PHYSICAL CHEMISTRY

E. I. Balabanov, A. A. Berlin, V. P. Parini, V. L. Tal’roze,
E. L. Frankevich, and M. I. Cherkashin

ELECTRICAL CONDUCTIVITY OF POLYMERS WITH CONJUGATED BONDS

(Presented by Academician V. N. Kondrat’ev, June 14, 1960)

In connection with the problem of obtaining organic polymeric substances with various electrophysical properties, including the question of organic semiconductors, a broad study of the electrical properties of various types of polymeric substances with systems of conjugated bonds and heteroatoms in the conjugation chain is of interest \((^{1})\). The authors synthesized the classes of polymers listed below, studied their electrical conductivity \(\sigma\), and its dependence on temperature.

1. Polymers with an acyclic conjugation chain \((^{2,3})\): polyphenylacetylene (1), copolymers of polyphenylacetylene with hexine (2) and with paradiethynylbenzene (3).

2. Polymers with benzene nuclei in the conjugation chain: polyphenylene

\[ \left[ \mathrm{Cl}\!-\!\begin{matrix}\text{phenylene ring}\end{matrix}\!-\!\mathrm{Cl} \right]_n \tag{4} \]

polyphenylene azo compounds \((^{4-6})\) of the type

\[ \left[ \mathrm{Cl}\!-\!\begin{matrix}\text{phenylene ring}\\[-2pt] X\end{matrix} \!-\! \begin{matrix}\text{phenylene ring}\\[-2pt] X\end{matrix} \right]_m - \left[ -\mathrm{N{=}N}- \begin{matrix}\text{phenylene ring}\\[-2pt] X\end{matrix} \!-\! \begin{matrix}\text{phenylene ring}\\[-2pt] X\end{matrix} -\mathrm{Cl} \right]_n, \]

where \(X = H\) (5), \(\mathrm{CH_3}\) (6), \(\mathrm{COOH}\) (7), polymeric aromatic and fatty-aromatic compounds containing quinoid and amino groups \((^{7,8})\): polyphenyleneaminoquinones of the type

\[ \left[ -\begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} \!-\! \begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} -\mathrm{NH}- \begin{matrix} \mathrm{O}\\[-2pt] \text{quinone ring}\\[-2pt] \mathrm{O} \end{matrix} -\mathrm{NH}- \right]_n, \]

where \(X = H\) (8), \(\mathrm{Cl}\) (9) at \(R = H\), and \(X = H\) (10) at \(R=\mathrm{COOH}\); poly-\(p\)-phenylenediaminoquinone (11), polyhexamethylenediaminoquinone (12)

\[ \left[ -\begin{matrix}\text{phenylene ring}\end{matrix} -\mathrm{NH}- \begin{matrix} \mathrm{O}\\[-2pt] \text{quinone ring}\\[-2pt] \mathrm{O} \end{matrix} -\mathrm{NH}- \right]_n \tag{11} \]

\[ \left[ \begin{matrix} \mathrm{O}\\[-2pt] \text{quinone ring}\\[-2pt] \mathrm{O} \end{matrix} -\mathrm{NH}-(\mathrm{CH_2})_6-\mathrm{NH}- \right]_n \tag{12} \]

polyphenyleneazoquinones of the type

\[ \left[ -\begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} \!-\! \begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} -\begin{matrix} \mathrm{O}\\[-2pt] \text{quinone ring}\\[-2pt] \mathrm{O} \end{matrix} - \right]_m - \left[ -\begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} \!-\! \begin{matrix}\text{phenylene ring}\\[-2pt] R\end{matrix} -\mathrm{N{=}N}- \begin{matrix} \mathrm{O}\\[-2pt] \text{quinone ring}\\[-2pt] \mathrm{O} \end{matrix} \right]_n, \]

where R = H (13) and COOH (14); polymeric triazene (15), a substance containing quinone-imine groups (16)

\[ \left[ \text{—cyclohexyl—cyclohexyl—NH—N=N—} \right]_n \qquad (15); \]

\[ \left[ \text{—cyclohexyl—N=phenylene=N—} \right]_n \qquad (16); \]

polymeric chelate compounds \({}^{(9)}\) of polydiphenylaminoquinone with metals (for example, copper) (17)

\[ \text{[structural formula of a copper chelate of polydiphenylaminoquinone]} \]

Molecular complexes of acenaphthene with chloranil (18) and with the pyridinium derivative of polyphenyleneaminoquinone (19) were also synthesized.

