Aggregation-mode-dependent Optical Properties of Cationic Gold Clusters: Formation of Ordered Assemblies in Solution and Unique Optical Responses

In this Article, we report the results of studies on the aggregation of a diphosphine-ligated [Au-6](2+) cluster in aqueous organic solvents, and we highlight the formation of ordered cluster assemblies associated with the J-aggregate-like absorption response and photoluminescence enhancement. Upon aggregation in methanol/water (10/90 v/v), the nitrate salt exhibited a definite red shift of the visible absorption band from the monomeric form. X-ray diffraction profiles of the aggregate in solution gave sharp signals, which revealed the formation of ordered cluster assemblies with particular orientation. The short intercluster distances, as estimated from the diffraction results, imply the interpenetrative packing of the ligand layers, which may contribute to the excitonic coupling of the cluster transition dipoles. The aggregation at the water-rich conditions also caused the enhancement of the near-IR photoluminescence emission, which can be ascribed to the restriction of molecular motion/vibration resulting from the close packing of the cluster molecules in the aggregates. The detailed inspections on the effects of the water contents and counteranions indicated that there are several aggregation modes with different optical features. This work showcases the unique properties of "cluster of clusters" and shows the potential utility of coordinated metal clusters in the design of aggregation-based functional materials.