Electrophysical properties of polymeric phthalocyanines

ELECTROPHYSICAL PROPERTIES OF POLYMERIC PHTHALOCYANINES * YE. I. BALABANOV, YE. L. FRANKEVICtt and L. G. CHERKASI-IINA I n s ti tu te of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 11 April 1962)

POLYMERS with a system of conjugate bonds are known to have increased electrical conductivity [ 1-4]. I t has already been found [5] that the polymeric phthalocyanines of copper, prepared on pyromellitic acid base, have electrical conductivity of around aa00oK -----10- v - 10-s ohm -1 cm -1 at an activation energy of 7-9 kcal/mole. The present work presents the results obtained in a study of the electrical conductivity of polymeric phthaloeyanines prepared on pyromellitic acid base, with emphasis of some questions of the effect of gases on their properties. One of the aims of the work was to further the development of methods for synthesizing polymeric phthalocyanines with high conductivity. SYNTHESIS AND METHODS OF TREATING THE POLYPHTHALOCYANINES A phthalocyanine was fused with pyromellitie acid at 180-200 ° in the presence of urea, cuprous chloride as the complex-former, and a m m o n i u m molybdate as the catalyst in the molar ratio pyromellitie a c i d : c u p r o u s chloride== 1 : 1.8. The catalyst was found to have a positive effect in increasing the yield of reaction product, also intensifying its colour. The product of the fusion was treated with hot water, dilute hydrochloric acid, water, ammonia solution and water, in t h a t order. The products separated were dark coloured powders, insoluble in dimethylformamide and easily soluble in concentrated sulphurie acid which was what we used to reprecipitate the polyphthalocyanines (PPC). This was the method used to refine away any impurities. To remove traces of cuprous chloride the reprccipitated and washed polymer was extracted for 60 hours with boiling pyridine. Tile t r e a t m e n t ended with a long (80 hours) period under a v a c u u m at 250°/10 -1 m m until no more impurities were liberated by sublimation. A more detailed description of the synthesis of PPC has already been given in [5]. Copper P P C ' s synthesized with pyromellitic acid appear, from the elementary analysis data given in the Table, to have the same properties as those described in [5].

STUDY OF THE ELECTRICAL PROPERTIES

The principal measurement made on the PPC specimens concerned electrical conductivity. Depending on the resistivity of the specimens these were made either on a MOM-4 megohmmeter or the usual kind of measuring bridge. The * Vysokomol. soyed. 5: No. 11, 1684-1689, 1962. 797

798

YE. I. B A L A B A N O V

et al.

specimens were tablets 10 mm dia. and 0.5-1 m m thick. They were compacted from the synthesized powder at a pressure of 10,000 kg/cm 2. The first measurements showed that the results were closely dependent on the conditions of measurement. In a vacuum the resistivity of the specimens was different from that found when they were studied in air. This persuaded us to conduct subsequent measurements in vacuo, using specimens which had been thoroughly degassed, which gave reproducible measurements. Before s t a r t i n g t h e investigation the specimens were p u t into special molybdenum glass ampoules where they were between two flat metal contacts held together b y springs. To provide the required electrical contact the flat surfaces of the specimens were coated with alkodag. Degassing was conducted in the same ampoule. It was placed in a cylindrical furnace and heated up to 250 ° with a diffusion pump working all the time.'After 10-30 hr heating the vacuum in the ampoule had reached 5 × 10-6 ram. The temperature dependence of the electrical conductivity was measured in the same ampoules. They were placed in a thermostat where their temperature could be fixed in the range 18-95. The results of the measurement of the temperature dependence of the electrical conductivity of different specimens are nearly always described b y a formula of the a----a0 exp (--E/kT) type. The Table gives the a0, E and as00oK values found before and after degassing the specimens. I t can be seen that the resistivity increases if air is allowed to reach the specimen and falls if they are in a deep vacuum. I f the specimens spend some time in air after the vacuum treatment, however, the resistivity rises again. For P P C prepared with intentionally high conductivity, measured in air, it is typical for the volume resistivity to be usually less sensitive to the period of contact with air after the vacuum treatment (Table, specimens 2 and 4), Apparently the combination of treatment used first of all impart greater perfection to t h e structure of the PPC, reducing its adsorptive capacity. I t is emphasized that the physical meaning of the values a. and E cannot be established on the basis of only a few measurements of conductivity. It is natural to expect that the activation energy of two processes will determine the electrical conductivity in the final analysis: i.e. the processes of the creation and movement of current carriers. To find out the characteristics of each of the processes we tried to measure the Hall effect on polymeric specimens. In the apparatus used the specimen was in a variable magnetic field (50 e/s) with an alternating current (70 e/s) passing through it; the Hall potential was recorded at the difference frequency. During the measurements the specimen was in a vacuum of 5 × 10-6 mm and could be heated up to 250 °. No Hall effect was found on a n y of the P P C specimens. An assessment of the response of the apparatus led to the conclusion that the carrier mobility was not more than 0-3 em~/v, sec in the specimens at temperatures between 18-250 °. This means that the comparatively high conductivity of the specimens must be due to a concentration of current carriers of more than 2 × 101~ era -a (for a300o~=10-4 ohm -1 vm-1).

