GEOMETRICAL AND ELECTRONIC-STRUCTURES OF A BENZIMIDAZOBENZOPHENANTHROLINE-TYPE LADDER POLYMER (BBL)

Variation of the geometrical and electronic structures of a benzimidazobenzophenanthroline-type ladder polymer (BBL) was theoretically investigated. The effect of charge transfer and protonation at the imine N sites revealed fundamental differences with respect to POL-type ladder polymers. Semiempirical (MNDO) solid-state theory was employed for geometry optimization, and the electronic structures were calculated from a modified extended Huckel energy band theory. Neutral BBL is predicted to have aromatic naphthalenic and benzenoid units and alternating bond lengths of imines in the ground state. This results in a band gap of 2 eV, in agreement with optical absorption experiments. The lowest empty band of the neutral form is flat. As charge transfer occurs, the naphthalenic unit becomes quinoid-like and the bond lengths around an imine N become almost equal to each other. The geometrical and electronic structures of neutral BBL are not much affected upon protonation, which induces creation of stable carbonium ions between two nitrogens along the N-imino amide units. Therefore, acid doping may increase the conductivity of BBL films not just through protonation but also through charge transfer. A symmetry of the effect of donor and acceptor doping is interpreted in terms of molecular orbital theory. The absence of a red shift in the absorption spectrum upon doping in BBL is different from most conducting polymers, and is interpreted as being a consequence of the robustness of the geometry and the flatness of the conduction band.