Oberflächen und Schalter

 

Subgroup Surfaces and Switches:

 


Dipl.-Chem. Hanne Jacob, Dipl.-Chem. Sven Olaf Schmidt, Dipl.-Chem. Holger Naggert, M. Sc. Michaela Klaß, M. Sc. Hannah Brandenburg, M. Sc. Benedikt Flöser, B. Sc. Alexander Schlimm and Prof. Dr. Felix Tuczek

 

 

I) Spin-state switching in solution

 

 

 

Octahedral transition-metal complexes with d4 to d7 electronic configurations exist in high spin (HS) and low spin (LS) states. The strength of the ligand field determines the spin state of the complex (Figure 1). If the energy difference between the two states is comparable to the thermal energy, a transition from one state to the other is possible. This temperature dependent spin transition has been discovered on an Fe(II)-complex in the 1930's and the phenomenon was named spin crossover (SCO).[1] Still, more than 90% of all SCO-compounds contain an Fe(II)-ion as central atom.[2]

 

 

SCO_Fig1.jpg

Figure 1. Schematic illustration of spin state switching in an Fe(II)-complex.

 

 

 

A spin transition cannot only be induced by a temperature change, but also by pressure and - most widely studied - by light. The latter effect is called Light-Induced Excited Spin State Trapping (LIESST)[3], but the metastable HS-state can only be trapped at sufficiently low temperatures, above which the relaxation process to the LS-state becomes too fast.

 

 

 

Another way of light-induced spin transition is the Ligand-Driven Light-Induced Spin Change (LD-LISC). This effect results from a cis-trans-isomerization of coordinated ligands that contain a photoswitchable unit, like a C=C- or N=N-double bond (styrene or azobenzene derivatives, respectively).[4] The change in configuration alters the ligand field strength and thus results in a spin transition (Figure 2). In contrast to SCO and LIESST this effect is not limited to the solid state.

 

 

Lately, we reported about Light-Driven Coordination-Induced Spin State Switching (LD-CISSS) in Nickel-porphyrines based on a change in the coordination geometry (Figure 2).[5-7] The system consists of a tetradentate square planar ligand and additional, photoswitchable ligands which may coordinate in axial position depending on their configuration. Another strategy we pursue is to combine these two units and connect the photoswitchable ligand to the tetradentate ligand. Then the complex resembles a record-player.[8] If the photoswitchable ligand is in the trans-configuration, the metal center of the complex is in a square-planar environment. The photoinduced isomerization of the ligand to its cis-configuration leads to a square pyramidal coordination of the metal center and in this way to a spin switch of the metal center. This is the first example for light-induced spin state switching in solution at room temperature.

 

 

SCO_Fig2.jpg

Figure 2. Schematic figure of spin switching in solution: LD-LISC (top) and LD-CISSS (bottom).

 

 

 

Another research target of our group is the synthesis of new SCO-compounds for spin switching in solution based on iron(II) and iron(III). Recently, we reported experimental and theoretical studies of photoisomerizable ligands in [Fe(salten)]-complexes (Figure 3).[9] Further we synthesized and investigated [Fe(o-bpy)(X)2] with X = NCS, pyridine and imidazole for potential surface fixation.[10]

 

 

 

SCO_EurJic2012.jpg  SCO_obpy_bipy.jpg

Figure 3. Photoswitchable Ligands (L) in [Fe(salten)L] (left) and [Fe(o-bpy)(X)2] with X=NCS, pyridine and imidazole (right).

 

 

 

II) Fixation of transition-metal-compounds on Au(111)


Self assembled monolayers (SAMs) of organic molecules on gold surfaces are nowadays studied intensively.[10] The molecules, which are fixed on the surface and build up SAMs, consist of three parts:

 

 

 

sas-fig5

Figure 4. Self assembled monolayers on a gold surface.

 

 

 

  • BLUE: The head group, by which the molecule gets covalently attached to the gold surface. This group usually is a thiol-unit, because of the sulfur’s ability to form covalent bonds with gold.
  • GREEN: The spacer group is responsible for the orientation of the molecules on the surface. It is the so called backbone of the SAM.
  • VIOLET: The most important aspect of the surface modification is the possibility to choose different types of end groups.

 

 

 

The monomolecular layers on the gold surfaces are prepared by immersed an Au(111)-substrate in an ethanolic solution of the molecule and characterized by Infrared-Reflection-Absorption-Spectroscopy (IRRAS), STM, XPS and NEXAFS. In the IRRAS-spectrum only vibrations are observed that have a transition dipole moment perpendicular to the surface.[11] Another method for the vibrational characterization of surfaces is gap-mode Raman spectroscopy.[12] Recently, we reported about the fixation of [Fe(salten)]-complexes on Au(111).[13]

 

 

SCO_Hanne2013.jpg

Figure 5. Schematic figure of [Fe(salten)]-complexes on Au(111).

 

 

 

Spin switching on the surface proceeds according to the same principles as spin switching in solution. Through the photoinduced isomerization of the azo ligand the metal center changes its spin state.

 

 

 

 

III) Thin films of SCO-compounds

 

 

 

The third research topic of our surfaces and switches group is the analysis of thin films of SCO-compounds for the application in spintronics. Thin films are obtained by thermal vacuum deposition and characterized by AFM, STM, UV/vis- and vibrational spectroscopy as well as magnetic susceptibility measurements. Figure 6, left shows a thermal deposited film on a quartz-disc of SCO-compound [Fe(bpz)2phen] in two different spin states, induced by temperature and light at low temperature.[14] We demonstrated in cooperation with Workgroup Prof. Dr. R. Berndt Electron Induced Excited Spin State Switching (ELIESST) by the STM-Tip (Figure 6, right).[15] Link to press release in german/english.

