Stable Radical Cations and Their π-Dimers Prepared from Ethylene- and Propylene-3,4-dioxythiophene Co-oligomers: Combined Experimental and Theoretical Investigations.

Co-oligomers composed of two 3,4-ethylenedioxythiophene (EDOT) units and two or three 3,4-propylenedioxythiophene (ProDOT) units, i.e., 2E2PEt and 2E3PEt , were newly synthesized together with the ProDOT trimer 3PMe . On the basis of cyclic voltammetry, the gaps between the first and second oxidation potentials (ΔE1-2 ) of 2E2PEt and 2E3PEt were found to be larger than that of the previously synthesized ProDOT tetramer 4PHex . These co-oligomers gave the fairly stable radical cations 2E2PEt •+ and 2E3PEt •+ by chemical oxidation with AgSbF6 . The disproportionation of 2E2PEt •+ and 2E3PEt •+ into neutral and dicationic species, which was observed for 4PHex •+ , was inhibited in accord with the larger ΔE1-2 . Additionally, the formation of the π-dimers (3PMe )2 2+ , (2E2PEt )2 2+ , and (2E3PE )2 2+ was clearly observed in dichloromethane solution at low temperatures with UV-vis-NIR spectroscopy. Furthermore, the π-dimerization enthalpies of 2E2PEt •+ and 2E3PEt •+ were greater than that of 3PMe •+ , suggesting the formation of fully π-contacted structures. The structures of the π-dimers were optimized at the B97D3 method, and the calculated absorption spectra of the π-dimers obtained using TD-DFT methods were in reasonable agreement with the observed ones, supporting the reliability of the calculated structures.