Theoretical analysis of effects of π-conjugating substituents on building blocks for conducting polymers

Geometries of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene 1 and its C=O, C=S, C=CH2, C=CF2, and C=C(SR)2 analogues were optimized using density functional theory. Three of the above groups, C=C(CN)2, C=O, and C=S, were also examined on dipyrrole, difuran, dicyclopentadiene, and diborole. Electronic structures were analyzed with respect to their suitability as building blocks for conducting polymers with the natural bond orbital (NBO) method. All bridging groups investigated decrease HOMO-LUMO gaps compared to the unsubstituted parent dimers. Substitution affects HOMO and LUMO energies. Energy gap reduction is caused by a stronger decrease of LUMO energies compared to HOMO energies. The C=S group leads to even smaller energy gaps than the dicyanomethylene group since the HOMO is lowered less in energy with C=S. Compared to unsubstituted dimers, the strongest substituent effects are found with pyrroles and furans. Boroles and thiophenes are least affected. The smallest HOMO-LUMO gaps are obtained for electron-poor systems such as boroles followed by cyclopentadienes. This is analogous to the trend for the unsubstituted parent systems. All of the bridging groups are potential π-acceptors due to their low-lying π*-orbitals, and the corresponding polymers are predicted to be n-dopable. In aromatic structures, the LUMO is localized around the bridging substituent and the coefficients at the α-carbon atoms that reflect electron density are small. This might contribute to the poor conductivity of the n-doped form of poly-1. Electron- poor monomers and polymers tend to switch to quinoid structures. In quinoid repeat units, the HOMO is localized but not as strongly as the LUMO in the aromatic repeat units. The LUMO in quinoid repeat units is delocalized with large coefficients at the α-carbon atoms. Quinoid polymers could therefore be good conductors in the n-doped state.