P hotoswitching of Nitroprussides V . Rusanov

1. irradiated part of the crystal populated with SI and SII crystal in ground state Photoswitching of Nitroprussides V. Rusanov Department of Atomic Physics, University of Sofia, Bulgaria Sv. Stankov ESRF, Grenoble, France H. Paulsen, A. X. Trautwein University of Lübeck, Germany The cover page of the journal Laser und Optoelektronik with visible information written in single crystal cut of sodium nitroprusside Na2[Fe(CN)5NO]2H2O, as a congratulation to its 25th “birthday” anniversary. Through a model the crystal was first irradiated with light of wavelength 457.9 nm and then for the second time with light of wavelength 1064 nm. Expected population: GS 70 %, MSI 15% and MSII 15 %. Laser und Optoelektronik, 25(6) 18 (1995).

2. The discovery ! On Optical Dispersion in Transparent Molecular Systems Ulrich Hauser, Volker Oestreich, and Heinz Dieter Rohrwecke Physikalisches Institut, Universität zu Köln, Köln, Germany Received September 17, 1976

3. Mössbauer investigation of the FWHM of the metastable states in SNP GS: QS=+1.716(3) mm/s IS= 0.00 mm/s MSI: Pop. 47.1(4) % QS=+2.755(3) mm/s IS=+0.178(3) mm/s MSII: Pop. 35.5(5) % QS=+2.862(3) mm/s IS= 0.194(3) mm/s Th. Woike, W. Kirchner, Hyung-sang Kim, S. Haussühl, V. Rusanov, V. Angelov, S. Ormandjiev, Ts. Bonchev, A. N. F. Schroeder, Hyperfine Interactions 77, 265 (1993).

4. 600-900 nm MS I 350 - 580 nm GS 900 - 1200 nm 350 - 580nm 350 - 580 nm MS II 600-1200 nm Wavelength range of transfers between GS, MSI and MSII A maximum of about 45 % of the molecules can be transferred into the first metastable state, MSI, by irradiation with blue light of 457.9 nm from an Ar+-laser and only a few percent into the second state, MSII. The latter state can, however, be populated up to about 30 % when a crystal with the first state MSI already populated at maximum is irradiated a second time with infrared light from a Nd-YAG-laser with wavelength 1064 nm. Both states can be completely and reversibly transferred to the ground state GS by irradiation with light of the red spectral range, e.g. of 632.8 nm from a He-Ne-laser. A summary of the Mössbauer parameters for all three states and of the population conditions can be found in: V. Rusanov, Sv. Stankov, and A. X. Trautwein, Hyperfine Interactions, 144/145, (2002) 307.

5. 2 0 m m t w e l v e l i g h t - e m i t t i n g d i o d e s g-beam s i n g l e c r y s t a l c u t b e t w e e n t w o m y l a r f o i l s Irradiation of the GNP single crystals by Light-Emitting Diodes (LED) G = guanidinium l=450±35nm at 297 K V. Rusanov, H. Winkler, C. Ober, and A. X. Trautwein, Eur. Phys. J. B 12, 191 (1999).

6. Explanation of the nature of MSI and MSII by strong structural changes of the NP anion, proposed by M. Carducci et al. MSII: Side-on bonding MSI-Isonitrosyl bonding Energy GS of the NP anion M. D. Carducci, M. R. Pressprich, P. Coppens, J. Am. Chem. Soc. 119, 2669 (1997).

7. temperature control 2 cm kapton window GNPcrystal LED synchrotron beam NFS detector NIS detector NIS investigation of the MSI populated in GNP (a) Simulated NIS spectrum for a crystal populated with GS (75 %), MSI (18 %) and MSII (7 %). New Fe–O isonitrosyl bond stretching vibrational mode of MSI at 72 meV has been predicted by DFT calculations ( ); (b) NIS spectrum of the crystal before the illumination; (c) NIS spectrum of the crystal illuminated for 24 h by two light emitting diodes (LED), 450 nm wavelength. A new line is observable in the NIS spectrum at 73 meV ( ). The inset shows schematically the illumination cell. H. Paulsen, V. Rusanov, R. Benda, C. Herta, V. Schünemann, C. Janiak, T. Dorn, A. I. Chumakov, H. Winkler, A. X. Trautwein, J. Am. Chem. Soc. 124, 3007(2002).

