Oligothiophenes End-Capped by Nitriles. Preparation and Crystal Structures of α,ω-Dicyanooligothiophenes NC(C4H2S)nCN (n = 3−6)

The synthesis of dicyanooligothiophenes NC(C4H2S)nCN (n = 3−6) is reported. For n = 3, 4, and 5 the dinitriles are generated by treatment of the corresponding dibromo compounds with copper(I) cyanide in quinoline. For n = 6 the preparation involves the Ni-catalyzed coupling of 5-bromo-2,2‘:5‘,2‘‘-terthiophene-5‘‘-carbonitrile. The structures of all four dicyano derivatives have been determined by X-ray crystallography. For n = 3, the space group is monoclinic C2/c, with a = 18.363(7), b = 11.8356(9), c = 30.666(4) Å, β = 102.15(2)°, V = 6515(3) Å3, Z = 20. For n = 4 the space group is triclinic P1̄, a = 7.3254(9), b = 7.8658(6), c = 8.1813(8) Å, α = 64.706(8), β = 76.059(8), γ = 76.692(8)°, V = 409.29(7) Å3, Z = 1. For n = 5 the space group is monoclinic C2/c, with a = 13.633(4), b = 11.706(5), c = 37.073(8) Å, β = 90.22(2)°, V = 5929(3) Å3, Z = 12. For n = 6, the space group is monoclinic P21/a, with a = 13.8962(14), b = 5.9100(16), c = 14.0798(16) Å, β = 98.446(4)°, V = 1143.8(4) Å3, Z = 2. In all four structures the molecules are approximately planar, with all-trans thiophene rings. The oligomers are linked into ribbonlike arrays by intermolecular CN- - -H contacts. For n = 3, 4, and 5 these ribbons are stacked in approximately coplanar layers (π-stacks). For n = 6 the ribbons are packed in a herringbone motif very similar to that observed in H(C4H2S)6H (α-6T). The UV−visible spectra of NC(C4H2S)nCN (n = 2−6) have been recorded, and shifts in the π−π* excitation energies of these compounds relative to those found in other α,ω-disubstituted oligothiophenes are interpreted in the light of MNDO calculations. Extended Hückel band structure calculations on NC(C4H2S)nCN (n = 4 and 6) indicate substantial intermolecular interactions; both structures have well-developed 2-dimensional electronic structures.