Spin-crossover (SCO) system integrated with photochemical [2 + 2] reaction is firstly reported. The dimerization of azastilbene ligands dramatically modulated SCO properties, providing a new strategy for exploiting photo-responsive SCO materials.

The first example of phototunable SMMs has been reported. Upon UV irradiation, variations of the coordination sphere around Dy(III) ions actually affect the magnetic behaviour of the compound via [2+2] cycloaddition reaction, leading to a magnetic transformation from the SMM behaviour to a field-induced slow relaxation.

Metal−organic frameworks (MOFs) provide versatile platforms to construct multi-responsive materials. Herein, by introducing the neutral tetradentate ligand and the linear dicyanoaurate(I) anion, we reported a rare cationic MOF [Fe II (TPB){Au I (CN) 2 }]I•4H 2 O•4DMF (TPB = 1,2,4,5-tetra(pyridin-4-yl)benzene) with hysteretic spin-crossover (SCO) behavior near room temperature. This hybrid framework with an open metal site (Au I ) exhibits redox-programmable capability toward dihalogen molecules. By means of post-synthetic modification, all the linear [Au I (CN) 2 ] − linkers can be oxidized to square planar [Au III (CN) 2 X 2 ] − units, which results in the hysteretic SCO behaviors switching from one-step to two-step for Br 2 and threestep for I 2 . More importantly, the stepwise SCO behaviors can go back to one-step via the reduction by L-ascorbic acid (AA). Periodic DFT calculations using various SCAN-type exchange-correlation functionals have been employed to rationalize the experimental data. Hence, these results demonstrate for the first time that switchable one-/two-/three-stepped SCO dynamics can be manipulated by chemical redox reactions, which opens a new perspective for multi-responsive molecular switches.

Herein, we report the first example showing the reversible on-off switching of spin-crossover (SCO) property by solid state photochemical [2+2] reaction. The ultraviolet (UV) light-induced [2+2] cycloaddition reaction of 3-spy ligands in a twodimensional (2D) Hofmann-type framework [Fe(3-spy) 2 {Pd(CN) 4 }] (1, 3-spy=3-styrylpyridine), which shows a hysteretic twostep SCO behavior, gives a 3D Hofmann-type framework [Fe(rctt-3-ppcb){Pd(CN) 4 }] (2, rctt-3-ppcb=rctt-1,3-bis(3-pyridyl)-2,4-bis(phenyl)cyclobutane, r=reference group, c=cis and t=trans) accompanied with the disappearing of SCO properties. Moreover, upon heating at 250 °C for 12 h, the rctt-3-ppcb ligand in 2 could be partially dedimerized to 3-spy with 68% completion through single-crystal-to-single-crystal (SCSC) transformation, giving the annealing complexes [Fe(3-spy) 1.36 (rctt-3-ppcb) 0.32 {Pd(CN) 4 }] (3) which display an incomplete SCO behavior. The 2 ⇌ 3 interconversion is successfully achieved via continuous UV irradiation and thermal annealing, demonstrating the effectiveness of photochemical [2+2] reaction on switching on-off SCO properties.

The photochemical [2 + 2] reaction on a mononuclear Dy(iii) single-molecule magnet leads to the higher energy barrier after UV irradiation, along with the shorter magnetic relaxation time that is due to the acceleration of Raman process at low temperatures.