EFFECT OF STRUCTURE ON ELECTRICAL CONDUCTIVITY OF ORGANIC COMPOUNDS
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The Effect of Structure on the Electrical Conductivity of Organic Compounds.
Polyazophenylenesl
by Donald M . Carlton, D. K. McCarthy, and R. H. Genz Sandia Corporation, Sandia Base, Albuquerque, NEWMexico
(Received May 6, 1964)
The effect of molecular structure on the electrical conductivity of organic compounds has been examined by synthesizing a series of nine polymers and measuring the electrical conductivity as a function of temperature. These data have confirmed the influence of subtle structural alterations am the electrical conductivity. In addition, the degree of r-orbital overlap along a conjugated chain has been shown to be the most important structural characteristic; the role of the electronic effect of substituents is more difficult to interpret.
Introduction The phenomenon of the electrical conductivity of organic compounds has evoked a great deal of interest in recent years. In spite of this intense research interest, however, the mechanism of conduction remains largely obscure. Previous research in this field has been greatly influenced by the work on inorganic semiconductors. Theoretical and experimental success with this type of semiconductor has caused many workers to apply these same techniques t o organic compounds. Many authors have attempted theoretical descriptions of organic semiconductors by using various modifications of band theory.2 Though these attempts have met with limited success, a general mathematical description of the phenomenon continues to be elusive. Experiments in this area have been mostly concerned with the measurement of carrier mobilities, energy gaps, etc., and have dealt, only cursorily with the possible effects of molecular structure on the electrical conductivity. This paper presents a study of the effects of such structures on electrical conductivity. The conductivity mechanism is generally thought of as being an intermolecular phenomenon; i e . , because the aromatic nucleus is a superconductor and yet exhibits considerable resistance in the bulk material, the main problem seems to be one of transferring the charge carrier from molecule to molecule. In an intermolecular mechanism of this kind, it would seem that the structure of the molecules should be of
paramount importance. Because an electrical phenomenon is involved, the electron donating or withdrawing properties of substituents on an aromatic system should affect the electrical conductivity, and since the availability of T-orbitals seems important to the conductivity mechanism, the electron-withdrawing 'substituents should be detrimental to the conductivity. The conjugated system is probably the most universally recognized structural parameter in this area of research. All of the so-called organic semiconductors are conjugated systems. In addition, Pohl and Engelhardt3 have discussed the importance of the degree of conjugation. It would seem, therefore, that if, in a linear conjugated system, groups more or less capable of transmitting electronic effects were substituted in the backbone, there should be a corresponding change in the electrical conductivity. In these compounds, groups less capable of transmitting electronic effects should retard conductivity. To conduct an investigation of the effects of the structural parameters described above, it was neces(1) Presented in part at the 19th Southwest Regional Meeting of the American Chemical Society, Houston, Texas, December 5-7, 1963. (2) Some examples are: (a) W. C. MoCubbin, Trans. Faraday Soe., 59, 26 (1963); (b) 0. H. LeBlanc, Jr., Proceedings of the InterIndustry Conference on Organic Semiconductors, Chicago, Ill., 1961; and (0) S. H. Glarum, J. Phys. Chem. Solids, 24, 1577 (1963). (3) H. A. Pohl and E. H. Engelhardt, J. Phys. Chem., 66, 2085 (1962).
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sary to choose a suitable system. Several criteria were established. (1) The system should be conjugated for the aforementioned reason. ( 2 ) The system should be linear in order to reduce complexity. (3) The system should have an unambiguous structure, and the structure should be capable of variation. (4) The system would not have to be what might be called a "good" conductor, but it should have a conductivity in a range where measurements could be made with confidence. (5) Because higher molecular weight compounds were of interest, the compounds should have increased molecular weight over the dimer or trimer.4 It must be realized, however, that free radical polymerization reactions yielding a conjugated system inevitably result in low molecular weight compounds. This seems to be due to the stabilization of the radical intermediates. The system which best met the criteria was the polyazophenylene system reported by Berlin.6 This polymer is composed of a polyphenylene backbone, interrupted by azo linkages, and is synthesized by the cuprous ion decomposition of the bisdiazonium salt. Figure 1 shows the reaction scheme and the positions of the substituents. No compound incorporating both a backbone substituent and a side-chain substituent was synthesized. Each compound synthesized was purified (vide infra), and the conductivity was measured as a function of temperature. The plots of log conductivity ( u ) us. 1/T are shown in Fig. 2 and 3. All of these measurements were made under rigidly controlled conditions. One possible source of error which received special attention was the impurity problem. Each compound was washed for 48 hr. with constant-boiling hydrochloric acid, 24 hr. with water, dried, and washed for 48 hr. with xylene. I n spite of this exhaustive treatment and the satisfactory elemental analyses, there remained some possibility of error being introduced into the data by impurities. Polyazophenyl ether was therefore intentionally doped with 1% and 3% of the expected major impurity (CuC12) and the conductivity was measured as a function of temperature. No change in curve shape and only a very slight shift toward higher conductivity were observed. This experiment indicated no significant contribution from metallic impurities at this level to the investigation. Crude and purified samples of each compound were examined by electron paramagnetic resonance (e.p.r.) in order to determine whether low levels of paramagnetic impurities were being removed in the purification. The e.p.r. spectra of the crude samples were observed to be very complex and "dirty," whereas the spectra The Journal of Physical Chemistry
D. M. CARLTON, D. K. MCCARTHY, AND R. H. GENZ
r C1 0
HONO _c
8' R
R R
A. x . s o z , s,o,-c=c-,
! N
R = NOz, H , O C H 3 , C I
Figure 1. Reaction scheme for the synthesis of the various polyazophenylenes used in this study: A. A. Berlin, et al., J. Polymer Xci., 55, 675 (1961).
