Magnetic resonance of conduction electrons in fluoranthene radical cation salts

Synthetic Metals, 19 (1987) 355-360

355

MAGNETIC RESONANCE ON CONDUCTION RLWCl~ONS IN F

~

RADICAL CATION SALTS

G.DENNINGER, E.DORMANN and M. SCHWOEKER Physikalisches Institut and BIMF, UniversifAt Bayreuth, Postfach 101251, D-8580 Bayreuth (F.R.G.)

ABSTRACT The Overhauser shift method is a very sensitive technique to investigate the hyperfine interaction of conduction electrons with m--netic nuclei in organic conductors with a sufficiently narrow ESR line [1,2]. The high sensitivity is due to its double resonance character and is used to measure details of the proton N~R in single crystals. Despite an unresolved hyperfine structure, the average hyperfine tensor is accessible. Extension of the method uses m~-netic nuclei like *aC, 19F and ~H in deuterated crystals as a local probe of the conduction electron spin density. Results for the (Fluoranthene)i+PFe - radical cation salt are reported.

INT~ION The conduction electron ESR of a whole class of organic conductors consists of a narrow single line. Due to the rapid motion of the electrons and the correspondingly short correlation times interactions like the hyperfine coup)ling are motionally n a r r o w e d and a r e not resolved in the spectrum. One consequence of the hf interaction is the Knight shift commonly observed in metals which has been measured for the lsC in (FA)~ + PF,- [3]. A further consequenoe is a shift in the ESR-line position which is known as the '0verhauser shift'. It has been postulated by 0verhauser [4] and was subsequently observed in Li at 10w temperatures [5]. In ordinary metals this shift is small, but in the organic conductors the shift is enhanced by the Dynamic Nuclear Polarization (DNP) and is com~erable to the linewidth of

10rag in (FA)~*PF,-. It can be measured with high precision and is

the method of choice to investigate the hyperfine interaction in organic conductors with sufficiently narrow ESR-lines.

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356

ESR signol

5 :

!3

LSov - -

4

~ B0

Fig. 1. ESR-spectrumwith an additionsl radiofrequency field as a perameter. Yp: Larmor frequency of the protons.

EXP~IMENT The basic idea of the experiments is shown in fig. I. Using conventional ENDOR-equipment an additional radio frequency field (rf) is applied to the sample durinEew-ESR.

Sweeping the rfacross the ranEeoontaininE the proton

Larmor frequency one observes the shift which is shown schematically in fig.1. By locking the spectrometer to the centre of the resonance, one can easily observe the shift as a function of radiofrequeney, temperature and crystal orientation. The observed shift

~Bov is due to the longitudinal nuclear field ~Bp being

switohedoffbysaturatinE

the proton NMR. This longitudinal nuclear field ~Bp

is produced by the averaEeA,, of the proton hyperfine tensors Az,

/~.B~ = I/2"A,,.N'~'I/(g,~U.,) where N is the number of protons per electron,

(I) ~ is the proton spin polarization

and z is the direction of Bo. If the proton spin polarization ~D was the thermal equilibrium value ~

= I.I$I0 -e for RT, the nuclear field would be of the

o r d e r o f 10/~3. However, due t o t h e Dynamic N u c l e a r P o l a r i z a t i o n

via the Overhanser effect,

~o is enhanced by a factor V which can reach Vn,, = 658 for pure isotropic couplinE. The product

~[,z"V is directly determined by the experiment.

For the protons the enhancement factor V was determined by measurinE the shift for very low microwave power. V depends on orientaticm: with the field Bo

357 perpendicular to the needle axis _a V = 525 + 40. From the value ~ B o , we obtain a value ~,, = -l.16GauB per proton typical for an organic radical. Switching the radio-frequency field off after saturatinE the proton hNR yields an exponential decay of the observed shift z~B,v. The time constant is the relaxation time T, . Since the shift =B0,(t) can be measured with high sensitivity, the relaxation time of the protons in tiny single crystals can be obtained. Fig. 2 shows an example for two orientations of the field Bo: the T, time depends on orientation and has been analyzed in terms of an additional orientation dependent T, process not involving the conduction electrons [6].

0 > o

m

<3 > o

nn <3 o

-2

0.0

1.0

2.0

3.0

tls Fig. 2. Anisotropy of the *H

T, time in (FA)=PF6 for T=20OK.

By application of pulse sequences for the rf-field the proton T= time is accessible via the Overhauser shift method, too. No orientation dependence has been observed within experimental error.

The method is not restricted to protons. Since one expects the conduction electrons to be mainly distributed on the aromatic stack, the obeerved shifts from 19F and 3,p should be small. This is indeed the case: for *IF the shift amounts to 0.5% of the proton value and we were t~able to measure the shift induced by 3*p. More interesting however is 13C, as the conduction electron wavefunction is derived fr~u C-p=-type orbitals.

