PARAMAGNETISM AND SEXICONDUCTIVITY IN DOPED CHARGE-TRANSFER COXPLEXES
1849
Paramagnetism and Semiconductivity in Doped Charge-Transfer Complexes by Mufaro J. Hove, Brian M. Hoffman,*l and Robert J. Loyd Department of Chemistry and ;Materials Research Center, Northwestern University, Evanston, Illinois 60201 (Received August 23, 1971) Publication costs assisted by the Advanced Research Projects Agency of the Department of Defense
We have studied the epr and conductivity of the nonionic charge-transfer (D-A) complexes carbazole- tetracyanoquinodimethaneand carbazole-2,3-dichloro-j,B-dicyanobenzoquinone. These complexes are intrinsically nonionic and therefore diamagnetic, but they exhibit weak, temperature-dependent paramagnetism due to the presence of paramagnetic impurities. Interactions among unpaired impurity spins result in a nonmagnetic ground state and paramagnetic excitations. The complexes behave as semiconductorswith activation energy for conduction greater than that for the creation of the paramagnetic spins, but doping the complexes with A-, analogous to n doping in inorganic semiconductors,suggests that both the conductivity and the paramagnetic properties are associated with the unpaired electrons on the A- anions.
Introduction The continued interest in the conductivity of organic charge-transfer complexes has involved attempts to establish some relationship between the paramagnetic electrons and the charge carriers responsible for the observed conductivity, but up to now no universal relationship has been found between the epr and electrical parametem2 There are two types of organic donor-acceptor (D-A) complexes: paramagnetic ionic (DfA-) and diamagnetic nonionic (DA).3 I n this notation D represents an electron donor and A an electron acceptor. The resistivities of these systems are in the range of 1014-10-2 ohm em, and most of the complexes show a temperature-dependent conductivity characteristic of semiconducting materials. I n general, the ionic complexes show higher conductivities than the nonionic ones. Our report deals with the epr and conductivity studies on the charge-transfer complexes of the nonionic type (DA), carbazole-tetracyanoquinodimethane and carbazole-2,3 - dichloro - 5,6 - dicyanobenzoquinone. These complexes are intrinsically nonionic and therefore diamagnetic, but they exhibit weak paramagnetism due to the presence of paramagnetic impurities. Temperature-dependent studies of the epr signal intensity show that interactions among the unpaired impurity spins result in a nonmagnetic ground state and paramagnetic excited state. The complexes behave as semiconductors with activation energy for conduction greater than that for the creation of the paramagnetic spins, but doping the complexes with A-, analogous to n doping in inorganic semiconductors, suggests that both the conductivity and the paramagnetic properties are associated with the unpaired electrons on the anions.
Experimental Section ( a ) Preparation of Complexes. Carbazole (CARB), tetracyanoquinodimethane (TCKQ), and 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) were purified by sublimation. The complexes (CARB-TCKQ and CARB-DDQ) were prepared by mixing equimolar solutions of the donor (CARB) and acceptor (TCKQ, DDQ) in boiling chloroform. The dark solution was allowed to stand at room temperature for several hours. CARB-TCNQ was obtained as a dark blue crystalline powder and CARB-DDQ was obtained a s - a green crystalline powder. Elemental analysis indicated that both complexes (CARB-TCKQ and CARB-DDQ) are 1: 1. Li+DDQ- and Li+TCKQ- m r e prepared by mixing hot solutions of Li+I- and DDQ and TCNQ in hot acetonitrile. The doped complexes were prepared by adding varying small amounts of Li+A- to the hot reaction mixkure immediately after the donor and acceptor solutions were mixed. ( b ) Measwements. Infrared-Visible Absorption. Absorption spectra of the complexes in the infrared region were made on the Beclzman IR-5 infrared spectrophotometer in KBr pellets. Visible absorption spectra were cxamined in KBr pellets and dilute acetonitrile solution, using a Cary 14 recording spectrophotometer. The lithium salt of the acceptors (Li+TNCQ- and Li+DDQ) and the neutral molecules of the donor and acceptors were used as references. Electron Paramagnetic Resonance ( E p r ). Epr measurements mere carried out with a Varian Model V4500-10A X-band spectrometer at 100-kHz field modulation. The temperature was controlled by passing heated or cooled nitrogen gas about the sample tube and measured with a thermocouple with a precision of 1°K. (1) Alfred P. Sloan Fellow. (2) (a) A comprehensive review is F. Gutmann and L. E. Lyons, “Organic Semiconductors,” Wiley, New York, N. Y., 1967; Y. Okamoto and Walter Brennen, “Organic Semiconductors, Reinhold, New York, N. Y., 1964. (3) H. AM.McConnell, B. M. Hoffman, and R. $1. Metzger, Proc. Kat. Acad. Sci. li. S., 53, 46 (1965).
