Weak-Ferromagnetism and Ferromagnetism in

Chapter 19

Weak-Ferromagnetism and Ferromagnetism in Tetrafluorotetracyanoquinodimethanide Salts 1

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Toyonari Sugimoto , Kazumasa Ueda , Nobuko Kanehisa , Yasushi Kai , Motoo Shiro , Nobuyoshi Hosoito , Naoya Takeda , and Masayasu Ishikawa 2

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Downloaded by MONASH UNIV on September 14, 2013 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch019

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Research Institute for Advanced Science and Technology, University of Osaka Prefecture, Sakai, Osaka 593, Japan Department of Applied Chemistry, Osaka University, Suita, Osaka 565, Japan Rigaku Corporation, Akishima, Tokyo 196, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo 106, Japan 2

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Several salts of the radical anion of tetrafluorotetracyanoquinodimethane (TCNQF ) (tetrafluorotetracyanoquinodimethanide, 4

TCNQF -) exhibited very unique magnetisms, i.e., weakferromagnetism and ferromagnetism. The weak-ferro-magnetism appeared below 12 K in the Li salt of T C N Q F - . On the other hand, the ferromagnetism was observed in the charge-transfer (CT) complexes of T C N Q F - with pyridinium-substituted imidazolin-14

+

4

4

oxyls as well as in the N(CH3) + salt of TCNQF - with a half molecule of TCNQF . The ferromagnetic phase-transition temperature (Curie temperature, Tc) was in the range of 0.4 to 0.55 K in the former salt, which can be declared as a first purely organic ferromagnet based on a well-characterized CT complex. Very interestingly, the latter TCNQF /TCNQF - mixed salt revealed a remarkable high T close to room temperature. 4

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c

Tetracyanoquinodimethane (TCNQ) (1) and its tetrafluoro-substituted derivative (TCNQF4) (2) are well known as common electron acceptors in the formation of charge-transfer (CT) complexes. Most notably, the first synthetic metal utilized TCNQ as the organic acceptor (3,4). This discovery has triggered off a great advance in electrical conduction in organic materials, and at last led to a first organic superconductor in 1980 (5).

0097-6156/96/0644-0276$15.00/0 © 19% American Chemical Society

In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

19. SUGIMOTO ET AL.

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Ferromagnetism and TCNQF

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Salts

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Ferromagnetism is another target to be acieved in organic materials. Since the first discovery of ferromagnetic interaction in a galvinoxyl crystals (6), similar phenomena have also been recognized in almost 20 organic radical crystals (7). Very recently, a purely organic ferromagnet has been synthesized in a p-nitrophenyl nitronyl nitroxide crystal (8) and successively in several nitroxide crystals (9-72). However, the highest Tc value is only 1.48 K (9). In addition, there has so far been almost no progress in the achievement of ferromagnetism even in purely organic CT complexes, which have the advantage of different compositions between donors and acceptors, high stability, strong spin-spin coupling by aid of Coulombic interaction between the positive and negative charges, and coexistence of local spins and conduction electrons interacting with each other. This article presents weak-ferromagnetism in the L i salt of the radical anion of TCNQF4 (TCNQF4-) and ferromagnetism in the C T complexes of T C N Q F 4 " with pyridinium-substituted imidazolin-l-oxyls as well as in the


+

tetramethylammonium salt of TCNQF4/TCNQF4"'. These results bring an expectation that TCNQF4 (and also TCNQ in future) might make a great contribution to the development of high T purely organic purely ferromagnets. c

Weak-Ferromagnetism in the Li+ salt of T C N Q F 4 " (13) Prior to studying the magnetic properties of the CT complexes with T C N Q F 4 " , the +

spin-spin interaction between TCNQF4" molecules was investigated by using the L i , +

+

+

N a , K , Rb+, Cs+ and N(CH3)4 salts Previous works indicate that in salts other +

than Li -TCNQF4"* strong antiferromagnetic interaction occurs as a result of dimer formation between TCNQF4"' molecules, but in the L i

