Hierarchical Assembly and Aggregation-Induced Enhanced Emission of a Pair of Isostructural Zn₁₄ Clusters

Information of solid-state and solution structures is crucial in the characterization of molecular clusters and in advancing the understanding of their diverse properties. [Et₃NH]₂[Zn₁₄(hmq)₈(OH)₄X₁₀] [X = Cl and Br; H₂hmq = 2-(hydroxymethyl)quinolin-8-ol] consist of a peanut-shaped Zn₁₀O₁₂ core, in which the Zn atoms occupy the faces and corners of an octahedron and are protected by bonded halogen atoms and bulky organic ligands. Observation of the [Zn₁₄(hmq)₈(OH)₄X₁₀]^2- fragment in electrospray ionization mass spectrometry (ESI-MS) suggests that the cluster is stable in solution. ESI-MS analyses from dissolved crystals and mother liquors reveal that Zn(hmq) self-assembles to Zn₅(hmq)₄Cl, then dimerizes through four [OH]^- bridges to Zn₁₀(hmq)₈(OH)₄Cl₂, and progressively captures four ZnCl₂ one-by-one to [Zn₁₄(hmq)₈(OH)₄Cl₁₀]^2-. Because the supramolecular interactions between the anion and cation in the solid suppress the rotation/vibration of the halogen atoms and confine the movable organic ligands on the rigid Zn-O core, both crystal phases exhibit intense photoluminescence, much stronger than that in solution. This is the first coordination cluster to exhibit "aggregation-induced enhanced emission". In addition, preliminary tests indicate that these coordination clusters are promising for organic-light-emitting-diode applications.