Magnetic properties of spin-labelled polyacetylene

Synthetic Metals, 36 (1990) 353 - 365

353

MAGNETIC PROPERTIES OF SPIN-LABELLED POLYACETYLENE H. WINTER, G. SACHS and E. DORMANN* Physikalisches Institut and Bayreuther Institut fiir MakromolekiH-Forschung (BIMF), Universita't Bayreuth, Postfach 101251, D-8580 Bayreuth (F.R.G.)

R. COSMO and H. NAARMANN Kunststofflaboratorium (ZKT-B1), BASF AG, D-6700 Ludwigshafen (F.R.G.)

(Received November 16, 1989; accepted February 2, 1990) Abstract Polyacetylene has been chemically modified by the incorporation of spin-labelled moieties. The magnetic properties were characterized by static magnetic susceptibility measurements and ESR spectroscopy. In spite of the very low spin concentrations of about 10 -3 per CH unit employed, magnetic spin-spin interactions were observed. Magnetically correlated units consisting at least o f pairs of spins with ferromagnetic exchange coupling (J/kB t> 8 K) were found in galvinoxyl-modified polyacetylene.

1. Introduction Spin labelling has been established as a very powerful technique in the study of biological systems and has allowed molecular flexibility or reorientational motion to be studied by ESR spectroscopy or its advanced variants [ 1 - 3 ]. Even if added to immobilized molecules, spin labels can give interesting information: t h e y can be used as reporters of the preferential orientation of the molecules to which t h e y are attached, or of their own spatial distribution, which eventually deviates from a statistical one, or at least on their mutual spin-dipolar or exchange interactions [ 1 - 5 ]. In the present investigation, we added small concentrations (typically about 10 -3 per CH unit) of well-known spin labels or stable free radicals (Fig. 1) to unstretched standard polyacetylene films [6]. The chemical modification and its influence on the mechanical properties or the electrical conductivity of the films {after or w i t h o u t doping with iodine) have been reported before [7]. Thus we focus here on ESR and magnetic susceptibility studies. We show that ESR allows a reliable distinction between those spin labels which are irreversibly chemically bound to the polyacetylene skeleton

*Author to w h o m correspondence should be addressed. 0379-6779/90/$3.50

© Elsevier Sequoia/Printed in The Netherlands

354

o O N~ I

la o

CONH2

O-

o.

2a

2b

3a

lb

N-CH2CH2CH2NH-~'~N_O . IC 0

3b ld

Fig. 1. Spin labels used in the present investigation: l a - l d were introduced by DielsAlder cycloaddition reaction; 2a and 2b by indirect addition (using maleic anhydridemodified polyacetylene films); whereas the stable free radicals 3a and 3b were mainly incorporated by diffusion.

and those which are left mobile. Furthermore, we analyse how reliably can weak magnetic interactions of the radical spins be discerned with the help of ESR and SQUID magnetometry. We show one example where such interactions occur in spite of the very low spin-label concentration of a b o u t 10 -3 per CH unit present. 2. Experimental

2.1. Sample preparation The chemical modification of unstretched standard polyacetylene films [6] by the incorporation of spin labels and their subsequent characterization has been described earlier [7 ]. Three different techniques have been applied: the spin labels l a - l d were added directly by Diels-Alder cycloaddition reaction, since they contain a Diels-Alder active maleimide fragment bound to the stable nitroxyl free radical. The spin labels 2a and 2b were added indirectly by adding them to polyacetylene films which had been previously modified b y the incorporation of maleic anhydride (2.5 mol%). It is assumed that the stable free radicals 3a (saturated) and 3b (galvinoxyl) were incorporated mostly by diffusion into the porous structure of the film [4, 5]. Galvinoxyl acts probably as ~r-acceptor (forming charge-transfer complexes); it increases the electrical conductivity of the film by four orders of magnitude [7 ].