250 l " " I
150
200 " " I
'
TEMPERATURE 100 1
"
I
'
OC
'
'
50 I
,
'
I
COND~CTIYITY Mhdcrn
(31 POLYAZOFLUORENE
Figure 2. Plot of electrical conductivity us. 1 / T for compounds listed in group A, Table I.
(4) The molecular weights of the compounds used in this study are of the order of 2000. This estimate is based on the end-group analysis of Berlin6 and on osmometric measurements of soluble polymer fractions. (6) A. 8 . Berlin, V. I. Liogon'kii, and V. P. Parini, J . Polymer Sci 55, 675 (1961).
EFFECT OF STRUCTURE ON ELECTRICAL CONDUCTIVITY OF ORCAI~IC COMPOUNDS
TEMPERATURE ‘C I50
IO0
50
CONDUCTIVITY
Mhokm
2663
material between the rods. A glass envelope wais sealed around the rods while they were held under a specified pressure. The cell was then evacuated by means of an oil diffusion pump and sealed to eliminate water and to prevent oxidation of the sample. Kovar rods were used because the thermal expansion is similar to glass. Another holder in which not only the temperature and voltage gradient could be varied, but also the pressure, was built. This holder, made of metal, included a screw by which pressure could be applied. Strain gauges were used to monitor the pressure. The electrodes were a guarded system made of Kovar; glass was melted between the guard and guarded electrode to function as an insulator. The glass was then ground flat. The surfaces of the electrodes were rhodium plated. A glass sleeve around the electrodes was also used with this holder to retain the material between the electrodes. The temperature dependence of conductivity of the polyasophenylenes was exponential, which is characteristic of organic semiconductors. The temperature dependence is described by the equation u = urnexp(-AE/kT)
Figure 3. Plot of electrical conductivity vs. l / T for compounds listed in group B, Table I.
of the washed sample8 were uncomplicated and “clean.” These data showed .the absence of paramagnetic inipurities in the washed samples.
Experimental Conductivity Measurements. The apparatus used to measure the electrical conductivity consisted of a Hewlett Packard (711A) regulated power supply connected in series with the sample and a Keithley (610A) electrometer. The conductivity was computed from the equation
I t V A
g = - -
where u is the conductivity, I is the measured current, V is the applied voltage, t is the thickness of the sample, and A is the effective area of the sample. Conventional electrical conductivity sample holders could not be used because all of the samples were in a powder form. Therefore, a special holder had to be made. One type of holder used was made of two Kovar rods; a glass sleeve was used to hold the powder
where u is the conductivity, urn is the conductivity at temperature, A E is the activation energy, k is Boltzmann’s constant, and T is temperature in “E.. From this equation, the activation energy and urn were calculated (Table I). The conductivity at 100’ is also listed. In order to compare the conductivity data for each polyazophenylene listed in Table I, the voltage gradient
Table I : Experimental Conductivity Data and Calculations urn,
u100,
Material
AB,
mho/om.
mho/om
e.v.
