Novel oligothiophenes surrounded by bicyclo[2.2.2]octene (abbreviated as BCO) frameworks ranging
from dimer to hexamer, 1(nT) (n = 2, 3, 4, 6), were prepared, and their structures and electronic
properties were investigated. Dimer 1(2T) was synthesized by oxidative coupling of the 2-lithiated
monomer generated from 4,5-BCO-annelated 2-bromothiophene 8 with CuCl2 in 76% yield. Trimer
1(3T) and tetramer 1(4T) were synthesized by Stille coupling of 2,5-dibromo-3,4-BCO-annelated
thiophene 4 and of the 5,5‘-dibromo derivative of bis(3,4-BCO-thiophen-2-yl) 10 with 2-stannylated
4,5-BCO-annelated thiophene 9 in 41% and 46% yield, respectively. Hexamer 1(6T) was synthesized
by oxidative coupling of terthiophene 12, tris-annelated with BCO units, in 81% yield. X-ray
crystallographic studies showed that the thiophene rings in 1(2T) and 1(3T) are rotated around
the inter-ring C−C bond(s) with the CC−CC dihedral angles of −174.3(5)° for 1(2T) and −149.7(3)° and 34.4(3)° for 1(3T). In the crystal structures of 1(2T) and 1(3T), no π-stacking was observed
as expected from the steric effect of the BCO units. Theoretical calculations for 1(2T) and 1(3T) at
the B3LYP/6-31G(d) level indicated that the annelation with BCO units either at the 2,3- or 3,4-positions of thiophene rings raises both the KS HOMO and LUMO levels. In the electronic absorption
spectra of 1, the longest wavelength absorption band corresponding to the π−π* transition is
bathochromically shifted with the increase in absorption intensity as the number of thiophene rings
increases, and the absorption of the polythiophene 1 with infinite length was predicted to be 419
nm. The cyclic voltammetry of 1 in CH2Cl2 at −78 °C (2T) or at room temperature (3T, 4T, 6T)
showed two reversible oxidation waves, indicating that the radical cation and dication of 1 are
stable under these conditions.