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.我们报道了罗丹明6G腙-吡啶席夫碱（rhodamine 6G hydrazone-pyridine Schiff base）配体（L1: R = H；L2: R = CH3）及其过渡金属配合物的合成与全面表征，其中包含3d-4f超分子配合物[Fe(L1)2][Dy(NO3)5]·CH2Cl2·2H2O·CH3OH（1）、[Ni(L1)2][Dy(NO3)5]·2CH2Cl2·CH3OH（2）以及[Fe(L2)2](NO3)2·2CH3OH（3）。单晶X射线衍射（single-crystal X-ray diffraction）分析显示，Fe(II)/Ni(II)离子通过与开环中性配体L1或L2螯合，形成八配位N4O2配位环境。配合物1至3均存在π-π堆积相互作用，形成一维超分子链状结构。磁学研究表明，配合物1在室温以上表现出自旋交叉（spin crossover, SCO）行为，伴随脱溶剂化（desolvation）过程与客体效应（guest effect）；而配合物3则呈现溶剂不敏感的自旋交叉行为，其拐点约为280 K。变温荧光测试结果显示，配合物1和3在自旋交叉过程中，均在558 nm处出现呫吨基团（xanthene group）的反常发射，该现象源于Fe(II)的金属到配体能量转移（metal-to-ligand energy transfer）。值得注意的是，配合物1中的[Dy(NO3)5]2−阴离子展现出慢磁弛豫行为，使其成为首个兼具自旋交叉-荧光耦合特性与单分子磁体（single-molecule magnet, SMM）行为的3d-4f材料。