Photoinduced magnetic coupling between 4f systems and π–π* excited states: An overview in 1:1 lanthanide–porphyrin complexes

Lanthanide-porphyrin complexes are gaining renewed interest as molecular systems where light and magnetism can be coupled at the excited-state level. Building on the foundation of single-molecule magnetism in lanthanide coordination chemistry, recent studies have uncovered a new magnetic interaction between the angular momentum generated by the localized 4f electronic system (J) and the orbital angular momentum generated by the cyclic [Formula: see text]-conjugated electronic system (L). This phenomenon, called J-L interaction, occurs only upon photoexcitation and has been experimentally confirmed through variable-temperature, variable-magnetic-field magnetic circular dichroism (VTVH MCD) spectroscopy. In particular, terbium and dysprosium complexes show temperature- and magnetic-field-dependent MCD A-terms in the range of Soret and Q-bands region of porphyrin, providing direct evidence of magnetically active excited states. Systematic comparisons among lanthanide ions and capping ligand environments indicate that the magnitude and sign of the J-L coupling are sensitive to the metal identity and ligand field symmetry. Lower-symmetry capping ligands consistently enhance the interaction, and theoretical simulations (using RASSCF/RASSI methods) support the experimental results. These findings may open the door for photoresponsive magnetic materials, where light can control molecular magnetization. This brief review highlights recent experimental and computational advances in this field, focusing on design principles and future prospects for utilizing excited-state magnetism in lanthanide coordination complexes.