 

 

Holger_figure8newsas-fig9

Figure 6. Spin-switching of [Fe(bpz2)phen] in films by temperature, light and single molecular switching by the STM tip.

 

 

 

In cooperation with Workgroup Prof. Dr. W. Kuch further investigations by Near-edge x-ray absorption fine spectroscopy (NEXAFS) and STM indicate phen-dimers in the first and intact SCO-complex [Fe(bpz)2phen] in the second layer at Au(111).[16]

 

 

 

SCO_Holger3.jpg

Figure 7. phen-dimers in first and intact SCO-complex [Fe(bpz)2phen] in the second layer at Au(111) versus fragmentation of [Fe(bpz)2phen] into four-lobed [Fe(bpz)2]-complex at GaAs(110)

 

 

 

Recently, we demonstrated in cooperation with Workgroup Prof Dr. L. Kipp by Ultraviolett Photoelectron Spectroscopy (UPS) light induced spin state switching in thin films by vacuum-UV and green light stimulus.[17] Link to press release in german/english.

 

 

 

In cooperation with Workgroup of Prof. Dr. I. Parchmann we developed a smart and easy synthesis of the SCO-complex for education in coordination chemistry, magnetism and thermochromism at high-school and university.[18]Link to video release in german at SFB 677 Podcast.

 

 

 

We provide:

 

 

 

 

Research proposals for Bachelor, Master and PhD Theses in an exciting and relevant high-impact field of photoswitching and surface chemistry.

 

 

 

 

We expect:

 

 

 

 

 

  • Sound skills in organic and organometallic chemistry (Schlenk techniques)
  • Knowledge of physical chemistry (optical, vibrational, 57Fe-Mößbauer-, NMR-spectroscopy, kinetics)
  • Interest in Spincrossover-phenomena
  • Soft skills and teamwork in an interdisciplinary field

 

 

 

Acknowledgements


We thank the DFG for funding in SFB 677 “Funktion durch Schalten”.

 

 

 

 

 

References:

 

 

 

  • [1] L. Cambi, L. Szegö, Ber. Deutsch. Chem. Gesell. 1931, 64, 167.
  • [2] P. Gütlich and H. A. Goodwin, Eds., Spin Crossover in Transition Metal Compounds, Vol. I – III (Springer-Verlag Berlin-Heidelberg-New York, 2004).
  • [3] S. Decurtins, P. Gütlich, C. P. Köhler, H. Spiering, A. Hauser, Chem. Phys. Lett. 1984, 105, 1.
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  • [7] S. Thies, H. Sell, C. Bornholdt, C. Schütt, F. Köhler, F. Tuczek, R. Herges,Chem. Europ. J. 2012, 18, 16358 DOI: 10.1002/chem.201201698.
  • [8] S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sönnichsen, F.Tuczek, R. Herges, Science 2011, 331, 445-448.
  • [9] A. Bannwarth, S. O. Schmidt, G. Peters, F. D. Sönnichsen, W. Thimm, R. Herges, F. Tuczek, Eur. J. Inorg. Chem. 2012, 16, 2776 DOI: 10.1002/ejic.201101227.

  • [10] S. O. Schmidt, S. Kisslinger, C. Würtele, S. Bonnet, S. Schindler, F. Tuczek, Z. Anorg. Allg. Chem. 2013, 639(15), 2774 DOI: 10.1002/zaac.201300425.
  • [11] L. Hallmann, A. Bashir, T. Strunskus, R. Adelung, V. Staemmler, C. Wöll, F. Tuczek, Langmuir 2008, 24, 5726-5733.
  • R. Ketheeswari, L. Hallmann, T. Strunskus, A. Bashir, C. Wöll and F. Tuczek, Phys. Chem. Chem. Phys. 2010, 12(17), 4390-4399.
  • [12] U. Jung, M. Müller, N. Fujimoto, K. Ikeda, K. Uosaki, U. Cornelissen, F. Tuczek, C. Bornholdt, D. Zargarani, R. Herges, O. Magnussen, Journal of Colloid and Interface Science 2010, 341, 366-375.
  • [13] H. Jacob, K. Kathirvel, F. Petersen, T. Strunskus, A. Bannwarth, S. Meyer, F. Tuczek, Langmuir 2013, 29(27), 8534 DOI: 10.1021/la400663y.
  • [14] H. Naggert, A. Bannwarth, S. Chemnitz, T. van Hofe, E. Quandt, F. Tuczek, Dalton Trans. 2011, 40, 6364,DOI:10.1039/C1DT10651A.
  • [15] T. G. Gopakumar, F. Matino, H. Naggert, A. Bannwarth, F. Tuczek and R. Berndt,Angew. Chem. 2012, 124(25), 6367 DOI: 10.1002/ange.201201203.
  • [16] T. G. Gopakumar, M. Bernien, H. Naggert, F. Matino, C. F. Hermanns, A. Bannwarth, S. Mühlenberend, A. Krüger, D. Krüger, F. Nickel, W. Walter, R. Berndt, W. Kuch, F. Tuczek, Chem. Europ. J. 2013, 19(46), 15702 DOI: 10.1002/chem.201302241.
  • [17] E. Ludwig, H. Naggert, M. Kalläne, S. Rohlf, E. Kröger, A. Bannwarth, A. Quer, K. Rossnagel, L. Kipp and F. Tuczek, Angew. Chem. 2014, 126, 3063–3067 DOI: 10.1002/ange.201307968. Angew. Chem. Int. Ed. 2014, 53, 3019–3023 DOI: 10.1002/anie.201307968.
  • [18] J. Rudnik, H. Naggert, S. Schwarzer, F. Tuczek und I. Parchmann, CHEMKON 2014, 21(2), 85-88. DOI: 10.1002/ckon.201410222