The electrical conductivity of some TCNQ complexes derived from 2,2′-dichlorodiethyl ether and poly(epichlorhydrin)

The Electrical Conductivity of Some TCNQ Complexes Derived from 2,2'-Dichlorodiethyl Ether and Poly (ep ichlorhydrin) J. MALCOLM BRUCE and J. R. HERSON The chloromethyl groups of 2,2'-dichlorodiethyl ether and poly(epichlorhydrin) have been used to quaternize pyridine, and the quaternary compounds have been converted into simple and complex salts with TCNQ. The resistivities of these salts have been measured.

COMPARATtVEL¥ high electrical conductivities have been observed for simple organic donor-acceptor systems, especially those involving nitrogenous bases (B) and TCNQ [7,7,8,8-tetracyanoquinodimethane (I)].

~ N)2(I) C(CN)2 Three types of system have been prepared: complexes B,TCNQ, simple salts RB + TCNQ "-- in which R is usually alkyl, and complex salts RB + TCNQ ~ TCNQ °. The simple salts are obtained from quaternary salts RB + X - by metathesis with the lithium salt, Li + TCNQ ~, of the anion radical of TCNQ, and the complex salts by incorporation of neutral TCNQ into the simple salts. The complex salts show the highest conductivities I'~. Attempts have recently been made 3 to prepare conducting polymeric systems by using the pendant pyridyl groups in poly(2- and 4-vinylpyridines) as sites for the attachment of TCNQ residues, but the salts obtained had much lower conductivities than those of analogous monomeric systems, and, further, the maximum condu~ivity of the complex systems occurred when the quotient [TCNQ ]/[TCNQ °] was considerably greater than unity. The present paper describes some studies with poly(epichlorhydrin) in which the chloromethyl groups were used to quaternize pyridine, and the resulting pyridinium chlorides were then converted into simple and complex salts. Ethyl iodide and 2,2"-dichlorodiethyl ether were used as models in order to determine appropriate reaction conditions, and to provide compounds of defined composition for use as spectroscopic Standards. EXPERIMENTAL

Solvents, ethyl iodide, and 2,2"-dichlorodiethyl ether were dried and fractionally distilled; it is important that solvents should not contain readily reducible impurities since these can oxidize TCNQ'-- to TCNQ °. 619

J. MALCOLM BRUCE and J. R. HERSON Poly(epichlorhydrin) was prepared using a ferric chloride-propylene oxide catalyst ~, and had My ,-~ 70 000. TCNQ was sublimed at 150°/10 -4 mm Hg. Li + TCNQ was prepared 1 from TCNQ and lithium iodide in acetonitrile. Reactions were carried out under nitrogen, and, unless water was involved in the preparation, the products were handled in a dry-bo~ filled with nitrogen. Ultra-violet-visible spectroscopy was used to differentiate between simple and complex salts, and for the determination 1 of the extent of incorporation of TCNQ and TCNQ ° in the simple and complex salts respectively; carefully purified solvents, usually acetonitrile or dimethyl formamide (DMF), were required in order to obtain reproducible spectra. TCNQ had k ..... (in MeCN) 392 m/z (e 39 000), and k~ax. (in DMF) 338 m/~ (E 27 000). Li + T C N Q = had k .... (in MeCN) 392 (intl.) 406, 418 m/~ (~ 21 500), 25 000, 26 000), and h .... (in DMF) 395 (intl.), 408, 420 m/~ (E 19 500, 22 000, 23 000). Infra-red spectroscopy of Nujol mulls provided further information: the simple salts had a sharp band at 850 cm -~ which was absent from the complex salts. Resistivity measurements were made using the four-probe method s on compressed discs prepared from samples which had been dried at 20°/10 -2 m m Hg over phosphorus pentoxide for 60 hours. The probes were gold-plated. Activation energies were determined from Arrhenius plots obtained by measuring the resistivities at 5 ° intervals from - 2 0 ° to +20 ° inclusive; good straight-line plots of l o g p versus 103/T were obtained. All measurements at 20 ° and those at other temperatures required for the determination of the activation energies quoted in Table 2 were made in vacuo; measurements for the activation energies quoted in Table 1 were made under a stream of dry nitrogen maintained at the required temperature. Compounds were prepared as follows, using equimolar proportions of reactants unless stated otherwise : (a) A boiling solution of N-ethylpyridinium iodide (0.13 g) in ethanol (3 cm 8) was added to a filtered solution of Li + TCNQ = (0" 11 g) in boiling ethanol (30 cm3), the mixture was boiled for a few minutes, and allowed to cool to room temperature. The precipitate was collected, and washed with ice-eold ethanol and then with ether to give (N-ethyl pyridinium) + TCNQ "(0.065 g, 30 per cent) as purple microcrystals, decomp. 225 ° to 235 ° (Found: N, 22"8. C19H~3N5 requires N, 22.5 per cent). It had k .... (in MeCN) 392 (intl.), 406, 418 m/.~ (e 21 500, 24 500, 25 500), and )~.... (in DMF) 395 (intl.), 408, 420 m ~ (E 19 000, 21 000, 22 000). (b) A solution of the foregoing simple salt (0.074 g) in acetonitrile (4 cm 3) was treated with a boiling .solution of TCNQ (0.067 g) in acetonitrile (5 cm3), the mixture was allowed to cool to room temperature, and the precipitate was collected and washed with ether to give (N-ethyl pyridinium)+ TCNQ-:', TCNQ ° (0.078 g, 55 per cent) as deep purple needles, decomp. 195 ° to 235 °. It had h ..... (in MeCN) 393 m/~ (E 59 000) and k ....... (in DMF) 338, 395 (intl.), 408, 420 m/z (~ 26 500, 19 500, 20 5013, 2~ 500). 620

