Luminescence, structural, and bonding trends upon varying the halogen in isostructural aurophilic dimers

Abstract

The preparation of three isonitrile complexes (p-tosyl)CH₂NCAu^IX (X = Cl, Br, and I) along with their structural, spectral, and computational characterization are reported. X-Ray crystallography reveals that these complexes all crystallize in the same space group, C2/c, and have closely related supramolecular structures. The three complexes exhibit crossed-dimer structures with short Au⋯Au aurophilic distances of 3.0634(4) Å, 3.1044(7) Å, and 3.1083(5) Å, for X = Cl, Br, and I, respectively. These distances are among the shortest ligand-unassisted Au⋯Au interactions reported. While RNCAuX complexes that we reported earlier associate as anti-parallel, one-dimensional aurophilic polymers with long Au⋯Au distances (∼3.6 Å) and exhibit orange–red phosphorescence, the analogous aurophilic dimers herein show seemingly counter-intuitive blue–green emissions despite having much shorter Au⋯Au distances. DFT computations are used to augment experiment and study the T₁ phosphorescent excited state of [RNCAuX]_n in parallel, anti-parallel, and staggered conformations. Excimeric bonding and large Stokes shifts are predicted for all models, the extent of which is sensitive to both “n” and conformation with trends commensurate with experimental luminescence data. Calculations for the three [MeNCAuX]₂ dimeric complexes reveal blue–green phosphorescence with a red shift as a function of increasing halide softness, consistent with experimental data for (p-tosyl)CH₂NCAu^IX (Cl > Br > I). The overall experimental and theoretical work signifies the central role of ground-state aurophilic bonding and excited-state excimeric bonding on the electronic structure, hence facilitating development of structure–luminescence relations that may assist in the rational design of novel optoelectronic devices.