New low-gap polymers from 3,4-ethylenedioxythiophene-bis-substituted electron-poor thiophenes. The roles of thiophene, donor-acceptor alternation, and copolymerization in intrinsic conductivity

New low-gap thiophene-based regular copolymers are produced by anodic coupling of 3,4-ethylenedioxythiophene-2,5-substituted thieno[3,4-b]pyrazine (TP), cyclopenta[2,1-b;3,4-b']dithiophen-4-one (CO), and 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b']dithiophene (CN). The copolymers are characterized by cyclic voltammetry, FTIR reflection-absorption and UV-vis spectroscopy, electrochemical quartz crystal microbalance analysis, and in situ p- and n-conductivity measurement. The copolymers show low optical gaps (measured at the maximum absorption) and electrochemical gaps (measured from redox potentials) in the range 0.8-1.3 eV. The CN-based polymer displays the lowest reported electrochemical gap (0.8 V). Random copolymers of CO and 3,4-ethylenedioxythiophene (EDT) have also been produced and compared with the relevant regular copolymer. Copolymerization of CO with increasing amounts of EDT decreases the gap. From an analysis of redox potential as a function of EDT fraction, it is found that the gap is limited by the redox potentials of the individual homopolymers. Localization of n-doping carriers in the polythiophene chains is progressively increased by donor-acceptor alternation and then by copolymerization till the expected intrinsic conductivity is made completely p-type.