Magnetic and conductive properties of quinoidal oligothiophenes

The magnetic and conductive properties of a series of quinoidal oligothiophenes, that is, bis- (dicyanomethylene)oligothiophenes (TnCN4, n ) 2-4), have been investigated in the solid state and (for magnetic analysis) in solution, and the results have been compared with those from density functional theory (DFT) calculations. Solution electron spin resonance (ESR) spectra of radical cations and anions are characterized by hyperfine structures due to coupling with nitrogen and thiophene hydrogen atoms. Neutral solutions are ESR active, indicating a significant presence of diradical species with no hyperfine structure. ESR spectra of powder samples at room temperature give a concentration of radical species up to 1 mol % (T3CN4), that is, 0.5% of the compound exists in a diradical state. The percentage decreases dramatically from T3CN4 to T2CN4, as a result of the decreased number of aromatic rings. The diradical concentration increases with temperature according to activation energies which are higher for the shorter members and in very good agreement with DFT calculations. Tetrahexyl-substituted T4CN4 is redox conducting at the neutral-polaron mixed oxidation level with a maximum conductivity of 0.03 S cm-1, about two orders of magnitude higher in comparison with an aromatic tetrahexyl-substituted octathiophene.