Element-Specific Sensitivity of Lanthanoid Ions to Chiral Crystal Fields in K3[Ln2(NO3)9] Probed by 4f–4f Circular Dichroism

Abstract

For chiral inorganic crystals composed of achiral components, chiral crystal fields around metal ions are expected to play a key role in the emergence of chiroptical properties. Circular dichroism (CD) spectra can serve as useful probes of such chiral crystal fields. Here, we report the optical resolution of K3[Ln2(NO3)9], a family of chiral potassium lanthanoid nitrates, and systematically investigate chiral crystal fields associated with 4f electrons in the solid state. The K3[Ln2(NO3)9] series (Ln = La, Ce, Pr, Nd, Sm, Eu, and Gd) crystallizes in the chiral space groups P4132 and P4332, and both enantiomorphs are obtained by spontaneous resolution. Among the series, K3[Pr2(NO3)9], K3[Nd2(NO3)9], K3[Sm2(NO3)9], and K3[Eu2(NO3)9] show sharp CD bands arising from 4f–4f transitions, demonstrating the presence of chiral crystal fields around lanthanoid ions in a fully inorganic environment. The distorted, noncentrosymmetric coordination environment around the lanthanoid ion partially relaxes the Laporte-forbidden nature of 4f–4f transitions, leading to enhanced absorption and CD intensities. Notably, Sm3+ exhibits the largest absolute CD anisotropy factor (|gCD|), suggesting that it is particularly sensitive to the chiral crystal field among the lanthanoid ions investigated.