Lanthanide molecular nanomagnets featuring reversible photoresponsive characteristics and photomediated magnetic behavior are highly appealing for developing next-generation optics and memory devices though the rational assembly of such materials remains elusive. Herein, we demonstrate a step-wise approach, which employs a photochromic dysprosium–viologen compound {[Dy(ipbp)₂(H₂O)₃]·NO₃·2H₂O}_n (1) (H₂ipbp·Cl = (1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium chloride)) as the building block and 2,5-pyrazinedicarboxylate dianions (2,5-pzdc²⁻) as bridges, giving rise to a new framework {[Dy(ipbp)₂(2,5-pzdc)_0.5(H₂O)]·5H₂O}_n (2) exhibiting a photochromic effect and slow magnetic relaxation under a zero dc field. This finding is notable and exciting, as the starting compound 1 shows no magnetic relaxation on its own. The ab initio calculations demonstrate that by replacing water molecules at the equatorial plane in 1 with the 2,5-pzdc²⁻ bridge, the longer Dy–N bond length and the less charged pyrazine nitrogen atom result in a weaker equatorial field strength, and the coordination geometry around the Dy³⁺ ion in 2 is thus closer to the ideal triangular dodecahedral geometry than in 1, leading to stronger axial magnetic anisotropy of the Dy³⁺ ion and switch-on of slow magnetic relaxation under a zero dc field. Further magnetic analysis of compound 2 before and after UV light irradiation reveals the weak ferromagnetic coupling between the photogenerated radicals and the Dy³⁺ ions, a slight inhibition of quantum tunnelling of magnetization, as well as a modest enhancement of the energy barrier for the slow magnetic relaxation, highlighting the photomodulation of the dynamic magnetic behavior.

中文翻译：

---

在零直流场下具有光开关慢磁弛豫的镝-紫精框架的逐步组装


镧系元素分子纳米磁体具有可逆的光响应特性和光介导的磁行为，对于开发下一代光学和存储器件极具吸引力，尽管此类材料的合理组装仍然难以捉摸。在此，我们展示了一种逐步方法，该方法采用光致变色镝-紫精化合物 {[Dy（ipbp）₂（H₂O）₃]·NO₃·2H₂O}_n （1） （H₂ipbp·Cl = （1-（3,5-二羧基苯基）-4,4′-联吡啶氯化铵）） 作为结构单元，2,5-吡嗪二羧酸二离子 （2,5-pzdc²⁻） 作为桥，产生了一个新的框架 {[Dy（ipbp）₂（2,5-pzdc）_0.5（H₂O）]·5H₂O}_n （2） 在零直流场下表现出光致变色效应和缓慢的磁弛豫。这一发现值得注意且令人兴奋，因为起始化合物 1 本身没有表现出磁弛豫。 从头计算表明，通过将 1 中赤道平面上的水分子替换为 2,5-pzdc²⁻ 桥，较长的 Dy-N 键长和带电的吡嗪氮原子较少，导致赤道场强较弱，因此 2 中 Dy³⁺ 离子周围的配位几何形状比 1 更接近理想的三角形十二面体几何形状，导致 Dy³⁺ 离子的轴向磁各向异性更强，并在零直流场下开启缓慢的磁弛豫。对化合物 2 在紫外光照射前后的进一步磁性分析揭示了光生自由基和 Dy³⁺ 离子之间的弱铁磁耦合，对磁化量子隧穿的轻微抑制，以及对缓慢磁弛豫的能垒的适度增强，突出了动态磁行为的光调制。