X-RAY STRUCTURAL STUDIES OF VARIOUS OCTYL-SUBSTITUTED POLYTHIOPHENES

The structure of poly(octylthiophene)s and alternating octylthiophene-thiophene-copolymers with a varying degree of intrachain regularity was studied by means of X-ray diffraction and optical absorption measurements. Although these polymers show a varying degree of crystallinity, evidence is found that they all tend to pack as relatively straight parallel chains. The lateral packing distance depends strongly on the regularity and the relative amount of alkyl side chains. Generally poly(thiophene)s with a larger relative amount and higher stereoregularity of the octyl side chains tend to pack more ordered. On the other hand, a large conjugation length (high degree of chain planarity), as revealed by optical absorption spectroscopy, and a crystalline packing of the polymer chains are not found to be interdependent. For the polymer with highest degree of crystallinity two surprisingly different crystalline structures were found for the two versions with different regularity, viz. poly(3-octylthiophene) and poly(3,3'-dioctyl-2,2'-bithiophene). For the latter we find evidence of a polymer chain twist between adjacent head-to-head coupled thiophene rings, and we propose a structural model for its crystalline packing. Our new model can explain previously measured differences in spectroscopic and transport-related properties for these two stoichiometrically identical polymers.