-IHI:. EI.ECTRICAL CONDUCTIVITY OF SOME TCNQ COMPLEXt.;~, (c) A solution of 2,2'-dichlorodiethyl ether (1"29 g) in pyridine (5 cm ') was refluxed for 12 h, and the excess of pyridine was then removed at 60°/10 -~ mm Hg to leave 2,2'-dipvridiniumdiethyl ether dichloride as a white solid (2-82 g, 100 per cent) (Found: C, 55"6; H, 6-3; CI, 23"5; N, 9.1. CI~Hls CLN20 requires C, 55"8; H, 6.0; CI, 23-6; N, 9.3 per cent). It had ..... (in EtOH) 261 mp, (E 8 400). (d) Treatment of the foregoing pyridinium salt with Li ÷ TCNQ ~ as described under (a) gave (2,2'-dipyridiniumdiethyl ether) 2+ (TCNQ:)~ (75 per cent yield) as purple microcrystals, decomp. 205 ° to 225 ° (Found: N, 21 '8. C38H.~rNI00 requires N, 21-9 per cent). It had ,~...... (in MeCN) 392 (intl.), 406, 418 m/z (¢ 44 000, 51 000, 53 000), and )~...... (in-DMF) 395 (intl.), 408, 420 m/x (E 39 000, 44 000, 46 000). (e) Treatment of the foregoing simple salt with TCNQ as described under (b) gave (2,2'-dipyridiniumdiethyl ether) 2+ (TCNQ : )5 TCNQ ° (80 per cent yield) as deep purple needles, decomp. 230 ° to 245 ° (Found: N, 22.9. C~0H30NI~O requires N, 23"3 per cent). It had ,k..... (in MeCN) 393 m/z (e 80 500), and )~...... (in DMF) 338, 395 (infl.), 408, 420 m/z (E 52 500, 39 500, 41 000, 42 500). An attempt to introduce a further tool, of TCNQ ° by using an excess of TCNQ and a reaction time of one hour was unsuccessful. (1) A solution of poly(epichlorhydrin) (1.58 g) in pyridine (5 cm 3) was reftuxed for 24 h (a precipitate was formed), and the excess of pyridine was removed at 60°/10 -2 mm Hg to give the polymeric pyridinium compound as a light-brown solid [Found: C, 53'5; H, 6"1; N, 7"7. (CsHIoC1NO),~ requires C, 56.0; H, 5-9; N, 8-2 per cent] which had )~..... (in EtOH) 261 m/z (e 3 800). These data indicate that c a . 90 per cent of the chloromethyl groups have reacted with pyridine. Attempts to obtain a higher extent of reaction by prolonging the reflux period led to decomposition, and the use of solvents in attempts to prevent the precipitation of the quaternary compound resulted in lower incorporation of pyridinium groups. (g) The foregoing quaternary compound (2.55 g) in ethanol (80 cm ~) was added to Li + TCNQ (3'5 g) in DMF (80 cm~), the mixture was reftuxed for 2 h, cooled and diluted with water (700 cm3). The precipitate was collected, washed Successively with water, ethanol, and ether, and then refluxed with benzene (150 cm 3) for 4 h to remove any neutral TCNQ. The polymeric simple salt (3.5 g, 70 per cent) was a purple powder [Found: C, 67"5; H, 4-9; N, 18"3; C1, 3"5. (C~H1j',I~O), requires C, 69.5; H, 4"1; N, 20'0 per cent] which decomposed in D M F : the absorption at 420 m/z, characteristic of TCNQ , decayed, and new absorptions, due to a decomposition product, appeared at 330 and 483 m/z, and progressively increased in intensity until, when the peak at 420 m/~ had disappeared, that at 483 m/z had ~ 19 000. The relative intensities of the peaks at 420 and 483 m/z showed an almost linear relationship, and extrapolation back to zero decomposition gave ,k.... (in DMF) 395 (intl.), 408, 420 m/z (~ 15 600, 16 800, 17 600), indicating that 80 per cent of the pyridinium chloride residues had reacted. Solid samples kept in the air decomposed similarly, although much less rapidly. 621

