Preparation, Photophysics, and Electrochemistry of Segmented Comonomers Consisting of Thiophene and Pyrimidine Units: New Monomers for Hybrid Copolymers

An efficient coupling route to novel π-conjugated comonomers consisting of pyrimidine, thiophene, and
bithiophene units was developed. The novel π-donor−acceptor−donor and π-donor−acceptor−acceptor−donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman
homocoupling reactions. Photophysical investigation of these alternating π-donor and acceptor compounds
indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results
in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%.
Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its
triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation
and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly
reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended
conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy
and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a
hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative
to its metal-free counterpart.