EPR of Spin Transitions in Complexes of Cu(hfac)2 with tert-ButylPyrazolylnitroxides

Irina Drozdyuk

International Tomography Center, Novosibirsk, Russia

Cu(hfac)2LBu0.5C7H16

Cu(hfac)2LPr

Cu(hfac)2LBu0.5C7H8

Cu(hfac)2LEt

Cu(hfac)2LBu0.5C8H10

Cu(hfac)2LBu0.5C7H16

N

NR

N

N

O

O

O

Cu

O

O

O

CF3

CF3F3C

F3C

LR Cu(hfac)2

• Very sensitive to the radical structure and the organic solvents

Family of Cu(hfac)2LR

T=293 K T=203 K T=115 KStrongly-coupled state (SS)

Weakly-coupled state (WS)

J »kT

J «kT

• The flip of the Jahn-Teller axes in the triads

Structural rearrangements and spin transitions

Izv. An. 11 (2004) 2304

Why do we need it?

Applicable facilities Features of Cu(hfac)2LR:

Nanometer scale

Room temperature

Ability to control and switch transition

Combining of properties

Nanosecond time of relaxation

Light-induced transition

Molecular switch

Dense storage of information (STT-RAM)

Logical schemes (10-17 J, 1 ns)

Spin transistor (100% polarisation)

Accumulator of energy

low-temperature spectrum high-temperature spectrum

S=3/2

S=1/2

S=1/2

gc=(2gR+gCu)/3

gb=gCu

ga=(4gR-gCu)/3

J

2J

EPR of Cu(hfac)2LR

Inorg. Chem. 46 (2007) 11405

TriadCu2

TriadCu2

Magnetic field, Т

1.0 1.1 1.2 1.3 1.4

Magnetic field, Т

1.0 1.1 1.2 1.3 1.4

• Finding new functional ligands • New radicals absorbe less light

Background and motivation

Angew. Chem. Int. Ed. 47 (2008) 6897

400 500 600 700 800 9000.00

0.25

0.50

0.75

1.00

Wavelength, nm

Ab

sorb

an

ce

Cu(hfac)2LR

Cu(hfac)2LR

tert

590=2230

590=420

N

NR

N

N

O

O

O

Cu

O

O

O

CF3

CF3F3C

F3C

LR Cu(hfac)2

NN

N O

R

LRtertLR

Background and motivation

??

?

JACS 132 (2010) 13886

• Finding new functional ligands • New radicals absorbing less light

• Tuning various intercluster exchange interactions

Experimental dependence μeff(T), magnetic field = 1 Т 

0 50 100 150 200 250 300 3501.8

2.0

2.2

2.4

2.6

eff

(B.M.)

T (K)

(●) [Cu(hfac)2LtertPr ]n

(▼) [Cu(hfac)2LtertEt ]n

(■) [Cu(hfac)2LtertMe ]n

N

N

N

O

N

N

N

O

N

N

N

O

Researching compounds

Installation

Bruker Elexsys E580 EPR-spectrometer X/Q band

• Cryostat

• Helium cooling system

• Temperature control system

• Q-band

• T= 4-293 К

• Polycrystalline powder samples

• Averaging over axial angle

Conditions of experiment

• CW - mode

200 К

150 К

120 К

100 К

75 К

Magnetic field, Т

293 К

240 К

200 К

160 К

120 К

80 К

Magnetic field, Т

293 К

200 К

145 К

100 К

75 К

Magnetic field, Т

250 300 350 400 450

B / мТ

T=260 KT=140 KT=90 K

Typical EPR-spectrum of Cu(hfac)2LR

• Principal changing of EPR-spectra of Cu(hfac)2LtertR

Experimental results

Cu(hfac)2LtertPrCu(hfac)2Ltert

EtCu(hfac)2LtertMe

Jinter=0

Jinter0

Magnetic field, mТ

Jinter=0

Jinter0

Magnetic field, mТ

Triad

Cu2

-∑2JinterSiCu2Si

R1,2

Intercluster exchange

• An approach of the modified Bloch equations

• Spin Hamiltonian of the systemJ

2J

WSSS

Theoretical modeling

TriadCu2 TriadCu2

, where

erCuCuCuRRCuCuRRR HSBgSSSJSBgSSBgH int

22111 ˆ2ˆ 2121

ABBAAA

A MiGGidtdG

012

11

BAABBB

B MiGGidtdG

012

11

yxMiMG BA 2,12,1,

2

002 ,

34,||Cutriad

Cutriad

CuRa

MM

gggkTJ

2

002 3,33

32,||Cutriad

Cutriad

CuRc

MM

gggkTJ

200 К

150 К

120 К

100 К

75 К

293 К

240 К

200 К

160 К

120 К

80 К

293 К

200 К

145 К

100 К

75 К

Results of theoretical modeling

Compound T, K t, T2triad, T2

Cu2, q

´10-11 s ´10-11 s ´10-7 s

Cu(hfac)2LtertMe 75 2 30 4 77

Cu(hfac)2LtertMe 200 10 11 40 54

Cu(hfac)2LtertEt 80 1.1 22 40 77

Cu(hfac)2LtertEt 293 10 8 4 54

Cu(hfac)2LtertPr 75 1.1 15 40 77

The set of parametersEstimation of intercluster exchange J inter :

• observed line shape

|Jinter|>0.1 cm-1|gA,C – gCu2|>1/ 1/2Jinter

Magnetic field, Т Magnetic field, Т Magnetic field, Т

Compound |Jinter|, cm-1

Cu(hfac)2LtertMe  0.15

Cu(hfac)2LtertEt  0.15

Cu(hfac)2LtertPr - 

Compound |Jinter|, cm-1

Cu(hfac)2LtertMe 0.8

Cu(hfac)2LtertEt 1.5

Cu(hfac)2LtertPr 1.5 

• theoretical modelingWS SS

Cu(hfac)2LtertPrCu(hfac)2Ltert

EtCu(hfac)2LtertMe

• The first EPR-study of new thermo-switchable molecular magnets [Cu(hfac)2LtertR ]n

• These compounds differ from those studied previously by the structure of a nitroxide ligand. Replacement of the nitronyl nitroxide substituent in LR by tert-butylnitroxide one supresses the intercluster exchange pathway between different polymer chains, and leads also to an ehancement of intercluster exchange interaction (up to a few cm-1)

• Despite the exchange narrowing of the spectra due to the stronger intercluster interaction in these complexes, observed line shapes are significantly different depending on the spin state of a triad (WS or SS).

• Theoretical modeling has confirmed the assigment of the observed spectra to the complexes with the triad in the one of these two spin states.

• This investigation explains the main trends of EPR applied for the caracterization of phase spin transitions in new compounds and creates the basis for their future studies.

Conclusion

Thank you for your attention!