355

According to their IR spectra, all the modified polyacetylene films investigated were eis--> trans isomerized during the chemical modification (requiring prolonged heating in refluxing toluene [7]). Different pieces of the same film have been used for ESR or static magnetic susceptibility measurements; the latter were performed more than 6 months later. 2.2. ESR measurements and data analysis

In a glove b o x under a nitrogen atmosphere, the respective spin-label modified polyacetylene (SLMPA) samples were transferred in ESR probe tubes also containing a thermocouple close to the sample site for temperature reading. Nitrogen (or helium for measurements at lower temperatures) was used for sample protection and as a thermal exchange gas. The ESR measurements were obtained with a commercial X-band ESR spectrometer (Bruker ER 200D-SRC) at 9.4 GHz. At each temperature setting, realized with the help of an Oxford Instruments gas-flow variable temperature cryosystem, the signal of the sample and the signal of an NBS ruby standard were recorded together. Thus absolute values of the ESR susceptibility were determined. As is shown in Fig. 2 for t w o examples, and as is well known from spin-label ESR [ I - 3], no simple (Gaussian or Lorentzian) lines were observed. This is due to the powder distribution caused by the

t-

ry

(/1 LJJ

Id T = 280K

_J

S e"

W

_J

lOG

2b T = 2BOK

i j S

=B

10(; ----~ B 2b

T:

1OK

-B

T:

1OK

°B

Fig. 2. Absorption-derivative ESR spectra (9.4 GHz) for two SLMPA samples at temperatures of 280 K and 10 K. The pronounced low- and high-field edge singularities directly reveal the axial values 14Azz and gz of the nitroxyl radical spin Hamiltonian from the powder average (S = 1/2, I( 14 N) = 1). The centre region o f the spectra results predominantly from the x and y orientations (where gx and gy are larger, but 14Axx and 14Ay~ substantially smaller (e.g. 1/5 ) compared to the z-components).

356

anisotropy of the g tensor and 14N (I = 1 ) hyperfine interaction (14,4 tensor) of the nitroxyl free-radical spin S = 1/2. It was therefore necessary to determine the area of the absorption signal at each temperature by double numerical integration of the ESR signal (the differentiated absorption signal). The 150 G field range, shown in Fig. 2, was considered to be sufficiently large for that purpose. At the end of the ESR measurements, each individual sample was weighed, so that ESR susceptibilities could be given in units of emu/g (or converted into units of emu/mol). Figure 3 shows the ESR spectrum of an SLMPA sample that, after preparation, was wetted with toluene for an extended period of time. In comparison with the line shapes familiar from Fig. 2, three additional equidistant narrow lines (marked b y arrows) can be distinguished at room temperature. They are caused b y mobile spin labels l a with a short rotational correlation time and correspond to the isotropic average values of the spin-label's g and 14,4 tensors. These lines can no longer be distinguished at low temperatures, where the reorientational motion is slowed down (Fig. 3). This example indicates that a quantitative ESR analysis of this type, the time dependence of the spectrum after adding of solvent to the film, may be used to distinguish the portion of spin labels that does not react with the polyacetylene skeleton at all, b u t is only introduced b y diffusion in porous regions

l

-6

Cla) T = 296K

c"

n~ (/1 W

-----

"6 c

B

(la) T = 222K

rv, 0'1 IJJ

°B Fig. 3. E S R s p e c t r u m for a n S L M P A sample wetted with toluene for a l o n g time. T h e three E S R lines (giso, Z4Aiso, S = 1/2, I(Z4N) = 1), d u e t o spin labels in t h e s o l v e n t , c a n b e dist i n g u i s h e d b y t h e i r s h o r t r o t a t i o n a l c o r r e l a t i o n time at high temperatures ( T = 2 9 6 K), a n d are m a r k e d .