Electrophysical properties of polymeric phthalocyanines

799

T h e p r o b l e m o f the n a t u r e a n d role of the gas which is liberated d u r i n g t h e v a c u u m t r e a t m e n t a n d s u b s t a n t i a l l y alters t h e c o n d u c t i v i t y o f the specimens, was studied in r a t h e r m o r e detail. T h e mass s p e c t r o m e t e r seemed to be the best approach. W i t h a n o r d i n a r y mass s p e c t r o m e t e r gas a m o u n t s can be a n a l y s e d right d o w n to 10 -4 cm 3, which, for a v o l u m e o f 0.2 cm a o f t e s t specimen, would c o r r e s p o n d to a c o n c e n t r a t i o n o f l012 cm -z gas molecules in the specimen, which could w i t h d r a w on heating. T h e ampoules with the specimens were c o n n e c t e d u p to the i n p u t s y s t e m o f the mass s p e c t r o m e t e r t y p e ~/IKh-1302, to the t u b e passing d i r e c t l y t o w a r d s the ion source. T h e y were e v a c u a t e d with a diffusion p u m p , t h e e v a c u a t i o n was h a l t e d a n d the gases liberated were analysed. P e a k s were r e c o r d e d in t h e spectrometer, which rose w h e n the specimens were heated. The following were f o u n d to show a t i m e increase: m/e=16; 17; 18; 28 a n d 44, a n d also peaks with m/e 16. Ar~alysis o f the mass s p e c t r u m showed t h a t these peaks increased as a result of t h e f~llowing gases: hTHa, t t 2 0 , N 2 a n d CO 2. Figure 1 shows the kinetics o f the gas liberation f r o m P P C specimen No. 5 (see Table). T h e s p e c t r o m e t e r was g r a d u a t e d in such a w a y t h a t the t o t a l a m o u ~ t s o f the different gases l i b e r a t e d could be determined. C~lculated per 1 cm 3 v o l u m e o f c o m p a c t e d specimen, these a m o u n t s were: Molecules Concentration

CO2 2.1 × l019

N2 2 × 10is

HzO 1.1 x 1019

NH 9 3.7 x 10is

Before degassing t h e resistivity o f the specimens at r o o m t e m p e r a t u r e was 65 K o h m (a specific conductivity---- 1.4 × 10 -5 o h m -1 cm-1). After liberation of t h e a b o v e gases it was 33 K o h m .

0

20

40

60

80

Tfme,rnin

100

"120

FIG. 1. Kinetics of gas liberation from specimens of copper polyphthalocyanine immediately after preparation: 1--ttzO; 2--CO2; 3--N~; 4--NH3. I t seems to us t h a t an i m p o r t a n t question is, w h a t is the effect u p o n t h e c o n d u c t i v i t y o f each o f t h e gases which we o b s e r v e d being liberated ? To answer this, we c o n d u c t e d the following e x p e r i m e n t . T h e degassed specimen whose gas

No.

C

A n a l o g of No. 4; s y n t h e s i s and treatment in inert m e d i u m (At)

20.43 20.30 21"18 21.20

7"77 8"00 9"28 9"23

51"96 2"59 20"69 9.57 51 "88 2.59 20'51 9-31

2.55 2'32 2.33 2-36

1 x 10-5

l 0 -4"~ 10 -4.7 lO-4.s 10-5-~ 10-4.3

I0 -~

10 -s'5 10 -5.9

vacuum, Q-i cm-i

1.83 22.95 8"15 2.05 23.05 8"35

Cu

10 -s 10 -7.9 10-9 10-8

N

aa00OKbefore treatment in deep

2-17 20"99 9"44 1.94 21-34 9"34

H

sition. %

Elementary compo-

P o l y p h t h a l o c y a n i n e o f copper synthesized from pyro51-62 m e l litic acid 51"70 S y n t h e s i s as a b o v e ; t r e a t m e n t - w i t h o b v i o u s l y ~oo little v a c u u m t r e a t m e n t a t 250°/10 -1 m m As a b o v e , b y p h t h a l o c y a n i n e s y n t h e s i s w i t h o u t a catalyst; ordinary treatment, including reprecipitation f r o m c o n c e n t r a t i o n H2SO 4, pyridine extraction and 49"65 long vacuum treatment at 49.87 250°]10 -1 ram. A n a l o g o f No. 2; s y n t h e s i s a n d t r e a t m e n t in A r atmosphere and a medium 49"96 b u t possible t o e l i m i n a t e 50"13 contact with air 51"33 As No. 1; t r e a t m e n t as 51 "42 No. 2

