Thermal and Optical Switching of Iron(II) Complexes

Abstract

Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged

Picosecond X-ray Absorption Spectroscopy of a Photoinduced Iron(II) Spin Crossover Reaction in Solution

“…6 Since the discovery of spin equilibria in the 1930s, 7,8 considerable knowledge has been gained about the physicochemical properties of SCO complexes using various physical methods. 4,9,10 The kinetics of the SCO transition has been studied using a number of techniques, most notably lasertemperature jump and optical pump-probe spectroscopies. 4,11 However, the structural dynamics that accompany the change in electronic configuration during the course of the SCO transition have never been directly measured.…”

Temperature and pressure effects on the spin state of ferric ions in the [Fe(sal2-trien)][Ni(dmit)2] spin crossover complex

“…Spin-crossover (SC) complexes of transition metal ions (3d 4 -3d 7 ) have attracted much attention from the fundamental point of view [1][2][3][4]. These complexes represent one of the best examples of molecular bistability.…”

Tuning the magneto-structural properties of non-porous coordination polymers by HCl chemisorption

“…In this area, coordination chemistry is having a key role by providing remarkable examples of molecule-based materials in which magnetic properties can be reversibly tuned at low temperatures through a physical stimulus, such as light 1,2 or pressure 3,4 . With the recent explosion in research on metal organic frameworks (MOFs) [5][6][7][8][9][10] , the use of a chemical stimulus such as gas sorption to affect the magnetic properties of a porous coordination polymer has also been explored 11 .…”