Synthesis and properties of novel oligothiophenes surrounded by bicyclo[2.2.2]octene frameworks.

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 C=C-C=C dihedral angles of -174.3(5) degrees for 1(2T) and -149.7(3) degrees and 34.4(3) degrees for 1(3T). In the crystal structures of 1(2T) and 1(3T), no pi-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 pi-pi 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 degrees 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.