2 0 x 10-l2 7 5 X 10-l8 15 X
8 0 x loo 18X 1 2 x 10-2
18 15 18
x
2 6
5 5X
8 0 X
2 5
2 0 X 4 0 X 10-14 6 0 X 4 5 X lO-’S 10 X
8 0 X 100 6 0 X 6 0 X loo 1 2 X 100 3 0 X
1 8 18 2 2 2 2 2 6
A. Side-chain substituents Polyazophenylene Polyazofluorene Polyazomethoxyphenylene Polyazochlorophenylene Polyazonitrophenylene
10
x
5 0
B. Backbone substituents Polyazophenglene Polyazostilbene Polyazophenylsulfone Polyazophenylether Polyazophenylsulfide
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was held constant a t 1100 v./cm. and the pressure at 90 kg./cm.2. Also, to assure that the results were reproducible, each compound was checked at least twice in the glass holder and also with the guarded electrode system. That the results from the two sample holders were within experimental error indicated that surface conduction was not a problem. To ensure that the materials were exhibiting ohmic conduction, all materials were checked for conductivity as a function of potential gradient a t a pressure of 90 kg./cm.2 and a temperature of 100'. The conduction of all inaterials was ohmic below approximately 1100 v./cm. and deviated from ohmic above 1100 v./cm. The conductivity as a function of pressure was also measured. For pressures slightly above 80 kg./cm.2, the variation of conductivity with pressure was negligible. Syntheses. A 0.1-mole sample of diamine starting material was placed, together with 27.8 ml. of concentrated hydrochloric acid (sp. gr. 1.19), 50 ml. of water and a quantity of cracked ice, in a 500-ml. erlenmeyer flask wrapped with aluminum foil. The ice was added to reduce the temperature of the resulting amine hydrochloride suspension to 0-5". The reaction mixture was stirred with a magnetic stirrer. A solution of 13.8 g. of sodium nitrite in 30 nil. of water was prepared and added by drops to the chilled amine hydrochloride solution (60-90 drops/min. from a dropping funnel). After the addition was complete, the mixture was allowed to react for 15-20 niin. during vigorous stirring. The temperature was held a t less than 5' by adding ice as required. A solution of 60 g. (0.6 mole) of cuprous chloride in 2 1. of concentrated ammonium hydroxide (29% ammonia) was prepared in a 4-1. beaker and chilled to less than 10' by the addition of approximately 500 g. of cracked ice. The beaker was placed on a magnetic stirrer, and stirring of the solution was initiated. The bisdiazoniuni salt solution was transferred from the erlenmeyer flask to a foil-wrapped dropping funnel, together with approximately 20 g. of cracked ice, and was added by drops to the cuprous complex solution. A precipitate formed at once, and gas was evolved with the formation of a stable foam. The rate of diazonium salt addition and the stirring rate were adjusted to prevent an overabundant accumulation of the foam. Where these measures proved insufficient, small quantities of ether were added as a deflocculant. After the addition of the diazonium salt was completed, stirring was discontinued and the temperature of the reaction mixture was allowed to rise slowly t o ambient. The precipitate was filtered from the The Journal of Physical Chemistry
D, M,CARLTON, D, K, MCCARTHY, AND R. H. GEM
mother liquor, digested with 600 nil. of acid solution (1 : 1 ratio of concentrated hydrochloric acid to water), refiltered, and thoroughly washed with water. The material was then dried a t 160'F. and set aside, pending purification. The above constitutes a general method for preparation of polyazophenylenes. Individual deviations from this method will be described in the discussions of products and starting materials. Products and StaTting Materials. Each material was synthesized a t least twice to allow checks for reproducibility. Two of the materials were synthesized once under nitrogen atmosphere to determine the effect, if any, of atmospheric oxygen upon the reaction products. KO effect of oxygen was observed. All products were recovered as solid powders. (1) From the starting material benzidine, m.p. 127-129', polyazophenylene, a dark brown solid powder, was obtained. Run I1 was done under a nitrogen atmosphere; yields: I (54%), I1 (47%). (2) From 3,3'-diniethoxybenzidine, m.p. 134.5-1 35 ', polyazomethoxyphenylene, dark brown in color, was recovered; yields: I (72.5y0), I1 (71%). ( 3 ) Polyazonitrophenylene, dark red in color, was synthesized from the starting material 3,3'-dinitrobenzidine which melts with decomposition beginning a t 208.5'; yields: I (65%), I1 (58%). (4) In the synthesis of polyazophenyl sulfide, a brown solid powder, run I11 was done under nitrogen atmosphere, and 0.05 mole starting material (4,4'dianiinodiphenyl sulfide m.p 105-107') was used for this run. Other quantities were scaled accordingly; yields: I (49.2%), I1 (52%), I11 (41%). (5) Polyazophenyl ether was recovered as a black solid with yields of 80.1% (run I) and 68% (run 11) from the starting material oxydianiline, which decomposes starting a t 158'. (6) From the starting material 4,4'-diaminodiphenyl sulfone, m.p. 158.5-160', polyazophenyl sulfone, dark red in color, was synthesized; yields: I (64%), I1 (69%). (7) I n the synthesis of polyazostilbene, a black solid, run I1 was performed using 0.05 mole of starting material (4,4'-diaminostilbene, m.p. 228.5-228'). All quantities were adjusted accordingly; yields: I (%%), I1 (61.5%). (8) Polyazofluorene, dark red-brown'in color, was obtained from the starting material 2,7-dianiinofluorene, 1n.p. 161-162'. Runs were limited to 0.05 mole. All quantities were adjusted accordingly; yields : I (52%), 11 (59.5%), (9) Polyazochlorophenylene, a brown solid powder, was synthesized from the starting material 3,3'-dichlorobenzidine which decomposes a t 145' ; yields: I (73%), I1 (67%). All yields reported were for crude product based on diamine starting material.
