THE SEMI-CONDUCTIVITY OF ORGANIC SUBSTANCES PART 4.-*SEMI-QUINONE TYPE MOLECULAR COMPLEXES BY D. D. ELEY, H. INOKUCHI~ AND M. R. WILLIS University of Nottingham, Nottingham, England Received 14th May, 1959 The electrical conductivities of some molecular complexes of the donor-acceptor type between aromatic amines and halogenated p-benzo-quinones have been examined by both a.c. and d.c. methods. The complexes were found to behave as semiconductors with an energy gap of approximately 0 5 eV. Complexes with a stronger electron donor show a much enhanced conductance and marked deviation from Ohm’s law. These results are discussed in relation to the optical and magnetic properties of these compounds. The semiconducting properties of organic molecular crystals have been ex- tensively studied in the last decade.In resonating aromatic systems the conduction has been attributed to activated rr electrons.1 The electrons and/or positive holes have been considered as moving in energy bands common to the whole crystal, on tunnelling through the potential barriers between molecules.2s The inter- molecular forces in this type of crystal are of the van der Waals type and result in intermolecular distances of about 3.5 A. Molecular complexes, on the other hand, are substances in which the bonding is frequently stronger than pure van der Waals forces. Such interaction has been treated quantum mechanically by Mulliken.4 Complexes between halogen molecules and polycyclic hydrocarbons show a very high electronic conductivity and semi-conduct with very low 59 6 activation energies (A€ = 0.1-0.2 eV).The formation of these complexes is accompanied by changes in magnetic properties.7~ 8 Analogous results for complexes between anthracene and alkali metals and for sodium and bromine with nitrogen heterocyclic compounds have been obtained by Ubbelohde.9~ 10 Weiss 11 has considered the possibility of ionization in complexes between quinones or nitro compounds and aromatic hydrocarbons. The magnetic im- plications of such bonding have been investigated by paramagnetic resonance methods 12 and by the magnetic balance.13 These findings are in accordance with a low energy ionic state in favourable cases. This paper describes an investigation into the energetics and mobility of elec- trons in charge transfer complexes between halogenated quinones (acceptors) and aromatic amines (donors), by electrical methods. The effect of the strong inter- molecular (or ionic) forces and the crystal structure 14 are discussed in relation to the electron mobility.It is hoped that investigations of this kind will lead to a better understanding of the mobility and spin-resonance of electrons in organic crystals and their bearing on catalytic activity.15 In addition, the mobility of electrons resulting from the charge transfer process has been suggested as one of the processes of energy transfer in living organisms.16 A preliminary publication of some of the work reported in this paper has been made elsewhere.17 * part 1 : ref. (1) ; part 2 : ref. (2) ; part 3 in Faraday SOC. Discussions on “ Energy transfer with special reference to Biological Systems ”, Nottingham, April, 1959.-f present address : Department of Chemistry, University of Tokyo, Japan. 54D . D . ELEY, H. INOKUCHI AND M. R. WILLIS EXPERIMENTAL RESISTANCE/TEMPERATURE MEASUREMENTS BY THE D.C. METHOD 55 Preliminary experiments showed that the solid complexes decomposed rapidly under vacuum due to the evaporation of the more volatile component (the base). The effect was also very marked in air, as shown by a rapid increase in resistance, and a change of colour to that of the quinone. To obviate this effect, the freshly prepared crystals A (fig. 1) were compressed in a tube of insulating material B between two tightly fitting electrodes C. The sample was then effectively in a closed system in which it can reach equilibrium with its vapour.The sample attained a constant resistance in 4-2 h. Measurements on the NN-dimethyl aniline complexes were made in the cell described by Inokuchi.5 Measurements on the NNN’N’-tetramethyl p-phenylene diamine complexes were made in a slightly modified cell, as shown in fig. 1. A pressure of 80 kg cm-2 was appiied by means of a nickel Monk spring D to minimize intercrystalline resistances. The cell, together with a thermometer, was placed in a screened glass tube and surrounded by a Dewar vessel. Owing to the low melt- ing points and instability of the complexes, resistance measurements were made as the samples