Organic radical clusters with ferromagnetic intermolecular interactions

Solid State Communications, Vol. 57, No. 6, pp. 453--456, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Press Ltd.

ORGANIC RADICAL CLUSTERS WITH FERROMAGNETIC INTERMOLECULAR INTERACTIONS Kunio Awaga, Tadashi Sugano and Minoru Kinoshita The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

(Received 26August 1985, in revised form 5 October 1985 by W. Sasaki) The magnetic properties of the mixed crystal of galvinoxyl radical (2,6-dit-butyl-4-(3,5-di-t-butyl-4-oxocyclohexa-2,5-dienylidenemethyl)-phenoxyl) and its precursory compound, hydrogalvinoxyl, in 6:1 ratio have been studied. The magnetic susceptibility of the mixed crystal follows the Curie-Weiss law in the range of 2 - 3 0 0 K with a positive Weiss constant of 7 K. Thus the ferromagnetic interactions are maintained over the whole temperature range in the mixed crystal, in contrast to the pure galvinoxyl in which the paramagnetism disappears below the phase transition temperature of 85 K. The magnetization curves for the mixed crystal measured at 2 and 5 K seem to follow closely along the theoretical curve for S = 3. The ferromagnetic interactions thus extend over six radicals on an average. However, the observed magnetization could more reasonably be explained by assuming an assembly of segments consisting of galvinoxyl radicals ferromagnetically coupled with each other and having various spin multiplicities in proportion to the segment size.

INTRODUCTION

galvinoxyl was almost completely suppressed for the mixed crystals prepared from the 6:1 mixture of galvinTHE MAGNETIC BEHAVIOR of organic radicals has oxyl and hydrogalvinoxyl. In this mixed crystal, the been interested in and a good number of radical ferromagnetic intermolecular interactions are confirmed compounds have been studied until now. Among these, to survive down to 2 K. Furthermore, the field depengalvinoxyl (2,6-di-t-butyl-4-(3,5-di-t-butyl-4-oxocyclodence of the magnetization at low temperature indicates hexa-2,5-dienylidenemethyl)-phenoxyl, see Fig. l(a)), that the spin state in the mixed crystal is in a septet one of the stable free radicals, is known to exhibit state on an average. In this report, we describe the quite distinguished magnetic behavior [ 1 - 3 ] ; the temmagnetic behavior of the 6:1 mixed crystal and discuss perature dependence of the magnetic susceptibility of the origin of the spin multiplet state of the mixed galvinoxyl above 85 K follows the Curie-Weiss law with crystal. a positive Weiss constant. Therefore, this radical is considered to have ferromagnetic intermolecular interEXPERIMENTAL actions between adjacent molecules in contrast to antiferromagnetic interactions in most organic radicals. Galvinoxyl [5] and hydrogalvinoxyl [6] were However, a phase transition occurs and most part of synthesized according to the methods given in the the paramagnetic susceptibility disappears below the literature. Mixed crystals were crystallized from a solution of a mixture of both compounds in a mixture transition temperature of 85 K. For opening up the possibility of suppressing the of ethanol and diethyl ether. We made several kinds of phase transition and keeping the intermolecular inter- mixed crystals of different mixing ratios varying from actions ferromagnetic down to a lower temperature 5:1 to 12:1. All of these were blue needles and were range, we have initiated a study of the effect of not different from galvinoxyl itself in appearance. We modifying the phase transition and the spin system in have not determined the actual compositions of the galvinoxyl by substituting a small amount of precursory mixed crystals. However, judging from the infrared compound, hydrogalvinoxyl (2,6-di-t-butyl-4-(3,5-di-t- spectra of the mixed crystals, the intensity ratios of the butyl-4-oxocyclohexa-2,5-dienylidenemethyl)-phenol, components seem to be in good proportion to the see Fig. l(b)). Hydrogalvinoxyl has the molecular and mixing ratios within the series of mixed crystals. The static magnetic susceptibility and the magnetcrystal structure similar to those of galvinoxyl and is ization were measured by using a Faraday balance. The known not to exhibit a phase transition [4]. We have found f r o m magnetic susceptibility magnetic susceptibility was measured over the continuous measurements that the magnetic phase transition of temperature range of 2 - 3 0 0 K . The details of the 453

454

FERROMAGNETIC INTERMOLECULAR INTERACTIONS

Vol. 57, No. 6

1 010' i

(b)

0

OH

0J 2

"--. c)_ 01

i

t

O~3

(~

= -C(CH3)3

200

300

T/K ~~XZlIII~III~

0

Fig. J. The molecular structures of galvinoxyl (a) and hydroga|vinoxy] (b).

apparatus have been described previously [7]. The diamagnetic susceptibility of galvinoxyl was experimentally determined to be - - 0 . 6 1 0 x 10-6emug -l from the analysis of the temperature dependence of the magnetic susceptibility of galvinoxyl itself. The correction of diamagnetic contribution to the susceptibilities of the mixed crystals was made by using this value commonly for galvinoxyl and hydrogalvinoxyl. The radical concentration of galvinoxyl is estimated to be ca. 95% from the susceptibility at room temperature.

