Photophysics and Bonding in Neutral Gold(I) Organometallic Complexes with an Extended Aurophilic Supramolecular Structure

This work represents a synergistic experimental/computational study of the molecular spectroscopy
and bonding in Au(CO)Cl in solution and the solid state. The luminescence behavior for crystalline
solids is similar for Au(CO)Cl and related (RNC)AuCl complexes that likewise stack in infinite chains,
and both exhibit orange-red unstructured phosphorescence bands with extremely large Stokes shifts ((15−20) × 103 cm-1). The long aurophilic distances computed for the ground state (∼3.2 Å) are contracted
in the phosphorescent excited state (∼2.6 Å), demonstrating excimeric Au−Au covalent bonds. The
spectral data suggest phosphorescent species in which the excimeric Au−Au bonding is extended beyond
two adjacent molecules in the solid state. Controlling the concentration in frozen solutions attains
phosphorescent bands due to dimeric species for which the emission energies are higher (in the blue
region) than those for crystalline solids and are reproduced by ab initio calculations. The spectral findings
herein suggest that predictive information about the supramolecular structure may be obtained by the
luminescnce behavior. This is exemplified by crystals of (1,1,3,3-Me4BuNC)AuCl, whose red-orange
luminsecence anticipated an extended-chain supramolecular structure, which was later verified crystallographically, as the molecules were found to pack in zigzag chains with alternating short (3.418 Å) and
long (4.433 Å) aurophilic distances. Solutions of Au(CO)Cl exhibit negative deviation from Beer's law
for, higher-energy monomer bands with the appearance of lower-energy bands at high concentrations
due to the dimerization of molecules. Time-dependent density-functional theory (TD-DFT) calculations
for monomer and dimer models show good agreement with the experimental spectra and account for the
major absorption bands. Ab initio calculations (CCSD(T)/cc-pVTZ) show a blue shift of ∼23 cm-1 in
the νC⋮O frequency upon complexation, thus providing the first computational evidence of this anomalous
blue shift known experimentally for Au(CO)Cl.