π-Dimerization ability of conjugated oligomer dication diradicaloids composed of dithienylpyrrole and benzodithiophene units

Stable conjugated oligomer dications 1²⁺ and 2²⁺ composed of 2,5-di(2-thienyl)-N-methylpyrrole (DTP) units at the both ends of the π-systems with methoxy substituents at the 3-position of thienyl units and methylthio end-capping group and benzo[2,1-b:3,4-b′]dithiophene (BDT) unit(s) at the central part with sterically demanding phenyl substituents were synthesized. From DFT calculations and ESR experiments, it was concluded that the diradical character in 1²⁺ and 2²⁺ was reduced in comparison with the radical character of their reference compound 3^·+ composed of one DTP and one BDT units. In addition, the shorter 1²⁺ was considered to have a further lower diradical character than that of 2²⁺. Nevertheless, judging from the results of VT-UV-vis-NIR measurements, both 1²⁺ and 2²⁺ showed a comparable π-dimerization ability at the DTP moiety in dichloromethane solution with 3^·+.