100

z

It z Z Igll~zll

i

0

100

200 '

--

30 0

T/K Fig. 2. The temperature dependence of the magnetic susceptibility of the 6 : 1 mixed crystal. The inset shows that of pure galvinoxyl. i

2

z (b) x

E

IlJ i:1. 1 RESULTS AND DISCUSSION The temperature dependence of the paramagnetic susceptibility Xp of pure galvinoxyl was reexamined over the range of 2 - 3 0 0 K in our laboratory for the purpose of comparison with that of the mixed crystals. The result is shown in the inset of Fig. 2. Above 85 K, the temperature dependence of Xp follows the Curie-Weiss law with a positive Weiss constant o r paramagnetic Curie point. This behavior agrees wel! with the reported results [1, 2]. Below the phase transition temperature very weak paramagnetic remainder, reduced by more than an order of magnitude in comparison with that reported [2], is seen. The temperature dependence of Xp of galvinoxyl in the range of 4 0 - 1 0 2 K is extracted and given in an enlarged scale in Fig. 3(a). The susceptibility changes suddenly at 83 K in the cooling process and at 89 K in the heating process, resulting in a hysteresis loop. This observation is in accordance with the first order nature of the phase transition shown by the heat capacity measurements [4]. The temperature dependence of Xp of the 6:1 mixed crystal is shown in Fig. 2 and Fig. 3(b). The susceptibility follows the Curie-Weiss law over the whole temperature range excepting a slight decrease of Xp observed at about 70 K, which would be due to a trace of the phase transition of galvinoxyl. Figure 4 shows the temperature dependence of

t.o

o

0

i

i

60

80

100

T/K Fig. 3. The comparison between the temperature dependence of the magnetic susceptibility of the pure galvinoxyl (a) and that of the 6 : 1 mixed crystal (b).

1/Xp of galvinoxyl and the 6:1 mixed crystal. In galvinoxyl, while the Weiss constant for the high temperature phase is 18K, that for the low temperature phase is nearly OK. The unpaired electrons no longer have ferromagnetic interactions in the low temperature phase. On the other hand, the mixed crystal shows little difference in the Curie and the Weiss constants above and below the temperature where the small anomaly is observed. The Weiss constant is kept positive even in the low temperature range and found to be 7 K from the data in the range of 20 K < T < 60 K. Besides the fact that the phase transition is almost completely suppressed, the ferromagnetic interactions are still kept down to 2 K as a result of addition of hydrogalvinoxyl. In the range of T < 20K, the plots of 1/Xp turn

Vol. 57, No. 6

FERROMAGNETIC INTERMOLECULAR INTERACTIONS

150

Wee

%

'~=lOOr

/

®

f

x

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]

x'j"

/

(o),.,;,"

'ao

,

',io

=E

0.2 0.0

I

0

5

10

15

20

25

10-3HT-I / Oe K-1

o

Fig. 5. The field dependence of the magnetization of the 6:1 mixed crystal at 2.1 and 4.9K. See the text for theoretical curves (solid and broken).

T/K Fig. 4. The temperature dependence of the inverse of the magnetic susceptibility; (a-l) the high temperature phase of pure galvinoxyl, (a-2) the low temperature phase of pure galvinoxyl, (b) the 6 : 1 mixed crystal. aside the straight line. The phenomenon is not due to partial saturation of magnetization, because the static magnetic field was set at as low as 3000 Oe for these measurements (cf. Fig. 5). We preferably think that this could be the phenomenon similar to that observed near the Curie point in a ferromagnetic compound. From the susceptibility measurements, we have learned that the ferromagnetic interactions in the 6:1 mixed crystal are surviving over the whole temperature range covered. Thus it would be of great interest to examine the field dependence of magnetization particularly at low temperature. We measured the field dependence of the magnetization of the mixed crystal at about 2 and 5 K. Figure 5 shows the plots of the magnetization M vsH/T. The magnetization tends to saturate rather fast, but does not show hysteresis. Therefore, the spin system in the mixed crystal is considered to be in a paramagnetic state with high spin multiplicity. Provided that only the states in the lowest term are thermally populated, magnetization of paramagnetic species is given by M = M s a j ( x ),

