Oligothiophene tetracyanobutadienes: Alternative donor-acceptor architectures for molecular and polymeric materials

Oligothiophene-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2 + 2] cycloaddition reactions between tetracyanoethylene and oligothiophene alkynes. The TCBD moiety is compared to other electron acceptors attached to dibutylterthiophene including dicyanovinyl (DCV) and tricyanovinyl (TCV). These donor-acceptor molecules (TCBD-3T, DCV-3T, and TCV-3T) show red-shifted absorption spectra relative to the unsubstituted oligothiophene as a result of intramolecular charge-transfer (ICT). Monosubstituted terthiophenes bearing the electron acceptors show both oxidation and reduction processes as characterized by cyclic voltammetry. Density functional theory (DFT) calculations are used to explain the electronic and redox properties of the materials. Electrochemical oxidation of a bis(terthienyl)-substituted TCBD molecule (3T-TCBD-3T) yields a conducting polymer exhibiting balanced ambipolar redox conduction with similar values for the oxidized and reduced states of the polymer (1 × 10^-3 S cm^-1). Raman spectra of the asymmetric donor-acceptor materials are characterized by two intense bands characteristic of the aromatic and quinoidal regions in the conjugated π-system of the oligothiophene.