Molecular design and density functional theory investigation of novel low-band-gap copolymers between quinoid acceptors and aromatic donors

The structures and electronic properties of furo[3,4-b] pyridine-based alternating donor and acceptor conjugated oligomers, in which furan and pyrrole are used as donors, and their periodic polymers were investigated using density functional theory at the B3LYP/6-31G(d) level. The bond lengths, bond length alternation, bond critical point (BCP) properties, nucleus-independent chemical shift (NICS) and Wiberg bond index (WBI) were analyzed and correlated with conduction properties. The changes of bond length, BCP properties, NICS and WBI all show that the degree of conjugation increases with main chain extension. The changes of NICS also show that the conjugation is stronger in the central section than in the outer section. Hydrogen bonding interactions and nitrogen atom substitution in the acceptors play very important roles in the geometries, electronic structures and energy gaps. The theoretical results suggest that pyrrole-based polymers are good candidates for conducting materials, compared with furan-based polymers. With an increase of nitrogen atom substitution in the acceptors in these polymers, the intermolecular charge transfers along the polymeric axes are enhanced, and the bond length alternations and HOMO-LUMO energy gap for these polymers are decreased. The results suggest that the six polymers studied all have lower energy gaps (in the range 0.81-1.26 eV), which indicate that these proposed polymers are good candidates for n-doping conductive materials, especially poly(7-(furan-2-yl)furo[3,4-e][1,2,4] triazine) and poly(7-(1H-pyrrol-2-yl) furo[3,4-e][1,2,4] triazine). (C) 2011 Society of Chemical Industry