Synthesis, structure and band gap energy of covalently linked cluster-assembled materials

We have synthesized a series of cluster assembled materials in which the building blocks are As ₇ ^3- clusters linked by group 12 metals, Zn, Cd and Hg, to investigate the effect of covalent linkers on the band gap energy. The synthesized assemblies include zero dimensional assemblies of [Zn(As ₇) ₂] ^4-, [Cd(As ₇) ₂] ^4-, [Hg ₂(As ₇) ₂] ^4-, and [HgAsAs ₁₄] ^3- in which the clusters are separated by cryptated counterions, and assemblies in which [Zn(As ₇) ₂] ^4-, [Cd(As ₇) ₂] ^4- are linked by free alkali atoms into unusual three-dimensional structures. These covalently linked cluster-assembled materials have been characterized by elemental analysis, EDX and single-crystal X-ray diffraction. The crystal structure analysis revealed that in the case of Zn and Cd, the two As ₇ ^3- units are linked by the metal ion, while in the case of Hg, two As ₇ ^3- units are linked by either Hg-Hg or Hg-As dimers. Optical measurements indicate that the band gap energy ranges from 1.62 eV to 2.21 eV. A theoretical description based on cluster orbital theory is used to provide a microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy.