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

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) CAu^I₆ cluster protected by N,N’-dibenzylbenzimidazolylidene (BIBn) ligands is reported. Two types of crystals emitting blue (λ_em^max = 479 nm) and turquoise (λ_em^max = 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 CAu^I₆ core makes this possible. Furthermore, when these crystals are grounded with a spatula, they are found to emit green PL (λ_em^max = 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.