Does the Donor−Acceptor Concept Work for Designing Synthetic Metals? 2. Theoretical Investigation of Copolymers of 4-(Dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene and 3,4-(Ethylenedioxy)thiophene

Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4 H -cyclopenta[2,1- b:3,4- b ‘]dithiophene (CDM), and co-oligomers (CDM/EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λ max from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λ max of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction “band”. It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.