POLYMORPHIC MODIFICATIONS OF PHTHALOCYANINES
2295
On the Polymorphic Modifications of Phthalocyanines
by Jacques M. Assour R C A Laboratories, Princeton, N e w Jersey
08640
(Received January 16, 1966)
X-Ray powder diffraction patterns, infrared absorption spectra, and electron spin resonance data show undisputable evidence for the existence of only two phthalocyanine crystalline modifications defined as a and p. Experimental results for the alleged y-polymorph strongly suggest that the a- and y-phases differ only in particle size.
I. Introduction For years phthalocyanines have been investigated by the dyestuff and pigment industries to determine their tinctorial and color strength, dispersibility and solubility in organic solvents, crystallization, and flocculation of pigments.' A striking result obtained from these investigations is the capability of the molecular phthalocyanines to exist in a t least three alleged polymorphic forms-a, @, and y . Recently, in our laboratories we have been engaged in the study of the electronic properties of phthalocyanines and their potential use in the field of molecular electronics. Detailed investigations of the electronic properties of the phthalocyanine complexes are dependent on the accurate knowledge of the molecular and crystal structures. The dependence on crystallography becomes more significant if the organic complex is known to exist in more than one crystalline modification, a result which might give rise to different physical and electrical properties. Wihksne and Newkirk,2 and Eley and Parfitt3 have already reported on the sharp difference between the electrical conductivities of the a- and @-phthalocyaninepolymorphs. Recent electron spin resonance experiments have also shown4 an increased perturbation on the energy levels of the cobalt atom when the cobalt phthalocyanine molecule is embedded in the a- or 0-lattice. Although the subject of phthalocyanine polymorphs has been considered by several workers, the investigations reported in the literature were mainly confined to the identification of only two crystalline phases. In the present paper, we report our experimental results on the identification Of the third form defined the phase.
'-
11. Earlier Identification of the Phthalocyanine Polymorphs According to the nomenclature5 first reported by Andrews and co-workers,6 the /3-crystalline modification is the most stable phase since it does not undergo a polymorphic transition when stored in organic solvents, while the a and y are metastable polymorphs. @-Phthalocyanines are the only polymorphs prepared by sublimation a t about 550°, and well-defined nionoclinic single crystals can be easily grown. Experimentally, we have found that the a-phase is completely converted to the @-phasewhen heat-treated above 300'. The molecular orientation and crystal structure of the @-modificationhave been thoroughly studied by Robertson.' Detailed X-ray analyses of the a- and y-phthalocyanines have not been performed because it has not been possible to grow single crystals of these polymorphs. However, Robinson and Kleins have determined the lattice constants of a-CuPc from the Xray diffraction pattern of a polycrystalline specimen.
(1) For a detailed discussion of these properties see F. H. Moser and A. L. Thomas, "Phthalocyanine Compounds," Reinhold Publishing Corp., New York, N. Y., 1963. (2) K. Wihksne and A. E. Newkirk, J . Chem. Phys., 34, 2184 (1961). (3) D. D. Eley and G. D. Parfitt, Trans. Faraday SOC.,51, 1529 (1955). (4) J. M.Assour and W. K . Kahn, J . Am. Chem. SOC., 87, 207 (1965). (5) Unfortunately, this nomenclature has not been adopted universally and several workers have switched t h e a- and @-notation. (6) D. B. Andrews, et al., Fiat Final Report No. 1313 PB 85172, Feb. 1, 1948. (7) J. .1.I. Robertson, J . Chem. ~ o c . 615 , (1935); 1195 (1936); 219 (1937). (8) M. T. Robinson and G. E. Klein, J . Am. Chem. Soc., 74, 6294 (1962).