1. Compounds with nonbenzenoid rings in the conjugation chain: tetrasalicylferrocene (20) and its polymeric chelate complexes \({}^{(10)}\) with \(\mathrm{Fe}^{2+}\) and \(\mathrm{Be}^{2+}\) (21, 22)

\[ \text{[structural formula of a polymeric chelate complex of a ferrocene derivative with metal } M\text{]} \]

Polymeric chelate complexes of perianthene with \(\mathrm{Cu}^{2+}\) (23) and \(\mathrm{Fe}^{2+}\) \({}^{(11,12)}\).

The synthesis and properties of some of the compounds listed above (for example, 8, 10, 11, 13, 14) have not yet been covered in the literature. Special publications will be devoted to them in the near future.

Of considerable interest are polymers containing quinoid nuclei in the conjugation chain (10, 14), and especially compounds in which the quinoid structure is incorporated into the chain through a nitrogen heteroatom (16). In such substances one may expect a sharp decrease in the excitation energy to the triplet state and, in some cases, the formation of ion-radical structures.

The samples studied were, for the most part, tablets 10–12 mm in diameter.

In the present communication we shall confine ourselves to a general characterization of the regularities obtained. In all cases, with increasing temperature the electrical conductivity increased according to the law

\[ \sigma = \sigma_0 \cdot e^{-E/kT}, \]

where \(\sigma_0\) and \(E\) are constants for the given sample.

Deviations from this law occurred only near the decomposition temperature of the substance. The obtained values of \(E\) vary from 4.6 kcal/mol (0.2 eV) for substance 16 to 49.5 kcal/mol (2.1 eV) for polyphenylacetylene, and even up to 92 kcal/mol for the acenaphthene complex with chloranil. The values of \(\sigma_0\) vary from \(10^{-12}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\) for polyphenylene to \(6\cdot10^{51}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\) for the acenaphthene complex with chloranil.

The nature of the treatment of the sample has a great influence on these parameters. Thus, for example, the pre-exponential factor for polyphenylacetylene decreases by 22 orders of magnitude on going from a film obtained from solution to a tablet pressed at \(200^\circ\mathrm{C}\).

At the same time, however, there is also a decrease in the “activation energy” \(E\), so that the electrical conductivity of both samples at room temperature

turns out to be approximately the same. Such a phenomenon of symbatic change of the pre-exponent and the activation energy is often called the compensation effect (c. e.) and has a number of analogies in chemical kinetics and catalysis, and for electrical conductivity it has been observed in metal oxides (¹³). The nature of the c. e. is still not clear; one of the theoretical approaches to solving this question was considered recently (¹⁴).

It turned out that in our case the c. e. is a regularity that encompasses all, or almost all, of the substances obtained. This is especially clearly seen from Fig. 1, where the data of Table 1 are plotted in the coordinates $\lg \sigma_0$—$E$.

Table 1

* Samples for which the straight line $\lg\sigma—1/T$ underwent a break. Numbers with one and two primes refer to the same sample before and after the break.

We have here an entirely exceptional manifestation of the c. e.: over a range of sixty (!) orders of magnitude of change in the pre-exponent and a twentyfold change in the activation energy for substances differing in structure.

A number of the samples studied possess electrical conductivity several orders of magnitude greater than the conductivity of ordinary organic dielectrics. This applies above all to samples 16, 21, and 22, which in electrical conductivity approach certain organic semiconductors known from the literature \((^{15-17})\).

Fig. 1. Relationship between the pre-exponential factor and the activation energy of electrical conductivity

Of great interest is the very strong dependence of \(\sigma\) on \(T\), corresponding to high values of \(E\), in the case of polyphenylacetylenes (which at room temperature are typical insulators). In combination with a large \(\sigma_0\), this leads, as the temperature is raised, to the fact that the \(\sigma\) of polyphenylacetylene “catches up” with the \(\sigma\) of a whole series of polymers that exhibit high electrical conductivity at room temperature.

It may be expected that further investigations will make it possible to establish a relationship between the electrophysical properties and the structure of individual polymer molecules and of the materials obtained from them.

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
11 VI 1960

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