1. Thermal and light-induced spin transition in iron compounds P. Gütlich et al. Institut für Anorganische Chemie und Analytische Chemie Johannes Gutenberg-Universität Mainz Staudingerweg 9, D-55099 Mainz, Germany E-mail: guetlich@uni-mainz.de http://ak-guetlich.chemie.uni-mainz.deguetlich@uni-mainz.de

2. Thermal spin transition (spin crossover) is a widely observed phenomenon in coordination chemistry of 3d transition metal ions with d 4 up to d 7 electron configuration. It was first observed in iron(III) compounds around 1930 by the italian researchers Cambi and coworkers, but is now well established and extensively studied in many iron(II) compounds, last but not least by application of the powerful tool of Mössbauer spectroscopy. Spin crossover in iron(II) compounds may be observed if the ligand field strength is such that the difference between the lowest vibronic energy levels of the high spin state 5 T 2 and the low spin state 1 A 1 state is comparable with thermal energy k B T (k B = Boltzmann constant). The spin transition behaviour can be influenced by chemical alteration, e.g. ligand replacement, change of non-coordinating anion and solvent molecule, substitution of spin state changing metal by another metal, e.g. substitution of iron by zink. It can also be influenced by physical perturbation such as irradiation with light, application of pressure or a magnetic field. For a comprehensive coverage of spin crossover research see: - Gütlich, P.; Hauser, A.; Spiering, H.Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. - Gütlich, P.; Goodwin, H.A. (Eds.), Spin Crossover in Transition Metal Compounds, Top. Curr. Chem. Vol 233, 234, 235, Springer, Berlin Heidelberg New York The influence of the ligand molecules on the spin state of the central iron(II) ions is demonstrated with the following two examples and their temperature dependent Mössbauer spectra shown in the next picture.

3. Tuning the Ligand Field by Ligand Replacement [Fe II (phen) 2 (NCS) 2 ] 300K 77 K 180 K 184 K 186 K 188 K HS LS [Fe II (phen) 3 ]X 2 LS 300 K 240 K 80 K 5 K LS

4. [Fe II (phen) 3 ]X 2 (phen = 1.10-phenanthroline) is a typical low spin compound with the characteristic Mössbauer spectra as shown on the left; the isomer shift is ca. 0.2 mm s -1 (relative to -iron) and the quadrupole splitting ca. 0.5 mm s -1, nearly independent of temperature. If one of the relatively strong phen ligands, which occupies two coordination positions of the octahedron, is replaced by two monofunctional NCS - groups, the average ligand field strength becomes weaker than the mean spin pairing energy and the compound [Fe II (phen) 2 (NCS) 2 ] adopts high spin (HS) character at room temperature. The Mössbauer spectrum at 300 K shows the typical features of an iron(II) HS compound with isomer shift of ca. 1 mm s -1 and large quadrupole splitting of ca. 3 mm s -1. However, the compound [Fe II (phen) 2 (NCS) 2 ] fulfils the condition for thermal spin crossover to occur, viz. ΔE HL k B T. On lowering the temperature, the compound changes spin state from high spin to low spin near 180 K as is well documented by the Mössbauer spectra as a function of temperature, which was first reported by I. Dezsi et al. in 1967. Since then more than 200 spin crossover compounds of iron(II) and iron(III) have been studied by Mössbauer spectroscopy (see e.g. P. Gütlich, H.A. Goodwin (eds.), Spin Crossover in Transition Metal Compounds, Springer Series Topics in Current Chemistry, Vol. 233, 234, 235, Berlin Heidelberg 2004). [Fe(ptz) 6 ](BF4) 2, where ptz stands for the ligand molecule 1-propyl-tetrazole as shown in the picture, is another iron(II) coordination compound exhibiting thermal spin crossover with a spin transition temperature T 1/2 of ca. 135 K. The HS phase is nearly colourless, the LS phase is red. Thus the thermally induced spin transition can be easily followed by optical spectroscopy in the uv/visbile range. The 57 Fe Mössbauer spectra clearly indicate the transition near 135 K between the HS phase (quadrupole doublet shown in red) and the LS phase (singlet shown in blue). Whereas the quadrupole doublet is typical for iron(II) in the HS state, the singlet of the LS state is rarely observed and points at a quasi regular octahedral surroundings at the iron centre.