357 of the film (short time response), from the reversibly added portion, and also from the irreversibly addition-reacted portion (long time response). The results of the ESR data analysis are compiled in Table 1. The value of go has been derived from the zero crossing of the absorption-derivative ESR signal, which corresponds to the maximum (not the centre of gravity) of the asymmetrical and structured ESR absorption line. Thus go cannot be related to the g tensor of the free-radical spin in a simple way, but also contains the influence of the anisotropic hyperfine interaction. Similarly, the absolute value of ABpp, the peak-to-peak width of the absorption-derivative ESR signal (included in Table 1), measures details of the powder-averaged line shape (e.g. the gx, Ax~ versus gy, Ayy anisotropy) in addition to the strength of spin-spin interactions. For all the samples analysed, the ESR-derived susceptibility approximately obeys a Curie law X~sR

(1)

= CEs~/T

The high-temperature Curie constants CESR have been expressed as relative concentration of radical spins S = 1/2 per CH unit of polyacetylene (nESR in Table 1). Generally, the ESR-derived spin concentration is smaller than the nominal one. A weak curvature could usually be seen, however, in the dependence of the inverse susceptibility on temperature, as shown in Fig. 4(a). The size of the low-temperature deviations from eqn. (1) could be better visualized from a plot of the product ×ESR X T as a function of temperature, as is given in Fig. 4(b) for the same sample. Instead of a horizontal line corresponding to CEsa of eqn. (1), deviations in the low-temperature range have generally been observed. Usually, such deviations would n o t necessarily have to be of 'physical' origin, indicating a temperature dependence of CESR or the relevance of spin-spin interactions. Since the thermocouple must be situated outside of the microwave field, and thus outside of the cavity, though as close to the SLMPA sample and the ruby standard as possible, a temperature error (with the sample at lower temperature than the r u b y standard) could n o t be absolutely excluded. By varying the thermocouple's position for switched-off microwave power, the error was estimated n o t to exceed 1 K, however. In order to quantify the deviation from the Curie law, a Curie-Weiss law Xzsa

= C' /( T -

0zsR)

(2)

was fitted to the data in the low-temperature range (see Fig. 4(c) for an example). The respective asymptotic Curie temperatures 0ESR are given in Table 1. The sign of 0ESR is systematically 'ferromagnetic'. Whereas most 0ESR values are only of the same order of magnitude as the T-error bar estimated above, the 0~s R value of galvinoxyl-modified polyacetylene (3b) is clearly larger, in spite of the relatively small spin-label concentration used.

TABLE 1

3.4 + 0.4 2.00360 14.4 17.7 +1.8 -+ 1.1 --2.0 + 0.7

n x ( 1 0 - 3 p e r CH) go ( + 0 . 0 0 0 0 5 ) ARpp (298 K) (G) ~kBpp( 1 0 K) ( G ) 0EsR(K) 0 x (K)

