Tuning Photochromism and Photomagnetism via Diverse Bimetal-cyanido Viologen Hybrid Materials

Abstract

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 (MVII)0.5[MIIIMII(CN)6]·(H2O)n (i.e., FeMn, FeZn, CrMn, CrZn; MVII = 1,1'-dimethyl-4,4'-bipyridine dication; MIII = FeIII, CrIII; MII = MnII, ZnII), by incorporating diverse polycyanidometallates and viologen moieties into the donor-acceptor (D-A) hybrid molecular-based materials. FeZn and CrZn both exhibit visible photoinduced electron transfer (PET) processes upon Xe lamp irradiation at room temperature. However, while CrZn undergoes a reversible photochromism, FeZn displays irreversible photochromism, which originates not only from photoinduced radicals but also from Fe reduction. Interestingly, the change in 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 behaviors upon irradiation due to the magnetic interactions between photoinduced viologen radicals and low-spin (LS) FeIII 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 to photo-responsive multifunctional materials using cyanidometallic viologen hybrid compounds.