Reversible SC‐SC Transformation involving 4+4 Cycloaddition of Anthracene: A Single‐Ion to Single‐Molecule Magnet and Yellow‐Green to Blue‐White Emission

In search of magneto-optic materials, the mononuclear compounds LnIII (depma)(NO3 )3 (hmpa)2 (Ln=Dy, Gd) were synthesized. The anthracene moieties undergo 4+4 dimerization when irradiated at 365 nm without loss of crystallinity. The Dy compound switches from a single-ion to a single-molecule magnet with doubling of the spin reversal barrier energy and from yellow-green to blue-white emission. The dimerization is reversed by heating at 100 °C or partially on light irradiating at 254 nm. The results suggest that lanthanide phosphonates with anthracene are promising smart materials displaying synergistic magneto-optic property.