Multinuclear CunSm clusters encapsulated by aromatic micelles as aqueous red-to-NIR phosphorescent ink

Abstract

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_nS_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.