Anion effects of fluorescent spin crossover Fe(II) rhodamine hydrazone complexes

we report the synthesis, and comprehensive characterizations of rhodamine 6G hydrazone-pyridine Schiff base ligands (L1: R = H; L2: R = CH3) and their transition metal complexes, including 3d-4f supramolecular complexes [Fe(L1)2][Dy(NO3)5]·CH2Cl2·2H2O·CH3OH (1), [Ni(L1)2][Dy(NO3)5]·2CH2Cl2·CH3OH (2), and [Fe(L2)2](NO3)2·2CH3OH (3). Single-crystal X-ray diffraction analysis reveals that the Fe(II)/Ni(II) ion adopts a octahedral N4O2 environment via chelation with ring-opened neutral ligands L1 or L2. Complexes 1-3 all exhibit π-π stacking interactions forming one-dimensional supramolecular chain structures. Magnetic studies demonstrate that complex 1 exhibits spin crossover (SCO) above room temperature accompanying desolvation (guest effect), whereas complex 3 shows solvent-insensitive SCO with an inflection point at c.a. 280 K. Temperature-dependent fluorescence measurements show that both complexes 1 and 3 exhibit anomalous emission at 558 nm (xanthene group) during SCO, attributed to Fe(II) metal-to-ligand energy transfer. Notably, the [Dy(NO3)5]2− anion in complex 1 displays slow magnetic relaxation, making it the first 3d-4f material with combined SCO-fluorescence coupling and single-molecule magnet (SMM) behavior.