Influence of the substituent on the chiroptical properties of poly(thieno[3,2-b]thiophene)s

A series of chiral 3,6-substituted poly(thieno[3,2-b]thiophene)s (PTTs) were synthesized: 3,6-dialkoxy-substituted PTTs, 3,6-dialkylthio-substituted PTTs, and alternating copolymers of 3,6-dialkoxythieno-[3,2-b]thiophenes and 3,6-dialkylthieno[3,2-b]thiophenes. The polymers were prepared by a Stille-coupling reaction and their (chir)optical properties were investigated in solution as well as in film. The substituent appeared to play a decisive role in the polymer's macromolecular structure and supramolecular organization. The very small steric hindrance between the substituents and the polymer backbone of dialkoxy-substituted PTTs, in addition to the strong chalcogen-chalcogen attractions, allows these polymers to adopt a rigid rod-like conformation in good solvents. Upon transition to nonsolvents and film, the polymer strands chirally stack but no macroscopic order could be obtained. If the steric hindrance between adjacent monomer units is increased and the strength of the chalcogen-chalcogen attraction is decreased, as is the case in dialkylthio-substituted PTTs and the alternating copolymers, the polymers are present as random coils in solution. In films or in poor solvents, they planarize and stack and macroscopic order is present. Very large Cotton effects in both poor solvents and film were observed. In film, the CD spectra of some of the (semi-crystalline) polymers is a combination of real CD and other contributions.