Linear ladder-type π-conjugated polymers composed of fused thiophene ring systems

A novel carbon-sulfur (C2S) analogue of polyacene has been synthesized and characterized. The polymer is composed of a nondegenerate trans-cisoid polyacetylene backbone in which all hydrogen atoms are replaced with sulfur atoms to form thiophene rings that are fused into a linear cross-conjugated ladder-type structure. The newly developed synthetic route of the ladder-type polymer is based on an intramolecular condensation of a prepolymer consisting of 3-(alkane-1-sulfinyl)thiophen-2,5-diyl units in triflic acid. The geometric and band electronic structure of the ladder-type polymer were calculated and analyzed at the PM5 level of theory. The fused ring structure imposes strong quinonoid-type character to the originally benzenoid polyene backbone, which destabilizes the HOMO and stabilizes the LUMO and thus reduces the band gap. The shift of absorption and emission maxima to longer wavelengths responding to lower pi-pi* transition energies than those of polythiophene and the smaller gap between electrochemical p- and n-doping potentials are consistent with the reduction of the band gap. The microstructure of the isolated oligomers in their neutral, singly oixidzed, and doubly oxidized states were computed for short and intermediate chain length. Various energetic and structural aspects of the convergence behavior from the properties of small cations or dications to those of an isolated polaronic or bipolaronic defects on a sufficiently long chain were monitored and are discussed. The geometric structures of the p-doped ladder-type polymer in the form of either polaron or bipolaron were calculated by adopting suitable boundary conditions to represent charged unit cells. The geometry relaxation process for the charged ladder-type backbone induces the appearance of a stronger quinonoidic character than that in the case of polythiophene, which suggests that the fused rings more easily adopt quinonoid forms than thiophene rings. Calculated energetic aspects of the doping behavior and the paramagnetic resonance spectra of p-doped polymers revealed that the ladder-type polymer possesses polaron as the prevailing charged species, in contrast to many other pi-conjugated polymers where bipolaron is the lowest-energy charge storage configuration.