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International Tomography Center, Institutskaya 3a, 630090, Novosibirsk, Russia

Elena Bagryanskaya. International Tomography Center, Institutskaya 3a, 630090, Novosibirsk, Russia. Thermal and Optical Switching of the Exchange Interactions in Nitroxide-Copper(II)-Nitroxide Clusters as Studied by EPR.

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International Tomography Center, Institutskaya 3a, 630090, Novosibirsk, Russia

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  1. Elena Bagryanskaya International Tomography Center, Institutskaya 3a, 630090, Novosibirsk, Russia Thermal and Optical Switching of the Exchange Interactions inNitroxide-Copper(II)-Nitroxide Clusters as Studied by EPR

  2. Molecular magnetics based on polymer-chain heterospin systems (transition metal ions and stable nitroxide radicals) Potential application as new components in electronics Dielectric light Transparent

  3. “Breathing” crystals Cu(hfac)2LR and Cu(hfac)2LR0.5solv Pr(-C3H7) Bu(-C4H9) Et (-C2H5) Me (-CH3) Bu +benzene Bu +octane Bu +toluene Bu +octene Bu +orthoxylene Bu +heptane Bu +hexane

  4. Structural rearrangements and spin transitions • reversible structural rearrangements T=293 K T=115K T=203 K weakly-coupled state strongly-coupled state  Novel functional magnetic materials with a potential for applications in spintronics, magnetic data storage etc.

  5. A diversity of magnetic anomalies R=Bu R=Bu heptane R=Bu octane R=Bu o-xylene R=Bu octene Bu toluene R=Bu hexane R=Bu benzene R=Et R=Pr R=Me ?  Exchange interactions and their temperature evolution  Magneto-structural correlations  Optical switching of spin states – is that possible?

  6. The EPR study: outline General trends and effects predominant population of the ground multiplet  dynamic mixing (spin exchange) processes Classification of spin transitions and estimations of J different manifestations of spin transitions  correlations with magnetic susceptibility Measurement of temperature dependence of J  different approaches Light-Induced Excited Spin State Trapping (LIESST) on Cu(hfac)2LPr

  7. “g<2 signals” in spin triads 2.2 2.0 1.8 g value 2.6 2.4 1.6 T=260 K T=140 K T=90 K 250 300 350 400 450 Magnetic field / mT

  8. EPR spectrum at T below the spin transition >N-O-Cu-O-N< g >N-Cu-N< g g|| >N-O-Cu-O-N< g|| >N-Cu-N<

  9. Anomalous values g<2 of a three-spin cluster X-band Q-band experiment simulation 200 250 300 350 400 450 1000 1100 1200 1300 1400 Magnetic field / mT g<2 g<2 • EPR at two frequencies shows the appearance of signals with g<2 • Spectra at X- and Q-band can be simulated using the same values g<2

  10. Explanation of the “g<2 signals” Cu J12 J23 O O J13 J’ J’ Cu O O • Octahedral geometry of the three-spin cluster >O-Cu-O< excludes g<2 for the copper ion • the nitroxides are also known to have g>2 >N-O-Cu-O-N< (linear geometry)

  11. Solution of the spin-Hamiltonian S1=S2=S3 =1/2 |J|>>H

  12. “g<2 signals” in spin triads -2J -2J S=3/2 Cu O O J S=3/2 S=1/2 J 2J S=1/2 S=1/2 1050 1100 1150 1200 1250 1300 1350 2J S=1/2 1350 1050 1100 1150 1200 1250 1300 Magnetic field / mT Very strong J>kT leads to a static spin polarization: only the lower S=1/2 state with g<2 is populated, and only this EPR line is observed

  13. “g<2 signals” in spin triads triad Simulated Experimental 0.9 1.0 1.1 1.2 1.3 1.4 2.8 3.0 3.2 3.4 3.6 Magnetic field / T Magnetic field / mT Magnetic field / T Ref. M. V. Fedin et al., // J. Phys. Chem. A 110 (2006) 2315-2317 EPR at X-, Q- and W-band: g<2 [g||=1.91, g=1.97] X-band (9 GHz) Q-band (35 GHz) W-band (94 GHz) triad triad T=90 K 200 250 300 350 400 450 • sign of J : “g<2 signals” indicate a strong antiferromagnetic exchange • the effective g-factor of a spin triad is less than 2 indeed!

  14. Expected high temperature spectra -2J -2J Cu O O S=3/2 J S=1/2 1050 1100 1150 1200 1250 1300 1350 2J S=1/2

  15. Dynamic mixing process 220 K 200 K 190 K 180 K 170 K 160 K 150 K 140 K 130 K 120 K 110 K 90 K 80 K 70 K 60 K T T 2.8 3.0 3.2 3.4 3.6 3,1 3,2 3,3 3,4 3,5 3,6 Magnetic field / T Magnetic field / T Cu(hfac)2LPr at W band Disordered powder Oriented powder

  16. Electron spin exchange process S=3/2 J S=1/2 2J S=1/2 Cu O O M. V. Fedin, et al.// J. Phys. Chem. A 111 (2007) 4449-4455 Ref. • modulation of the exchange interaction by lattice vibrations Fast exchange between several lines center frequency linewidth

  17. Dynamic mixing processesin Cu(hfac)2LBu0.5C8H10 122 GHz 34 GHz 244 GHz fast exchange Intermediate/slow exchange slow exchange

  18. The EPR study: outline General trends and effects predominant population of the ground multiplet  dynamic mixing (spin exchange) processes Classification of spin transitions and estimations of J different manifestations of spin transitions  correlations with magnetic susceptibility Measurement of temperature dependence of J  different approaches Light-Induced Excited Spin State Trapping (LIESST) on Cu(hfac)2LPr

  19. Classification of spin transitions in Cu(hfac)2LR Pr(-C3H7) Bu(-C4H9) Et (-C2H5)  Me (-CH3)  Bu +benzene  Bu +octane Bu +toluene  ? Bu +octene Bu +orthoxylene Bu +heptane Bu +hexane  ?    

  20. Theory C B S=3/2 A J S=1/2 2J S=1/2 Effective magnetic moment Boltzman populations of corresponding multiplets Effective g-tensor of a spin triad (fast exchange limit) Boltzman factors, number and probabilities of EPR transitions within each multiplet A, B and C

  21. Estimations of the exchange interaction value geff eff • |J|kT corresponds to geff 2 • for |J|>2kT the evolution of geff(T) is essentially completed

  22. Large amplitude spin transition in Cu(hfac)2LBu0.5C7H16 2,5 2,4 2,3 eff m 2,2 2,1 2,0 70 K 1,9 70 K 1,8 0,0 0,5 1,0 1,5 2,0 2,5 3,0 2,04 2,02 110 K eff g 110 K 2,00 130 K 1,98 160 K 160 K 1,96 220 K 0,0 0,5 1,0 1,5 2,0 2,5 3,0 220 K | J |/ kT J>150 cm-1 at T<110 K geff=[1.993; 1.983; 1.905] gCu=[2.048; 2.078; 2.314] 250 300 350 400 1000 1100 1200 1300 1400 Magnetic field / mT X-band Q-band

  23. Three-step spin transition in Cu(hfac)2LMe 2,5 2,4 2,3 eff m 2,2 40 K 40 K 2,1 2,0 1,9 1,8 0,0 0,5 1,0 1,5 2,0 2,5 3,0 65 K 65 K 2,04 2,02 90 K eff 90 K g 2,00 1,98 160 K 160 K 1,96 0,0 0,5 1,0 1,5 2,0 2,5 3,0 | J |/ kT J80 cm-1 at T<56 K geff=[1.996; 1.993; 1.916] gCu=[2.040; 2.050; 2.280] 1.0 1.1 1.2 1.3 1.4 2.8 3.0 3.2 3.4 3.6 Magnetic field / T Q-band W-band

  24. Inverse spin transition in Cu(hfac)2LEt 2,5 2,4 2,3 eff m 2,2 2,1 100 K 2,0 1,9 1,8 0,0 0,5 1,0 1,5 2,0 2,5 3,0 200 K 2,04 2,02 eff g 2,00 240 K 1,98 1,96 2.8 3.0 3.2 3.4 3.6 0,0 0,5 1,0 1,5 2,0 2,5 3,0 | J |/ kT Inverse transition is also explained Magnetic field / T Ref. M. V. Fedin, et al. // Inorg. Chem. 46 (2007) 11415 W-band

