Reversible Luminochromism of an N‐Heterocyclic Carbene‐Protected Carbon‐Centered Hexagold(I) Cluster by Solvent and Mechanical Stimuli

Abstract

Luminochromic gold clusters have potential applications in sensing due to the unique optical properties arising from their geometric and electronic structures. However, the rational design of such clusters is extremely challenging, especially when the goal is to impart reversible stimuli‐responsive properties to gold clusters. Herein, the reversible multi‐stimuli responsiveness of a carbon‐centered hexagold(I) CAuI6 cluster protected by N,N’‐dibenzylbenzimidazolylidene (BIBn) ligands is reported. Two types of crystals emitting blue (λemmax = 479 nm) and turquoise (λemmax = 497 nm) photoluminescence (PL) are obtained and found to be interconvertible by the addition and removal of solvent. Single‐crystal X‐ray structure analysis has revealed significant differences between the crystal structures, including the conformation of the BIBn ligands. Theoretical calculations suggest that these conformational differences directly affect the photophysical properties and that the flexibility of the BIBn ligands attached to the CAuI6 core makes this possible. Furthermore, when these crystals are grounded with a spatula, they are found to emit green PL (λemmax = 520 nm) following a partial loss of crystallinity. This is a remarkable example of both vapoluminochromism and mechanoluminochromism in an N‐heterocyclic carbene (NHC)‐protected gold cluster and holds promise as a model compound for understanding the principles governing stimuli‐responsiveness in metal clusters.

Chemical Flexibility of Atomically Precise Metal Clusters

“…Solvents often play a key role in the synthesis of metal clusters, significantly affecting their structure, stability, and reactivity, much like other materials prepared from solution or processed with solvents. − Solvent effects − encompass factors such as polarity, coordination ability, solvation, and boiling points. For example, certain solvent molecules, including DMF (N,N-dimethylformamide), THF (tetrahydrofuran), MeOH (methanol), MeCN (acetonitrile), and acetone, have the capability to stabilize metal clusters through coordination with the metal atoms. − This coordination aids in lowering the overall energy of the cluster, consequently influencing the geometry of the cluster.…”