Study of IF5 chemical doping of aluminium polyfluorophthalocyanine: (PcAlF)n

Synthetic Metals, 16 (1986) 227 - 233

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STUDY OF IF s CHEMICAL DOPING OF ALUMINIUM P O L Y F L U O R O P H T H A L O C Y A N I N E : (PcAIF)n D. DJURADO*, A. HAMWI, C. FABRE**, D. AVIGNANT and J. C. COUSSEINS

Laboratoire de Chimie des Solides, U. A. 444, Universitd de Clermont-Ferrand II, B. P. 45, 63170 Aubi~re (France) ( Received December 30, 1985 ; in revised form May 2, 1986; accep ted June 9, 1986 )

Abstract Chemical IF5 doping of aluminium polyfluorophthalocyanine, (PcA1F)~, has been studied by thermogravimetric analysis, infrared spectroscopy, elemental analysis and electrical conductivity measurements. An optimum electrical conductivity (o = 6 × 10 -2 ~2-1 cm -1) is achieved for a doping rate lying in the range 0.5 - 0.8 mole of IFs per (PcA1F) ring. The decrease of electrical conductivity of the doped material measured for higher doping rates is shown to be related to the disappearance of the eclipsed configuration in the stacking of the rings, induced by excess inserted dopant species. These results confirm those previously published concerning the AsF s doping of the polymer and can be explained with the help of the same structural model.

Introduction Because of their numerous physicochemical properties, molecular crystals of metal-centred phthalocyanines (PcM with Pc = C32H16Ns and M = metal) have been the subject of many studies [1 - 3]. Some polymers have also been synthesized [4, 5], in particular those with the general formula (PcMX), with M = Si, Ge and X = O [6] or M = A1, Ga and X = F [7], which exhibit a good electronic conductivity when partially oxidized by an electron acceptor such as iodine [8, 9] or by fluorinated anions such as PF 6- and BF4- [10]. The remarkable thermal and chemical stabilities of polymers doped in this way make them possible condidates as electrodes in solid-state galvanic cells having the F- ionic conductor as separator. The study of IF s chemical doping of (PcA1F)n (Fig. 1) described here may bring some information about the possible behaviour of iodine-doped (PcA1F)n used in this way. Moreover, this study states experimentally the *Author to whom correspondence should be addressed. **Present address: L. E. D. S. S. V I - Chimie USMG, B. P. 68, 38402 Saint Martin d'H~res C6dex, France. 0379-6779/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

228

Fig. 1. Structure of (PcA1F)n.

correlation existing between the structural arrangement of rings of conducting chains and the final electrical performances of doped (PcA1F)n.

Experimental The material synthesis and all the experiments were carried out using the process and apparatus described in ref. 11. Prior to each doping experiment, the reaction cell was outgassed for 24 hours under high vacuum (5 × 10 -6 Tort) in order to remove any trace of moisture. Three different IF s (Matheson) vapour pressures (1, 4.45 and 20 Torr) were set up by fixing the temperature of the liquid dopant source. Electrical conductivity measurements were performed using Van der Pauw's method [11] on pressed samples provided with four electrical contacts made with Aquadag Eccocoat 256 (Emerson and Cummings).

Results and discussion

Dopant species Generally, some iodine was released during the doping experiments. This phenomenon has also been observed by other authors who doped polyacetylene with IF s [ 12] or inserted this pentafluoride in graphite [ 13]. This can be understood by considering that two reactions were responsible for this behaviour:

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(i) the reduction:

polymer

oxidation

3IFs + 2e-

> 2IF6- + IF3

occurs with

concomitant

IFs partial

(ii) a decomposition reaction of thermodynamically unstable IF 3 takes place, yielding free iodine: 5IF 3 = 12 + 3IF s Elemental analyses performed on two samples and reported in Table 1 partially confirm this hypothesis, giving an I:F ratio close to 1:6. The infrared spectrum of a heavily-doped sample (Fig. 2(c)) exhibits a band in the region close to 700 cm -1. This may be assimilated into the (IF6-) characteristic band, the existence of which has been revealed at 625 cm -1 [14] w i t h o u t any specification, as far as we know. Moreover, it must be noticed that i.r. spectra do n o t clearly exhibit characteristic C--F bands (1100 - 1200 cm -1) [15], proving that no fluorination reaction takes place either during doping or during heating corresponding to the undoping step (Figs. 2(b)(c)(d).) All d o p e d samples exhibit i.r. spectra with broadened bands and shifted baselines (Figs. 2(b)(c)), which is certainly due to an electronic absorption

[8].

