Gold Au(I)₆ Clusters with Ligand-Derived Atomic Steric Locking: Multifunctional Optoelectrical Properties and Quantum Coherence

An atomically precise ultrasmall Au(I)₆ nanocluster where the six gold atoms are complexed by three sterically interlocking stabilizing ligands is reported, allowing a unique combination of efficient third harmonic generation (THG), intense photoluminescence quantum yield (35%), ultrafast quantum coherence, and electron accepting properties. The reaction of 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol (TRZ) with HAuCl₄ leads to complexation by thiolation. However, intriguingly, another reduction step is needed to form the centrosymmetric Au(I)₆TRZ₃ clusters with the multifunctional properties. Here, ascorbic acid is employed as a mild reducing agent, in contrast to the classic reducing agents, like NaBH₄ and NaBH₃CN, which often produce mixtures of clusters or gold nanoparticles. Such Au(I)₆ nanocluster films produce very strong THG response, never observed for nanoclusters. The clusters also produce brilliant single and multiphoton luminescence with exceptional stability. Density functional theory calculations and femtosecond transient absorption studies suggest ultrafast ligand-to-metal charge transfer, quantum coherence with long decoherence time 200–300 fs, and fast propagation of excitation from the core to the surrounding solvent. Finally, novel electron-accepting ground state properties allow p-doping of 2D field-effect transistor devices. Summarizing, the potential of ultrasmall sterically interlocked Au(I) clusters, i.e., complexes allowed by the new sequential reduction protocol, towards multifunctional devices, fast photoswitches, and quantum colloidal devices is shown.