Volume 69,Xumber 7 J u l y 1966
2296
According to their interpretation, the crystal structure of a-CuPc is presumably tetragonal. The work by Susich9 on the identification of organic dyestuffs by X-ray powder diffraction has attracted several workers in the field, and X-ray powder patterns for both the a- and @-phthalocyanineswere measured by Tarantino and co-workers,'O Ebert and Gottlieb," Karasek and Decius,12 and Shigeniit~u.'~A single Xray powder diffraction pattern for the alleged ycrystalline modification has been reported by Eastes. l 4 Thus, identification of the phthalocyanine polymorphs was invariably achieved by comparing their characteristic X-ray powder patterns. A second well-established and more sensitive method for differentiating between the phthalocyanine polymorphs has been the application of infrared spectroscopy.I5 Indeed, nicely resolved spectra for the aand @-phthalocyanine derivatives were measured by Ebert and Gottlieb," Kendal1,'j and Sidorov and Kotlyar.lG The distinct spectral differences between the frequencies and intensities of the infrared absorption bands observed for both polymorphs enable one to identify each crystalline phase quite readily. To our knowledge, no infrared spectrum has been reported for the y-phthalocyanine. 111. Experimental Methods The investigations reported here were carried out primarily with copper phthalocyanine (CuPc) because experiniental results for this complex, with which we can compare our data, are readily available in the literature. These studies, however, were also extended to other phthalocyanine derivatives such as H Z c , CoPc, NiPc, and ZnPc. The phthalocyanine compounds were synthesized by the niethod of Barrett and co-workers.17 The products were purified chemically to remove excess acid and organic impurities. The resultant powder was then sublimed in a nitrogen atmosphere at T = 500'. The sublimation process which lasted for 4 days yielded large single crystals in the shape of thin needles. The crystal structure was identified by X-ray measurements and agreed with the nionoclinic structure determined by R o b e r t ~ o n . ~The a-CuPc polymorph was prepared by dissolving the @-phase crystallites in 98% sulfuric acid. The solution was then poured slow-ly into crushed ice, stirred well in order to niaintain the temperature constant, causing the a-CuPc phase to precipitate. The product was separated by Centrifugation, digested several times with NH,OH and CH30H, and dried under vacuum. The y-CuPc complex was prepared according to the method described by Eastes.14 p-CuPc powder was slurried in aqueous sulfuric acid of GO% concentration for a The Journal of Physical Chemistry
JACQUES M. ASSOCR
period of 4 hr. The precipitate was then extracted by centrifugation, washed thoroughly to remove excess acid, and dried under vacuum. 1. X - R a y Difraction Pattems. Powder diffraction patterns were recorded by a Norelco X-ray diffractometer employing nickel-filtered copper radiation. Samples in the form of fine powder were packed into a 1 x 2-cin. aluminum groove. The entire scan from 5 to 50' was swept using a 1' divergence slit and a receiving slit of 0.015 cm. The diffractoriieter traces were also supplemented with X-ray powder photographs recorded by a Philips powder camera. 2. Infrared Spectra. Infrared spectra were measured with a Perkin-Elmer double-beam spectrophotonieter JIodel 221. A standard infrared spectrum of a polystyrene thin film was employed for calibration, and the errors found were less than i2 cni.-l. Samples of a- and @-CuPcwere prepared and measured in the form of KBr pellets and evaporated thin films on NaCl disks. Samples of the y-CuPc phase were prepared by suspending the powder in Sujol and Fluorolube, respectively. The infrared spectra were measured at room temperature. 3. Electron Micrographs. Thin films of a- and 7CuPc were prepared by dispersing fine powder in a 1% solution of collodion. Micrographs of the phthalocyanine crystallites were taken with an RCA electron microscope Model 3B a t 28,700 direct magnification. The photographs shown in Figure 1 were enlarged four times.
IV. Experimental Results and Discussion Micrographs of the a- and y-CuPc crystallites are shown in Figure 1. The y-form particles have an apparent shape of an elongated flat plate, and those of the a-form have a rod-like structure. The scale of 0.5 p affords a direct coinparison between the average size of the primary crystallites.18 The average length of each a-particle is of the order of 0.1 p or less, whereas (9) G. Susich, Anal. Chem., 2 2 , 425 (1950). (10) F. R. Tarantino, D. H. Stubbs, T. F. Cooke, and L. A. Melsheimer, A m . I n k Maker. 2 9 , 35 (1951). (11) A. -4. Ebert, Jr.. and H. B. Gottlieb, J . A m . Chem. Soc., 74, 2806 (1952). (12) F. W. Karasek and J. C. Decius, ibid., 74, 4716 (1952). (13) M. Shigemitsu, Bull. Chem. SOC.Japan, 32, 607 (1959). (14) J. W. Eastes, U. S. Patent 2,770,620 (1956). (15) D. N. Kendall, Anal. Chem., 2 5 , 382 (1953). (16) A. N. Sidorov and I. P. Kotlyar, O p t . i Spektroskopiya, 11, 92 (1961). (17) P. A. Barrett. C. E. Dent, and R. .'1 Linstead, J . Chem. SOC., 1719 (1936). (18) It should be noted that the collodion solution used t o disperse the fine powder of each phase may alter the size of the agglomerates, but it should have no effect on the size of the primary particles.