J. M A L C O L M B R U C E and J. R. H E R S O N

(h) Typically, the foregoing polymeric simple salt (0-46 g) and TCNQ (0"058 g) were dissolved in DMF (25 cm3), and the DMF was removed at 20°/10 -'~ mm Hg to leave the polymeric complex salt as a shiny black solid film which had hm,. (in DMF) 338, 395 (intl.), 408, 420 m/z (e 10 000, 17 500, 20 000, 22 000 by extrapolation). Like the simple salt, it decomposed in DMF, finally to hm~,. 347, 418 and 481 m/z. Polymeric complex salts containing various proportions (see Table 2) of neutral TCNQ were prepared analogously. When these samples were pressed into discs for resistivity measurements, green patches, suggesting the presence of crystals of neutral TCNQ, appeared on the surface of some of them, and a steady current could not be maintained during the resistivity determinations. RESULTS

AND

DISCUSSION

Treatment of 2,2'-dichlorodiethyl ether with pyridine gave the bis-quaternary compound from which the simple salt (II) was obtained, but only one tool. of neutral TCNQ could be introduced into (II) when attempts were made to prepare the :oniplex salt. Interaction of the neutral TCNQ with both TCNQ anion radicals in (II), possibly by formation of a sandwich compound such as (III), may account for this; possibly related compounds, but containing inorganic cations, e.g. (Cs+)~(TCNO ~-)~(TCNQ°), have been observed previously1.

!HzCH2--O--CH2NCH2

z N•H2CH2-O--CHzCH "r ~ TCNO, ~

TCNQOi~7 - 'T' + C N Q v

(11)

(Hi)

Resistivities and activation energies for the simple (II) and complex (III) salts prepared under different conditions are given in Table 1. The resistivities of the samples prepared under nitrogen were higher than those of samples prepared in air, and the resistivities of the samples prepared under nitrogen fell when they were exposed to air, despite subsequent drying prior to the electrical measurements. The decrease in resistivity of the samples exposed to air may be the result of any increase in resistivity due to oxidative decomposition being more than offset by a decrease due to absorption of moisture which could not be removed by drying. The activation energies do not show a parallel trend. Table 1. Resistivities a n d activa,tion energies for c o m p o u n d s (II) and (III) Compound

Conditions of preparation

p (ohm cm at 20°C)

II II II III III III

4"40× 2"52× 1.25 x 2"23 x 1-21 x 2"95×

106 106 106 10 106 10

E (eV)

0-413

0"385 0'170 0"206

(a) In air. (b) Under nitrogen. (c) Under nitrogen, then exposed to sir. All samples dried prior n~easurement of resistivity.

