Thermal- and light-induced SCO effect in Fe(II) complexes and coordination polymers

The review presents several families of spin crossover (SCO) active Fe(II) coordination compounds with photoactive N-donor heterocyclic ligands, in which the photoinduced structural changes can activate reversible change of spin state and thus control magnetic properties under isothermal conditions. Detailed description of structural, spectral, and magnetic behavior for selected examples of photoisomerizable coordination compounds are provided. From the application point of view, light is an excellent tool to control SCO properties. The first and best known approach called Light Induced Excited Spin State Trapping (LIESST) has a significant technological limitation due to low temperatures (< 120 K) required for the trapping and existence of photoexcited metastable states. The second and more elegant approach known as Ligand-Driven Light-Induced Spin Crossover (LD-LISC) seems to be a very suitable strategy utilizing light-induced structural changes to control the spin. Isomerization of photoswitchable groups, such as azobenzenes or stilbenes, can cause reversible transformation between two isomeric forms after exposition to selective wavelengths at ambient temperature. A very recent third approach, the Guest-Driven Light-Induced Spin Crossover (GD-LISC) effect employing the photoisomerizable guest molecules to control the spin state has also been introduced.