Synthetic routes to solution-processable polycyclopentadithiophenes

Three synthetic protocols have been employed to prepare solution-processable poly(4,4-dialkylcyclopentadithiophenes). These polymers are fused-ring analogues of poly(3-alkylthiophenes) and structural analogues of the polyfluorenes. Oxidative polymerization of 4,4-dialkylcyclopenta[2,1-b;3,4-b']dithiophenes using iron (III) chloride gave high-molecular-weight polymers, whereas nickel-catalyzed polymerization of the 2,6-dibromo derivatives via in situ-generated thienylmagnesium and thienylzinc reagents gave moderate-molecular-weight polymers. The structures of the polymers were confirmed by NMR spectroscopy and MALDI-TOF mass spectrometry. In the polymerization using organomagnesium intermediates, some chain capping by methyl end groups was observed. This side reaction can be avoided by polymerization of the organozinc reagents. Post-functionalization of the halogen termini of these polymers can be readily achieved using a cross-coupling with an aryl Grignard reagent. The main chain conjugation in these polymers can be directly correlated to the length of the polymer backbone because the UV-vis spectra of fractionated samples show absorption maxima that increase with the average degree of polymerization as determined by GPC, up to a limiting value of similar to20. UV-vis spectroscopy suggests that poly(4,4-dialkylcyclopentadithiophenes) do not self-assemble in the solid state, as has been previously observed for regioregular poly-3-alkylthiophenes.