  25. Approaches for the measurement of J(T) Problem: gCu (and thus gA, gB, gC) are functions of T • Solutions: • obtain isotropic geff(T) from the powder spectra and then simulate it •  use single-crystal orientations where • this effect is minimum T EPR or magnetic susceptibility? J(T) can be obtained by fitting experimental geff(T)

  26. J(T) measurement for Cu(hfac)2LPr0.5C8H18 S.L. Veber, et al.// JACS 130 (2008) 2444 Ref.  J changes by an order of magnitude! Ref.

  27. Motivation: LIESST on iron(II) spin crossover compounds dark, 15 K  55 K,  15 K S=2 light, 15 K  97 K  55 K,  15 K  148 K S=0

  28. Motivation: LIESST on iron(II) spin crossover compounds So far LIESST has been observed on spin-crossover complexes of iron(II/III), and never on copper(II)

  29. Can LIESST occur in breathing crystals Cu(hfac)2LR ? • Some “prerequisites” for LIESST observation: • spin transition • significant geometry (bond length) • difference between HS and LS states • (Cu-O)~0.3 A • UV/near-IR absorption band • slow relaxation between HS and LS states - ???

  30. Experimental 1.4 W 14 mW • the crystals are very dark – the light is absorbed within a thin outer layer • solution: a mixture of grinded crystals with glycerol (suspension) • precaution: particles size should be adjusted experimentally polycrystalline powder of Cu(hfac)2LPr  suspension Cu(hfac)2LPr/ glycerol • no chemical influence of glycerol

  31. EPR-detected LIESST on Cu(hfac)2LPr T=180 K T=7 K, dark 7 K, light 5 min 3 hours 20 K -> 7 K SS=strongly-coupled state (|J|>>kT) WS=weakly-coupled state (|J|<<kT)

  32. LIESST of Cu(hfac)2LPr vs. time and T 13 K 10 K 7 K • before illumination after illumination, waiting, lifting T up to 20 K and cooling back to 7 K • no photochemistry/photodecay • The conversion into a weakly-coupled state is certainly not due to a heating by light

  33. Supposed mechanism J 2J Intermediate excited state |J|>>kT |J|<<kT Weakly-coupled state Strongly-coupled state

  34. Conclusions General trends of EPR applied to the strongly exchange-coupled spin triads are studied. Specific characteristics originate from: - predominant population of ground spin multiplet (“polarization”) • dynamic mixing (electron spin exchange) processes •  Spin transitions in the family of “breathing” crystals Cu(hfac)2LR are studied and classified. The correspondence with the magnetic susceptibility data is established •  The approaches for the measurement of temperature dependences of exchange interaction J(T) are developed and applied for the compounds Cu(hfac)2LR • Effective “magnetic dilution” due to the phase spin transition in two-spin complexes (Cu(hfac)2LMe) allows us to obtain the value of dipole-dipole interaction in exchange-coupled spin pair as well as the value of exchange interaction between neighboring pairs • The light-induced spin state conversion (LIESST effect) is found in the breathing crystals Cu(hfac)2LR. The excited state lives for hours-to-days at cryogenic temperatures

  35. THANKS  Dr. Matvey Fedin, Sergey Veber (ITC-Novosibirsk)  Prof. Victor Ovcharenko, Dr. Ksenia Maryunina, Dr. Sergey Fokin (ITC- Novosibirsk)  Dr. Igor Gromov and Prof. Arthur Schweiger(ETH-Zürich)  Prof. Daniella Goldfarb, Alexey Potapov (Weizmann Inst.Sci., Rehovot)  Prof. Wolfgang Lubitz, Dr. Edward Reijerse (Max-Planck-Inst. für Bioanorganische Chemie, Mülheim/Ruhr) Financial support:   RFBR, AvH, INTAS, SB RAS, Council at RF president, FASI

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