraction analysis of HT-P3RThs and HH-P4RTzs reveals that they take a face-to-face stacked structure with an end-to-end packing mode, except for HT-P3MeTh (R = Me). HT-P3MeTh forms a face-centered lattice with an interlayer distance of 3.51 Å. An alternative copolymer of bithiazole and 4,4‘-dibutylbithiazole is packed in an interdigitation mode. At temperatures below 0 °C, the HT-P3HexTh molecules are π-stacked in CHCl3, and the 1H NMR spectrum shows a severe magnetic effect on the thiophene ring. Solid 13C NMR data are also consistent with the π-stacking.