Construction and Characterization of Several New Lanthanide−Organic Frameworks: From 2D Lattice to 2D Double-Layer and to Porous 3D Net with Interweaving Triple-Stranded Helixes

Five novel lanthanide−organic frameworks: [Ln(NTA)(H2O)2]·H2O (Ln = Eu (1)) and [Ln(NTA)(H2O)] (Ln = Tb (2), Ho (3, 4), and Tm (5); NTA = nitrilotriacetate anion) have been constructed by a rational synthetic strategy. 1 and 5 were successfully obtained by tuning the number of coordinated water molecules to lanthanide ions under different temperatures, while 2 and 3 were prepared under the same synthetic temperature as that of 5. Structural analyses indicate that they belong to three different space groups. 1 displays an irregular two-dimensional (2D) lattice consisting of alternative L- and R-type single-chain helixes, and both 2 and 3 are 2D double layer structures, while 4 and 5 are porous three-dimensional (3D) frameworks. Surprisingly, although 3 and 4 have identical molecular formulas, their structures exhibit significant differences originating from different pH values used in the synthesis. Interestingly, one-dimensional (1D) channels exist in the 3D frameworks of 4 and 5; each channel is fabricated through interweaving left- and right-hand triple-stranded helixes. The explorations of magnetic properties of 2 and 5 were carried out. The luminescent investigations reveal that 2 exhibits characteristic luminescence of Tb3+, and the lifetime of the transition 5D4 → 7F5 is about 0.78 ms when excited at λex = 296 nm.