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

Abstract

Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-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 λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax 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.