were cooled below room temperature with solid C02. The resist- ance was measured using a valve voltmeter whilst the specimen was cooled at the rate of approximately 1 deg./min.The specimen was recovered in the form of a tablet and its dimensions determined with a micrometer. RESISTIVITY BY THE A.C. METHOD The method used was that described by Eley and Parfitt 2 using a Marconi Q-meter. This depends on the fact that the equivalent circuit of a powder may be regarded as a resistance p3 (of the crystals themselves) in series with a parallel arrangement of a condenser C2 and a resistance p2 (typical of the intercrystalline gaps). A measurement of the full and empty cells was taken at a range of frequencies between 50 kc/sec and 20 Mc/sec. The calculated resistance of the powder itself was then found to fall to a constant value p3, at high frequencies, characteristic of the crystal.A typical result is shown in fig. 2 Owing to the gradual decomposition of the samples and to the difficulty of maintaining specimens at constant temperatures below room temperature, it was not found possible to do temperature/resistance measurements. However, as a comparison with the d.c. results, resistivities at room temperature were measured. The complex between iodanil and NNN’N’-tetramethyl p-phenylene diamine was unstable at room temperature and so the measurement was made at a lower temperature (7°C) and the value corrected to room temperature. The cell shown in fig. 1 was again used. For the run on the empty cell, the electrodes were separated by a tablet of Perspex of the same dimensions as the sample. This Perspex disc had a resistance some 106 times that of the specimen.l--The conductivity Cell for a.c- and d-c. measurements on molecular complexes. PREPARATION OF MATERIALS CHLoRANIL.-The commercial product was recrystallized twice from benzene. BROMAN1L.-The commercial product was recrystallized twice from benzene. IoDANrL.-Prepared by the method of Jackson and Bolton 18 and recrystallized twice NN-DIMETHYL ANILINE (DMA).-The commercial product was distilled twice under NNN’N’-TETRAMETHYL p-PHENYLENE DIAMINE (TMPD).-The dihydrochloride was The free base was obtained by treating an aqueous solution of from benzene and once from acetone. reduced pressure to give a colourless liquid. obtained from B.D.H.56 SEMICONDUCTIVITY OF ORGANIC SUBSTANCES dihydrochloride with dilute NH40H. The white precipitate was filtered, washed with distilled water and dried overnight in a vacuum desiccator.The white powder was then dissolved in ether, filtered and distilled in ‘uacuo. COMPLEXES OF “-DIMETHYL ANILINE.-The quinone was dissolved in dimethyl aniline and one-fifth of its volume of alcohol added to precipitate the crystals. The product was dried at room temperature. loglo frequency FIG. 2.-A.c. measurements on the TMPD-bromanil complex. (a) corresponds to measurements on the empty cell, (6) to the cell plus sample, and (c) to the sample alone. COMPLEXES OF NNN”’-TETRMTHYL p-PHENYLENE DImIm.-The base was dis- solved in benzene and a solution of the quinone in benzene added.27 The crystals were filtered and washed with benzene and ether. With the iodailil complex, the low solubility of it in benzene made this method unsuitable and acetone was used.All the complexes were shown by analysis to contain a 1 : 1 ratio of components. RESULTS RESISTIVITES AT ROOM TEMPERATURE BY THE A.C. METHOD COMPARED WITH THOSE BY D.C. Both measurements were made on the same specimen in order to provide the most direct comparison. The resistivities include a packing factor of 0.5 to correct approx- imately for the volume fraction of voids in the packed polycrystalline samples.15 RESISTANCE/TEMPERATTJRE MEASUREMENTS for intrinsic semiconductors, The variation of resistance with temperature was found to obey the usual equation p = po exp (Ac/2kT). Typical runs are shown in fig. 3 and 4. DEVIATIONS FROM OHM’S LAW Fig. 5 and 6 show the variation of current with potential gradient for one complex from each series, namely, bromanil-NN-dimethyl aniline and bromanil-NN”N-tetra- methyl p-phenylene diamine.Owing to the low resistances of the samples, high potential- gradients were not employed as they would lead to heating effects. To minimize thermalD. D. ELEY, H . INOKUCHI AND M. R . WILLIS 57 1/T ( x lO4), Tin O K dimethyl aniline, (c) iodanil-dimethyl aniline. FIG. 3.-The d.c. conductivity of complexes. (a) chloranil-dimethyl aniline, (b) bromanil- 8 p! Y 9 6 .- rn 4 FIG. 4.