3 5.9+0.6 (0.7 + 0.1) 2.3 + 0.4 2.00567 13.1 (7.8) (+1.3 -+ 1.0) --1.7 + 0.5

Ib

3.3 + 0.3 2.00362 13.1 16.2 +2.5 + 1.3 --1.7 +- 0.4

2 2.2+0.2

lc

2.3 + 0.2 2.00285 3.7 7.6 +0.8 + 1.3 --2.2 -+ 0.5

2 0.44+0.04

Id

7.0 + 1.2 2.00402 18.1 20.7 +2.0 + 1.6 +1.8 + 0,4

8 3.5+0.4

2a

3.3 + 0.2 2.00321 4.9 9.6 +1.6 + 1.2 0.0 -+ 0.2

5 0.65+0.07

2b

1.1 + 0.1 2.00280 4.4 6.1 +2.4 + 1.3 --1.8 -+ 0.5

3.4 0.34+0.03

3a

1 0.54+0.05 (0.66 + 0.07) 1.2 + 0.1 2.00395 8.0 (11.1) +4.6 + 1.0 +4.1 -+ 0.5

3b

B o t h t y p e s o f data were o b t a i n e d f r o m d i f f e r e n t small p o r t i o n s o f t h e same films ( t h e X d a t a r o u g h l y 6 m o n t h s later t h a n t h e E S R data). T h e spin c o n c e n t r a t i o n s n n o m ( n o m i n a l ) , nES R ( E S R , h i g h - t e m p e r a t u r e l i m i t ) a n d n x are given in relative u n i t s , t h e values in b r a c k e t s () were o b t a i n e d f r o m a separate m e a s u r e m e n t , go c o r r e s p o n d s t o t h e zero crossing o f t h e a b s o r p t i o n - d e r i v a t i v e E S R signal. T h e p e a k - t o peak w i d t h o f t h e a b s o r p t i o n - d e r i v a t i v e E S R signal, zkBpp, c h a r a c t e r i z e s o n l y t h e c e n t r a l p a r t of t h e E S R line (see e.g. Fig. 2). T h e a s y m p t o t i c Curie t e m p e r a t u r e 0ESR was o b t a i n e d b y f i t t i n g a C u r i e - W e i s s law ( e q n . (2)) t o t h e E S R s u s c e p t i b i l i t y d a t a for t e m p e r a t u r e s below 60 K. In c o n t r a s t , 0 x h a s b e e n o b t a i n e d b y f i t t i n g a C u r i e - W e i s s law i n c l u d i n g d i a m a g n e t i c c o r e c o n t r i b u t i o n (eqn. (3)) t o t h e static m a g n e t i c s u s c e p t i b i l i t y d a t a over t h e w h o l e t e m p e r a t u r e range.

3 2.0+0.2

la

nnora ( 1 0 - 3 per CH) nESR ( 1 0 - 3 p e r CH)

Spin label

Magnetic p r o p e r t y data derived for spin-labelled p o l y a c e t y l e n e films b y E S R a n d s t a t i c m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s

O0

359 800 2a

? 0

g~

°~°

°o

~p.,° 0

(a)

o

i

i

100

200

T/K

0.60 2Q

0

E

V

0.40

°°° ,.°,

° ° . , °

°

°

•

•

I"" 0.20 i-Q

O.OC

(b)

i 100

i 2O0

30O T/K

1S0

I

-0

I00

E

0

(c)

20

4o

llo

T/K

Fig. 4. The different ways of analysing the ESR-derived susceptibility data for the SLMPA sample 2a: 1/X (a) or xT (b) in the high-temperature range and 1/X in the lowtemperature range (c). For further details see text.

2.3. Magnetic susceptibility measurements The static (total) magnetic susceptibilities of the differently spinlabelled polyacetylene films have been measured with a SQUID magnetometer (Quantum Design: magnetic property measurement system) for magnetic field strengths of 500 Oe up to 55 kOe in the temperature range of 1.8 K to 300 K. During the measurements, the sample was surrounded by a low pressure helium atmosphere. Absolute accuracy of the temperature reading in the low-temperature range was at least one order of magnitude higher than for the ESR experiments (i.e. AT < 0.1 K). Except for the low-

360

temperature high-field range, where a Brillouin function had to be used, the data were analysed by fitting of the relation X = Xdia +

C×/(T- 0x)

(3)

Figure 5 shows the results for the SLMPA sample l c as an example. The relevant parameters are given in Table 1. Again the Curie constant is expressed as number of spins S = 1/2 per CH unit of the polyacetylene skeleton. For most samples, n× is found to be close to the nominal concentration nnom, b u t different from nESa. One more parameter had to be adjusted in the total-susceptibility analysis with eqn. (3) than that using eqn. (2): the strength of the molecular diamagnetism. Nevertheless, the derivation of n x b y the static-susceptibility analysis is considered to be more reliable than that of nESR: it cannot be influenced by the temperature dependence of the relaxation time and the linewidth of an eventual unrecognized very broad c o m p o n e n t of the ESR spectrum, which may be included in nESa, to a temperature-dependent portion, thus altering the ESR-intensity analysis. We discuss below how this error for t/ESR also influences the derivation of 0ESR. The disagreement between 0ESa and 0 x is evident; the values generally even differ in sign, being 'antiferromagnetic' for most of the latter. As mentioned above, the modification of a polyacetylene film with galvinoxyl (3b) gave the largest deviation from a simple Curie law in the ESR analysis (Table 1). Furthermore, for this film, ~ESR and 0 x agree reasonably. Therefore we investigated the low-temperature magnetic field dependence of the magnetic properties for this SLMPA sample in particular detail. Figure 6 shows the magnetic m o m e n t as function of H/T for different temperatures, in comparison with the variation that is predicted b y the Brillouin function, for different values of the spin-quantum number [8]. The experimental data have been corrected for the diamagnetic contribution, derived i

"•

0.16-

w

.,.=i

spin label l c

0.12.

