This feature article summarizes the most up-to-date advances of lanthanide–radical compounds with single-molecule magnet behavior, including the design, synthesis and magnetic properties.

The pursuit of spin crossover (SCO) materials with photoswitchable multistability is driven by fascinating perspectives toward light-response switches and opto-magnetic memory devices. Herein, we report a 3D Hofmann-type metal–organic framework [Fe(bpn){Ag(CN)₂}₂]·1.5(Fnaph) (bpn = 1,4-di(pyridin-4-yl)naphthalene, Fnaph = 1-fluoronaphthalene), which displays an incomplete two-step SCO behaviour. The asymmetric host–guest interactions induce an intermediate HS–LS state at low temperature. Photomagnetic measurements exhibited an extremely rare bidirectional light-induced excited spin-state trapping (LIESST) effect with green and near-IR light irradiation, in which both metastable HS* and hidden LS* states were revealed at 10 K. The light-induced tristability paves the way for designing optomagnetic memory devices.