Homochiral lanthanide metal-organic frameworks based on amino acid-functionalized terpyridyl ligand: synthesis, structure determination, and optical properties

Abstract Utilizing chiral terpyridine derivative ligand (S)-2-((4-([2,2':6',2''-terpyridin]-4'-yl)benzyl)amino)propanoic acid (HL) and corresponding Ln(NO3)3, five new isostructural homochiral lanthanide metal-organic frameworks [{[Ln2(HL)2(NO3)4(H2O)4]·NO3·Cl·7H2O}n [Ln = Nd (1), Pr (2), Sm (3), Eu (4), Tb (5)] comprised of left- and right-handed helical chains were solvothermally synthesized. Various means such as IR spectroscopy, elemental analysis, powder X-ray diffraction, and thermal analysis were used to characterize the compounds, wherein compounds 1 and 2 were crystallographically characterized. In the structure of representative 1, zwitterionic ligands HL bridge Nd3+ ions through the monodentate coordination of their carboxylates and the tridentate chelation of their terpyridyls, leading to the formation of left- and right-handed helical chains. The adjacent left- and right-handed helical chains are further connected into a 2D helical layer through interchain hydrogen-bonding interactions, and these layers stack up through interlayer π···π interactions to form an interesting three-dimensional homochiral cationic architecture with one-dimensional triangular channels wherein guests anions and water are trapped. The compounds were systematically studied with respect to their solid-state optical properties, including CD spectra, and nonlinear optical and luminescent properties, and the results showed that the sensitization effect of HL ligand on the emission of Tb3+ and Eu3+ was better than that of Sm3+.