-The d.c. 38 39 4 0 41 33 34 35 36 104/T, Tin O K TMPD, (c) iodanil-TMPD. conductivity of [complexes. (a) chloranil-TMPD, (b) bromanil-9 8 7 6 5 4 3 2 I 0 I f 1'1 10 I 1 L I ~ 100 200 300 400 500 600 700 000 - - potential gradient, V/cm FIG.5.-Variation of current with potential gradient for bromanil-DMA. Curve (b) shows the effect of reaching electrical and thermal equilibrium at each point. W 2 s X n U / / V I I I I 0 100 200 300 400 500 600 potential gradient, V/cm bromanil-TMPD. FIG. 6.-Variation of current with potential gradient forD . D . ELEY, H. INOKUCHI AND M. R. WILLIS 59 effects, the sample was allowed to reach equilibrium at the highest voltage used. The complete run was then rapidly performed. Polarization was quite marked, and the effect of allowing the resistance to reach equilibrium at each potential is shown in fig. 5. Me-N-Me A II I \/ Me-N-Me I I Me-N-Me TABLE SP SPECIFIC RESISTANCE OF THE COMPLEXES c1- Cl Br -FBr 5 x 107 9 x 107 3 x 107 8.1 x 108 1.5 x 109 1.7 x 108 1.3 x 104 4 2 x 104 1.1 x 105 2.0 x 104 1.3 x 105 1.5 x 106 parameter resistivity p a.c., l2 cm at 22°C resistivity p d.c., Q cm at 22°C resistivity p ax., f2 cm at 22°C resistivity p d.c., L2 cm at 22°C TABLE 2.-ENERGY GAP, AND PRE-EXPONENTIAL FACTOR Me-N-Me 1 0.47 0-45 0.43 AE, eV (1 0-93 X 104 2.1 x 105 2-9 x 104 PO, Q Cm \ Me-N-Me 0-5 3 0.59 0.56 2-21 0.59 14.15 DISCUSSION The complexes have been shown to be relatively good organic semiconductors. Complexes of this type have been investigated by X-ray diffraction and shown to have crystal lattices built up of stacks of alternate D and A molecules.~4~ 19 Here the electron donor D is the amine, the acceptor A the quinone.A rough relation- ship between energy gap and number of 7~ electrons allows us to predict a A€ of about 5 eV for crystalline benzene, the resistivity being immeasurably large.;! By contrast, the alternate arrangement of aromatic quinone and halogenated quinone molecules have energy gaps of about 0.5 eV and resistivities at 22°C (measured by ax.) of 104 to 107 i2 cm.The complexes must therefore fall into a different class of semiconductors from the aromatic molecules. In fact, their energy gap and resistivity approach that of the solid monoradical, diphenyl picryl hydrazy1,Z. 15 which has h e = 0.16-0-26 eV and p 15°C = 1.7 x 108 L? cm and it seems probable60 SEMICONDUCTIVITY OF ORGANIC SUBSTANCES that the conduction mechanisms are similar. The hydrocarbon + halogen com- plexes are even better conductors, e.g. violanthrene-iodine 5.6 has A6 = 0.14 eV and resistivity at room temperature 45 i2 cm.Both the amine-quinone and the hydrocarbon-halogen complexes fall into the class of charge transfer complexes recently investigated in detail by Mulliken.20~ 21 This author describes these complexes (in solution) in term of a ground state, wave function $p~, largely made up of the van der Waals complex # (A,D) but with a small amount of ionic resonance hybrid # (A-D+), i.e., $N = a#(A,D) + b$(A-D+), a > b. The characteristic colour is attributed to excitation to the first excited state of the complex, $E, mainly the ionic state, plus a small amount of van der Waals resonance hybrid. $IZ = a*$(A-D+) - b*#(A,B), a* > b*. This description may apply to the three dimethylaniline complexes in solution, and possibly in the solid state, since the crystals have been shown to be non- paramagnetic in a sensitive electron-resonance spectrometer.22 On the other hand, the three tetramethyl p-phenylene diamine solid complexes show a strong electron resonance, the percentage free radical content being 0.2 % for the chlor- anil, 2.0 % for the bromanil and 20 % for the iodanil complex.12922 This corresponds to a high degree of stability of the ionic state, which has been stated to predominate in certain complexes.13'23 These results may be explained in terms of two completely independent ions, behaving as a diradical (A-224, D+ 2,&), or in terms of a certain overlap of electronic wave functions giving rise to a triplet state (A-D+3&).However, the population of the triplet state should depend upon temperature, while the percentage of diradicals has been found independent of temperature 22 (by comparison of the electron-resonance signal with that from a stable solid monoradical).The Mulliken theory has been developed for DA pairs in solution, and the extension to the crystalline state is not obvious. It is possible that increased neighbour contacts will favour Coulombic interactions and tend to stabilize the ionic diradical state. We note that in going from the van