-
POLYMORPHIC MODIFICATIONS OF PHTHALOCYANINES
2297
I
I
d
the average size of the y-particle is approximately 1 p. For X-ray powder measurements the particle size of the a-polymorph is too sinall to yield sharp diffraction insxima. Crystallites with average diameter in the range of to IO-' r m - ' will invariably give rise to a powder pattern whose lines are diffused into broad diffraction bands which are difficult to rharacterize accurately. This effect has severely limited the application of X-ray powder diffraction in identifying two similar crystalline modifications. On the other hand, crystallites of the y-polymorph have the proper size to produce well-resolved diffraction maxima. The above two effects are clearly illustrated in Figures 2 and 3. The X-ray powder diffraction patterns for three crystalline modifications, prepared in our laboratories, are shown in Figures 2 and 3. Before we compare our data with the experimental results reported previously, it is of interest to summarize briefly the characteristir features of the powder diffraction patterns published thus far. For example, in four out of the five patterns recorded*.l"--" for the a-phase, only one diffractioii line of maximum intensity was observed a t a11 interplanar sparing of approximately 12.8 A. in the region from 11 to 14 11. In the fifth diffraction pattern: this line is split into two almost equally intense peaks. The interplanar spacing (dht,) corresponding to this line differs in each pattern by as much as 0.36 A.
Figure 3. X-ray powder diffraction of copper phthalocyanine polymorphs.
(-3%). Furthermore, by comparing the diffraction lines recorded in the middle-angle region, we find that the interplanar spacings in all the a-patterns are in Volume 89. Nu&r
7
J u l y 1M6
JACQUES M. ASSOUR
2298
poor agreement. Another striking feature of the CuPc diffraction patterns is apparent from the pattern of the alleged y-p~lymorph'~;the two lines of maximum intensity a t 11.95 and 13.5 8. occur in the same region as the broad intense line at 12.8 8. observed in the aphase, and the lower interplanar spacings for y C u P c can be easily correlated to those of the a-CuPc. Since X-ray diffraction patterns are characteristic of the atomic arrangement in a given crystal, the discrepancies noted previously for the interplanar spacings clearly demonstrate the uncertainties in the interpretation of the experimental results obtained for the phthalocyanine polymorphs. The powder patterns (Figures 2 and 3) recorded for our samples have the general features described above; ie., the a-pattern has a single intense broad line at and is in agreement with the patterns reported 12.83 9. by Tarantino and co-workers,I0 Ebert and Gottlieb," and Shigemit~u.'~The pattern of the alleged yCuPc has the same splittings as those observed by Eastes14 for the y-phase and by Robinson and Klein8 for the a-phase although the interplanar spacings are different. The major differences in the d,,, values occur in the region from 11 to 14 8.)but they do not exceed 4%. The causes which might give rise to these errors can be numerable; however, if one assumes that all the samples investigated by several workers were "pure" phthalocyanines and contained each only one polymorph, the errors amounting to 4% in the region of low Bragg angle diffraction might occur in powder diffraction experiments since in this region absorption effects become significant. Other factors which are known to contribute to the above discrepancies are sample purity, particle size effects, preferred orientation, crystal texture, and experimental techniques. The distinct characteristics of both the a- and ppowder diffraction patterns strongly indicate that these two complexes have indeed two different crystalline structures. The existence of these two phthalocyanine crystalline modifications has been also confirmed by infrared spectroscopy and electron spin resonance experiments to be discussed below. E a ~ t e s . on ' ~ the
crystalline modification which is based on one X-ray powder pattern is questionable. Although, as mentioned earlier, Robinson and Klein8 have interpreted the crystal structure of a-CuPc as tetragonal, our X-ray powder data on the same polymorph imply a preferred orthorhombic structure. It is also interesting to note that the occurrence of 6 molecules/unit cell for an orthogonal crystal structure is rare. Several tetragonal structures of the analogous tetraphenylporphine chelates, which are known to exist in more than one polymorph,lg have only 2 or 4 molecules/unit cell. A definite identification of the crystal structure of the a-polymorph must therefore await a careful X-ray analysis similar to that performed by Robertson' on the P-phase. The average size of the primary crystallites of the aand y-phases, which is believed to be responsible for the difference in the X-ray patterns, was found to be critically dependent on the method of pigment preparation. Since the a-polymorph is usually prepared by pouring the concentrated sulfuric acid solution of phthalocyanine on crushed ice, it is believed that the sudden contact at low temperature considerably limits the growth of the nuclei of the crude pigments, and particles 0.1 p long are invariably obtained. On the other hand, when the sulfuric acid solution was poured into water which was maintained a t room temperature, different-size particles were obtained which gave rise to a distorted powder diffraction pattern. The y-phase, however, is prepared a t room temperature with a 60YG sulfuric acid solution. This method apparently provides favorable conditions necessary for the increased growth of the primary particles to sizes exceeding 1 p. Several strictly controlled methods for the preparation of different phthalocyanine pigments have been discussed by Andrews and co-workers.6 There are two other important reasons to question the identification of the y-crystalline modification. The first is apparent from the interpretation of the infrared spectra shown in Figure 4 . For the sake of this discussion we have adopted the notation of Sidorov and Kotlyar.16 The vibrational bands are numbered
of resolution leads us to conclude that these differences are insufficient to identify the y-phase unequivocally. Therefore, the identification of the alleged y-copper
86, 2342 (1964).
