Chiroptical properties of cyclopentadithiophene-based conjugated polymers

A series of chiral, substituted conjugated polymers containing the cyclopenta[2,1-b:3,47b']dithiophene unit were synthesized: poly(cyclopenta[2, 1-b:3,4-b']dithiophene)s, poly(thieno[3,2-b]thiophene-alt-cyclopenta[2,1-b:3,4-b']dithiophene)s and poly(thiophene-alt-cyclopenta[2,1-b:3,4-b']dithiophene)s. The polymers were prepared by a Stille-coupling reaction and their (chir)optical properties were investigated in solution as well as in film. UV-vis and fluorescence spectroscopy indicated that the polymers are present as rigid, highly conjugated strands in solution. Although the homopolymers (poly(cyclopenta[2, 1-b:3,4-b']dithiophene)s) do not self-assemble in solution as well as in film, the alternating copolymers (chirally) stack upon transition to nonsolvents and solid state. The (chir)optical properties in film appeared to depend on the annealing conditions: initially, the chiroptical response increases, but at higher temperatures, the Cotton effects irreversibly disappear. Moreover, the CD spectra of films of poly(thieno[3,2-b]thiophene-alt-cyclopenta[2,1-b:3,4-b']dithiophene)s with chiral substituents on the thieno[3,2-b]thiophene moiety appeared to be a superposition of two effects, which had a different annealing dependency. In poly(thiophene-alt-cyclopenta[2, 1-b:3,4-b']dithiophene)s, only chiral exciton coupling was found to be present.