der Waals type dimethyl- aniline complexes to the ionic diradical type tetramethyl-p-phenylene diamine type there is a 102-104 times decrease in resistivity, in spite of a small increase in A€, That is to say, changing to the stronger donor molecule increases the conductance at the same time as the fraction of diradicals present.It is interesting that Matsunaga 8 has recently reported 14 % of free radical character for the violanthrene-iodine complex, with similar results for related systems. This supports our suggestion to associate free radical character of the crystal with the observed high conductivity. However, this model alone cannot account for differences between the specific complexes; thus the order of in- creasing conductivity, namely, TMPD-iodanil (20), < TMPD-bromanil (2), < TMPD-chloranil (0.2), < violanthrene-iodine (1 4) clearly does not correspond to the percentage free-radical character, given in the brackets. THE FREE ELECTRON MODEL We shall now adapt the model already used to explain conductivity in aromatic molecules and D.P.P.H.2 to charge transfer complexes.In this model the con- ductance process is separated into 2 stages. (a) A molecule containing n r electrons possesses on molecular orbital theory n energy levels, the lower n/2 levels being filled each with 2 electrons with paired spins. An electron is first excited from the highest filled ( 4 2 ) level to the lowest unfilled ( 4 2 + 1) level, and the excitation energy AE is equated to the observed energy gap. Considering 4 adjacent molecules, only, for simplicity, AAAA -+ AA*AA.D . D. ELEY, H. INOKUCHI AND M. R. WILLIS 61 (b) From this level, the electron may tunnel through the intermolecular potential barrier to the corresponding level in the crystal. The tunnelling process determines the electron mobility, and hence the pre-exponential resistivity factor PO.AA*AA -+ AA+A-A -+ AA+AA-. Charge separation may occur against the Coulomb attractive energy, since the charges will be stabilized by polarization of the surrounding T electrons, the 7-r electrons effectively screening the positive and negative changes from each other. Where there is a single unpaired electron in the uppermost filled level, as in the solid monoradical DPPH, tunnelling occurs without the need for excitation energy At-, which in fact is very small. D+ A- (4 FIG. 7.-The free electron model for molecular complexes. This model is adapted to crystalline donor-acceptor complexes in fig. 7. This gives a diagrammatic representation of adjacent donor and acceptor molecules, treated as one-dimensional potential energy boxes, the T electron energy levels for simplicity being drawn as equally spaced although this is never actually the case (cf.a treatment of complex formation by Shuler25). In (a) we show the van der Waals D,A molecule pair. The zero of energy is taken as the electron at infinity and the two boxes are placed side by side in accordance with this convention. Let us now imagine an electron transferred from D to A, to give a D+A- pair (b). The lowering of energy due to Coulombic interaction e2/r is expressed by lowering the energy box corresponding to A. Taking I D for the ionization potential of the donor, and EA for the electron affinity of the acceptor, and r as the separation of the molecules, we have : DMA acceptor, I,, - EA - e2/r = 7.2 - 1.0 - 4.1 = 2.1 eV, (r = 3-40A); TMPD acceptor, ID - EA - e2/r = 6.7 - 1-0 - 4.5 = 1.2 eV, (r = 3*26&.The ID values are from Foster 26 and the Y values from Wallwork.14 On this62 SEMICONDUCTIVITY OF ORGANIC SUBSTANCES view there will be a greater tendency for the TMPD-complexes to be ionic. We have assumed an electron affinity of 1.0 eV for the halogenated quinones, but this will of course vary over the series. If we knew the Madelung constant 2, we could estimate the Coulombic energies in the lattice. A value of 1.75 as for the ordinary rock salt lattice would give Coulombic energies of 7.2 eV for the DMA complex and 7.9 eV for the TMPD complex, giving overall energies of formation of - 1.0 eV and - 2.2 eV respec- tively, and making the acceptor level lower than the donor level in both cases.In fact, it must remain higher than the donor level to explain the diamagnetism of the DMA complexes. The situation for the two types of complex is shown in (c) and (d). For the DMA complexes (c), the electrons remain paired in the highest level of the donor and the energy gap A€ for semiconductivity is the energy to raise the electron to the lowest unfilled acceptor level. In