THERMAL AND OPTICAL SWITCHING OF BISTABLE IRON COMPOUNDS AND POSSIBLE APPLICATIONS

Iron(II) coordination compounds with ligand field strengths close to the mean spin pairing energy may undergo thermal spin transition in the solid as well as in the liquid state. The thermodynamically stable state is high spin (HS) 5T2 (under O(h) symmetry) with (t2)4 e2 electron configuration above the critical transition temperature T(c), and low spin (LS) 1A1 with (t2)6 electron configuration below T(c). The spin transition curve gamma(HS)(T) (molar fraction of HS molecules as function of temperature) can be abrupt or gradual, with or without hysteresis, or even with a plateau near T(c). T(c) itself can vary over the whole temperature scale between the liquid helium region up to above room temperature, depending on the nature of the ligand molecules which may be chosen such as to ''fine-tune'' the ligand field strength via steric and electronic effects. Such bistable systems exhibiting thermal spin-crossover have the potential to be used as temperature sensors and thermal switches. The phase detection (HS or LS) can be carried out by Mossbauer or optical spectroscopy, magnetic measurements, and other techniques. Bistable iron(II) compounds of the above mentioned nature can also be switched by irradiation with light. Green light switches the LS(1A1) state to the HS(5T2) state, which surprisingly may have practically infinitely long lifetimes at sufficiently low temperatures. Red light switches from HS to LS. The switching is faster than nanoseconds, and the quantum yields are very high. This phenomenon of ''Light-Induced Excited Spin State Trapping'', which we have abbreviated as LIESST, occurs in single crystals, polymer foils, and KBr pellets. Such bistable materials exhibiting the LIESST effect show promising features for possible applications in optical information technology. First holography experiments have been successfully carried out in our laboratory.