TABLE 1 Doping p a r a m e t e r s , c o n d u c t i v i t y and e l e m e n t a l analysis o f t h r e e d i f f e r e n t samples IFs vapour pressure (Torr)

~ I

4.45

20

Exposure time

Doping rate a IFs x - --(in PcA1F

o (~-1 cm-1) mole)

5 min 20 h o u r s

not measurable 0.3

6

84 h o u r s 5 min 20 h o u r s

0.56 0.15 0.4

0.8 6 x 10 - 4 4 x 10 -2

5 min 20 h o u r s 40 h o u r s

0.55 1.56 2.4

1 × 10 -1 3 ×10 -3 2.4 × 10 - 4

a D e t e r m i n e d b y mass u p t a k e m e a s u r e m e n t .

E l e m e n t a l analysis

10 -4

× 10 - 3

C 57.47% H 2.73% N 15.82% I 4.62% F 6.76% Al 4.04% ~ (PcA1F) (IFs.Ts)0.24

C 54.72% H 3.07% N 15.1% I 6.81% F 9.14% AI 4.1% (PcA1F)(IF6.3)o.3s

230

I

I

1800

I

t

I

I

1400 1000 WAVE NUMBERS (cm -1 )

I

[

600

Fig. 2. Infrared spectra of: (a) pristine and unsublimed (PcA1F)n; (b) (PcAIF) (IFs)0.a; (c) (PcAIF) (IFs)I.s;(d) (PcAIF) (IFs)I.S after TGA.

Doping rate dependence of the electrical conductivity This section deals exclusively with results relative to doped powdered samples. After each doping experiment, the powder was pressed under argon to carry out conductivity measurements. Results obtained for different IFs vapour pressures and various exposure times are reported in Table 1 and displayed in Fig. 3. The curve shown in Fig. 3 exhibits a maximum for x = 0.5 - 0.8 (IFs): (PcA1F) (in mole) doping rate values.

Undoping All samples could be undoped by heating to 500 °C under an inert atmosphere {argon or nitrogen) or under vacuum. If only an IFs loss was assumed, TGA results were in good agreement with those deduced from mass uptake measurements. The most heavily
231

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I

log (~)

3 F

°/

I

\\ Ii

I I

i

i

I ¢ I

i

I I

I

I

I I

I

3° ~i° 19° 2~35°~3~5e5¢67 ° 2e

2[

ip

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II

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Fig. 3. Electrical conductivity as a f u n c t i o n of the doping rate. (~d = conductivity of doped material; a0 = initial conductivity of pristine material (6 X 10 - s ~ - 1 cm-1). Fig. 4. X-ray powder patterns of: (a) pristine u n s u b l i m e d (PcA1F)n; (b) (PcA1F)(IFs)0.1% (c) (PcA1F)(IFs)I.S; (d) (PcA1F)(IFs)I. s after TGA.

However, all undoped samples exhibited an electrical conductivity of the order of 10 -7 ~2-1 cm -1 and i.r. spectra similar to that of a pristine polymer. Doping and undoping effects on the polymer structure The X-ray powder pattern of a lightly
232 This behaviour is quite similar to that exhibited by the polymer when it was heavily doped by AsFs [11]. It is thus possible to use the same qualitative model to explain the observed features. The (PcGaF)n structure has been determined from a single crystal by Wynne, and exhibites triclinic s y m m e t r y [16]. From comparison of their X-ray powder patterns, it is reasonable to assume that both (PcAIF)n and (PcGaF). are isostructural. Different X-ray powder patterns clearly show that the inter-ring distance ((100) reflection corresponding to d = 3.58/~) remains unchanged after doping and undoping. This implies that IF5 molecules lie preferentially between the polymer chains rather than phthalocyanine rings. Furthermore, the effective inter-ring distance has been evaluated as 3.66 ~ [7]; remembering that the minimum calculated diameter of an IF s (or IF6- ) molecule is 4.6 ~ [17], it is possible to insert compact columns of dopant between polymer chains up to 3.66/4.6 ~ 0.8 (IFs) per (PcA1F), ring. Beyond this limit, the eclipsed stacking of rings can no longer be conserved, and a staggered configuration with distortion in the b - c plane arises. This feature is corroborated by the important modifications of the peaks of the X-ray powder pattern except that at 20 = 24.85 ° (d = 3.58 /~) of a heavily
Conclusion The electrical conductivity o f (PcA1F), can be enhanced by four orders of magnitude by doping with IFs gas. The optimal doping rate to obtain the highest-conducting material is x ~ 0.5 - 0.8 IF 5 molecule per (PcA1F), ring. For higher values of x, the conductivity decreases because of the structural rearrangement induced by the dopant insertion, which leads to a lower orbital overlap between consecutive rings. However, heavy doping does not result in either a major chemical modification of the system possibly

233

expected from fluorination reaction of cycles, or in a breakdown of the (PcA1F)n molecular skeleton. A good conducting system, corresponding to a well-defined composition, is achieved only if the doping is carried out slowly, i.e., under low dopant pressure for sufficiently long exposure time. Using IFs as a dopant has allowed us to confirm and specify results previously obtained with the similar pentafluoride, AsFs. Finally, we have shown that for such dopants, the essential parameters to take into account are electronic affinity and the size of the dopant molecules.

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