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)22+, (2E2PEt)22+, and (2E3PE)22+ 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.