Spectroscopic insight into spin states of lanthanide double-decker single-molecule magnets

Bis(phthalocyaninato) lanthanide (LnPc₂) double-decker complexes represent a premier class of single-molecule magnets (SMMs) and have emerged as promising molecular building blocks for next-generation information storage, molecular spintronics, and quantum computing devices. Their potential stems from the large magnetic moments and significant magnetic anisotropy originating from the shielded 4f orbitals of the central lanthanide ion. Leveraging the sub-nanometer spatial resolution and spectroscopic capabilities of scanning tunneling microscopy (STM), recent investigations have provided unprecedented insights into the precise characterization of the electronic structure and magnetic properties of individual LnPc₂molecules at the single-molecule level. STM uniquely combines real-space imaging within situmanipulation, enabling site-specific spin control and the precise addressing of 4f moments at the single-molecule level. This review systematically examines recent progress in STM-based investigations of LnPc₂complexes, focusing on five pivotal aspects: (1) the on-surface adsorption geometries and molecular conformations; (2) the electronic signature of the delocalizedS= 1/2 ligand spin; (3) the breakthroughs in probing the localized 4f magnetic moment; (4) the modulation of spin states through external stimuli; and (5) the emergent interplay between molecular spins and Cooper pairs in superconductor-SMM hybrid systems. Finally, by synthesizing advances in STM methodology and lanthanide molecular magnetism, we highlight the key challenges in harnessing LnPc₂complexes for future research.