-%

6 0

'~" 0 4

1'

50

8 "~C~

0.8 o~ ~" 0.6

z *~

i .-.:';:°°'I

"~t'~

10

1.0

g

(a-l)

455

(1)

with Jgi~BH kB T ' where Ms is the saturation magnetization, J is the quantum number of the total angular momentum, Bj(x) is the Brillouin function for J, #B is the Bohr magneton, g is the Lande g-factor and kB is the Boltzmann constant. The broken curves in Fig. 5 are theoretical ones for J = S = 1,2, 3, 4 and 5 (with L = 0) by setting Ms = 9.6 erg Oe -1 g-l. The observed magnetization

seems to correspond approximately to the curve for S = 3 or septet state. It appears that the ferromagnetic interaction extends over about six radicals. However, watching closely the experimental plots and the theoretical curves, we could recognize a slight difference between them. The spin multiplicity seems to become gradually lower with increasing field. Such field dependence of the spin multiplicity can be reproduced, as shown later, by assuming that the spin system in the mixed crystal is an assembly of clusters which have various spin multiplicities. The saturation magnetization given above corresponds to the spin concentration (S = 1/2) of ca. 70% of molecules contained in the mixed crystals. The decrease from 100% would be attributed to the decrease of Xp at about 70 K as well as to the mixing of hydrogalvinoxyl. In the crystalline state [8], the galvinoxyl molecules stack along the c-axis to form one-dimensional columns. In the mixed crystal, the stacking chains of galvinoxyl should be partitioned into the segments by hydrogalvinoxyl. The temperature dependence of Xp of the 6 : 1 mixed crystal indicates that the interactions between neighboring radicals are still kept ferromagnetic as in pure galvinoxyl but the radicals in the segments become hard to give rise to the phase transition. The number of radicals in segments may well have a distribution as a matter of course. Therefore, the field dependence of the magnetization of the 6:1 mixed crystal in Fig. 5 could be interpreted by taking account of the distribution. When some of the galvinoxyl radicals in an infinite stacking chain is site-randomly replaced by hydrogalvinoxyl, the occurrence probability of the segments containing n radicals is given by

/.

= c"/7. c',

(2)

where c is the radical concentration and corresponding to 6/7 for the 6:1 mixed crystal. The segment having

456

FERROMAGNETIC INTERMOLECULAR INTERACTIONS

n radicals would take the spin quantum number of n/2. If the interactions between the segments are ignored, the magnetization of this chain is written by

n

M]Ms = ~, fn ~ Bnn(x)

//y

n

fn -~.

(3)

The result calculated with this equation by summing up to n = 5 0 is shown in Fig. 5 by the solid curve. Quantitative agreement between this curve and the experimental plots is rather poor, but the curve reproduces fairly well the tendency of the field dependence of the magnetization of the 6:1 mixed crystal. The discrepancy between them would be attributed to the following reasons. (1) Possible modification of the segment distribution from site-randomness or quenching of some spins in long segments at the anomaly of Xp near 70 K. (2) The discrepancy between the mixing ratio and the actual composition in the mixed crystal. (3) The presence of weak interactions between segments and]or between chains. From these, it is concluded that the mixed crystal is the assembly of the clusters of various spin multiplicities in proportion to their size which follows the statistics similar to equation (2). To our knowledge, this is the first experimental evidence for formation of clusters consisting solely of organic radicals coupled

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with positive effective exchange interaction and having the high spin multiplet state. To make clear the effect of impurity concentration on the phase transition and the spin state, a study of the magnetic behavior of the mixed crystals of various mixing ratios is now in progress.

Acknowledgement - This work was supported in part by the Grant-in-Aid for the Special Project Research on the Properties of Molecular Assemblies (No. 59112003) from the Ministry of Education, Science and Culture.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

K. Mukai, H. Nishiguchi & Y. Deguchi, J. Phys. Soc. Japan, 23,125 (1967). K. Mukai, Bull. Chem. Soc. Japan, 42, 40 (1969). K. Mukai, K. Ueda & K. Ishizu, J. Chem. Phys. 77, 1606 (1982). A. Kosaki, H. Suga, S. Seki, K. Mukai & Y. Deguchi, Bull. Chem. Soc. Japan, 42, 1525 (1969). P.D. Bartlett & T. Funahashi, J. Amer. Chem. Soc. 84, 2596 (1962). K. Mukai & A. Sugabe, J. Chem. Phys. 72, 598 (1980). M. Takahashi, T. Sugano & M. Kinoshita, Bull. Chem. Soc. Japan, 57, 26 (1984). D.E. WiUiams,Mol. Phys. 16, 145 (1969).