622

to

THE ELECTRICAL CONDUCTIVITY OF SOME TCNQ COMPLEXES In view of these observations, the polymer samples were handled entirely under nitrogen• Abou~ 90 per cent of the pendant chloromethyl groups in the poly(epichlorhydrin) reacted with pyridine, and about 80 per cent of the resulting pyridinium units underwent metathesis with Li + TCNQ -:. The resistivities of five polymers (approximately as IV) containing different proportions of TCNQ ° are given in Table 2. Activation energies were determined for two of the samples.

CH 2

/

\

Table 2. Resistivi.tiesand activation energies for polymeric tom flex salts (IV) TCNQ ° content [TCNQ ":"] P (ohm cm at20°C) E (eV) (% by wt) •[TCNQ~I

1.5x107 7.5Xl& 3-2×1~ 2.6×1~ 8.8×10

0

5 15 22 33

I0 3.0 1.9 1.1

0.36

0.11

As expected, the resistivity falls as the proportion of TCNQ ° is increas'ed, and extrapolation of a plot of log p versus [TCNQ~-]/[TCNQ °] to [TCNQ -: ] / [TCNQ °] = 1 indicates a limiting resistivity of about 60 ohm cm. In contrast to the model simple salt '(II), the polymeric simple salt (IV; m = 0 ) is capable of taking up almost the theoretical amount of neutral TCNQ required to yield a normal complex salt. However, the resistivities of (III) and (IV; n = m) were not significantly different, indicating that the proximity of potentially conducting groups attached to an essentially insulating chain does not in this case confer conductivity properties which cannot be achieved through the crystal lattice of a system of much lower molecular weight, although a more highly conducting polymer might have resulted had it been possible to cause all the chloromethyl groups to react. These results contrast with ~hose ~ mentioned above for systems based on poly(vinylpyridine). The polymeric TCNQ compounds decomposed in air, in solution and in the solid state, considerably more rapidly than the model compounds. This may be due to attack of oxygen at the polymer backbone leading to oxidative elimination of cyano groups (cf. ref. 6) from the TCNQ units; such a reaction could be catalysed by traces of iron remaining from the catalyst used for the preparation of the poly(epichlorhydrin). Accelerated decomposition of other polymeric salts has been independently observed 7. W e thank Dr E. P. Goodings and his colleagues of Imperial Chemical Industries Ltd, Petrochemical and Polymer Laboratory, Runcorn Heath,

623

J. MALCOLM BRUCE and J. R. HERSON

for helpful discussions and for the conductivity measurements, and the S.R.C. /or the award of a Research Studentship (to J.R.H.). Department of Chemistry, The University, Manchester, 13 (Received February 1967) REFERENCES I MELBY, L. R., HARDER, R. J., HERTLER, W. R., MAHLER, W., BENSON, R. E. and MOCHEL, W. E. J. Amer. chem. Soc. 1962, 84, 3374 2 MELBY, L. R. Canad. J. Chem. 1965, 43, 1448 CrmSTrqUT,D. B. and PHILLIPS,W. D. J. chem. Phys. 1961, 35, 1002 HATANO, M., NOMORI, H. and KAMBARA,S. Polymer Preprints, Amer. chem. Soc., Div. of Polymer Chem., Sept. 1964, p 849 LUPINSKI, J. H., KOPPLE, K. D. and HERTZ, J. J. Preprints, I.U.P.A.C. Symposium on Macromolecular Chemistry, Prague, Aug. 1965, p 196 PRUITT,M. E. and BAGGEa~r,J. M. U.S. Pat. No. 2 706 181 (1955) PRUITT, M. E., BAGGEI'T,J. M., BLOOMFIELD,R, J. and TEMPLETON,J. H. U.S. Pat. No. 2 706 182 (1955) 5 VALDES,L. B. Proc. Inst. Radio Engrs, 1954, 42, 420 UHL1R, A. Bell Syst. tech. J. 1955, 32, 105 6 HERTLER, W. R., HARTZLER,H. D., ACKER, D. S. and BENSON, R. E. J. Amer. chem. Soc. 1962, 84, 3387 7 GOODINGS,E. P. and WRIGHT,P. Personal communication

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