Synthesis and characterization of poly(3,5-didodecyl-cyclopenta[2,1-b;3,4-b′]dithiophen-4-one)

•The first synthesis of a soluble 4H-cyclopenta[2,1-b;3,4-b′]dithiophene-4-one (CPDTO) homopolymer is reported.•Both Ullmann coupling and oxidation polymerization of didodecyl CPDTO gave oligomers.•A mechanism is provided for oxidation polymerization using FeCl3.•Palladium-catalyzed reaction of C12-diBrCPDTO and hexamethylditin was found effective to polymerize the monomer.•The C12-CPDTO polymer has high dihedral angles by DFT calculation. The poor coplanarity leads to wide band gap.

Different methods have been explored to synthesize the first soluble cyclopenta[2,1-b;3,4-b′]dithiophen-4-one (CPDTO) homopolymer, poly(3,5-didodecylCPDTO), C12-PCPDTO. Oxidation of the monomer C12-CPDTO by FeCl3 only gave oligomers. Elemental and NMR analyses indicated that oligomers were chlorinated at the both ends. A mechanism is proposed to explain why chlorination is so problematic for oxidation polymerization of C12-CPDTO in comparison to 3-alkylthiophene. Dedoping of the oligomers was also investigated. Pd-catalyzed reaction of dibromo monomer and hexamethylditin yielded C12-PCPDTO-Sn with an Mw of 22,816. This polymer exhibits a bimodal distribution of molecular weight, which is rationalized by an analysis of relative reactivities of species present in the polymerization process. The optical bandgap is 1.85 eV in both chloroform solution and film, much larger than that of electrochemically synthesized unalkylated PCPDTO film. This is explained by poor backbone planarity as predicted by DFT calculation of C2-CPDTO oligomers. The efficient synthesis of the monomer and the polymer paves the way for future synthesis of soluble planar CPDTO homopolymers with alternating alkylated and non-alkylated CPDTO units.

Graphical abstractDifferent methods have been explored to synthesize the first soluble CPDTO homopolymer, poly(3,5-didodecyl-cyclopenta[2,1-b;3,4-b′]dithiophen-4-one), C12-PCPDTO. Chemical oxidation of monomer by FeCl3 only gave oligomers, but Pd-catalyzed reaction of dibromo monomer and hexamethylditin yielded C12-PCPDTO with a Mw of 22,816 and a polydispersity of 2.56. The polymer was characterized by 1H and 13C NMR spectroscopy, gel permeation chromatography, differential thermal analysis, cyclovoltammetry, and UV–vis absorption.Figure optionsDownload full-size imageDownload high-quality image (171 K)Download as PowerPoint slide