J. M. ASSOURAND S. E. HARRISON
872
On the Origin of Unpaired Electrons in Metal-Free Phthalocyanine
by J. M. Assour and S. E. Harrison R C A Laboratories, Princeton, New Jersey
(Received November 4, 1963)
Metal-free phthalocyanine single crystals were found to contain 10’’ unpaired electrons/cm.a. By heat treatments under vacuum and in oxygen and hydrogen ambients, the centers responsible for the “free radical” resonance were found to be oxygen impurities absorbed by the compounds. Experimental evidence indicates that the unpaired electrons may be either in the bulk or on the surface of the phthalocyanine. The absorbed oxygen may be associated with the electronically active centers in phthalocyanine semiconduction.
I. Introduction Free radicals in residues of diamagnetic phthalocyanine metal derivatives were first detected through e.s.r. experiments by Ingram and Bennett.’ Winslow, et u Z . , ~ observed similar free radicals in metal-free phthalocyanine (H2Pc) and suggested that the origin of the unpaired electron occurs as a result of a broken *-bond in the phthalocyanine matrix. FuYen, Erdman, and Saraceno3 believed that diradical structures of phthalocyanines might be responsible for the free radical concentration obtained in petroleum asphaltene. We have observed the “free radical” resonance both in metd-free phthalocyanine (H2Pc) powder purchased from du Pont and that synthesized in our laboratory. The strong e.s.r. signal of the radical indicated that its concentration is large. Quantitative estimates of unpaired spins were made by comparison with the absorption of l,l-diphenyl-2-picrylhydrazyl (DPPH) which contains lo2’unpaired electrons/g. Our initial experimental studies began with HzPc crystals grown by sublimation from du Pont crude powder. A semiquantitative chemical emission spectrograph analysis for the compound revealed Cu as the major metallic impurity (200-2000 p.p.m.). The e.s.r. spectra of the single crystals consisted of C U + ~ hyperfine and nitrogen superhyperfine structures. No evidence of the free radical resonance was observed. Howevcr, when the crystals were lightly crushed into powder, the free radical resonance was observed in addition to the Cuf2 and nitrogen lines. Subsequently, crystals were sublimed from “pure” Hd’c and no metallic resonances were found; only the free radical resonance was observed. Evaporated thin Thc Journal of Physical Chemistrv
films using single crystals as a starting material also revealed the free radical resonance. Since the phthalocyanine compound are being studied in our laboratory as organic semiconductors,6 the existence of free radicals in the system may prove to be significant in the semiconduction of the compound. Large concentrations of free radicals were shawn to increase the electrical conductivity of some organic compounds.2 The possibility of the radical contributing to the free carrier concentration6 as a donor or acceptor of free carriers will determine the polarity of the involved carriers. Furthermore, if the radical is situated on the surface, it may have an influence on the relative importance of surface and bulk conductivities. In the present paper, attempts to study the nature of the free radical, its location, as well as its chemical bonding and stability are reported. The investigations were carried out primarily by e.s.r. measurements with samples that were heat-treated under vacuum and in oxygen and hydrogen ambients. In attempting to
(1) D. J. E. Ingram and J. E. Bennett, Phil. Mag., 45, 545 (1954). (2) F. H. Winslow, W. 0. Baker, and W. A. Yager, J . A m . Chem. SOC., 7 7 , 4751 (1955). (3) T. FuYen, J. G. Erdman. and A. J. Saraceno, Anal. Chem.. 3 4 , 694 (1962). (4) S. E. Harrison and J. M. Assour, Proceedings of the First International Conference on Paramagnetic Resonance, Academic Press, New York, N. Y.,1963. (5) For a discussion of organic semiconductors, see C. B. G. Garett in “Semiconductors,” N . B. Hannay, Ed., Reinhold Publ. Corp., New York, N. Y., 1959. ( 6 ) L. S. Singer and J . Kommandeur. J . Chem. Phys., 34, 133 (1961).
ORIGINOF UNPAIRED ELECTRONS IN METAL-FREE PHTHALOCYANINE
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create radicals by irradiation, samples treated with y- and neutron rays were also examined.