(b!
The Journal of Physical Chemistry, Vol. 76, N o . IS, 1073
1850 The epr studies were done on powder systems, since the signal intensity was insufficient for single-crystal work. The g values of the complexes were measured using the dual-cavity technique with a dilute aqueous solution of potassium peroxylamine disulfonate (g = 2.0054, U N = 13.0 G) as reference. The microwave frequency was measured with a Hewlett-Packard X-532-B frequency meter. The g values, determined from the appropriate features of the powder spectrum, were checked by computer simulation with a program which assumes random orientation of spins and performs a double numerical integration over an octant of the unit sphere, utilizing a Gauss-Legendre integration in 0 and a GaussTschebyschev integration in @. The extrema1 features in the simulation are sensitive to relative changes of the input g values of 4 3 X lo+, which is smaller than the real accuracy of the measurement which is estimated to be *0.0005. The program could use either Lorcntzian or Gaussian line widths, which for simplicity were assumed to be isotropic. Studies of the temperature dependence of the epr signal intensity were performed over the range 150373°K. The g-value anistropy was removed by overmodulation and the intensity was taken to be proportional to the square of the width of the resulting line times its height. The absolute number of spins was determined by comparing the integrated absorption intensity of the sample t o that of a known amount of DPPH dispersed in KBr. The effect of oxygen on the paramagnetism of the complexes was determined by observing the change in the epr signal intensity before and after introducing oxygen into the sample tube. Conductivity Studies. The resistance of compressed pellets was measured as a function of temperature. The pellets were made on B 0.5-in. diameter die, using an applied force of 10 tons. A suspension of colloidal graphite in methanol was applied to the surface of the pellets to ensure good contact between the electrode plates and the pellet. The apparatus consists of a, Victoreen n!todel 5010 operational amplifier, wired as a current-to-voltage transducer with a voltage gain of two. The feedback resistors were housed in a Keithley shielded switch, Model 3011, with all co-ax fittings removed, bolted directly to the amplifier. The feedback resistors were Victoreen glass units, ranging from 10ls t o lo6 ohms. Each resistor had a capacitor in parallel to give a time constant of 10 sec. The (-) input was connected to the high-impedance electrode with a rigid rod, and the low-impedance electrode voltage was determined by a potentiometer between ground and the regulated 15-V power supply of the operational amplifier. The resistance of the compressed pellet was found by subjecting the pellet to a 15-V potential and measurThe Journal of Physical Chemistry,VoE. 76,NO.IS,1978
M. J. HOVE,B. M. HOFFMAN, ANDR.J. LOYD CM’l 5000
2
3000
5
’300
8
1000
c1
11
800
650
14
,
,
.
._
Figure 1. Infrared spectra of (A) CARB-TCNQ, (B) L i T C N Q - , (C) TNCQ, and (D) CARB in KBr pellets.
Figure 2. Visible spectra of (A) CARB, (B) TCNQ, (C) CARB-TCNQ, and (D) Li+TCXQ- in KBr pellets.
ing the current from the potential drop across the known feedback resistance. Temperature-dependent measurements were carried out by placing the electrodes in a copper chamber which was then immersed in a slush bath. The copper chamber was continuously swept with dry nitrogen gas to avoid condensation of water vapor on the sample and electrodes.
