Here, we use a pyridinecarbaldehyde rhodamine 6G hydrazone ligand (L) to synthesize an Fe(II) complex 1 for the search of new fluorescent-spin crossover (SCO) materials. Single-crystal structural determinations suggest that the Fe(II) ion is chelated by two ring-opened ligands (L-o) to form a FeNO coordination environment, and intermolecular π---π contacts of the xanthene groups connect the adjacent molecules to form a supramolecular one-dimensional chain. Magnetic susceptibility measurements on complex 1 show that three-step SCO takes place in the temperature range of 120-350 K, and its desolvated form 1-d exhibits SCO around room temperature ( T↑ = 343 K and T↓ = 303 K) with a wide hysteresis loop of 40 K. Moreover, complex 1-d displays light-induced excited spin-state trapping phenomenon. Intriguingly, the fluorescence intensity of the maximum emission at 560 nm for complex 1-d displays discontinuous variation in the range of 250-400 K, indicative of the occurrence of synergetic fluorescence and SCO.

The magnetic properties of Fe(ii) pyridine-2-carbaldehyde rhodamine 6G hydrazone complexes are modulated by the substituents. The desolvated complex displays the correlation between the spin crossover and the fluorescence.

Two rhodamine 6G-based mononuclear dysprosium complexes, [Dysalicylaldehyde rhodamine 6G hydrazone, L A = 2-pyridylcarboxaldehyde benzoyl hydrazone), are synthesized, aiming at improving the magnetic behavior by modulating their coordination environment. Both complexes own one exclusive short Dy− O phenoxy coordination bond as the predominant bond and exhibit singlemolecule magnet behavior under zero dc field with the energy barrier (U eff /k B ) of 90 K (1) and 320 K (2) and apparent hysteresis at 1.9 K. The ab initio calculations indicate that the short Dy−O phenoxy bond determines the direction of magnetic anisotropic axis for 1 and 2. The quantum tunneling of magnetization (QTM) between the ground Kramers doublets (KDs) in 1 cannot be neglected, leading to an experimental U eff /k B much lower than the calculated energy of the first excited state (318.2 K). For 2, the stronger magnetic anisotropy and negligible QTM between the ground KDs guarantees that the energy barrier is close to the calculated energy of first KDs (320.8 K). On the other hand, the presence of ring-opened xanthene moiety makes complexes 1 and 2 in the solid state emit red light with emission bands of 645 and 658 nm, respectively.

Two new CoII complexes, namely bis{N-[(6-bromopyridin-2-yl)methylidene]-2-[6-ethylamino-3-(ethyliminiumyl)-2,7-dimethyl-3H-xanthen-9-yl]benzene-1-carbohydrazonate}cobalt(II) bis(perchlorate)–dichloromethane–methanol (1/1/2), [Co(C32H30BrN5O2)2](ClO4)2·CH2Cl2·2CH3OH or [CoII(L)2](ClO4)2·CH2Cl2·2CH3OH, (1), and the bis(tetrafluoridoborate) salt, [Co(C32H30BrN5O2)2](BF4)2·CH2Cl2·2CH3OH or [CoII(L)2](BF4)2·CH2Cl2·2CH3OH, (2) (L is commonly 6-bromopyridine-2-carbaldehyde rhodamine 6G hydrazone), have been successfully constructed and characterized. The crystal structure analysis revealed that complexes (1) and (2) are mononuclear and have a CoIIN4O2 distorted octahedral structure. The large π-conjugated xanthene moiety of the L ligand causes strong intermolecular π–π stacking interactions, yielding a supramolecular one-dimensional chain. Complexes (1) and (2) display an obvious fluorescence emission near 560 nm in the solid state. Magnetic investigations show that both (1) and (2) are paramagnetic, dominated by the structural distortion and spin-orbit coupling of CoII.