Poly(3-dodedyl-2,5-thienylenevinylene)s from the Stille coupling and the Horner–Emmons reaction

Regioregular (RR) conjugated polymers are critical for electronic and optoelectronic properties of polymer based semiconducting devices. Monosubstituted polythienylvinylene (PTV), a relatively low band-gap conjugated polymer, has been reportedly synthesized using the Stille coupling reaction between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene. Although a small fraction of the product shows good (∼90%) regioregularity, the whole product (S-C12-PTV) consists of mostly unidentified structures. To gain better control of the polymer structure and regioregularity, a difunctionalized C12-thiophene monomer, 3-dodecyl-5-formyl-thiophen-2-ylmethyl)-phosphonic acid diethyl ester, has been polymerized in near quantitative yield via the Horner–Emmons reaction. The full (100%) regioregularity of the resulting polymer (HE-C12-PTV) has been confirmed by its 1H and 13C NMR spectra. The full regioregularity is also reflected in its strong tendency to crystallize and practically no solubility in boiling hexane, in sharp contrast to S-C12-PTV. UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltammetry, thermal analysis (DSC & TGA) and X-ray diffraction have been used to characterize both polymers. UV-vis absorption spectra of the HE-C12-PTV chloroform solutions of different concentrations show well-resolved vibronic structures with an absorption maximum at 577 nm and a prominent shoulder at 614 nm. The optical bandgaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO energy levels of the HE-C12-PTV film were found to be at −4.98 eV and −2.88 eV, respectively. The electrochemical bandgap of the polymer in the film is estimated to be 2.1 eV. The DSC curve shows a pronounced melting peak at 205 °C. XRD study shows that a decent crystalline structure is formed without any annealing of the as-cast films.