Results The infrared and visible spectra of CARB, TCNQ, CARB-TCNQ, and Li+TCNQ are shown in Figures
PARAMAGNETISM AND SEMICONDUCTIVITY IN DOPED CHARGE-TRANSFER COMPLEXES
1851
I
I 4.0
3.0
5.0
10) I T
Figure 4. Plot of In arbitrary units.
I I
I
I I I
I I 1 I
I
1 and 2. The infrared bands of CARB-TCNQ and CARB-DDQ in the 2-12-p region (stretching modes) are a superposition of the spectra of the neutral donor and acceptor molecules and not of the ions, indicating that thcse complexes are nonionic. Because of the usual face-to-face stacking of the aromatic molecules in charge-transfer systems, the bending modes of the complexes are not expected to be the same as those of the neutral donor and acceptor molecules. The visible spectroscopic measurements also indicate that the bands appearing in the spectra of CARB-TCNQ are a superposition of the spectra of the individual donor and acceptor neutral molecules (Figure 2 ) . The one new feature, the band appearing a t 550 p in Figure 2, is the charge-transfer band,4 representing an electronic excitation from a nonionic (DA) t o an ionic (D+A-) state. These complexes, though nonionic, are weakly paramagnetic. Figure 3A gives a representative spectrum from a CARB-TCNQ sample, and Figure 3B gives the computer simulation using the g values listed in Table I and using a Lorentzian component line width of 0.11 G. .Although an anisotropic line width could
(OK)
[IT]us. 5"-' for CARB-TCNQ, I in
have improved the fit to the intensity of the signal a t gar the calculated field positions for the maxima agree. Simulation further shows that use of a Gaussian component line is inappropriate. For isolated spins of concentration n, the signal intensity would follow Curie's law, I a n / T . Organic crystals in which antiferromagnetic interactions between spins are important often exhibit a nonmagnetic ground state and magnetic excited states with creation energy E , and an excitation density P ( T ) . ~I n such a case
I Figure 3. Room temperature electron paramagnetic resonance spectrum of CARB-TCNA: (A) experimentall (B) computer simulation; 8 values from Table I, Lorentsian component line width 0.11 G.
6.0
a
P(T)/T
(1)
with P(T)
= P(a)
(2)
exP[--E,lkTl
when E, > kT. The validity of this equation for the system studied here is shown by linear plots of In [ I T ] vs. T-I, such as in Figure 4 for undoped CARBTCNQ. Observed values of E, are listed in Table I. The temperature-dependent studies thus show that the interactions of the unpaired electrons in these
Table I : Epr and Electrical Parameters CARB-TCNQ
E,, eV el eV P( a )/mol ula (ohm cm)-l 81 Sa 83 8av
Bsolnb
b
0.05 f 0.01 0.45 f 0.20 0.93-3.5 x 1020 1.9-7.1 X 10-10 2.0029 2.0026 2.0021 2.0025 2.0025
CARB-DDQ 0.040~0.01 0.45 f 0 . 0 2 1 . 8 - 7 . 8 X 10'' 1 . 5 - 6 . 5 X lo-'' 2.0074 2.0050 2.0019 2.0049 2.0050
a Dependent on doping; values are for room temperature. Li+A- in acetonitrile solution.
(4) J. Rose, "Molecular Complexes," Pergamon Press, Elmsford, N. Y . , 1967. (5) 2.G. Soos, J . Chem. Phys., 46, 4284 (1967) The Journal of Physical Chemijtry, Vol. 76, No. 18, 1972
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M. J. HOVE,B. M. HOFFMAN, AND R. J. LOYD
complexes lead to a nonmagnetic ground state, with thermally accessible paramagnetic excited states. The spin concentrations per mole were measured at room temperature for both complexes (Table I) and, together with the value of E,, allow us t o calculate p( m ) using eq 2. The small values of p( m ) prove again that these complexes are basically nonionic and nonmagnetic, since for a completely ionic system p( a) should be equal to two times Avogadro’s number. The impurity spins are not due to adsorbed oxygen, since introducing oxygen into a degassed sample resulted in an irreversible decrease of the epr signal intensity. Doping the complexes increased p( 05) without changing E,, the line shape, or line widths of the spectral lines. This suggests that A- impurity centers are responsible for the observed paramagnetism. This view is supported by the fact that the average g value of each complex is equal t o the solution g values of the corresponding LiA- (Table I). The temperature-dependent studies of the conductivity of compressed pellets show that these complexes are semiconductors u =
u(m)
exp(-@BT)
Figure 5. Plot of In u us. T-I for CARB-TCNQ (upper) doped, (lower) undoped.
