Steric Control in the π-Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[2.2.2]octene Units

A Two series of oligothiophenes 2(nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3ac, annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of -dimerization and the structureproperty relationships of the -dimers of oligothiophene radical cations. Their radical-cation salts were prepared through chemical one-electron oxidation by using nitrosonium hexafluoroantimonate. From variable-temperature electron spin resonance and electronic absorption measurements, the -dimerization capability was found to vary among the members of the 2(nT)+.SbF6 series and 3+.SbF6 series of compounds. To examine these results, density functional theory (DFT) calculations at the M06-2X/6-31G(d) level were conducted for the -dimers. This level of theory was found to successfully reproduce the previously reported X-ray structure of (2(3T))22+ having a bent -dimer structure with ciscis conformations. The absorption bands obtained by time-dependent DFT calculations for the -dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the -dimerization were divided into four factors: 1)SOMOSOMO interactions, 2)van der Waals forces, 3)solvation, and 4)Coulomb repulsion, and the effects of each factor on the structural differences and chain-length dependence are discussed in detail.