Crystal structures and spectroscopic characterization of radical cations and dications of oligothiophenes stabilized by annelation with bicyclo[2.2.2]octene units: Sterically segregated cationic oligothiophenes

The remarkably stable SbF6- salts of the radical cations of bithiophene 1(2T) and terthiophene 1(3T), completely surrounded by bicyclo[2.2.2]octene (BCO) units, were obtained by one-electron oxidation of the neutral precursors with NO+SbF6-, and their solid-state structures were determined by X-ray crystallography. In these radical cations, the presence of quinoidal character was apparent, as shown by the increased planarity and by comparison of the bond lengths with those of the neutral precursors. The shortest intermolecular pi-pi distances in the crystal structure of 1(2T).+SbF6- (distance between two Sp(2) carbon atoms, 4.89 Angstrom) and 1(3T).+SbF6- (distance between an Sp(2) carbon and a sulfur atom, 3.58 Angstrom) were found to be longer than the sums of the van der Waals radii of the corresponding atoms. In accord with this, no apparent change was observed in ESR and UV-vis-NIR spectra of solutions of 1(2T)(.+) and 1(3T)(.+) upon lowering the temperature, indicating that the pi- (or sigma-) dimer formation is inhibited in solution as well as in the solid state. The dications 1(2T)(2+) and 1(3T)(2+) were generated with the stronger oxidant SbF5 in (CHCI2)-C-2 at -40 degreesC and characterized by NMR spectroscopy. In the H-1 NMR spectra, two conformers were observed for each dication of both 1(2T)(2+) (transoid (t) and cisoid (c)) and 1(3T)(2+) (t,t and c,t) at room temperature due to the high rotational barrier around the inter-ring bond(s) between thiophene rings, which was caused by the enhanced double bond character of these bonds following two-electron oxidation. This is supported by DFT calculations (B3LYP/6-31 G(d)), which predicted the rotational barriers in the dications of unsubstituted bithiophene and terthiophene to be 27.6 and 22.9 kcal mol(-1), respectively. In the case of quaterthiophene and sexithiophene surrounded by BCO frameworks 1(4T) and 1(6T), oxidation with even one molar equivalent of NO+SbF6- afforded the dication salts 1(4T)(2+)2SbF(6)(-) and 1(6T)(2+)2SbFr(6)(-), which were isolated as stable single crystals and allowed the X-ray crystallography. In their crystal structures, the cationic pi-systems became planar again due to the great contribution of quinoidal resonance structures, and the pi-systems, which were arrayed in a parallel geometry, were also segregated by the steric effect of BCO units. The degree and tendency of changes in the bond lengths of thiophene rings of 1(4T)2+ and 1(6T)(2+) as compared with neutral precursors were similar to those of 1(2T).+SbF6- and 1(3T).+SbF6-, respectively, implying that the contribution of quinoidal character is modulated by the amount of positive charge per thiophene unit.