Design of novel donor-acceptor polymers with low bandgaps

Quantum chemical results on the electronic and geometric structures of some novel donor-acceptor polymers containing alternating electron-donating group X (X = CH₂, SiH₂ or GeH₂) and electron-accepting group Y (Y=>C=CH₂, >C=O, >C=CF₂ or >C=C(CN)₂) along the conjugated cis-polyacetylene backbone, obtained on the basis of the one-dimensional tight-binding self-consistent field-crystal orbital (SCF-CO) method at the MNDO-AM1 level of approximation, are reported. The optimized geometries of the polymers show a strong dependence on the nature of the electron-donating group X. Polymers derived from X = CH₂ or GeH₂ and Y=>C=C(CN)₂ are predicted to have bandgap values of less than 1 eV. An analysis of their π-bond order data and the patterns of their frontier orbitals shows these two polymers to have quinoid-like electronic structures, in contrast to the benzenoid-like electronic structures for the rest of the polymers.