Kinetically Controlled Self-Assembly of Phosphorescent Au Aggregates and Ligand-to-Metal-Metal Charge Transfer Excited State: A Combined Spectroscopic and DFT/TDDFT Study.

Metallophilic interactions in d-d(Au-Au)/d-d(Pt-Pt, Rh-Rh, Ir-Ir) complexes have been widely studied for decades, and metal-metal (M-M) bonding character has been revealed in both the ground and excited states. These M-M closed-shell interactions are appealing driving forces for the self-assembly of supramolecular/polymeric systems, providing luminescent properties distinctly different from those of the corresponding monomer. However, reports on attractive interactions between two Au complex cations are scarce in the literature. Herein is described a series of pincer-type cationic Au complexes with different auxiliary ligands, among which the Au-allenylidene complex displays a close Au-Au contact of 3.367 Å between neighboring molecules in its X-ray crystal structure; Au-isocyanide complexes show a broad red-shifted absorption band and prominent phosphorescence upon aggregation that was influenced by an attractive Au-Au bonding interaction in the excited state; and Au-acetylene complexes can undergo living supramolecular polymerization upon varying the counteranion. The nature of the emissive excited state(s) of the Au aggregates is assigned to a mixture of major [π-π*] and minor LMMCT (ligand-to-metal-metal charge transfer) states based on combined spectroscopic and DFT/TDDFT studies. The morphology of the Au aggregates is highly dependent on the concentration and nature of the counteranion. A qualitative model has been applied to account for the concentration- and counteranion-dependent kinetics of the supramolecular polymerization process.