Homochiral Metallohelicates Based on Zinc(II) and Cadmium(II) Clusters with Excellent Thermostability and Remarkable Second-Harmonic Generation

Two interesting homochiral coordination polymers based on in situ produced zinc(II)/cadmium(II) clusters, namely, {[Zn₄(L)₂(OH)₂(H₂O)₃]·3H₂O}_n (1) and {[Cd₃(L)₂(H₂O)₄]·4H₂O}_n (2), were hydro/solvothermally synthesized by the reaction of the designed enantiomerically pure chiral ligand (S)-5-((2-carboxy-5-oxopyrrolidin-1-yl)methyl)isophthalic acid (H₃L) with Zn(OH)₂ or Cd(OH)₂. Complex 1 is a three-dimensional (3D) metallohelicate constructed from novel six-fold helical chain substructures based on tetranuclear zinc(II) clusters. Complex 2 is a 2D double-layered metallohelicate containing interesting four-fold helical chain substructures based on trinuclear cadmium(II) clusters. Topologically, 1 can be described as an interesting (3,4,7)-connected net with a point symbol of (4²·6)(4²·6⁴)(4⁴·6¹⁵·8²), while 2 can be reducible to a (3,6)-connected kgd net. Thermal analyses disclosed that the dehydrated frameworks of 1 and 2 had excellent thermostability. Clearly, the in situ produced zinc(II)/cadmium(II) clusters are of great significance not only in the formation of novel helical frameworks of 1 and 2 but also in the excellent thermostability of their frameworks. Furthermore, their solid-state optical properties, including CD spectra and nonlinear optical and luminescence properties, were also carried out. In particular, 1 and 2 show remarkable second harmonic generation with efficiencies about 1.15 and 0.95 times that of potassium dihydrogen phosphate, respectively. Even more important, the high thermostability and remarkable second harmonic generation response of colorless transparent crystalline materials 1 and 2 are meaningful for further exploring their optical applications.