Tuning photochromism and photomagnetism via diverse bimetallic cyanido viologen hybrid materials†

The integration of photochromism and photomagnetism is of great significance for developing photo-responsive multifunctional materials. Herein, we successfully synthesized four cyanido-bridged bimetallic frameworks formulated as (MV^II)_0.5[M^IIIM^II(CN)₆]·(H₂O)_n (i.e., FeMn, FeZn, CrMn, and CrZn; MV^II = 1,1′-dimethyl-4,4′-bipyridine dication; M^III = Fe^III, Cr^III; M^II = Mn^II, Zn^II), by incorporating diverse polycyanidometallates and viologen moieties into donor–acceptor (D–A) hybrid molecule-based materials. FeZn and CrZn both exhibit visible photoinduced electron transfer (PET) processes upon Xe lamp irradiation at room temperature. However, while CrZn undergoes reversible photochromism, FeZn exhibits irreversible photochromism, which originates not only from photoinduced radicals but also from Fe reduction. Interestingly, the change in the absorption band of FeZn spans the UV-visible-near infrared (UV-Vis-NIR) region (ca. 200–2600 nm) after irradiation. Although FeMn and CrMn do not exhibit photoinduced colour changes, they behave as two-dimensional (2D) and three-dimensional (3D) ferrimagnets, respectively, with CrMn exhibiting long-range ordering below ca. 80 K. FeZn exhibits photomagnetic behavior upon irradiation due to magnetic interactions between photoinduced viologen radicals and low-spin (LS) Fe^III ions. Additionally, we investigate the remarkable influence of metal ions on photochromism and (photo)magnetism through detailed crystal structure analysis. This work provides a novel approach for synthesizing photo-responsive multifunctional materials using cyanidometallic viologen hybrid compounds.