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

Metallophilic interactions in d10-d10(AuI-AuI)/d8-d8(PtII-PtII, RhI-RhI, IrI-IrI) 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 AuIII complex cations are scarce in the literature. Herein is described a series of pincer-type cationic AuIII complexes with different auxiliary ligands, among which the AuIII-allenylidene complex displays a close Au-Au contact of 3.367 Å between neighboring molecules in its X-ray crystal structure; AuIII-isocyanide complexes show a broad red-shifted absorption band and prominent phosphorescence upon aggregation that was influenced by an attractive AuIII-AuIII bonding interaction in the excited state; and AuIII-acetylene complexes can undergo living supramolecular polymerization upon varying the counteranion. The nature of the emissive excited state(s) of the AuIII aggregates is assigned to a mixture of major 3[π-π*] and minor 3LMMCT (ligand-to-metal-metal charge transfer) states based on combined spectroscopic and DFT/TDDFT studies. The morphology of the AuIII 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.