11. Experimental Methods The microwave resonance equipment used was a Varian spectrometer. The magnetic field modulation was 100 kc. while the microwave frequency was 9.5 kMc. The magnetic field was determined by a Harvey Wells n.m.r. gauss meter in conjunction with a Hewlet t Packard 5241) counter. The samples were placed in quartz tubes purchased from Varian with 2.9-mm. i.d. A calibrated quartz rod was used for accurate positioning of the samples in the microwave cavity. The radiofrequency power was properly adjusted tlo prevent saturated e.s.r. signals while the amplitude modulation was set to be less than one-tenth of the free radical line width. I n all measurements reported here, the derivative of the absorption is recorded a t -25' and the line width AH is taken between two points of maximum slope. The actual spin concentration, spins/cm. 3, in the powdered samples was not established because only the relative change in the spin concentration was of interest. Therefore, the variation in the unpaired electron concentration in each sample as a function of heat-treatment temperature is expressed in arbitrary units. 111. Experimental Results Pure HzPc was synthesized according to the method of Linstead.7 Crystals grown from this powder contained the following metallic impurities by emission spectroscopy (in p.p.m.): Cu, 0.3-3; Li, 3-30; Al, 1-10; Si, 0.3-3; Rig, 3-30. Electron spin resonance of single crystals of 15-mm. length and 0.1-mm. width was examined a t 25' in air. The resonance spectra were recorded for the ac plane of the HzPc crystal to determine their angular dependence. The spectra consisted of one narrow line shown in Fig. 1. The line width AH = 4.3 0.5 gauss and the g-value was 2.0026 & 0.0003. Both AH and g were isotropic and no metallic resonances were found in these crystals. To our knowledge this is the first time that HzPc single crystals exhibited the free radical resonance. The concentration of the free radicals in the single crystals was determined by a comparison method using the DPPH as a standard sample. The unpaired electron concentration in the HzPc crystals was found to be approximately 101'/cm.3. Single crystals were lightly crushed into powder between two sheets of glassine paper and packed into a quartz tube which was then evacuated to low4torr and sealed. The tube was placed in a furnace for
W
I
I-
sa
+,+AH -4.3 GAUSS
w
a I
g = 2.0026
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Figure 1. Electron spin resonance of free radical in metal-free phthalocyanine crystal.
heat treatment, a t temperatures ranging from 25 to 500'. After each heat-treatment period which lasted for 2 hr., the spectrum of each sample was recorded. The spin Concentration increased slightly up to 300' and then decreased at 400'. The decrease in spin concentration was found to be caused by sublimation of H2Pc. At this temperature the powder began to sublime and small crystallites grew in the cooler zone of the quartz tube. At 600' the powder sublimed completely. Under vacuum, the total increase in spin concentration was negligible. The line width was AH = 5.3 f 0.5 gauss and g = 2.0026 f 0.0003. These values remained essentially constant during the entire heat-trea tmen t process. A second powdered sample was first evacuated and then filled with oxygen and sealed. The sample was heat-treated as above. Figure 2 represents the plot of the variation of the unpaired electron concentration as a function of heat-treatment temperature. The concentration remains almost constant to 200" and then increases linearly with temperature. In this case a significant increase in spin concentration is obtained. The line width AH = 5.4 & 0.5 gauss varied slightly with heat treatment while g = 2.0024 f 0.0003 remained constant. I n a third quartz tube containing HzPc powder, hydrogen was allowed to flow continuously while the sample was heat-treated at 300". The spectrum of this sample was recorded every hour. The spin concentration is seen to decrease in a hydrogen ambient as shown in Fig. 3. The decrease is seen to reach a (7) R. P. Linstead, et al., J . Chem. Soc., 1719 (1936).
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J. M. ASSOURAKD S. E. HARRISON
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ature and after 9 hr. of heat treatment no evidence of sublimation was observed. The entire procedure was repeated for two different samples and in each case the same results were obtained.
7 1
W
3 V
IV. Discussion
a
W
a
a
Figure 2 . Electron spin resonance of free radical in sublimed HlPc powder heat-treated in oxygen ambient.
[L
a 0.5
t TIM E ( HOURS)
Figure 3. Electron spin resonance of free radical in HZPC heat-treated at 300” in hydrogen ambient.
