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Molecular materials possessing phototunable fluorescence properties have attracted great interest owing to their potential applications in optical switches and storage. However, most fluorescence modulation is realized through light-responsive structural isomerization in solution. It is a formidable challenge to achieve phototunable fluorescence emission with high fatigue resistance and a fast response rate in the solid state for the development of devices. Here, a mononuclear compound was constructed via the coordination of fluorophores with Fe II ions, whose electronic configuration changed from low spin to high spin upon light irradiation. The photoinduced spin crossover of Fe II ions was accompanied by a 20% increase in the fluorescence emission intensity. A temperature-dependent spectroscopic study together with time-dependent density functional theory calculations revealed that the effective spectral overlap between the emission of the fluorophores and the absorption band of the Fe II ions differed between the low spin and high spin states. The photoinduced spin crossover switched the energy transfer from the fluorophore to the Fe II ion, resulting in fluorescence modulation. The presented results provide a novel approach for developing optical memory and sensors via electron rearrangement of photoinduced spin crossover.

A hydroxide-bridged centrosymmetric Dy III dimer with each Dy III being five-coordinated has been synthesized using bulky hindered phenolate ligands. Magnetic studies revealed that this compound exhibits a slow magnetic relaxation of a single-ion origin together with a step-like magnetic hysteresis of the magnetic coupled cluster. The thermal relaxation barrier of magnetization is 721 K in the absence of a static magnetic field, while the intramolecular magnetic interaction is very large among reported 4f-only dimers. CASSCF calculations with a larger active space were performed to understand the electronic structure of the compound. The thermal relaxation regime and the quantum tunneling regime are well separated, representing a good model to study the relaxation mechanism of SMMs with intramolecular Dy-Dy magnetic interactions.

The slow magnetic relaxation typical for single-ion magnets has been known for certain low-coordinate 3d metal complexes with d 6 , d 7 , and d 9 electronic configurations, but never for d 8 complexes. Herein, we report a study on two-coordinate d 8 cobalt(i)-N-heterocyclic carbene complexes, for which slow magnetic relaxation behavior was observed for [Co(IMes) 2 ][BPh 4 ] (IMes: 1,3-dimesitylimidazol-2-ylidene) under an applied dc field. The system represents the first d 8 single-ion magnet, and features a fitted energy barrier of U eff = 21.3 cm -1 and pre-exponential factor of τ 0 = 6.6 × 10 -6 s. The analog two-coordinate cobalt(i) complexes with different NHC ligands, [Co(sIMes) 2 ][BPh 4 ] (sIMes: 1,3-dimesitylimidazolin-2-ylidene) and [Co(IAd) 2 ][BAr F 4 ] (IAd: 1,3-dimesitylimidazol-2-ylidene; BAr F 4 : tetra(3,5-ditrifluoromethylphenyl)borate), do not show such single-ion magnet behaviour. Ab initio calculations imply that the dihedral angle between the two NHC planes and the degree of unsaturation of the NHC ligands can dramatically alter the D value of the two-coordinate cobalt(i)-NHC ions, possibly via changing of the Co-NHC π-interactions, and hence affect the spin-orbit coupling splitting.