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)[Zn-14(hmq)(8)(OH)(4)X-10] [X = Cl and Br; H(2)Hmq = 2-(hydroxymethyl)quinolin-8-ol] consist of a peanut-shaped Zn10O2 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 [Zn-14(hmq)(8)(OH)(4)X-10](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 Zn-5(hmq)(4)Cl, then dimerizes through four [OH](-) bridges to Zn-10(hmq)(8)(OH)(4)Cl-2, progressively captures four ZnCl2 one-by-one to [Zn-14(hmq)(8)(OH)(4)Cl-10](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.