Borole/Thiophene Cooligomers and Copolymers with Quinoid Structures and Biradical Characters

The geometrical structures and electronic properties of borole/thiophene cooligomers are studied employing the density functional theory with B3LYP functional. The borole-containing oligomers have been suggested to have the quinoid structure and distinct biradical character. The introduction of thiophene rings into the oligoborole will retard the appearance of the quinoid structure and consequently stabilize the borole-containing oligomers. Electronic structrues of polyborole and borole/thiophene copolymers are investigated by the periodic boundary condition (PBC) and the cyclic oligomer model. Both the PBC and cyclic models predict the quinoid structure of borole/thiophene copolymer. The PBC calculations give estimations of band gaps of around 2.21 eV for the polyborole and 2.22 eV for the borole/thiophene (1:1) copolymer, which are different from those (∼0.00 eV) obtained by the oligomer extrapolation schemes.