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

The preparation of three isonitrile complexes (p-tosyl)(CH2NCAuX)-X-I (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) angstrom, 3.1044(7) angstrom, and 3.1083(5) angstrom, 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 (similar to 3.6 angstrom) 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 T-1 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) CH2NCAuIX (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.