the TMPD complexes (d), one electron may pass over the higher D level to the lower A level and in fact one has stacks of ion radicals, all in 2 2 states. Departures from 100 % radical character may be associated with a small degree of overlap of wave functions, and a certain degree of electron pairing as a result. rhe electronic conduction in this system should be analogous to that in the solid monoradical DPPH.One may visualize electron transfer at the highest filled levels, for example, The energy gap should be related to AE'. On this picture, the energy gaps A€ and A& are really determined by the degree of matching of the levels in donor and acceptor which result from Coulombic interaction. A more significant factor for the conductance increase on passing from DMA to TMPD complexes may be the decrease in intermolecular spacing, estimated as from 3.40A to 3.26A. This should result in a more transparent barrier to electron tunnelling, and the marked decrease in po observed should result from this. It is possible also that the more marked non-ohmic behaviour of the TMPD complexes is to be associated with differences in barrier dimension, from-the DMA complexes. D+A-D+A- --f D+ADA- -+ D+AD+A".SURPACB PARAMAGNETISM In our preliminary publication 17 we gave rate constants for the ortho-para H2 conversion on the dimethylaniline complexes at - 183°C which were definitely indicative of surface paramagnetism. Thus, translating these figures into absolute rates, of molecules cm-2 catalyst sec-1, we have the figures DMA-chloranil 0.64 x 1010, DMA-bromanil 1.29 x 1010 and DMA-iodanil 1-61 x 1011. The corresponding figure 15 for DPPH is 6-47 x 1010 at - 183°C. (An error in the ref. (15) quoted is noted : the first-order constant for the conversion on DPPH at 17°C of 2.58 x 10-3 min-1 in fact should correspond to an absolute rate of 8.2 x 109 molecules cm-2 catalyst sec-1 and a collision efficiency of 1.14 x 10-12, not 3.3 x 1012 and 4-6 x 10-10,) Thus within the experimental error associated with estimation of surface areas, the DMA-iodanil surface is a 100 % free radical surface, although no bulk paramagnetism was detectable in electron resonance.In further work we have found these complexes and also the TMPD complexes (which latter do show bulk paramagnetism) ineffective in converting ortho to para hydrogen. We have concluded that the surface paramagnetism must be deter- mined by the conditions of preparation, and the matter is still under investigation. There is published evidence 3 for a variable degree of bulk paramagnetism depending on preparative conditions, so this suggestion is not unreasonable.The authors' best thanks are due to the Ramsay Memorial Fellowships Trust for the award of a Japanese Fellowship to H. Inokuchi, and to the British Petroleum Ltd., for a studentship awarded to M. R. Willis and for a grant for purchase of apparatus.D. D. ELEY, H. INOKUCHI AND M. R. WILLIS 1 Eley, Parfitt, Perry and Taysum, Trans. Faraday SOC., 1953, 49, 79. 2 Eley and Parfitt, Trans. Furuduy SOC., 1955,51, 152. 3 Mette and Pick, 2. Physik, 1953, 134, 566. 4 Mulliken, Rec. truv. chim., 1956, 75, 845. 5 Akamatu, Inokuchi and Matsunaga, Nature, 1954, 173, 168. 6 Akamatu, Inokuchi and Matsunaga, Bull. Chem. SOC. Jupan, 1956,29,213. 7 Matsunaga, Bull. Chem. SOC. Jupan, 1955, 28,473. 8 Matsunaga, J. Chem. Physics, 1959, 30, 856. 9 Holmes-Walker and Ubbelohde, J . Chem. Soc., 1954, 720. 10 Slough and Ubbelohde, J. Chem. SOC., 1957, 982. 11 Weiss, J. Chem. SOC., 1942, 245. 12 Kainer, Bijl and Rose-Innes, Naturwiss., 1954, 41, 303. 13 Kainer and Uberle, Ber., 1955, 8, 1147. 14 Wallwork, lecture to X-ray Analysis group (Inst. of Physics, Spring, 1958). 15 Eley and Inokuchi, 2. Electrochem., 1959, 63, 29. 16 Szent-Gyorgyi, Introductory lecture : Faraday SOC. Discussion on " Energy Transfer 17Eley and Inokuchi, 3rd Biennial Carbon Conference Bufalo (N.Y., 1957), to be 18 Jackson and Bolton, J. Amer. Chem. Soc., 1914, 36, 301. 19 Harding and Wallwork, Acta Cryst., 1955, 8, 787. 20 Mulliken, J. Amer. Chem. SOC., 1952, 64, 811. 21 Mulliken, J. Physic. Chem., 1952, 56, 801. 22 BijI, Kainer and Rose-Inns, J. Chem. Physics, 1959, 30, 765. 23 Kainer and Otting, Ber., 1955, 12, 1921. 24 Schneider, Radiospectroscopy Group Meeting (Southampton Univ., 1957). 25 Shuler, J. Chem. Physics, 1952, 20, 1865. 26 Foster, Nature, 1959, 183, 1253. 27 Schlenk and Knorr, Annalen, 1909, 368,277 63 with Special Reference to Biological Systems " (Nottingham, 1959). published by the Pergamon Press.