J. Am. Chem. Soc. 133, 16243–16250 (2011)

Some complexes of first-row transition metals with d-orbital occupancy from d4–d7 are capable of exhibiting the phenomena of 'spin crossover': switching between a high or low spin state. These different spin states have different magnetic properties and the switch is driven by an external stimulus such as heat or light. The latter has previously been used to induce spin crossover by altering the ligand field strength of photochromic ligands.

Now, Rainer Herges and colleagues from Christian-Albrechts University in Kiel have designed a system, also reliant on photochromic molecules, in which a change in the coordination number of the studied complex drives changes in its spin state and thus its magnetic properties. They used the square-planar complex Ni(II)-tetrakis(pentafluorophenyl)porphyrin — which is low spin and diamagnetic — and a 3-azopyridine ligand, which can be switched between cis and trans configurations using light. When the azopyridine ligand is in the trans configuration it binds well to the axial position of the square-planar nickel–porphyrin complex, causing it to switch to a high spin, paramagnetic state. When the complex is irradiated with a specific wavelength of UV light the azopyridine ligand isomerizes to its cis configuration. The ability of the ligand to bind to the nickel–porphyrin is now reduced — because of steric hindrance — allowing the complex to revert back to the low-spin, diamagnetic state.

The efficiency of the system was improved by adding bulkier groups to the 4 and 4′ positions, thus increasing the difference between the abilities of the cis and trans isomers to bind to the nickel–porphyrin complex.