Ultra-stable Ag₃₀ nanoclusters with metallic aromaticity show promise for optoelectronic devices

As ultra-small nanoparticles with uniform sizes and well-defined structures, ligand-protected coinage metal nanoclusters show distinctive electronic structures and physicochemical properties in contrast to bulk nanomaterials due to quantum size effects.

Nevertheless, most of them suffer from issues such as facile dissociation in solutions and poor thermodynamic stability, which greatly restrict their practical applications as optoelectronic materials.

Peripheral protection ligands are the key elements in coinage metal clusters, thus exerting a crucial impact on the chemical properties and stability.

In most cases, the protective effect of ligands with simply bridging character could not endow sufficient robustness and internal stability of nanoclusters. To provide adequate shielding and protecting effects for metal cluster kernels, bifunctional ligands having both bridging and chelating characteristics are better candidates for constructing robust metal nanoclusters with highly efficient luminescence.

In a study published in Science Advances, Prof. Chen Zhongning from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences and Prof. Sun Di from Shandong University designed a stable bifunctional diphosphine (2-Ph₂PC₆H₄PH₂) featuring primary and tertiary phosphorus donors as a unique chelator which shows synergistic bridging and chelating characteristics, and constructed ultrastable silver nanoclusters with metallic aromaticity.

Upon deprotonation, both -P^2- and -PPh₂ donors simultaneously bridged and chelated multiple metal ions to give m₅-P^2- capped M₅ pentagons through strong Ag-Ag interactions, thus resulting in arc surfaces or spherical caps, which serve as building blocks for the construction of spherical architectures with specific polyhedral nanoclusters.

By precisely controlling the deprotonation rate of 2-Ph₂PC₆H₄PH₂ and introducing organic or inorganic templates, researchers synthesized and characterized two novel silver nanoclusters (Ag₃₀ and Ag₃₂) possessing fascinating icosidodecahedral Ag₃₀ shells with an Ih symmetry.

The distinctive coordination geometry of 2-Ph₂P_αC₆H₄P_β^2- in m_5-η¹(P_β),η²(P_α,P_β) coordination mode endows Ag₃₀ and Ag₃₂ clusters with spherical metallic aromaticity, which plays a pivotal role in the exceptional stability of the two clusters.

As each 2-Ph₂P_αC₆H₄P_β^2- ligand possesses two negative charges and bonds to five silver(I) atoms via the -Pβ^2- donor to provide three lone pairs of electrons to form a 6c-6e^– delocalized bond, twelve such 2-Ph₂PC₆H₄P_2- ligands on the Ag₃₀ shell of Ag₃₀ or Ag₃₂ afford 72 bonding electrons (2×(5+1)²), which is consistent with the 2(N+1)² rule of spherical aromaticity.

Ag₃₀ exhibits high-efficiency near-infrared (NIR) electroluminescence in solution-processed organic light-emitting diodes (OLEDs) with peak external quantum efficiency (EQE) of 15.1%, representing a significant progress in the application of silver nanoclusters to NIR-emitting devices.

This study not only broadens the scope of protective ligands available for coinage metal clusters to get insights into synthesizing coinage metal clusters endowed with metal aromaticity and excellent stability, but also provides guides for the practical application of functional nanoclusters in optoelectronic devices.