EFFECTOF STRUCTURE ON ELECTRICAL CONDUCTIVITY OF ORGANIC COMPOUNDS
Results The data calculated from the conductivity information, plus u at 100" (ulo0), are listed in Table I. (The points of inflection exhibited by compound (4)in Fig. 2 and by compound ( 2 ) in Fig. 3 are due to the thermal decomposition of these compounds as confirmed by differential thermal analysis. A11 calculations are based upon data obtained at temperatures below these decompositions.) Xo apparent correlation between A&! or urnand structure is observed. This is not surprising in view of the uncertainty of the significance of either AE or urn. There does, however, seem to be a qualitative correlation between ulo0 and structure. The data listed in Table I are divided into two groups, according to whether the structural alteration is in the backbone (B) or the side chain (A): The B group, composed of compounds substituted in the backbone, exhibits conductivities in the order expected, i.e., groups which are poor transmitters of electronic effects are observed to conduct more poorly than do systems in which the backbone substituent is more capable of transmitting electronic effects. (The conductivity data for polyazophenyl sulfone must be considered as uncertain because of the inordinate dependence of the conductivity on the field strength a t the value of field strength used for the other measurements in this study.) It is not surprising that the unsubstituted compound is the best conductor of the series, or that uloo for polyazofluorene (arbitrarily listed with the sidechain substituted compounds) is very nearly within the estimated experimental error (*20%) of the value for polyazophenylenle. During the course of the reaction, some nitrogen is inevitably lost and phenylphenyl linkages occur. Thus, even though the orbital overlap between phenyl rings in the fluorene nucleus should be better than phenyl-phenyl linkages, this enhanced orbital overlap is not manifested in the conductivity data because of the limiting effect of the phenyl-phenyl bonds which are poorer transmitters. I n the case of the compounds listed in the A group of Table I (in which the structural alteration is in the side chain) the interpretation of the data is somewhat more difficult. There are two possible effects opera,tive. The first is that suggested in the Introduction:
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namely, that electron-withdrawing groups should inhibit the conductivity mechanism. The second effect arises from the fact that nitrogen is lost from the molecule during polymerization, and that this loss gives rise to phenyl-phenyl linkages a t which the ortho position is substituted with a rather bulky group. ortho-Substitution a t a phenyl-phenyl bond is known to inhibit the orbital overlap between phenyl groups, and since the conductivity seems to depend upon the linkage in the backbone least able to transniit electronic effects, this linkage could well be the structural parameter responsible for the differences in conductivity. Several observations should be made concerning the data listed in the first group and the possible influence of the two effects mentioned. First, on the basis of the known electron-withdrawing power of the various substituents used in this study ( i e . , the Hamniett (Tconstants), one would expect the nitro-substituted compound to differ considerably from the inethoxy or chloro compounds. That this is not observed indicates something other than a simple dependence upon the electron-withdrawing effect of the substituent. Second, the data do seem to correspond qualitatively with the size of the substituents. However, qualitatively, one can argue a siniilar relationship based upon electron-withdrawing effects since the nitro-substituted compound is a poorer conductor than the inethoxy compound. Because the data do not point exclusively to either effect, it seems reasonable to assume that both the steric and the electronic effects are simultaneously operative.
Conclusion In these experiments, the variation in the electrical conductivity of a particular series of organic coinpounds as a function of the niolecular structure has been studied. The experiments have shown that the variation Is a function of structure and that the most important structural parameter is the degree of r-orbital overlap along a conjugated chain. In addition, the data indicate a possible contribution of the electron-withdrawing effect of the substituents.
Acknowledgment. The authors are grateful to Jar. J. S. Mills of Stanford Research Institute for his assistance with the e.p.r. studies,
v o l u m e 68, ivumber 9 September, 1964
