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]...

Fig. 8.10 Schematic illustration of LIES ST and reverse LIESST oidicf complex. Spin allowed d-d transitions are denoted by arrows and the radiationless relaxation processes by wavy lines (from [21])...

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]. 

Figure 34 Schematic representation of the potential wells of the excited ligand field states in a spin-crossover system. Arrows indicate the mechanism for LIESST and reverse-LIESST. Reproduced with permission from ref. 23.

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. 

Figure 1 The electronic structure of iron(II) spin-crossover complexes. The mechanisms of LIESST, reverse-LIESST, and NIESST are indicated by curly arrows.

The spin-crossover phenomenon has gained renewed importance since the discovery of the LIESST for [Fe(ptz)s] (Bp4)2 (ptz = I -propyltetrazole) [ 10,11 ]. In this section, the LIESST and reverse-LIESST for [Fe(ptz)6](Bp4)2 are described. [Pe(ptz)fi](Bp4)2 has a highly symmetric rhombohedral structure (R3 Z= 3) [19]. The Pe(ll) ion, which is coordinated by six N4 atoms belonging to I -propyltetrazole (Scheme 9.1), shows the thermally induced spin transition from the LS state... 

Potential wells of the Aig ground state and the thermally accessible T2g state as well as the higher excited ligand field states with d electron configuration in the spin-crossover region. The mechanisms of LIESST and reverse-LIESST are indicated by arrows. (Reproduced from Ref. 

Fig. 2.14 Jablonski diagram depicting potential wells of the LS and HS sates for an Fe° complex, Illustrating the LIESST and reverse-LIESST mechanism. These phenomena refer to the possibility to reversibly address spin states of Fe complexes by light Irradiation...

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