5. Thermal spin transition in [Fe(ptz) 6 ](BF 4 ) 2 followed by optical spectroscopy N N N N C 3 H 7 E. W. Müller, J. Ensling, H. Spiering, P. Gütlich, Inorg. Chem., 22(14), 2074-8 1983 300 K 1 A 1 1 T 1 Six ptz molecules coordinate via N atoms to the iron center and form the twofold positive cationic complex ion. Two BF 4 ¯ serve as counter ions. 1 A 1 1 T 2 The complex compound [Fe(ptz) 6 ](BF 4 ) 2 is in the high spin (HS) state 5 T 2 (t 2g 4 e g 2 ) at room temperature down to ca. 135 K. The optical spectrum of the colorless crystal shows a weak absorption band for the 5 T 2 5 E transition. At 135 K, thermal spin transition to the low spin (LS) state 1 A 1 (t 2g 6 ) takes place on further cooling with dramatic color change from white (HS) to red (LS). The optical spectrum shows two spin-allowed (but parity-forbidden) singlet-singlet absorption bands. 80 K

6. v (mm/s) 155 K 136 K 98 K Rel. Transmission (%) LS HS Thermal spin transition in [Fe(ptz) 6 ](BF 4 ) 2 followed by Mössbauer spectroscopy and magnetic susceptilibility measurements E. W. Müller, J. Ensling, H. Spiering, P. Gütlich, Inorg. Chem., 22(14), 2074-8 1983 The 57 Fe Mössbauer spectra of [Fe(ptz) 6 ](BF 4 ) 2 recorded as a function of temperature clearly indicate the spin transition between HS state (quadrupole doublet shown in red) and LS state (singlet shown in blue) at ca. 135 K. The HS quadrupole doublet is mainly caused by noncubic electron distribution in the (Jahn-Teller-active) t 2g 4 eg 2 configuration. The singlet of the LS state arises from the t 2g 6 points at a quasi regular octahedral surroundings at the iron centre. The spin transition occurs with hysteresis as shown by the magnetic susceptibility measurements, from which the effective magnetic moment can be derived.

7. Mössbauer spectroscopy is ideally suited to follow the light-induced spin state conversion in this system as exemplified in the next viewgraph A polycrystalline sample of [Fe(ptz) 6 ](BF 4 ) 2 was cooled to 15 K. Before irradiation the sample is in the LS state and shows the typical Mössbauer spectrum of the LS state (upper left). After irradiating with green light (xenon 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 quadrupole doublet is due to the plate-like shape of the crystals (texture effect). Thermal relaxation on a 15 minute-timescale sets in at 50 K (lower left and upper right: the sample was heated for 15 minutes at 50 K and then cooled to the measuring temperature of 15 K in two runs). Thermal relaxation to the stable LS state is complete at 10 K. On further heating to 150 K the sample undergoes again thermal spin transition at 135 K to the (now stable) HS state. This photophysical phenomenon was first observed by S. Decurtins, P. Gütlich, C. P. Köhler, H. Spiering, A. Hauser, Chem. Phys. Lett., 105(1), 1-4 1984 and became known as Light-Induced Excited Spin State Trapping (LIESST). The processes involved in the LIESST effect are well understood on the basis of ligand field theory (Gütlich, P.; Hauser, A.; Spiering, H. Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. The next viewgraph explains the mechanisms of LIESST and reverse-LIESST.

8. Light-Induced Spin Crossover in [Fe(ptz) 6 ](BF 4 ) 2 Relative Transmission (%) 15 K no light 15 K green light 50 K ( 15 ') 150 K 50 K ( 30 ') 100 K 15 K green light 5 T 2 5 E LS HS

9. Light-Induced Excited Spin State Trapping (LIESST) egeg t2gt2g The mechanism of LIESST and Reverse-LIESST is explained on the basis of ligand field theory. Green ligth (e.g. 514 nm from Ar ion laser) affords the spin-allowed (but parity-forbidden) transition 1 A 1 1 T 1, 2. Preferred relaxation occurs via two subsequent intersystem crossing processes 1 T 1,2 3 T 1,2 5 T 2, which are favoured by spin-orbit coupling. The metastable HS state 5 T 2 has an extremely long lifetime (typically minutes near 50 K). Backconversion from the metastable HS state to the LS state can be achieved by irradiation with red light (e.g. 820 from Kr laser), whereby the first transition step 5 T 2 5 E is followed by two intersystem crossing processes 5 E 3 T 1,2 1 A 1 back to the stable LS state.