• Light-induced spin-state switching in the mixed crystal series of the 2D coordination network {[Zn1-xFex(bbtr)3](BF4)2}: optical spectroscopy and cooperative effects
    P. Chakraborty, C. Enachescu, A. Humair, L. Egger, T. Delgado, A. Tissot, L. Gune, C. Besnard, R. Bronisz and A. Hauser
    Dalton Transactions, 43 (47) (2014), p17786-17796
    DOI:10.1039/C4DT01728E | unige:42340 | Abstract | Article HTML | Article PDF
 
Depending on the iron(II) concentration, the mixed crystals of {[Zn1-xFex(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, 0.01 ≤ x ≤ 1, show macroscopic light-induced bistability between the high-spin and the low-spin state. In the highly diluted system with x = 0.01 and up to x = 0.31, the photoinduced low-spin state always relaxes back to the high-spin state independent of the initial light-induced low-spin fraction. In the highly concentrated mixed crystals with x = 0.67, 0.87 and 1, the strong cooperative effects coupled to a crystallographic phase transition result in light-induced bistability with decreasing critical light-induced low-spin fraction and increasing hysteresis width for increasing iron(II) concentrations. The lower limit for the light-induced bistability is estimated at x ≈ 0.5.
  
  • Light Induced Bistability in the 2D Coordination Network {[Fe(bbtr)3](BF4)2}: Wavelength-Selective Addressing of Molecular Spin States
    P. Chakraborty, S. Pillet, E.-E. Bendeif, C. Enachescu, R. Bronisz and A. Hauser
    Chemistry - A European Journal, 19 (34) (2013), p11418-11428
    DOI:10.1002/chem.201301257 | unige:29524 | Abstract | Article PDF
Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3](ClO4)2} (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3](BF4)2}, Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time and intensity. Crystallographic studies reveal a photo-induced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.
  • Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn1-xFex(bbtr)3](ClO4)2}: The Role of Cooperative Effects
    P. Chakraborty, C. Enachescu, C. Walder, R. Bronisz and A. Hauser
    Inorganic Chemistry, 51 (18) (2012), p9714-9722
    DOI:10.1021/ic301006c | unige:22920 | Abstract | Article HTML | Article PDF
 
The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn1-xFex(bbtr)3](ClO4)2} (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.201117, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model.
  
In the covalently linked 2D coordination network {[Fe(bbtr)3](BF4)2}, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, the iron(II) centers stay in the high-spin (HS) state down to 10 K. They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed 5dd band and back again by irradiating into the visible 1dd band. The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.
We investigated by optical microscopy the thermal spin transition in single crystals of [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl) butane). The growth of the low-spin phase was observed for different crystal orientations and sizes. The process always started from a corner of the crystal but its further development depended on the size, shape and thermal history of the crystal. In crystals of smaller size, under isothermal conditions, the low-spin phase developed in a continuous way, through the propagation of a rather well defined transformation front, with a higher propagation velocity inside the planes perpendicular to the c axis. In larger crystals the spontaneous occurrence of inhomogeneous stresses led to a stepwise propagation process.
  • Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [FexZn1−x(bbtr)3](ClO4)2
    I. Krivokapic, P. Chakraborty, C. Enachescu, R. Bronisz and A. Hauser
    Inorganic Chemistry, 50 (5) (2011), p1856-1861
    DOI:10.1021/ic102269y | unige:14980 | Abstract | Article HTML | Article PDF
 
Whereas the neat polymeric iron(II) compound [Fe(bbtr)3](ClO4)2, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [FexZn1−x(bbtr)3](ClO4)2x = 0.02 and 0.1, stay predominantly in the 5T2 high-spin state down to cryogenic temperatures. However, the 1A1 low-spin state can be populated as metastable state via irradiation into the spin-allowed 5T25E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spinhigh-spin relaxation is 1840(20) cm−1.
  • Significant variation of the singlet-quintet intersystem crossing rate constant in an iron(II) high-spin complex as a function of temperature
    I. Krivokapic, P. Chakraborty, R. Bronisz, C. Enachescu and A. Hauser
    Angewandte Chemie, 49 (45) (2010), p8509-8512
    DOI:10.1002/anie.201004500 | unige:14713
In the dilute mixed-crystal system [Zn1−xFex(bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K
  
  • The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations
    I. Krivokapic, C. Enachescu, R. Bronisz and A. Hauser
    Inorganica Chimica Acta, 361 (12-13) (2008), p3616-3622
    DOI:10.1016/j.ica.2008.03.064 | unige:3576 | Abstract | Article PDF
The thermal and the light-induced spin transition in [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin → low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy.
  • Spin transition and relaxation dynamics coupled to a crystallographic phase transition in a polymeric iron(II) spin-crossover system
    I. Krivokapic, C. Enachescu, R. Bronisz and A. Hauser
    Chemical Physics Letters, 455 (4-6) (2008), p192-196
    DOI:10.1016/j.cplett.2008.02.088 | unige:74 | Article HTML | Article PDF
The spin-crossover compound [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below Tc depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.

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