(3)
u( a ) is the conductivity a t infinite temperature and E is the activation energy for conduction (Figure 5 ) . An e value of 0.45 f 0.02 eV was obtained for complexes with both acceptors. The temperature-dependent studies were performed in the temperature range 178-298°K. Doping the complexes increased the conductivity without changing the activation energy for conduction, as is shown in Figure 6. As a means of relating the increases of both p ( m ) and u( a ) upon doping, Figure 6 gives a plot of log .(a) vs. log p ( a ) for CARB-TCNQ. The result is a straight line of slope 1.08, with standard deviation 0.07 (Figure 6, which suggests that the charge carriers and the magnetic spins have the same origin.
Discussion The Epr Results show that the impurity A- ions are the source of the observed paramagnetism. The proportionate increase in the epr signal intensity and conductivity on doping with Li+A- indicates that the A- ions are involved in both processes. Two possible models for the nature of the impurity centers are the distributive model and the cluster model. I n the cluster model the impurity centers would occur in relatively large domains. However, proportionality of .(a) and p ( a ) makes this model unattractive, since a linear increase of both quantities is not expected in this case. The distributive model views the interacting impurity spins as essentially randomly distributed throughout the solid. Spin exchange among the excited magnetic spins is small, since the system is dilute, and is therefore not expected to dominate the line width. The The Journal of Physical Chemistry, Vol. 76,No. IS, 1972
to3 T P K I
L N u (a)
Figure 6 : A plot of In p ( m ) us. In u( m ) on doped samples of CARB-TCNQ.
observed narrow line widths would primarily be due to the delocalization of the spins along a chain. The estimated line width 6w for an electron delocalized over N molecules is6 6w
-
A/N‘/~
(4)
-
where A is the line width for a localized spin. Calculations for CARB-TCKQ with A 10 G, roughly the total width of the TCNQ- hyperfine pattern, give an N of the order of lo3molecules. I t is not possible from the powder epr results t o build a convincing model for the interactions which cause the pairing of the impurity spins. Nevertheless, these results do show that it is possible to % dope” an organic semiconductor in a manner analogous t o that of inorganic semiconductors. Since “p doping” has been observed in organic charge-transfer com(6) D. D. Thomas, A . W. Merkl, A . F. Hildebrandt, and H. M. MoConnell, J . Chem. Phys., 40, 2588 (1964).
MUTUAL-DIFFVSION COEFFICIENTS IN AgK03-H20
1853
p o u n d ~ it, ~appears ~~ that the electrical properties of organic compounds are amenable to chemical alteration.
the Department of Defense through the Northwestern University Materials Research Center.
Acknowledgment. We wish to thank Dr. D. F, shriver for discussions‘ This work was ported by the Advanced Research Projects Agency of
(7) A. Rembaum, A. M. Hermam, F. E. Stewart, and F. Gutmann, J . Phys. Chem., 73, 513 (1969). (8) J. H. Lupinski, K. D. Kopple, and I. J. Hertz, J . Polym. A!%., 1561 (1967).