The Journal of Phusical Chemist?y
(19) E. B. Fleisher, C. K. Miller, and L. E. TT'ebb, J . A m . Chem SOC..
POLYMORPHIC MODIFICATIONS OF PHTHALOCYANINES
t
I 1100
1700
i
1700
-
L
I
1300
CM'I
I
I
1100
900
- l - _ _ - I
1100
1300
1100
1300
CM-,
D
I
700
I
1100
900
1100
900
700
1
2299
trum of the y-phase, shown in Figure 4, was found to be identical with that of the a-phase, and no variations between the vibrational bands were detected. Since the dissimilarity of the infrared vibrations observed in both the a- and &phases can be attributed to different packing, together with different intramolecular linkages, based on the infrared spectra, one can conclude that both the a- and y-phases have the same structure. A second reason to dispute the identification of the y-polymorph can be established by referring to the electron spin resonance (e.s.r.) study4of cobalt phthalocyanine. The remarkable feature of the e.s.r. data is the sensitivity of the spin-Hamiltonian parameters of the square-bonded cobalt phthalocyanine to the change in crystal structure and molecular environment. E.s.r. spectra of a- and P-polymorphs, which were prepared according to the known techniques and identified by X-ray diffraction and infrared spectroscopy, clearly revealed the existence of two crystalline modifications. Sfore significantly, we have observed the transition from the a-phase to the &phase through e.s.r. experiments. On the other hand, samples prepared according to the method of Eastes14for the y-polymorph yielded e.s.r. spectra which were identified as those of the a-polymorph. Several attempts to measure the e.s.r. of the alleged y-polymorph were unsuccessful; in each attempt the e.s.r. spectrum was typical of the a-pol ymorph.
I
1700
CM-'
700
Figure 4. Infrared absorption of copper phthalocyanine polymorphs : top, a-CuPc (KBr); middle, 0-CuPc ( t h i n film); bottom, y-CuPc ( N u j o l and Fluorolube).
phthalocyanine a t 300°, the @-polymorphwas obtained as shown by an infrared spectrum. The dissimilarity of the vibrational frequencies in both the CY- and pphases reveals not only their polymorphic character, but it also provides a reliable method for identifying these organic complexes. Since little is known of the polymorphic transition of the y-phase under the influence of high pressures or elevated temperatures, we have measured the infrared spectrum of the y-phase in the form of Nujol mulls. However, since Sujol has two strong bands in the region 1350 to 1500 cm.-', we have measured the phthalocyanine bands in this region in Fluorolube. The spec-
Summary The experimental results for the identification of three alleged copper phthalocyanine polymorphs show undisputable evidence for the existence of only two crystalline modifications defined as a and P. Data for the alleged y-copper polymorph are insufficient to allow unequivocal identification of this crystalline phase. Our results suggest that the X-ray spectral differences for the a- and y-phases can be attributed to differences in the average size of the primary particles. The determination of the crystal structure and molecular orientation of the a-polymorph must await a careful X-ray analysis.
Acknozcledgment. The author wishes to acknowledge the assistance of Ah-. L. Korsakoff, Dr. D. Ross, and SIr. S. Bennet. The help extended to us by the members of the X-ray department is gratefully acknowledged.
Volume 69, Number 7
July 1965
