Hierarchical Assembly and Aggregation-Induced Enhanced Emission of a Pair of Isostructural Zn14Clusters

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 Zn10O12core, 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 ZnCl2one-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.