358 13C O v e r h a u s e r s h i f t i

r

i

i

r

,

i

J

f

.10~ Q"

n,~ +20 <3 o

<3 0

0 -100 -50

-20( -30(

-100

-z,0( -50 -i,O -30 -20 -10 0 +10 +20 ,-30 *40 f-fres / kHz Fig.

3. O v e r h a u s e r

shift

of

* sC f o r

two orientations

o f Bo w i t h

respect

to a.

Contrary to the proton case, the observed shift of isC is highly anisotropic. With Be along the needle axis _a the shift is negative and large, whereas it changes sign and is an order of magnitude lower with Be perpendicular to _a. This can be explained by a corresponding anisotropy of the respective A tensor components. Since the shift ~Bo, is proportional to A,, and the conduction electron induced I/T, processes are governed by A,, and AT, for a particular orientation, one expeots a large anisotrol~/ of the relaxation rate. This is shown in Fig. 4 for the two components. The large negative shift relaxes with a T, of 5see, whereas the small positive shift has a T, of 0.4see. This is consistent with a relaxation process via the anisotropic hyperfine interaction.

0

os°

~1

i I 3C 0verhauser 0.5

1,0

I shift

t/s

1.5

-

20

-20° C~ . _ z m.,.,.

_ _

<:3

.v~,.,<:3 30

-60

20

-80

oj

OO

-120 '

Fig,

4. R e l a x a t i o n

of the

t/s

'30 Overhauser shift

in

(FA)2PFe,

T

=

20OK.

359 Neither in the ease of protons nor of * 3C do we have a resolution of individual nuclei. The observed linewidths of the shift signals are between 5 and 30 kHz and totally obscure the individual chemical shifts and Knight shifts. By substituting

the protons with =H ( perdeutereted crystals

) one observes

a resolved quadrupole splitting of the =H induoed Overhauser shift. This is shown in fig. 5 with B0 parallel to _a. Two resonances split by ~ 140 k}{z are

o 15C

g

'

en

'~ '

'v,.=9580'GHz

-~ ~oc

2!0

2!i

212

2'3

A vrf / MHz

Fig. 5. =H Overhsuser shift for B0 perallel to a_.

distinguished.

In a good approximation

depends only upon the angle

~

~EQ(e)

-1 )

where

=

~E,0.(

3.cos=e

t h e quadrupole splitting

m E e of a =H

between Be and the chemical bond axis

(2)

z~Ee o is a constant characteristic

for the C - =H bond. This constant

can be measured for E9 =

90 ° . This is the case for all =H in fig. 5,

and we deduce a value of

~ E Q o : 140 kHz.

With Bo in the molecular plane, the individual in frequency,

since the respective

=H - C bonds have individual angles with Bo.

The result for a perticular orientation

I

J

f o=--~-%f (2H)

=H positions are resolved

is shown in fig. 6. An analysis with

I

Bo

> o

rn

<3

I

-200

I

-100

I

o

I

100

I

200

f-fo / kHz Fig. 6. =H Overhauser shift with Bo in the molecular plane.

360

known X-ray structure data [7] shows that significant Overhauser shifts are obtained at atomic positions 3 and 8 . These are indicated in fig. 6 and are those positions, where the overlap of the C-p, orbitals is expected to be large in the dimer (FA)2. Since the shift due to an individual 2H is proportional to the conduction electron spin density on the respective C - atom, one concludes that the spin density is high at positions 3 and 8. For a quantitative analysis the individual Vi enhancement factors will be determined in experiments under way.

CONCLUSIONS The newly developed Overhauser shift method is a highly sensitive technique to measure the hyperfine interaction between conduotion electrons and magnetic nuclei in organic conductors. By substituting the protons with deuterium the individual molecular positions are resolved. The conduction electron spin density at the C - positions connected to 2H can be determined.

ACKNOWLEDGEmEntS We thank Brigitte Kraus and J.Gmeiner for growing the single crystals. This work is supported by Stiftm~g Volkswagenwerk.

REFERENCES 1

G.Denninger, W.St6cklein, E.Dormann and M.Schwoerer,

2

W.St6cklein and G.Denninger, Mol.Cryst.Liq.Cryst, (1988) in press.

3

M.Mehring and J.Spengler, Phys.Rev.Lett., 53 (1984) 2441.

4

A.Overhauser, Phys.Rev., 92 (1953) 41.

Che~.Phys.Lett.,

107 (1984) 222.

5

C.l%yter, Phys.Rev.Lett., 5 (1960) I0.

6

W.St6cklein, Ph.D. Thesis, Universit~t Bayreuth (1985).

7

V.Enkel,x~ul, B.S.MoITa, Ch.KrShnke, G.Wegner and J.Heinze, Chom.Phys.Lett., 66 (1982) 303.