Effect 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 the same line, [Fe(qsal)2][Ni(dmit)2].2CH3CN has also been proven to show a cooperative spin transition based on n-n interactions between ligands [105]. Moreover, this compound is also one of the rare Fenl complex to exhibit the light-induced excited spin state trapping (LIESST) effect. 

This compound is also remarkable since it also exhibits a LIESST effect. As claimed by the authors, [Fe(qsal)2][Ni(dmit)2]3.CH3CN.H20 can indeed be classified as a prototypal photoswitchable SCO molecular conductor. ... 

Experimental equipment for X-ray diffraction methods has improved enormously in recent years. CCD detectors and focusing devices (Goepel mirror) have drastically reduced the data acquisition time. Cryogenic systems have been developed which allow structural studies to be extended down to the liquid helium temperature range. These developments have had important implications for SCO research. For example, fibre optics have been mounted in the cryostats for exploring structural changes effected by light-induced spin state conversion (LIESST effect). Chaps. 15 and 16 treat such studies. 

The mononuclear hexakis(l-alkyl-tetrazole)iron(II) compounds with various anions have been extensively studied. It appears that the spin crossover characteristics of compounds with different alkyl substituents attached to N-1 of the tetrazole heavily depend on the crystal structure features. The transitions may be abrupt or rather gradual, complete or only involving a fraction of the Fe(II) ions, and the Tm values lie in the range 63-204 K [2c, 2f, 2g, 74-81]. Interest in these systems has focused on their suitability for detailed studies of the LIESST effect (A. Hauser, this volume). 

Fig. 19 LIESST effect observed by 57Fe Mossbauer spectroscopy for [Fe(btzp)3](Cl04)2 at 5 K, without light irradiation (top) at 5 K, after light irradiation (middle) at 125 K, after light irradiation (bottom). (Reprinted with permission from [87]. Copyright (2000) American Chemical Society)... 

Most interestingly, [Fe(btzp)3](Cl04)2 is the first one-dimensional Fe(II) spin crossover compound, which shows the LIESST effect, detected in this instance by 57Fe Mossbauer spectroscopy (Fig. 19). 

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]. 

Important information relating to the intramolecular interaction in dinuclear units of [Fe(phdia)(NCS)2]2(phdia) can be drawn from the study of the LIESST effect at low temperature. The Mossbauer spectrum recorded at 4.2 K after slow cooling reveals 16.0% of HS and 84.0% of LS species (Fig. 17a). The spectrum recorded after the irradiation (1=488 nm) of the sample for one hour at 4.2 K shows an increase of the intensity of the HS doublet up to 25.0% (Fig. 17b). The spectrum recorded subsequently in a magnetic field reveals no [HS-HS] pairs it consists of 50.0% of [HS-LS] and 50% of [LS-LS] pairs (Fig. 17c). This is further evidence of the inherent stability of mixed pairs in [Fe(phdia)(NCS)2]2(phdia). ... 

LIESST effect at 5 K. The BF4 derivative displays a similar TSCO behaviour, shifted by 20 K towards higher temperatures. The ID SCO compound [Fe(btze)3](BF4)2 (btze=l,4-bis(tetrazol-l-yl)ethane) with T1/2 140 K has also been reported [73]. 

The inter-conversion of the spin states in many instances is so rapid that the separate contributions to the 57Fe Mossbauer spectra are not resolved. Thus this technique, which has proved so diagnostic in iron(II) systems, is frequently less suited to the derivation of spin transition curves for iron(III). A further corollary of the faster spin state inter-conversion is the rarity of the LIESST effect among iron(III) systems, in contrast to its ubiquitous occurrence in iron(II). 

Despite these differences, the similarities predominate and virtually all the features noted for spin crossover in iron(II) are also found for iron(III). Because of the great emphasis on the cooperative aspects of the spin crossover phenomenon, iron(II) systems have tended to dominate more recent research. However, there are very striking examples among the iron(III) systems which are of strong relevance to these aspects and there is certainly scope for future work in this area. This is evident in much of the very recent work where it can be seen that specific strategies to increase the cooperativity have been successful and have led, for example, to solid iron(III) systems which display the LIESST effect [137, 138]. The generation of polymeric species as a means of increasing cooperativity, an approach which has been widely adopted for iron(II), has received relatively little attention for iro-n(III) and this is an area which can be expected to be exploited further. 

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. 

Fe(mtz)g](BF4)2, (mtz = methyltetrazole), which undergoes a photo-induced transition from low spin to high spin (LS FiS) at low temperature, and which is sufficiently stable at 10 K for data to be collected to determine the crystal structure corresponding to both the HS state, after irradiation (A = 514nm), and the LS state, obtained before photo-irradiation. The LIESST effect is evident via an extension of the Fe-N bond by 0.2A which corresponds to the electronic transition (LS) (HS). Complementary measurements of Mossbauer... 

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. 

The fact that irradiation of both the T g A g and T g A g bands for the LS Fe(ii) compound at 20 K iead to the observed LiESST effect impiies that the iower energy T g is an intermediate in the photochemicai process. Excitation of the higher iying, spin-aiiowed Tjg - A g transition in the LS form of the complex is followed by intersystem crossing and internal conversion to the active T g excited state, as shown in Figure 17.28. Likewise, irradiation of the spin-allowed Eg - T2g in the HS complex at 20 K is followed by intersystem crossing to... 

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

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