constant level after 6 hr. of heat treatment. To investigate the reversibility of the experimental results, the flow of hydrogen was stopped after 7 hr. of heat treatment and oxygen was allowed to flow through the same sample. Indeed, the spin concentration was found to increase a t a rapid rate. During the heattreatment process, the line width and the g-value did not vary a t all. In this case, the heat-treatment temperature, 300°,was below the sublimation temperThe Journal of Physical Chemistry
The spin concentration of the order of lo1’ unpaired electrons/cm. found experimentally in the HzPc compound is surprisingly high considering the fact that the molecule is diamagnetic and highly stable.8 The spectra recorded for the ac plane show that the line width AH and the g-factor are independent of the angular variation with respect to the applied magnetic field. The line width in the single crystal is approximately equal to that obtained in the crystalline powder and seems to be invariant to the changing ambients and to the changing spin concentration. The single crystals utilized in these experiments have a large surface-to-volume ratio. I n comparing the radical concentrati~n/cm.~ to the surface area of the crystals, it is found that a surface concentration of the order of 10l 4 unpaired electrons/cm. would manifest the same e.s.r. characteristic. Assuming that the crystal con, density of surface sites is tains loz1 ~ i t e s / c m . ~the -1014 sites/cm.2. Therefore, it appears possible that these unpaired electrons are located in surface sites rather than in the bulk of the material. The experimental results obtained from the heattreatment method show that the number of free radicals remained essentially constant under vacuum, increased with oxygen treatment, and decreased with exposure to a hydrogen ambient. During the entire heat treatment in different ambients, the only parameter which varied significantly was the unpaired electron concentration. These results are quite different from those obtained with similar experiments on carbon blacksg and charred hydrocarbons. lo I n these compounds, it was fouud that the presence of molecular oxygen severely broadens9 the unpaired electron resonance line and decreaseslo the unpaired spin concentration in the samples. The origin of the unpaired electron in the above compounds has been identified as a broken a-bond in the molecular matrix. Our experimental results clearly show that a reversible effect which gives an increase in spin concentration was obtained when a powdered sample was heat-treated in air. This result is in accord with that obtained by Winslow, et aL2 The mechanism for the reduction of spin concentra(8) J. M. Robertson, “Organic Crystals and Molecules,” Cornell University Press, Ithaca, N. Y.,1953. (9) R. I. Collins, M. D. Bell, and G. Krsus, J. A p p l . P h y e . , 30, 56 (1959). (10) J. Uebersfeld, Ann. Phys., 1, 393 (1956).
ORIGINOF UNPAIRED ELECTRONS IN METAL-FREE PHTHALOCYAN~NE
tion in the presence of a hydrogen ambient can be explained in terms of the experimental results obtained by Calvin, Cockbain, and Po1anyi.l1 Their studies indicate that HzPc and its metal derivatives act as catalysts in the formation of water from its elements. Their experiments were carried out by heating a mixture of hydrogen and oxygen between 250 and 370" in the presence of H2Pc crystals and observing the catalytic reaction. It seems reasonable to expect that in our experiments hydrogen may remove oxygen in much the same catalytic process. Therefore, our experimental results suggest that the unpaired spins may have their origin in reversibly absorbed oxyger. Oxygen is known to assume the role of a free radical. It may, in fact, exist as a diradical, though such a role is not implied by our spectra with a single sharp resonance. Our results indicate that oxygen rnay attach itself to the surface or diffuse into the bulk of the material. In the case of carbon blacks and similar compounds, in which free radical resonances have been found and to which the phthalocyanine resonance has been compared, l - the unpaired spins react differently. Wheli oxygen diffuses in these compourtds, which are assumed to contain free radicals, itj attaches itself to the free radicals and may annihilate them.l0 Consequently, a decrease in spin concentration is observed. Singer, Spry, and SmithI2 have noted that in sucrose charred a t 670", only the line width decreases and thLe total absorption remains constant a,s the oxygen preissure increases However, in sucrose charred a t 450", the resonance line width increases and the spin concentration decreases as a function of oxygen pressure. Now the effects of oxygen on charred carbons may be due to either the compensation of the broken a-bond or a broadening of the line due to the paramagnetisni of the oxygen. In our case, the presence of oxygen manifests itself in an increase of spin concentration which indicates the absence of free radicals due to broken a-bonds in the phthalocyanine matrix. Our results are then in disaccord with prevailing assumption1-3 that the unpaired electron responsible for the free radical is a delocalized electron from a broken a-bond in the phthalocyanine molecule. Additional evidence for the absorption of oxygen by phthalocyanines has been obtained from conductivity measurements in various ambients. In HZPc, conductivity measurements on single crystals without guard rings indicate a reversible 1000-fold increase in conductivity on exposure to an oxygen atmosphere a t room temperature along with a 15yo decrease in activation energy. l 3 With guard rings the same crystals show no change of conductivity. This indicates that the oxygen centers are effective on the
