LIESST

In chemistry and physics, LIESST (Light-Induced Excited Spin-State Trapping) is a method of changing the electronic spin state of a compound by means of irradiation with light.^[1]

Electron distribution for the two different spin states for a d⁶ metal in an idealized O_h geometry

Many transition metal complexes with electronic configuration d⁴-d⁷ are capable of spin crossover (and d⁸ when molecular symmetry is lower than O_h).^[2] Spin crossover refers to where a transition from the high spin (HS) state to the low spin (LS) state or vice versa occurs. Alternatives to LIESST include using thermal changes and pressure to induce spin crossover. The metal most commonly exhibiting spin crossover is iron, with the first known example, an iron(III) tris(dithiocarbamato) complex, reported by Cambi ^[3] et al. in 1931.

For iron complexes, LIESST involves excitation of the low spin complex with green light to a triplet state. Two successive steps of intersystem crossing result in the high spin complex. Movement from the high spin complex to the low spin complex requires excitation with red light.^[1]

References

1. Phillip Gūtlich, Light Induced Electronic Spin State Trapping (LIESST)
2. Phillip Gūtlich, Andreas Hauser, and Harmut Spiering, Thermal and Optical Switching of Iron(II) Complexes,Angew. Chem. Int. Ed. Engl. 1994.33, 2024-2054
3. Cambi, L.; Cagnasso, A. Atti. Accad. Naz. Lincei, CI. Sci. Fis., Mat. Nut., Rend. 1931, 13, 809. Cambi, L.: Szego, L. Ber. Drsch. Chem. Ges. B 1931, 64, 259.