**1. Introduction**

For the past decade, an intensive interest has been shown in the area of developing porous optical supertetrahedral clusters, such as for selenides [1–5]. The direct assembly of chalcogenides into the cluster framework will be a benefit approach in the development of crystalline porous chalcogenides. Recently, the Tn supertetrahedral clusters [2–4] dominate the advantages in the field for using porous materials in luminescence application. The reason is that the larger size of the Tn supertetrahedral clusters usually leads to a highly open frame work with porosity higher than 50% [1, 5, 6]. Such a property is suitable for studying optoelectronic properties induced by the quantum size cluster.

Another famous porous structure, the perovskite clusters with R3C space group, is known for its ternary ABX3 crystal structure as a light absorbent to be used in the solar cell area [7–9]. The porous spheres of perovskite SrTiO3 spheres present superior performance in photocatalytic oxygen evolution [10]. The porous structure existed in the R3C type structure could be envisioned as an alternating choice to study the optoelectronic properties due to quantum confinement. To date, the use of existed porosity in R3C structure for quantum size luminescence application

is very rare [11]. Here, we employ the Cd(SC6H4Me-4)2 as the starting material to prepare a novel ternary cadmium chalcogenide cluster [Cd6S18Se6] for studying the effect of crystalline porous frameworks in quantum size luminescence. The [Cd6S18Se6] cluster is potentially with porous R3C structure. The simpler atomic composition of R3C structure apparently allows one to manipulate the replaced site with more flexibility. Such an opportunity cannot be easily obtained from the larger Tn clusters [1, 4]. Thus, these ternary clusters will be more applicable in the study of the optoelectronic luminescent phenomena created by the embedded heteroatoms (i.e., Se) in the quantum size cluster. Therefore, the quantum confinement luminescence induced by the Se atom confined in [S3Se]2<sup>−</sup> tetrahedron is able to be observed in the ternary [Cd6S18Se6] cluster.
