Multinuclear Cu _n S _m clusters encapsulated by aromatic micelles as aqueous red-to-NIR phosphorescent ink

Sulfur-bridged copper clusters are important biocomponents, yet their artificial analogues have rarely been studied in solution due to poor solubility and stability. Here we report the preparation of red-to-near-infrared (NIR) phosphorescent solutions from multinuclear Cu _n S _m clusters upon encapsulation by aromatic micelles in water. For instance, whereas a 2-mercapto-6-methylpyridine-based Cu₆S₆ cluster shows no solution-state emission owing to its insolubility in common solvents, the encapsulated cluster emits strong red-to-NIR phosphorescence (Φ = 34%; λ = 550-850 nm) with high stability, even under aerobic and ambient conditions. Similar host-guest complexes are also obtained from analogous Cu₆S₆ clusters, displaying substituent-dependent red-to-NIR emission in water. The present method is applicable to larger Cu₁₂S₆ and smaller Cu₄I₄ clusters to generate aqueous, red, yellow, and green emissive solutions. Notably, the resultant host-guest solution can be used as aqueous colorless ink, for potential security applications, which exhibits strong emission when painted on paper in the red-to-NIR region upon UV-light irradiation.