1. Molecular Magnetic Switches
Dr. Marat M. Khusniyarov
Department of Chemistry and Pharmacy
Friedrich-Alexander-University of Erlangen-Nuremberg Erlangen, Germany

2. Spin-Crossover Metal Complexeseg eg
DT, Dp, hn
t2g
t2g
low-spin Fe(II)
high-spin Fe(II)
diamagnetic
paramagnetic LS HS
Switching:
Application:
magnetic properties,
electrical conductivity,
refractive index,
color, …
electronics,
spintronics,
sensors,
memory units, …
molecular

3. LIESST (Light-Induced Excited Spin State Trapping)LS HS LS HS
Problem:
photoinduced state is usually stable only at very
low temperatures T < 50K!
P. Gütlich, A. Hauser et al., Chem. Phys. Lett. 1984, 105, 1
A. Hauser, Top. Curr. Chem. 2004, 234, 155

4. LD-LISC (Ligand-Driven Light-Induced Spin Change)
trans- planar
cis- non-planar
first evidence for photoswitching at RT
M.-L. Boillot, J. Zarembowitch et al., Inorg. Chem. 1996, 35, 3975
spin change conversion  3%
H. Nishihara et al., Dalton Trans. 2009, 280
2. liquid phase
Limitations:
1. very low spin-change conversion

5. Spin-Crossover Molecular Photoswitch
open-ring isomer
closed-ring isomer
Photocyclization
UV, solution, RT
Photocycloreversion
visible light, solution, RT
Multiple photoswitching extinction at 376 nm

6. Spin-Crossover Molecular Photoswitch
Inorg. Chem. 2013, 52, 11585
HS S = 2
LS S = 0
NMR under UV irradiation solution, RT
Multiple photoswitching
HS  LS photoconversion: 40% at RT !

7. Reversible photocyclization in the solid state at RTUV
Photocycloreversion
visible light
Multiple photoswitching abs. at 375 nm
S 2p XPS
100% open-ring
68% open-ring
32% closed-ring UV
Reversible photocyclization in the solid state at RT

8. Spin-Crossover Molecular Photoswitch
NEXAFS at Fe L3
Angew. Chem. Int. Ed. 2015, 54, 12976
HS 100%
HS 68%
LS 32%
HS 84%
LS 16% UV
visible RT
HS S = 2
LS S = 0
room temperature !
solid state !
reversible !
molecular level !

9. Current Developments Modifications “1” “0” hn or electrically @ RT
Au(111)
Au(111)

10. Valence Tautomeric Metal Complexes strongly paramagnetic
Intramolecular Electron Transfer coupled with Spin-Crossover at Co center
LS-Co(III)
HS-Co(II)
weakly paramagnetic
strongly paramagnetic
St = 0 + 1/2
St = 3/2 + 1/2 + 1/2
meff = 1.73 mB
meffRT = 5.12 mB
A. L. Rheingold, D. N. Hendrickson et al., J. Am. Chem. Soc. 1993, 115, 8221
photoswitching requires T < 20 K!
Problem:

11. Valence Tautomeric Molecular Switches cis-trans isomerizable systems

12. Valence Tautomeric Molecular Switches
Variable temperature Evans NMR
Variable temperature UV-VIS
at RT:
HS-Co(II): 44%
LS-Co(III): 56%
LS-Co(III) HS-Co(II)
DG = DH – TDS
DH = 43(5) kJ mol–1
DH = 56(1) kJ mol–1
DS = 144(16) J mol–1 K–1
DS = 190(5) J mol–1 K–1
Both ΔH and ΔS are too large for a valence tautomeric equilibrium !

13. Equilibria in Solution
HS-Co(II)
HS-Co(II)
LS-Co(III)

14. Coordination-Induced Valence Tautomerism
Titration with 4-styrylpyridine
Chem. Eur. J. 2014, 20, 11149
UV-VIS-NIR
Evans NMR
IVCT
Ka = 4(1)·103 L mol–1
Ka = 6(2)·103 L mol–1
Coordination-Induced Valence Tautomerism (CIVT) RT
HS-Co(II)
LS-Co(III)

15. Photomagnetic Molecular Switch
Chem. Sci. 2015, 6, 4599
trans  cis photoisomerization (l = 320 nm)
UV-VIS-NIR
Evans NMR
EPR
Ligand-Driven Light-Induced Valence Tautomerism (LD-LIVT) RT
LS-Co(III)
HS-Co(II)
l = 320 nm
l = 272 nm

16. Overview Angew. Chem. Int. Ed. 2015, 54, 12976
Inorg. Chem. 2013, 52, 11585
Light-Induced
Spin-Crossover at RT
Chem. Eur. J. 2014, 20, 11149
Coordination-Induced Valence Tautomerism (CIVT)
Dalton Trans. 2013, 42, 5237
Chem. Sci. 2015, 6, 4599
Supramolecular Magnetic Networks
Ligand-Driven Light-Induced Valence Tautomerism (LD-LIVT)

17. Acknowledgements The Group: Magdalena Milek Alexander Witt
Dr. Ghulam Abbas
Prof. Rainer Fink
Dr. Benedikt Rösner FAU Erlangen-Nuremberg (XPS, NEXAFS)
Dr. Frank Heinemann FAU Erlangen-Nuremberg (crystallography)