0.08•

:D

~'

0.04-

0.00, 0

I00

2(30

300

T e m p e r a t u r e / Kelvin Fig. 5. Total static magnetic susceptibility of the SLMPA sample l c as f u n c t i o n o f temperature, measured for a m a g n e t i c field strength of 50 kOe (circles), and fitted with eqn. (3) (solid line; Xdia = - - ( 0 . 2 7 -+ 0.06) X 10 "~ emu/g).

361 8.0

I

.pi. 1.b~l

l

I

3b

I

I

/( . - S ' ~ ~

4.0

0.0 r.

E o

'~" /¢~/"

a T = 30.0 O

--

-8.0

-30.0

i

-20.0

-lho n / T

o'.o /

i~.o

T

=

4.4

K-

s = i/2

2~.o

30.0

kOe/K

Fig. 6. Molar magnetic m o m e n t as function of H/T for SLMPA sample containing galvinoxyl (3b), in comparison with the Brillouin functions for different values of the spin quantum number [8]. The experimental values have been corrected for the diamagnetic contribution, Xdia = --0.41 × 10 -6 emu/g, derived from the high-temperature range (eqn. (3)). The spin concentration derived from the static-magnetic susceptibility in the high-temperature range has been taken into account in the calculated field dependences.

above. It is evident that the magnetic m o m e n t saturates faster than independent galvinoxyl spins with S = 1/2 would do.

3. Discussion of the results

In principle, spin labels can interact with each other via classical electron-spin magnetic~lipole interactions and via exchange interactions. In the discussion, we will first concentrate on those SLMPA samples for which no clear indication o f exchange interactions has been found.

3.1. SLMPA samples l a - 3a At least in the case of a statistical distribution of the spin labels, it is not expected that considerable direct exchange interaction between the spin labels occurs in the SLMPA samples investigated here, since the spin concentrations employed are rather low. An average distance between the free radicals of above 25 • is estimated for a concentration of one spin label per 1000 CH units, using the lattice constants reported in ref. 9. Thus static exchange interactions, caused by direct overlapping of the orbitals of the unpaired electrons of the spin labels, can be excluded; t h e y occur usually only for distances of less than 7 - 8 A [1]. The same should be true for indirect exchange along the polyacetylene backbone, despite its expected larger range. Additional information concerning the mechanism of interaction between spin labels can generally be obtained by using labels