+

salt the temperature

dependence of paramagnetic susceptibility (Xp) follows the Curie law in the temperature range 110 to 300 K , indicating no significant spin interaction between TCNQF4"' molecules (14). It is of much interest to examine in the spin-spin interaction in the lower temperature region (ca. 2 K ) in the L i salt, where the dimer formation might also occur bringing about disappearance of the magnetic moment. However, contrary to this expectation weak-ferromagnetism was observed below 12 K . This magnetic phenomenon is well known in several inorganic systems involving C1-F2O3 (15) and NiF2 (16-18), but there has been no report in purely organic systems until this discovery. At almost the same time weak-ferromagnetism was also recognized in two organic radical crystals of 1,3,5-triphenylverdazyl (19) and 1,3,5triphenyl-6-oxoverdazyl (20). +

+

The temperature dependences of the Xp values in the L i and N a

+

salts of

TCNQF4"' are shown in Figure 1. The magnetic susceptibility was measured in the temperature range of 2 to 300 K at applied magnetic field of 500 Oe by using a SQUID magnetometer, and the Xp value was obtained after subtracting the diamagnetic contribution calculated by Pascal's method from the observed value. As seen in Figure 1, Na -TCNQF4"' has almost no contribution to Xp as result of strong antiferromagnetic interaction between the TCNQF4" radical pairs. A similar observation was also made for the K , R b , C s and N(CH3)4 salts. The energy difference between the singlet and thermally-accessible triplet states was estimated to be greater than 1500 K based on the analysis by use of a Bleaney-Bowers equation (21). +

+

+

+

+


278

MOLECULE-BASED MAGNETIC MATERIALS

C

NC R

N

R

R = H: TCNQ R = F: TCNQF

R = H: TCNQ-. R = F: TCNQF "'

4

4


Formula 1 0.10

0.08

I

0.06"

a 0.04

0.02

0.00 100

200

300

T(K) Figure 1. Temperature dependences of the paramagnetic susceptibility (Xp) obtained by subtracting the diamagnetic contribution from the magnetic susceptibility measured +

at applied magnetic field of 500 Oe in Li+-TCNQF4" (H) and N a - T C N Q F 4 " (•). Reprinted from T. Sugimoto, M . Tsujii, H . Matsuura, N . Hosoito/Weak ferromanetism below 12 K in a lithium tetrafluorotetracyanoquinodimethanide salt, 1995, pp. 183 - 186, vol. 235, with kind permission from Elsevier Science - N L , Sara Burgerhartstraat25, 1055 K V Amsterdam, The Netherlands (reference 13).

+

+

,

Contrary to the N a salt, for Li •TCNQF4• Xp increases as the temperature is lowered. The temperature dependence of Xp can be well-expressed by the Curie-Weiss law above 50 K with a Curie constant of 0.36*0.01 emu-K/mol and a negative asymptotic Curie temperature of -30*2 K (see Figure 2(a)). The radical concentration, as shown from the Curie constant, corresponds to one S=l/2 spin per molecule. The Xp shows a plateau around 25 K , and a sharp increase in Xp is observed near 12 K and In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

19. SUGIMOTO ET AL.

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Ferromagnetism and TCNQF " Salts 4

continues to about 5 K . There was no increase in Xp below 5 K . The anomalous magnetic behavior below 12 K can be more easily visualized in Figure 2(b), where the product of Xp and temperature (Xp-T) is plotted against temperature. +


behavior was reproducible for a fresh sample of L i - T C N Q F 4 " .