8. Angle-dependent Mössbauer study of the metastable states in SNP V. Rusanov, Th. Woike, H. Kim, V. Angelov, Ts. Bonchev, S. Haussühl, Solid State Commun. 126, 457 (2003). Th. Woike, M. Imlau, V. Angelov, J. Schefer,B. Delley, Phys. Rev. B 61, 18, 12249 (2000). Mössbauer parameters for MSI and MSII extracted from measured angle-dependent ratio of the left-hand to the right-hand line effective thickness t1/2/t3/2. Three parameters for MSII given in bold differ substantially from those measured for GS and MSI.

9. The dynamic properties of MSII Mössbauer spectra obtained at 83 K and 135 K from a SNP single crystal, b-cut, crystal thickness 490 m and natural isotope abundance of iron, populated up to about 30 % with MSII. In the inset the normalized areas for GS and MSII are given. In a temperature-dependent Mössbauer experiment new evidence in favour of the dynamic properties of MSII is found. The normalized area of MSII is reduced by about a factor of three when going from 83 to 135 K. This is not the result from a decay process of MSII to the GS (which occurs at about 150 K) because the normalized area of GS does not change significantly. The change of the Lamb-Mössbauer factor fLM for MSII, however, is dramatic and leads to a strong reduction of the normalized area.

10. Determination of the Mössbauer parameters of rare-earth nitrosylpentacyanoferrates (RNP) A light-induced colour change, which is stable at room temperature, has been observed in rare-earth nitroprussides RNP. This internal photochemical redox reaction has been discovered by O. Sato, T. Iyoda, A. Fujishima, K. Hashimoto, Science, 272,704 (1996) in Prussian blue analogs K0.2Co1.4[Fe(CN)6] ·6.9H2O. The irradiation induced an electron transfer from Fe to Co through the CN-bridges. The iron and the rare-earth cations in RNP are also connected via cyano bridges. The photoinduced valency, spin state and molecular magnetism can be observed because the rare-earth cation has a vacant electron state and could accept the electron transmitted via the cyano bridges. This is not the case for SNP in which this effect has not been observed. V. Rusanov, Sv. Stankov, and A. X. Trautwein, Hyperfine Interactions, 144/145 307 (2002).

11. Conclusions All nitroprussides can be switched by irradiation with blue light into two metastable states MSI and MSII, which are stable below 200 and 150 K, respectively. The ground state GS and the metastable states MSI and MSII are diamagnetic and, therefore, the photoswitching effects observed in nitroprussides can not be explained by light-induced spin-crossover. The metastable state MSI corresponds to the linkage isomer of the NO group, the so-called isonitrosyl configuration where the NO ligand is inverted and oxygen is bound to the central iron atom. For the metastable state MSII was a side-on bonded configuration of NO proposed. Mössbauer parameters of GS and MSI derived from Density Functional Theory calculations (DFT) are in quantitative agreement with the measured values. For MSII such agreement is lacking. This discrepancy is most likely due to the assumed static side-on configuration of the NO-group in the DFT calculations. The experimental Mössbauer parameters of MSII could be reproduced by calculation when taking into account the dynamic properties of the NO-group. In our opinion, in the liberation-rotation model of MSII, bending vibration as well as rotation of the NO-group occur, the latter around the main molecular axis. Population experiments were carried out on polycrystalline samples and confirmed theaccessibilityoflight-inducedmetastablestatesinthe nitroprussides of rare-earth elements. The colour of these nitroprussides can be changed (by light) and it remains stable at room temperature. Evidence for a new Light Induced Magnetic Excited State (LIMES), observed for the first time in rare-earth nitroprussides, is provided. Photo-induced changes in, structure, valency, spin and magnetization at low temperatures have been observed and are discussed. Acknowledgements We would like to thank the Alexander von Humboldt Foundation, AvH (V. R. and A. X. T.), the German Academic Exchange Serves, DAAD, (Sv. S.) and the German Research Foundation, DFG (H. P. and A. X. T.) for supporting this research.