362 with different lengths and flexibilities, like l a - l d . But an inspection of the OESR or ~x values compiled in Table 1 shows that there is no correlation between the ~ values and the radical-spin concentrations or the spin-label lengths. Thus exchange interaction between the spin labels can be neglected. There should be a detectable influence of the classical magnetic-dipole interaction between the randomly distributed spin labels, however: one spinlabel neighbour at a distance of about 25 A causes a dipole field of about 1 G. Indeed, Table 1 shows that the room-temperature ESR linewidth ABpp of the SLMPA samples is roughly correlated with the ESR-derived spinlabel concentration: the widths are larger for the samples with nESR above 2 × 10 -3 per CH unit than for the others. This shows that magnetic dipoledipole interactions with neighbouring spins can be distinguished from the isolated spin-label part (g and 14A anisotropy) of the linewidth, despite the relatively low spin-label concentration. The inhomogeneous line broadening increases with decreasing temperature (Table 1). Only part of this broadening can be due to inhomogeneous demagnetizing fields, increasing with decreasing temperature proportional to the total magnetic susceptibility. It is evident from a comparison of the 0Es R and 0 x values in Table 1 that the existence of a 'ferromagnetic' molecular field, that could be surmised from the ESR-susceptibility analysis (0Es a > 0), is not supported by the static-susceptibility analysis. We have ruled out the possibility of a systematic temperature error of about 2 K between sample and ruby standard in the ESR analysis, surpassing the estimate mentioned in Section 2.2 b y a factor of 2. We favour the following explanation, which is further supported by the results for galvinoxyl-modified polyacetylene, analysed in Section 3.2, and by the fact that the values of the spin concentrations n x are generally larger than nESR: the deviations from the simple Curie law (eqn. (1)), observed in the ESR-intensity analysis, that required the use of the Curie-Weiss law (eqn. (2)), are due to the interaction between the spinlabel spins that have been introduced into the polyacetylene film by the chemical modification, and 'defect' spins in the polyacetylene backbone. Such 'defect' spins are expected to exist in the SLMPA samples. It is well known that in standard undoped polyacetylene the unpaired-spin concentration changes with t r a n s - c o n t e n t during c i s - ~ trans isomerization. Typically, it increases from values of below 10 -4 to a b o u t 1 × 10 -3 per CH unit for lO0%-trans polyacetylene [9]. Comcomitantly, the room-temperature ESR linewidth decreases from a b o u t (ABpp)c~, = 6 - 10 G to (ZS~Bpp)trans = 0 . 2 - 2.5 G [9]. The polyacetylene films used in the present investigations had defect-spin concentrations of n× ~ 0.3 × 10 -3 per CH unit prior to the chemical modification. For comparison purposes, we furthermore investigated a polyacetylene film that was chemically modified like 3a and 3b, using the ~-acceptor 3,3',5,5'-tetramethyl-4,4'-diphenoquinone, that normally carries no spin (incorporation by diffusion, with nno m = 5 × 10 -3 per CH unit}. This film had the largest electrical conductivity of the SLMPA samples analysed here [7]. It showed a symmetrical

363 ESR line with F/ESR = ( 1 . 0 ± 0 . 5 ) X 1 0 - 4 per CH unit (go = 2.00288 -+ 0.00005, ABpp = 5.6 G at T = 298 K (9.7 G at 10 K)). By staticsusceptibility analysis, we derived for this polyacetylene film n x = 1 X 10 -3 per CH unit. In the SLMPA samples, the ESR linewidths of the defect spins on the polyacetylene skeleton will be strongly increased because of the magnetic interaction with the spin labels, b u t there is no indication of the absence of such backbone defect spins. We assume that the mutually interacting spinlabel and backbone spins are included in the derivation of XESR at low temperature to a larger extent than at high temperature, because of the temperature dependence of their linewidths; this can explain the deviations observed for example in Fig. 4(b). 0ES R is thus representative of the temperature dependence of CESR or, more precisely, of the varying portion of the number of spins counted by ESR. We emphasize that the n-values in Table 1 prove that not all spins have been observed by ESR (hightemperature nESR) that have been 'counted' by the static-susceptibility analysis. 3.2. Galvinoxyl modified polyacetylene

For galvinoxyl-modified polyacetylene, ESR and static-magnetic susceptibility analysis consistently indicate a 'ferromagnetic' asymptotic Curie temperature of a b o u t 0 ~ 4 - 5 K. The Curie-Weiss law, eqn. (2) or eqn. (3), is reasonably o b e y e d for temperatures above 10 K. At lower temperatures, the susceptibility remains smaller than according to this hightemperature approximation. No indication of long-range magnetic order is observed for temperatures above 1.8 K, however. Evidently, exchange interaction in galvinoxyl-modified polyacetylene is non-negligible in spite of the very low spin concentration of a b o u t 10 -3 per CH unit and the correspondingly large average galvinoxyl-galvinoxyl separation. This observation is, nevertheless, in agreement with our reasoning in Section 3.1, provided that the relevant exchange interaction takes place between spin-label and d e f e c t / b a c k b o n e spins and not between the spins of different spin labels. We call to mind that galvinoxyl is supposed to act as a n-acceptor, forming charge-transfer complexes, and that it increases the electrical conductivity of the modified polyacetylene film by four orders of magnitude [7 ]. The low-temperature magnetization curves as a function of H / T for galvinoxyl-modified polyacetylene (Fig. 6) prove that the sample contains a large portion of ferromagnetically coupled pairs of spins or even larger units of ferromagnetically correlated spins. For ferromagnetic exchange interaction between t w o spins S 1 = $2 = 1/2 with 3(ex = --2JSIS2

