Red-Shifts upon Metal Binding: A Di-Gold(I)-Substituted Bithiophene

5,5′-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bithiophene was metalated with (triphenylphosphine)gold(I) at the 5,5′-positions to yield a di-gold(I)-substituted bithiophene. The resulting complex was characterized by X-ray diffraction crystallography, optical spectroscopy, and elemental analysis. The digold bithiophene complex emits green, structured luminescence in chloroform. Its absorption profile is red-shifted from those of 2,2′-bithiophene or the boronate ester starting material. Density-functional theory (DFT) calculations indicate that the frontier orbitals of the digold complex concentrate on the bithienyl bridge. Time-dependent DFT calculations find that the LUMO←HOMO promotion mostly accounts for the absorption onset in all three compounds and that configuration interaction with vacant orbitals on gold modulates this transition. This interaction contributes to red-shifted absorption profiles of σ-aurated gold(I) organometallics.