Effect reverse LIESST

Fig. 16. Schematic potential energy diagram of the low-lying ligand field states for d spin-state transition compounds. Full lines indicate the mechanism of forward and reverse LIESST effect. According to Ref. [135]...

In 1984, Decurtins et al. discovered that the compound [Fe(ptz)6](BF4)2 (ptz=l-propyltetrazole) can be converted from the stable LS state to the metastable HS state by irradiation with green light at sufficiently low temperatures [14]. This phenomenon has become known as light-induced excited spin state trapping (LIESST) and is dealt with in detail by A. Hauser in a separate chapter in this series. Later, Hauser reported the reverse-LIESST effect, whereby red light is used to convert the compound back into the LS state [15]. 

Spin crossover (SC) was observed for the first time by Cambi and Cagnasso [1]. This phenomenon was reviewed in a number of excellent reviews [2-7]. Because of its possible technological utilisation [8,9], spin crossover is widely studied at present. The usual induction of spin crossover is based on temperature variation but pressure and concentration variations may lead to the same effect. Recently, optical induction has been involved and such experiments were termed the LIESST (Light Induced Excited Spin State Trapping) and the reverse LIESST effects [7]. 

When the colourless crystal in the LIESST-generated HS state is subsequently irradiated by a monochromatic red light of v= 12200 cm-1, the excited state 5E of the HS complex is generated. Its decay proceeds through the SE 37 1 - 1 A pathway and results in the reappearance of the purple LS system. This is referred to as the reverse LIESST effect. The electron spectrum of the reverse-LIESST-generated LS state is almost identical to that of the starting LS system at the beginning of the experiment [7]. 

A large number of different factors influence spin crossover. In addition to temperature-induced spin crossover, light-induced spin transition is also known this is the basis of the LIESST and reverse-LIESST effects, respectively. Of great interest is the utilisation of thermal hysteresis for data recording and construction of display units. 

Jablonski diagram illustrating the role of the T,g in the LIESST and reverse-LIESST effect of spin crossover Fe(ll) coordination compounds. 

Both photoinduced LS —> HS and HS > LS transitions involve transition through a 3Ti state, from which the system can relax into the LS and HS ground states via intersystem crossing processes. This reversible state switching has been summarized as light-induced excited spin state trapping (LIESST) effect,29 and especially for Fe-based compounds it can be conveniently traced by Mossbauer spectroscopy. 

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