Abstract
(English)
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Octahedral transition metal complexes with 5 to 7 d-electrons can have either a high-spin ground state with the maximum number of unpaired electrons allowed by the Pauli principle, or a low-spin ground state with the maximum number of paired electrons. The nature of the ground state depends critically upon the nature of the ligand. For certain ligands the zero-point energy difference between the two possible ground states may take on a value on the order of thermal energies. In this case the phenomenon of spin-crossover, i.e. a transition from the low-spin state at cryogenic temperatures to an almost quantitative, thermal population of the high-spin state at elevated temperatures may be observed. In addition, it is possible to populate the high-spin state at cryogenic temperatures as metastable state by way of irradiation in the visible part of the electromagnetic spectrum. The phenomenon of this light-induced spin crossover, termed 'light-induced excited spin state trapping (LIESST)' forms the basis of our research project.
For more than 10 years [Fe(ptz)6](BF4)2 remained the only compound for which a quantitative determination of the quantum efficiency for LIESST had been performed (A. Hauser, J. Chem. Phys. 94 (1991) 2741). Subsequently, the value of 0.8 reported in this paper was regarded as typical and no further investigations were undertaken until a very recent report by Ogawa et al (Phys. Rev Lett. 84 (2000) 3181) on [Fe(pic)3]Cl2.EtOH. These researchers reported a quantum efficiency, that is the number of complexes converted from the low-spin state to the high-spin state per absorbed photon, which depends upon the intensity of the irradiation and which can reach a value of 34. Based on thermodynamic arguments, this value looks incredible. The quantum efficiency of LIESST being a parameter of primordial importance for any application of the phenomenon, we thought it wise to check and double check the results of Ogawa et al. Our own experiments, performed with the utmost care, do in no way confirm these results. Rather, we find a quantum efficiency for LIESST independent of the irradiation intensity and on the order of unity, corroborating the previous results on [Fe(ptz)6](BF4)2. For [Fe(pic)3]Cl2.EtOH there is, however, a small but non-negligible dependence of the quantum efficiency on the fraction of molecules already converted to high-spin state. This is not surprising and is in line with the rather strong cooperative nature of the thermal spin transition in this compound.
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