Sulfur Multicenter Bonding and Distortions in a Hypothetical Polydithioquinone

The electronic properties of polydithioquinone, a hypothetical C 4 S 2 carbon−sulfur polymer related to both tetrathiotetracene and thiothiophthene, are examined using approximate molecular orbital calculations. We focus on a curious pattern of sulfur−sulfur bonding interactions suggested by the band structure calculations, a pattern observed in thiothiophthenes and other molecular analogues of our polymer. The π system of each C 4 S 2 repeat unit contains eight electrons, consistent with a thiothiophthene-like rather than localized polydithioquinone valence structure. A pairing distortion in the carbon sublattice is implied; furthermore, the half-filled sulfur σ bands suggest that the sulfur sublattice is also likely to undergo distortions. Restricting our consideration to those distortions that retain planarity of the polymer, we find deformations of the carbon backbone considerably less favorable energetically than those of the sulfur sublattice. Four nicely bond-localized isomers result from the distortions of both carbon and sulfur sublattices; these all are found to be low bandgap semiconductors. We also examine helical distortions and find a stable isomer with an approximately 10-fold helical axis.