Mutual-Diffusion Coefficients at 25” in the System Silver Nitrate-Water1 by John G. Albright and Donald G. Miller” Chemistry Department, Lawrence Livermore Laboratory, Livermore, California 9.4660
(Received June 7, 1971)
Publication costs assisted by Lawrence Livermore Laboratory, U.S.Atom.ic Energy Commission
Volume-fixed mutual diffusion coefficients D, have been determined by the Rayleigh method for aqueous AgN03 at 25’ from 0.05 to 8 mol/l. A laser light source was successfully used to provide sharp fringes at all concentrationseven with bath water in the reference path of the Tiselius cell. Our results are in good agreement with previously reported optical data in the region of overlap (0.1 to 1.5 M)but differ significantly from two discrepant series of diaphragm cell measurements for higher concentrations.
I. Introduction Application2r3 of irreversible thermodynamics to electrolyte solutions has stimulated interest in obtaining activity, conductance, transference number, and diffusion data for electrolyte solutions from which ionic transport coefficients may be calculated. This paper is concerned with diffusion data for the system Agi\‘O3H 2 0 for which t+ has recently become a ~ a i l a b l e . ~ Harned and Hildreth5 obtained good experimental data for this system in the dilute concentration range (0.0030.06 M ) by the restricted-diffusion conductance method, where M is the concentration in moles per liter. Longsworth6n7obtained data in the moderate concentration range (0.1-1.5 M ) by the free-diffusion method with Rayleigh interferometric optics. Data extending to higher concentrations, 4 and 9 M , have been obtained with the diaphragm-cell method by Firth and TyrrelP and by Janz, et aL19respectively. Because of inherent uncertainties in the diaphragm-cell method and substantial inconsiirtencies found in the comparison of the two sets of data, it was decided to measure diffusion coefficientsto near saturation by the free-diffusion method with Rayleigh interferometric optics.
11. Experimental Section Preparation of Solutions. All solutions were prepared gravimetrically. Triply distilled water was used throughout. Baker Analytical reagent grade AgN03 rated at better than 99.9% purity was used without further purification. Sucrose and KC1 were used for
calibration. Sucrose was obtained from the National Bureau of Standards and rated at better than 99.99% pure. The KC1 was from a sample that had been purified by the method of Pinching and Bates. lo Densities for preparation of solutions were taken from the literature.11-16 (1) This work was performed under the auspices of the U. 8. Atomic Energy Commission. (2) D. G. .Miller, J . Phys. Chem., 70, 2639 (1966). (3) D. G. Miller, ibid., 71, 616 (1967). (4) M. J. Pika1 and D. G. Miller, ibid., 74, 1337 (1970). (5) H. S. Harned and C. L. Hildreth, Jr., J . Amer. Chem. Soc., 73, 3292 (1951). (6) L. G. Longsworth in “Structure of Electrolyte Solutions,” W. S. Hamer, Ed., Wiley, New York, N. Y . , 1959, Chapter 12. (7) Data for the concentration range 0.1-1.0 iM are given in ref 6 as part of the results of thermal-diffusion experiments. Further unpublished data for 0.1-1.5 M obtained by Rayleigh free-diffusion method were graciously sent to us in a private communication and are given in the text with Professor Longsworth’s permission. (8) J. G. Firth and H. J. V. Tyrrell, J . Chem. Soc., 2042 (1962). (9) G. J. Janz, G. R. Lakshminarayanan, M. P. Klotzkin, and G. E. Mayer, J . Phys. Chem., 70, 536 (1966). (10) G. D. Pinching and R. G. Bates, J . Res. Nat. Bur. Stand., 37, 311 (1946). (11) G. Jones and J. H. Colvin, J . Amer. Chem. Soc., 6 2 , 338 (1940). (12) A. N. Campbell and R. J. Friesen, Can. J . Chem., 37, 1288 (1959). (13) A. N. Campbell and K . P. Bingh, ibid., 37, 1959 (1959). (14) L. J. Gosting and M . S, Morris, J . Amer. Chem. Soc., 71, 1998 (1949). (15) L. J. Gosting, ibid., 72, 4418 (1950). (16) The densities of the solid reagents used for buoyancy corrections
were from the “Handbook of Chemistry and Physics,” 47th ed, Chemical Rubber Publishing Co., Cleveland, Ohio, 1968. The Journal of Physical Chemistry, Vol. 76, N o . 13, 1972
