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 π−π distances in the crystal structure of 1(2T)•+SbF6- (distance between two sp2
carbon atoms, 4.89 Å) and 1(3T)•+SbF6- (distance between an sp2 carbon and a sulfur atom, 3.58 Å) 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 π- (or σ-) 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
CH2Cl2 at −40 °C and characterized by NMR spectroscopy. In the 1H 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/6-31G(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+2SbF6- and 1(6T)2+2SbF6-, which
were isolated as stable single crystals and allowed the X-ray crystallography. In their crystal structures,
the cationic π-systems became planar again due to the great contribution of quinoidal resonance structures,
and the π-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.