D e t a i l s of p o l y m e r i c phthalocyanine (synthesis and treatment)

e

lO-S.1

10-1.6

10-1.o

3"7 4-14

10-1.6 lO-l.s

10-5.2

10-1.2

6.0

5.1 4.9

10-5.3

lO-O..~

2-8 × 10 -5

10-44 10-4 10-4.~ 10-a

10-4-2

10-4

10-3

10-5.6

10-4.2

10--2.5

cY300°K ~'2-1 c m - 1

4.4 5.2

,

UO, k c a l / m o l e f2-1 cm-X

E~

Electrical properties after vacuum treatment

5.1 5.18

6.3 8.3 2.8 7.25

0"300"K

lO-X.7

10-5.4

10-5.g

10--1.6

10-6-3

~2-1 em-1

10-e-8 10-5.~ lO-S.s

t

10-1.8 10-o.8 10-3.~ 10-2

0"0, k c a l / m o l e ~c~1 c m - i

E~

Electrical properties after letting in air

ELECTRICAL PROPERTIES OF POLYPHTHALOCYANINES OF COPPER AND PYROMELLITIC ACID

o

Electrophysical properties of polymeric phthalocyanines

801

liberation kinetics is s h o w n in Fig. 1 was placed in d r y air. The a t m o s p h e r i c moisture was frozen o u t a t - - 7 8 °, a n d t h e n t h e resistivity o f t h e specimen was measured. The t i m e c u r v e is s h o w n in Fig. 2. Two sections can be seen, one o f quite rapid, a n d one o f quite slow, change.

52

x

-

Y.

40

{ l

[

~

20

I

40

, 40

I

Time, rain 80 f

60

I

"120

I

80

T(me, hr

FIG. 2. Variation in resistivity of polyphthalocyanine specimen in dry air. The specimen was first degassed. The mimetics of the gas liberation are shown in Fig. 1. The initial section of the curve is given in a larger scale. The lower curve shows that the oxygen has no effect on the resistivity of the specimen, even after being ir~ air and vacuum treated afterwards. The vacuum treatment starts at point A with a diffusion pump, at 250 °. After a g o o d time (80 hr) in air the specimen was a g a i n c o n n e c t e d to the m a s s s p e c t r o m e t e r a n d degassed in a deep v a c u u m . The gas liberation kinetics is s h o w n in Fig. 3. F o r the curves in the left h a n d side o f the g r a p h the t e m p e r a t u r e was room. I t can be seen t h a t t h e following gases, O2, CO2, N 2 a n d H 2 0 are liberated. W h e n the specimen is h e a t e d liberation o f all the gases increases again,

100 8O

60

20I~',

0

~

20

,,

~0%1 60

80 Trine, min

NO

120

"!40

FIG. 3. Kinetics of gas liberation from specimen after being in air (see Fig. 2). For the curves on the LH side of the graph the temperature of the specimen was 18°; in the right, every 30 min after heating at 250°: 1--H20; 2--~2; 3--02; d--CO2.

802

YE. I. B A L A B A N O V et al.

and then falls. There is an important feature in the behaviour of the oxygen; if the temperature of the specimen is increased slightly (50-60°), the oxygen yield becomes negative, i.e. it begins to be rapidly absorbed (this was established b y the sharp reduction in the height of the oxygen peak); liberation of H20 and CO S continues after this. The vacuum treatment means that the resistivity of the specimen once more becomes the same as before the specimen spent some time in oxygen, or even a little less. Subsequent admissions of oxygen into the ampoule with the specimen had less effect than the first (lower curve in Fig. 2), b u t it is important to note that the vacuum treatment and heating of the specimen after the "oxidation" reduce its resistivity. With this kind of treatment we managed to reduce the resistivity of a specimen to a third. We also investigated the effect of adsorbed water on the electrical conductivity of copper P P C specimens. A specimen heated i n vacuo was brought into contact with steam (H20 pressure z 18 mm wt. c o l ) a t room temperature. Figure 4 hows the kinetics of th~ variation in resistivity. R,KD 240

o--

.

_

.