(4)

where J is the exchange integral, the triplet state, S = 1, lies by 2 J below the S = 0 singlet state. The high-temperature static magnetic susceptibility of a system consisting of such (S = 1 and S = 0) pairs can be described by eqn.

364

(3), where 0 = J/2kB [10]. A detailed fitting attempt for the observed magnetic m o m e n t versus H / T dependence indicated, however, that a superposition is observed, consisting at least of the contributions of isolated spins S = 1/2 and pairs S = 1 (and very probably also units with larger spin). We emphasize that the comparison of nESR and n× in Table 1 for sample 3b indicates that not all spins could be observed by ESR in the 150 G field range used for the intensity calculation. S = 1 pairs with large magnetic dipole-dipole interactions and thus substantial zero-field splitting are at best partially included in the derivation of nESR. In conclusion, the high- and low-temperature static-magnetic susceptibility data of galvinoxyl-modified polyacetylene indicate a certain portion of ferromagnetically correlated spins, with a ferromagnetic exchange integral of at least J/kB = 20 ~> 8 K. This exchange coupling seems thus more efficient than the interactions reported recently for concentrated c~-nitronyl nitroxides [11].

4. Concluding remarks We have presented a careful analysis of the magnetic interactions of spin labels which had been introduced in low concentrations by chemical modification of unstretched polyacetylene films, combining ESR and SQUID magnetometry. We have presented evidence for magnetic interactions between galvinoxyl spins and spins on the polyacetylene skeleton. These interactions couple at least two spins ferromagnetically, with an exchange integral of about 8 K. This coupling mechanism may be useful to produce long-range magnetic ordering, if concentrations of radical spins are introduced higher than the 10 -s per CH unit investigated here.

Acknowledgements We thank J. Gmeiner for his help in the sample handling in a glove box. Discussions with G. Denninger, M. Schwoerer and P. Strohriegl are acknowledged. This work was financially supported by the Bundesminister fiir Forschung und Technologie as part of Project 03-M-4019-7.

References 1 G. I. Likhtenshtein, Spin Labeling Methods in Molecular Biology, Wiley, New York, 1976. 2 L. J. Berliner (ed.), Spin Labeling H: Theory and Applications, Academic Press, New York, 1979. 3 L. R. Dalton (ed.), EPR and Advanced EPR Studies o f Biological Systems, CRC Press, Boca Raton, FA, 1985.

365 4 V. B. Stryukov, Dokl. Chem. Phys. (Engl. Transl. Dokl. Akad. Nauk S.S.S.R.) 179 (1968) 218. 5 E. G. Rozantsev, Free Nitroxyl Radicals, Plenum, New York, 1970, Ch. VII, p. 172. 6 H. Naarmann and N. Theophilou, Synth. Met., 22 (1987) 1. 7 R. Cosmo and H. Naarmann, Mol. Cryst. Liq. Cryst., in press. 8 K. H. Hellwege, Einfiihrung in die FestkiJrperphysik, Springer, Berlin, 1981. 9 J. C. W. Chien, Polyacetylene: Chemistry, Physics and Material Science, Academic Press, Orlando, FA, 1984. 10 C. Veyret and A. Blaise, Mol. Phys., 25 (1973) 873. 11 K. Awaga and Y. Maruyama, J. Chem. Phys., 91 (1989) 2743.