The Xp-T However,

enhancement of Xp slightly diminished when the samples were exposed to air for some time. A field-cooled magnetization experiment obtained at an applied field of 3 Oe is shown in Figure 3. The curve gradually increases below 12 K with decreasing temperature. At 2 K the applied field was switched off and the remanence was measured with increasing temperature. The temperature dependence of the remanence is similar to that of the field-cooled magnetization. The remanence disappeared at about 10 K . From these results a magnetic phase-transition of Li -TCNQF4"* is expected at around 12 K . Figure 4 shows the magnetization curves at 2,5 and 10 K , respectively. At 10 K the remanence was not observed and the magnetization was almost proportional to the magnetic field up to 55 kOe, the upper limit of the measurement. In contrast, the non-zero residual magnetization appeared at 2 and 5 K . The magnitudes of the residual magnetizations are 50 at 2 K and 30 emu/mol at 5 K . At these temperatures the magnetization curves seem to consist of two parts. The magnetization rapidly increases in the lower magnetic field, and then linearly increases in the higher magnetic field. Among the T C N Q F 4 - salts used in the present study only Li -TCNQF4"' has a magnetic moment coresponding to an S=l/2 spin entity. In the L i salt the spins interact antiferromagnetically, as is indicated from the negative asymptotic Curie +

+

+

temperature (-30*2 K ) as well as the decrease in Xp-T with a lowering temperature. The plateau in Xp observed around 25 K corresponds to the asymptotic Curie temperature. This suggests an onset of antiferromagnetic ordering, though it is not conclusive for the present. On the other hand, Figures 1-3 indicate definitive evidence of a magnetic phase transition around 12 K . Judging from the negative asymptotic Curie temperature, the magnetic ordering is antiferromagnetic. In antiferromagnets the magnetization is usually proportional to the applied magnetic field. Nevertheless, the L i salt has non-zero magnetization at 2 and 5 K , as is shown in Figure 4. This means that the cancellation of the magnetization by sub-lattice moments is not perfect. A possible reason of this occurrence is canting of sub-lattice moments (22, 23). The magnetization curve measurement in the lower field at 2 and 5 K can be reasoanbly interpreted as a magnetization process of the unbalanced moments. Assuming S=l/2 +

moment for each TCNQF4- molecule, the estimated canting angle at 2 K is about 0.7°. To understand the origin of weak ferromagnetism in Li+TCNQF4~\ information on the crystal structure is of critical importance. The crystal structure of the R b salt of TCNQ"* is known, and shows tight pairs of TCNQ"* molecules forming onedimensional stacks (24). Judging from the similar magnetic behavior in the other TCNQF4" salts, tight pairs of TCNQF4"' molecules are conceivable for +

U+-TCNQF4-. Ferromagnetism in Pyridinium-Substituted I m i d a z o l i n - l - o x y l / T C N Q F 4 - Salts (25, 26) The C T complexes of a series of 4,4,5,5-tetramethylimidazolin-l-oxyls substituted with 4-[tf-alkyl (R) pyridinium] groups (1(R) ': R=Me, Et, /i-Pr) at the 2-position +


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1000

100

200

300

200

300

T(K) 0.4 b 0.3

o

a

•

0.2

• •

•

3 3

Q

•

•

•

• •

0.1

0.0 100 T(K) Figure 2. Temperature dependences of (a) the reciprocal Xp (l/Xp) and (b) the product +

of Xp and temperature (X -T) in L i -TCNQF4"'. Reprinted from T. Sugimoto, M . p

Tsujii, H . Matsuura, N . Hosoito/Weak ferromanetism below 12 K in a lithium tetrafluorotetracyanoquinodimethanide salt, 1995, pp. 183 - 186, vol. 235, with kind permission from Elsevier Science - N L , Sara Burgerhartstraat25, 1055 K V Amsterdam, The Netherlands (reference 13).


19. SUGIMOTO ET AL.

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4

Salts

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o


a a

T(K) Figure 3. Temperature dependences of the field-cooled magnetization measured at the applied magnetic field of 3 Oe (•) and of the residual magnetization (•). The residual magnetization was measured with increasing temperature after the field-cooling. Reprinted from T. Sugimoto, M . Tsujii, H . Matsuura, N . Hosoito/Weak ferromanetism below 12 K in a lithium tetrafluorotetracyanoquinodimethanide salt, 1995, pp. 183 - 186, vol. 235, with kind permission from Elsevier Science - N L , Sara Burgerhartstraat25, 1055 K V Amsterdam, The Netherlands (reference 13).