"160 80 4G0

'

'

20

'

'

'

60

'

'

'

400

Dine,min

FIG. 4. Variation in resistivity of specimen in contact with steam at 18 mm Hg. This proceeds quite rapidly, the kinetic curve reaches saturation in practically the same time as that occupied b y the initial section of the curve taken in air. This similarity in the initial sections of the curves leads one to suppose that the variation in the resistivity of specimens in air is due to two processes: 1) adsorption of water and 2) partial oxidation of the specimen, which requires activation energy and therefore occurs more slowly than the former. I f the slopes of the initial sections of the curves in Fig. 2 and 4 are compared, the partial vapour pressure can be calculated for the " d r y " air used in oxidation. In this calculation we will assume that the initial rate of change in resistivity is proportional to the partial water pressure. The calculation gives a partial water pressure of 3 × 10-2 mm Hg. Such an amouut is quite consistent with the moisture content of air dried b y solid carbon dioxide. I t is emphasized that the use of air or oxygen dehydrated at --120 ° will go a long w a y to eliminating the "rapid" process of the rise in resistivity. The combination of experimental data above could be explained, for example, b y the following scheme. The synthesis produces a polymer in which, besides

Eleetrophysieal properties of polymeric phthaloeyanines

803

the usual "finished" p o l y m e r i c units o f c o p p e r p h t h a l o c y a n i n e , t h e r e are also "unfilled" ones containing surplus h y d r o g e n . These disturb t h e conjugation s y s t e m a n d lead to a n increase in t h e resistivity o f the p o l y m e r . T h e side groups o f molecules r e m a i n i n g f r o m the original a n d i n t e r m e d i a t e materials could be -COOH :

C()ONH, :

--CO/~ - CO

O:

-(10\

" : - CO +/N H

--CONH~ .•

These m a y be t r a p s for t h e c u r r e n t carriers; t h e i r existence causes a r e d u c t i o n in c o n d u c t i v i t y . I f t h e specimen is h e a t e d i n vacuo H20, H N a a n d CO s are liberated. These molecules seem to be l i b e r a t e d as a result o f the b r e a k d o w n o f the traps. I f the p o l y m e r spends some t i m e in a t m o s p h e r i c o x y g e n the b o u n d h y d r o g e n is oxidized a n d l i b e r a t e d as w a t e r on s u b s e q u e n t heating. The presence of w a t e r molecules in the p o l y m e r mass reduces its c o n d u c t i v i t y . H a v i n g a close affinity with the electron, t h e y act as carriers traps. O f course, the v a c u u m t r e a t m e n t o f the specimen a n d o x i d a t i o n o f t h e " e x c e s s " h y d r o g e n m e a n s t h a t t h e specimen will h a v e longer sections o f " c o n t i n u o u s " conjugation, which increases its c o n d u c t i v i t y . T h e a u t h o r s are highly i n d e b t e d to V. L. T a l ' r o z e a n d A. A. Berlin for their c o n s t a n t interest a n d useful discussions. CONCLUSIONS

(1) P o l y m e r i c copper p h t h a l o c y a n i n e s have been synthesized on pyromellitic acid base. A f t e r v a c u u m t r e a t m e n t t h e y h a v e electrical c o n d u c t i v i t y of aa00oK = 10-a+ 10-4 ohm-1 cm-1. (2) A mass s p e c t r o m e t e r has been used to a n a l y s e the composition o f the gas l i b e r a t e d during the v a c u u m t r e a t m e n t a n d it has been f o u n d t h a t t h e increase in specific electrical c o n d u c t i v i t y which occurs during this t r e a t m e n t is due to the r e m o v a l o f o x y g e n - c o n t a i n i n g molecules, which are electron accepters. (3) T h e r e d u c t i o n in t h e c o n d u c t i v i t y o f t h e specimens w h e n left in air is f o u n d to be due to a b s o r p t i o n o f w a t e r a n d the o x i d a t i o n o f t h e p o l y m e r b y atmospheric oxygen. Tra~mlated by V. ALFORI) REFERENCES

1. R. McNEILL and D. E. WEISS, Austral. J. Chem. 12: 643, 1959 2. A. EPSTEIN, and B. S. WILDI, J. Chem. Phys. 32: 321, 1960 3. Ye. I. BALABA~qOV, A. A. BERLIN, V. P. PARINI, V. L. TAL'ROZE, Ye. L. F R A N K E VICH and M. I. CHERKASHIN, Dokl. Akad. Nauk SSSI% 134: 1123, 1960 4. A . V . TOPCHIEV, M. D. GEIDERIKH, B. E. DAVYDOV, V. A. KARGIN, B. A. KRENTSEL,

I. M. KUSTANOVICH and L. S. POLAK, Dokl. Akad. Nauk SSSI=t 128: 312, 1959 5. A. A. BERLIN, L. G. CHERKASHINA and Ye. I. BALABANOV, Vysokomol. soyed. 4: 376, 1962