Strong NIR-II Magneto-Optical Activity of a Chiral Sm₁₅Cu₅₄ Cage

Abstract

The magneto-optical response of chiral materials holds significant potential for applications in physics, chemistry, and biology. However, exploration of the near-infrared (NIR) magneto-optical response remains limited. Herein, we report the synthesis and strong NIR-II magneto-optical activity of three pairs of chiral 3<i>d-</i>4<i>f</i> clusters of <i><b>R/S</b></i><b>-Ln</b>_<b>15</b><b>Cu</b>_<b>54</b> (Ln = Sm, Gd, and Dy). Structural analysis reveals that <i><b>R/S</b></i><b>-Ln</b>_<b>15</b><b>Cu</b>_<b>54</b> features a triangular prism cage with <i>C</i>₃ symmetry. Interestingly, magnetic circular dichroism (MCD) spectra exhibit remarkable magneto-optical response in the NIR-II region, driven by the <i>f-f</i> transition. The maximum <i>g</i>-factor of <i><b>R/S</b></i><b>-Sm</b>_<b>15</b><b>Cu</b>_<b>54</b> reaches 5.5 × 10^-3 T^-1 around 1300-1450 nm, surpassing values associated with Dy^III and Cu^II ions. This remarkable NIR-II magneto-optical activity may be attributed to strong magnetic-dipole-allowed <i>f-f</i> transitions and helix chirality of the structure. This work not only presents the largest Ln-Cu clusters to date but also demonstrate the key role of magnetic-dipole-allowed transitions on magneto-optical activity.