Tuning the Single-Molecule Magnet and Photoluminescence Properties of Binuclear Dysprosium Complexes by Light

Lanthanide-based single-molecule magnets (Ln-SMMs) showing a synergistic magnetic and photoluminescence change in response to external stimuli are very attractive due to their potential applications in information storage, spintronics, and devices. In this paper, we report two dysprosium complexes containing Dy2O2 motifs and anthracene units, namely [Dy2(SCN)4(L)2(dmpma)2(H2O)2] (1) and [Dy2(SCN)4(L)2(depma)2(H2O)2] (2), where HL stands for 2,6-dimethoxyphenol, dmpma is 9-dimethylphosphonomethylanthracene, and depma is 9-diethylphosphonomethylanthracene. Both exhibit slow magnetization relaxation at zero dc field, characteristic of SMM behavior. Interestingly, complex 2 shows two thermally activated relaxation processes, attributed to the presence of disordered anthracene groups. The effective energy barriers are 117.7 K for 1 and 48.4 and 110.8 K for 2. Under 395 nm UV light irradiation, compounds 1 and 2 undergo a [4 + 4] photocycloaddition reaction to form [Dy2(SCN)4(L)2(dmpma2)(H2O)2]n (1UV) and [Dy2(SCN)4(L)2(depma2)(H2O)2]n center dot nH2O (2UV) with linear chain structures, where the structural transformation of complex 2 occurs in a single-crystal-to-single-crystal (SC-SC) fashion. Synergistic changes in photoluminescence and SMM behavior were observed for both compounds before and after light irradiation. The results show that the photodimerization of anthracene groups not only inhibits the magnetic relaxation in 1 and 2 and increases the magnetization reversal barrier (172.0 K for 1UV, 120.5 K for 2UV) but also eliminates the anthracene group disorder in compound 2, leading to a significant change in the magnetic dynamics from a dual-relaxation to a single-relaxation process.