Chiral inversion of macroscopic helical structures in coordination polymers induced by lanthanide ions

Abstract

Chiral inversion is a common phenomenon in nature and has been observed in many artificial systems. For one-dimensional (1D) chiral coordination polymers (CPs), recent research has been limited to the mechanisms governing the formation of their macroscopic helical structures and the regulation of chirality by interchain weak interactions, without addressing the influence of metal ions. In this study, we demonstrate for the first time that different metal ions within the CP chains can induce the formation of helical structures with opposite helicity. By reacting lanthanide nitrates with R-(1-phenylethylamino)methylphosphonic acid (R-pempH₂) under similar hydrothermal reaction conditions except for the metal ions, we obtained isomorphous compounds Ln(R-pempH)₃·2H₂O (R-1h-Ln, Ln = Sm, Eu, Gd, Tb, Dy, Ho) featuring a macroscopic helical structure. We found that when the metal-to-ligand (M : L) molar ratio was kept at 1 : 6, the chirality of the superhelices depended on the specific lanthanide ions: the Sm and Eu helices were right-handed (P), while the others were left-handed (M). Interestingly, the helicity of heavy lanthanide compounds R-1h-Ln (Ln = Gd–Ho) was also influenced by the M : L ratio and the autoclave filling degree. Thus, by adjusting these two parameters, R-1h-Ln (Ln = Gd, Tb, Dy) superhelices with identical compositions and structures but opposite helicity can be obtained. Further studies indicated that the compounds R-1h-Tb(M) and R-1h-Tb(P), possessing opposite helical orientations, exhibited circularly polarized luminescence (CPL) properties. Although their signals are weak, they are nearly mirror images of each other, indicating that the macroscopic helicity significantly affects their chiroptical properties.