External heavy-atom effect of gold in a supramolecular acid–base π stack

Abstract

The nucleophilic trinuclear Au(I) ring complex Au₃(p-tolNCOEt)₃, 1, forms a sandwich adduct with the organic Lewis acid octafluoronaphthalene, C₁₀F₈. The 1·C₁₀F₈ adduct has a supramolecular structure consisting of columnar interleaved 1 ∶ 1 stacks in which the Au₃(p-tolNCOEt)₃ π-base molecules alternate with the octafluoronaphthalene π-acid molecules with distances between the centroid of octafluoronaphthalene to the centroid of 1 of 3.458 and 3.509 Å. The stacking with octafluoronaphthalene completely quenches the blue photoluminescence of Au₃(p-tolNCOEt)₃, which is related to inter-ring Au–Au bonding, and leads to the appearance of a bright yellow emission band observed at room temperature. The structured profile, the energy, and the lifetime indicate that the yellow emission of the 1·C₁₀F₈ adduct is due to monomer phosphorescence of the octafluoronaphthalene. The 3.5 ms lifetime of the yellow emission of 1·C₁₀F₈ is two orders of magnitude shorter than the lifetime of the octafluoronaphthalene phosphorescence, thus indicating a strong gold heavy-atom effect. The diffuse-reflectance spectrum of the solid adduct shows new absorptions that are red-shifted from the absorptions of the monomeric organic and inorganic components alone, indicating charge transfer. Luminescence excitation spectra suggest that these new absorptions represent the major excitation route that leads to the yellow luminescence of 1·C₁₀F₈, which is different from the conventional heavy-atom effect in which the phosphorescence route entails simply the enhancement of the S₁–T₁ intersystem crossing of the organic compound.