Reversible on-off switching of spin-crossover behavior via photochemical [2+2] cycloaddition reaction

Abstract

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.

Spin‐State Modulation in FeII‐Based Hofmann‐Type Coordination Polymers: From Molecules to Materials

“…Following the same trend, reversible on‐off switching of the SCO behavior was achieved in 2D HCP [Fe(3‐spy) 2 {Pd(CN) 4 }], which after UV irradiation transforms to a 3D framework connected through the corresponding cyclobutane adduct via a single‐crystal‐to‐single‐crystal manner [79] . Such change in the structure changed the hysteretic two‐step SCO behavior to complete HS states.…”

A Mononuclear Iron(II) Spin-Crossover Molecule Decorated by Photochromic Azobenzene Group

“…Molecules 2022, 27, x FOR PEER REVIEW 2 of 11 groups, showing cis-trans photoisomerization behavior [19,20], intramolecular photocyclization [21][22][23][24] and photochemical [2+2] cycloaddition [25,26], which were reported in recent years, and the corresponding photoresponsive SCO behaviors were further studied in the solid or liquid state. Moreover, a porous Hofmann-type SCO framework encapsulating trans-azobenzene photochromic guests [27] was reported by Tong et al recently.…”

“…For example, Zarembowitch et al put forward the ligand-driven light-induced spin crossover (LD-LISC) approach [ 15 , 16 , 17 , 18 ], showing the possibility to switch the spin state through generating different ligand fields around the SCO center from the two isomeric forms of photoreactive ligands. Based on this strategy, a series of SCO compounds decorated by photochromic groups, showing cis–trans photoisomerization behavior [ 19 , 20 ], intramolecular photocyclization [ 21 , 22 , 23 , 24 ] and photochemical [2 + 2] cycloaddition [ 25 , 26 ], which were reported in recent years, and the corresponding photoresponsive SCO behaviors were further studied in the solid or liquid state. Moreover, a porous Hofmann-type SCO framework encapsulating trans -azobenzene photochromic guests [ 27 ] was reported by Tong et al recently.…”

Topological structural transformation of a two-dimensional coordination polymer via single-crystal to single-crystal photoreaction

“…4–7 On the other hand, if a [2 + 2] photocycloaddition reaction takes place inside a solid compound, due to the significant changes in the internal organic compositions and intramolecular electronic interactions, the chemical and physical properties of the solid will also display significant differences. 8–10 Therefore, the [2 + 2] photocycloaddition reaction within solid-state compounds is a powerful tool to exploit photocontrolled intelligent materials and molecular/supramolecular devices.…”