with TCNQF4" were preapred by mixing aqueous solutions of the iodide salt of +

1(R) * and of the lithium salt of

TCNQF4" in equal concentrations at room

temperature (27). All CT complexes of 1(R)+' with T C N Q F 4 - were colored blue and blackish brown. The elemental analyses showed a 1:1 composition of the radical cation and the radical anion for all CT complexes. Of the CT complexes only l(Me)+-TCNQF4"* was obtained by recrystallization from acetone/petroleum ether as single crystals, whose crystal structure analysis was successfully performed. The crystal structure is shown in Figure 5a. The crystal has an alternating stacking of l(Me)+* and TCNQF4" layers along the b axis. Within each T C N Q F 4 - layer the two neighboring TCNQF4" molecules form a tight dimer structure as evidenced from the closer face-to-face contact (3.18 A) than a normal Ji-cloud thickness (3.54 A) as seen between the dimer units related by an inversion center. On the other hand, within each l(Me)+* layer one NO group is connected with the NO group of a neighbor along the a and c axes (see Figures 5b and 5c) As a result, each of the two-dimensional spin networks are completed within the a - c plane and furthermore extended to the b axis through the interaction with the neighboring spin networks by aid of the neighboring


282

MOLECULE-BASED MAGNETIC MATERIALS 400

300 "

0

1

200


a

100

10000

30000

20000

40000

50000

H(Oe) Figure 4. Magnetization (M) as a function of magnetic field (H) in the range of 0 - 55 +

kOe at 2 (o), 5 (•) and 10 K (n) in L i TCNQF4". Reprinted from T. Sugimoto, M . Tsujii, H . Matsuura, N . Hosoito/Weak ferromanetism below 12 K in a lithium tetrafluorotetracyanoquinodimethanide salt, 1995, pp. 183 -186, vol. 235, with kind permission from Elsevier Science - N L , Sara Burgerhartstraat25, 1055 K V Amsterdam, The Netherlands (reference 13).

+

1(R) ' ( R = Me, Et, /i-Pr) Formula 2

TCNQF4"' molecule (see Figure 5c). It should be noted that the oxygen atom of N O group is disordered. The preferential arrangement of oxygen atom is that as shown in Figure 5a, and the ratio is 83.7 : 16.3. ITiis fact must be taken into consideration in understanding the magnetic properties of this CT complex as well as the others. The situation of spin-spin interaction in the CT complexes of 1(R) -TCNQF4~* was shown by the magnetic measurement using a SQUID magnetometer (MPMS, Quantum Design) under an applied magnetic field of 500 Oe in the temperature range of +


19. SUGIMOTO ET AL.

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5 to 300 K . The molar paramagnetic susceptibility (Xp) at each temperature was obtained by correcting the diamagnetic contribution calculated from Pascal's constants The temperature dependences of the product of Xp and temperature (Xp-T) are shown in Figure 6 for the CT complexes of 1(R) " with TCNQF4". As is seen from the figure, the following characteristics can be pointed out. (1) For the CT complexes of +


a

+

Figure 5. The crystal structures of l(Me) * •TCNQF4"": (a) a whole view (reproduced with permission from reference 26); (b) a view projected down to the c axis; (c) a view projected down to the a xis.

Continued on next page



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+

+

l ( M e ) ' - T C N Q F 4 " and l(w-Pr) * TCNQF4" there was a steep increase in the X - T value with a lowering temperature below ca. 10 K , suggesting the dominance of ferromagnetic interaction. This was confirmed by investigating the field dependence of the magnetization at 2, 5 and 10 K , respectively, as shown in Figure 7. Thus, the lower the temperature, the more rapid the value approached the saturation value. The values of saturation magnetization are estimated ca. 4,000 and 5,200 emu/mol for l(Me)+'-TCNQF4- and l(n-Pr) '-TCNQF4-, respectively. These correspond to 72 p

+

+>

and 94% of

Weak-Ferromagnetism and Ferromagnetism in

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