Spin state conversion

Lifetime Limits and the Rate of Spin-State Conversion. 107... 

It is thus evident that the experimental results considered in sect. 4 above are fully consistent with the interpretation based on absolute reaction rate theory. Alternatively, consistency is equally well established with the quantum mechanical treatment of Buhks et al. [117] which will be considered in Sect. 6. This treatment considers the spin-state conversion in terms of a radiationless non-adiabatic multiphonon process. Both approaches imply that the predominant geometric changes associated with the spin-state conversion involve a radial compression of the metal-ligand bonds (for the HS -> LS transformation). 

On the other hand, NMR spectra appear in general as the average of the spectra of the two spin states [36, 153]. This observation determines an upper limit for the spin-state lifetime shorter than the nuclear spin relaxation time Tl = l/ktH < lO s. In general, therefore, either the superposition or the average of the particular spectroscopic properties of the two spin states is observed, subject to the relative magnitude of lifetime of the excited spectroscopic state and the rate of spin-state conversion. The rate /clh is thus estimated... 

Fe(6-Mepy)3tren](PF6)2- Within the series of iron(II) complexes of the type [Fe(6-Mepy)3 (py) tren](PFg)2 containing the hexadentate ligand tris[4-(6-R)-2-pyridyl]-3-aza-3-butenyl]amine, R = H or CH3, the compounds with X = 1, 2 show a similar HS <- LS transition in solution as well as in the solid state [170]. In solution, spin-state conversion rates have been obtained as shown in Table 3. The complex with x = 0 is pure HS in solution, whereas in the solid state a gradual transition is observed extending over the range... 

For iron(III) eomplexes, uic venues /vlh [Fe(aepa)2]BPh4 H2O and k = 6.7 x 10 s for [Fe(mim)2(salacen)]PF6 have been obtained [156, 166]. The rate constants derived from the line shape analysis of Mossbauer spectra thus vary between 2.1 x 10 and 2.3 x 10 s at room temperature, no significant difference between iron(II) and iron(III) being apparent. In addition, it is evident that the rates of spin-state conversion in solution and in the crystalline solid are almost the same. For iron(II) eomplexes, for example, the solution rates vary between /cjjl = 5 x 10 and 2 x 10 s , whereas in solid compounds values between kjjL = 6.6 x 10 and 2.3 x 10 s have been obtained. Rates resulting from the relaxation of thermally quenched spin transition systems are considerably slower, since they have been measured only over a small range of relatively low temperatures. Extrapolation of the kinetic data to room temperature is, however, of uncertain validity. 

As far as the solid complexes are concerned, the qbove conclusions are generally valid for gradual spin-state transitions, whereas additional features such as hysteresis effects are observed for transitions which show abrupt changes of physical properties. In fact, abrupt transitions seem to be formed if the volume change A V associated with the spin-state conversion of the molecules cannot be conveniently accommodated by the lattice. 

Fig. 8.16 Fe Mossbauer spectra of [Fe2 (PMAT)2](BF4)4-DMF at selected temperatures. At 298 K, the only quadrupole doublet is characteristic of iron(II) in the HS state. SCO from HS to LS occurs at one Fe(II) site of the dinuclear complex at ca. 225 K. The second Fe(II) site remains in the HS state, but feels the spin state conversion of the neighboring atom by local distortions communicated through the rigid bridging ligand, giving rise to a new quadrupole doublet (dark gray), i.e., HS in [HS-LS], in the Mossbauer spectrum. The intensity ratio of the resonance signals of HS in [HS-LS] to that of LS (black) in [HS-LS] is close to 1 1 at all temperatures (from [32])...

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. 

Adler, Hauser, Vef, Spiering and Giitlich (1989) Dynamics of spin state conversion processes in the solid state [228]. 

Salicylaldimine ligands often give stable Fe(III) complexes, so it is uncommon to meet Fe(II) complexes with such ligands. The dark blue-green complex [Fe 17] (17) shows an unusual thermally-induced, two-step spin-state conversion where two sharp transitions are separated by a plateau extending over 35 K in which 50% high-spin and 50% low-spin molecules coexist [41]. 

Ferraro and Takemoto68) have used FIR spectroscopy to follow the extent of spin state conversion under pressure in I, II, and III. In good agreement with Fisher and Drickamer78), who used Mossbauer spectroscopy to determine the relative changes of the fractions of the HS and the LS state in I and II and in other [Fe(phen)2X2] complexes under applied pressure, Ferraro and Takemoto observed... 

The SERRS of P-450 (rat Ever) adsorbed onto a phosphatidyl-choline/silver colloid (citrate-reduced) substrate leads to a low to high spin-state conversion. This spin-state marker band shift is ascribed to a strong interaction of P-450 with the phospholipid coating [17]. 

Mossbauer spectroscopy is ideally suited to follow the light-induced spin state conversion in [Fe(ptz)6]BF4)2 as exemplified in Fig. 2.18. A polycrystalline sample of [Fe(ptz)6](BF4)2 was cooled to 15 K. Before irradiation, the sample is in the LS state and shows the typical Mossbauer spectrum of Fe°-LS (upper left). After irradiating with green light (Xe lamp with filters or 514 nm band of an Ar ion laser) at 15 K the sample is quantitatively converted to the metastable HS state (middle left). The asymmetry in the intensity of the two components of the... 

V. Ksenofontov, A.B. Gaspar, J.A. Real, P. Gutlich, Pressure-induced spin state conversion in antiferromagnetically coupled Fe(II) dinuclear complexes. J. Phys. Chem. B 105, 12266-12271 (2001)... 

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