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

Abstract

Metallophilic interactions in d¹⁰-d¹⁰(Au^I-Au^I)/d⁸-d⁸(Pt^II-Pt^II, Rh^I-Rh^I, Ir^I-Ir^I) 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^III complex cations are scarce in the literature. Herein is described a series of pincer-type cationic Au^III complexes with different auxiliary ligands, among which the Au^III-allenylidene complex displays a close Au-Au contact of 3.367 Å between neighboring molecules in its X-ray crystal structure; Au^III-isocyanide complexes show a broad red-shifted absorption band and prominent phosphorescence upon aggregation that was influenced by an attractive Au^III-Au^III bonding interaction in the excited state; and Au^III-acetylene complexes can undergo living supramolecular polymerization upon varying the counteranion. The nature of the emissive excited state(s) of the Au^III 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 Au^III 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.