Chiral Silver-Lanthanide Metal-Organic Frameworks Comprised of One-Dimensional Triple Right-Handed Helical Chains Based on [Ln7(µ3-OH)8]13+ Clusters.

ABSTRACT

Three new isostructural chiral silver-lanthanide heterometal-organic frameworks [Ag3Ln7(µ3-OH)8(bpdc)6(NO3)3(H2O)6](NO3)·2H2O [Ln = Eu (1), Tb (2, Sm (3); H2bpdc = 2,2'-bipyridine-3,3'-dicarboxylic acid] based on heptanuclear lanthanide clusters [Ln7(µ3-OH)8]13+ comprised of one-dimensional triple right-handed helical chains were hydrothermally synthesized. Various means such as UV-vis spectroscopy, IR spectroscopy, elemental analysis, powder X-ray diffraction, and thermogravimetric/differential thermal analysis were used to characterize the compounds, wherein compound 3 was crystallographically characterized. In the structure of compound 3, eight µ3-OH- groups link seven Sm3+ ions, forming a heptanuclear cluster, [Sm7(µ3-OH)8]13+, and the adjacent [Sm7(µ3-OH)8]13+ clusters are linked by the carboxylic groups of bpdc2- ligands, leading to the formation of a one-dimensional triple right-handed helical chain. The adjacent triple right-handed helical chains are further joined together by coordinating the pyridyl N atoms of the bpdc2- ligands with Ag+, resulting in a chiral three-dimensional silver(I)-lanthanide(III) heterometal-organic framework with one-dimensional channels wherein NO3- anions and crystal lattice H2O molecules are trapped. The compounds were studied systematically with respect to their photoluminescence properties and energy-transfer mechanism, and it was found that H2bpdc (the energy level for the triplet states of the ligand H2bpdc is 21505 cm-1) can sensitize Eu3+ luminescence more effectively than Tb3+ and Sm3+ luminescence because of effective energy transfer from bpdc2- to Eu3+ under excitation in compound 1.