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surface in increasing the conductivity but not in the bulk. This result, however, does not rule out the diffusion of oxygen into the bulk of the compound, for it has been found, on the other hand, that oxygen will affect the bulk conductivity of CuPc and this oxygen can be identified with a deep-lying electron donor. Though the interaction of CuPc and H2Pc with oxygen may not be identical, the diffusion processes should be similar because of the congruence of the molecular and crystal structures of these two derivatives.8 In HzPc single crystals measurements of space-chargelimited currents reveal the existence of effective traps in the bulk of the crystal of the order of 1014/cm.3 a t a trap depth of 0.8 e.v. below the bottom of the conduction band. l5 Whether the oxygen active in the conduction processes gives rise to these traps and is the same a,s the oxygen that is responsible for the e.s.r. signal cannot be unequivocally established a t the present t,ime. It is difficult a t the present state of our knowledge to determine with certainty the site of the trapped oxygen. The absence of hyperfine structure suggests that the unpaired electron spin density on the surrounding nuclei is very small. Also, the narrowness of the resonance line shows that the free radical is an unpaired electron localized or possibly "trapped" in limited orbits which do not overlap with the phthalocyanine molecular orbitals. We have excluded exchange narrowing as the source for the narrow line width for two reasons. The first is that the spin density is so low compared to that of DPPH ( L e . , 1021 spins/ Secondly and more importantly, even if there were exchange narrowing with densities for less than loz1spins/cm.3, we would have expected the line width to vary as the spin concentration was altered. Further evidence that the resonance signal is not altered by exchange is given by the fact that LLPc dissolved in 1chloronaphthalene also manifests a resonance signal that is very similar in both AH and g-value4 to the HzPc signal. It may be that in phthalocyanine the free radical center is a complex of oxygen and some other unknown item in the phthalocyanine crystal. This unknown moiety of the free radical may be intrinsic to the (11) M. Calvin, E. G. Cockbain, and M. Polanyi, Trans. Faraday SOC.,32, 1436 (1936). (12) L. S. Singer, W. J. Spry, and W. H. Smith, Proceedings of the Third Conference on Carbon, Pergamon Press, New York, N. Y., 1959, p. 121. (13) J. M .Assour and S. E. Harrison, unpublished. (14) G. H . Heilmeierand S. E. Harrison, Phys. Rev.,132, 2010 (1963). (15) G. H. Heilmeier and G. Warfield, J . Chem. Phys., 38, 163 (1962).
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THOMAS B. HOOVER
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phthalocyanine crystals or a chemical or physical ( e . g . , lattice imperfection) impurity. The generation of free radical resonances on exposure to oxygen has been noted in experiments on biological materials by Miyagawa, Gordy, Watabe, and Wilbur. I n that work the oxygen was also reversibly absorbed. Even though the free radical resonances described in ref. 16 are somewhat asymmetrical whereas the resonances generated by oxygen in Pc are symmetrical and approximately half the width of the biological resonances, it may be that the centers that cause the oxygen to be absorbed in that material are electronically similar to the absorbing centers in Pc. Samples of fine HzPc powder were irradiated with y- and neutron rays. Preliminary studies of the ir-
radiated samples point out that the resulting e.s.r. spectra indicate that the free radical, as well as the phthalocyanine molecule, is stable under irradiation. The stability of phthalocyanines with regard to ionizing radiation may, as in the case of porphyrins,17 be due to the very high resonance energy of the compound.
Acknowledgments. The authors wish to acknowledge helpful discussions with Dr. X.E. Wolff and the assistance of L. Korsakoff in supplying pure metal-free phthalocyanine powder. (16) I. Miyagawa, W. Gordy, N. Watabe, and K. M. Wilbur, Natl. Aead. Sci., 44, 613 (1958). Pullman, N a t u r e , 196, 1137 (1962). (17) B. Pullman and 9.
Conductance of Potassium Chloride in Highly Purified N-Methylpropionamide from 20 to 40''
by Thomas B. Hoover 12'ational Bureau of Standards, Washington, D . C .
(Received Sovember 7 , 1968)
The conductance of potassium chloride in N-methylpropionamide was measured a t 5 O intervals from 20 to 40' and in the concentration range of 5 X to 3 x lo+ N . The Fuoss-Onsager conductance equation represents the data with only a small contribution from the term linear in concentration. The ion size parameter d J increases from 0.6 to 1.5 with increasing temperature, while the mean hydrodynamic (Stokes) radius is 3.1 8.
K-Methylpropionamide (YRIP) is of interest as a solvent for electrolytes because of its unusually high dielectric constant (176 a t 25 "), Dawson, Graves, and Sears2 have shown that a Kohlrausch plot represents the conductance data for potassium chloride fairly satisfactorily to much higher concentrations in N X P than in water. They found, however, that the slope of the plot differed by a fe\\T per cent from the theelimiting Of the Onsager equation' The present study was undertaken in order to obtain data The J o u r n a l of Physical Chem,istry
of sufficient precision to permit application of the extended conductance equation of Fuoss and O n ~ a g e r . ~
(1) Presented before t h e Division of Physical Chemistry a t the 145th National Meeting of the American Chemical New York, N Y., September 9-13. 1963. (2) L. R. Dawson, R . H. Graves, and P. G. Sears, J . Am Chem. Soc.; 79, 298 (1957). (3) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York. N. Y.. 1959, p. 195.
