On the nature of metastable high-spin states formed during spin transitions in iron(II) complexes

Abstract

The metastable high-spin (HS) state of iron is formed and fixed in iron(II) complexes during irradiation of a low-spin (LS) sample at low temperatures (~15 K) with green laser light at a wavelength of 514.5 nm [1]. This phenomenon was studied further in [2-4] (see also references in the above papers). In particular, it was shown that the metastable HS form is transformed back to the LS form by irradiation of crystals with red laser light at a wavelength of 752.7 nm. This effect may be of interest for molecular electronics, because molecular systems reversibly changing their state during irradiation with laser light may be used as optical keys in devices for processing and storage of optical data. M/Sssbauer spectroscopy is an essential tool for investigating light-induced spin transitions. The main results obtained by this method are the ratios between the LS and HS forms of iron, determined in various experimental conditions. The aim of this work is to draw attention to a new possibility offered by this method in studying the nature of the metastable HS state of Fe(II) formed in light-irradiated samples. Formulation of the problem will ftrst demand several explanations concerning the nature of the HS and LS forms of iron that are simultaneously observed in a particular temperature range during sharp spin transitions (ST) caused by temperature variations. In some papers (e.g., review [5]), it was shown that the HS form, which appears at increased temperatures, as well as the LS form, which appears during cooling, exist in crystals in the form of domains, i.e., certain regions in which all iron atoms are in the same spin state. An alternative form is chaotic distribution of iron atoms, which do not create regular regions. The domain structure of the LS and HS forms during ST is proved by hysteresis on the curves of the temperature dependences of the effective magnetic moment of the complex [5]. Let us consider the crystal in which the HS form appears during irradiation with light. It would be reasonable to ask whether this state exists in the crystal as dom~in~ or individual paramagnetic centers. The idea of using MiSssbauer spectroscopy for solving this problem is based oft the following fact. The HS state appears in the matrix of the LS state, in which iron atoms participate in cooperative interactions; hence the reaction of the atoms of the LS state to an admixture of the metastable HS state will depend on whether the atoms of the HS state form domains or individual paramagnefic centers in the low-spin matrix. We can assume that in the case of domains their influence on the structure of the LS state will be negligible due to the relatively low content of the neighboring pairs of atoms with different values of spins. The case is different when the HS centers are chaotically distribfited in the LS matrix. The changes in the spin states of iron atoms as a result of irradiation with light can lead to changes in the electronic states of the neighboring iron atoms of the LS matrix because of considerable differences in the metal-ligand distances of the LS and HS forms [7] and due to cooperative interactions in the LS matrix. The effect of the "large" paramagnetic admixture on the structure of the LS matrix will be more simaiflcant for crystals with strong cooperative interactions and for crystals with higher concentrations of admixtures. The "large" paramagnetic admixture loosens the structure of the LS matrix and decreases the covalence of the metal-ligand bond of the LS form, thus increasing the chemical shift c3LS of the M/Sssbauer line for the I~ form. Recently, we reported on a similar effect in [6], in which we studied the influence of iron substitution by zinc on the ST in the polynuclear Fe(ATr)3(NO3) 2 complex (ATr = 4-amino-l,2,4triazole). We found that the chemical shift of the LS form of the complex measured at 78 K monotonically increases with the concentration of zinc atoms. Tiffs effect was explained in terms of the model of steric strai-~ arising in crystals