We present the intriguing magneto-optical properties of the lanthanide complex [Tb(coum)₃(batho)]·[0.7EtOH], named Tb–batho, based on coum = 3-acetyl-4-hydroxylato-coumarin and batho = bathophenanthroline ligands. Tb–batho displays visible-range luminescence with a notable quantum yield (58%) upon sensitization of the “antenna” ligands. Employing a SQUID magnetometer equipped with a magneto-optic option, we conducted comprehensive in situ measurements of light-induced magnetization changes across varied magnetic fields, temperatures, and frequencies, utilizing light wavelengths ranging from 275 to 800 nm.

A reversible magnetic modulation of magnetization is observed upon toggling the light “on” and “off,” particularly pronounced at 380 nm excitation, resulting in a magnetization change ΔM(off/on) = 0.376μ_B fu⁻¹ (ca. 42.6% of the magnetization) at 1.8 K and 1 kOe. Although Tb–batho exhibits field-induced single ion magnet (SIM) behavior, characterized by a thermally-activated process with an activation energy of U_eff/k_B = 16.6 K @ 3 kOe and a slow direct process influenced by bottleneck effects, light irradiation does not noticeably alter its dynamic properties. All in all, Tb–batho emerges as a versatile multifunctional molecular material integrating SIM behavior, luminescence and light-induced magnetic switching, holding interest for diverse electronic applications, sensors, or quantum computing.