Frank H. Morer and Arthur 1. Thomas
Standard Ultramarine and Color CO. Huntington, West Virginia
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Phthalocyanine
The phthalocyanine class of organic compounds is the only new chromogen yet discovered in the twentieth century. Since elucidation of the structure of the phthalocyanine molecule and its definition by Sir Reginald P. Linstead and co-workers in 1933 (1)and in 1934 ( 2 ) ,more than 40 metal phthalocyanine compounds and several thousand phthalocyanine compounds have been reported. Although the subject of intensive investigation in the last thirty years, contemporary texts in organic chemistry mention the phthalocyanine compounds only briefly. The first description of them in book form is ACS Monograph KO. 157, "Phthalocyanine Compounds" (3). The structural formula for copper phthalocyanine is shown in Figure 1. The pioneer X-ray investigations by J. Monteath Robertson in the 1930's at the Davy Faraday Laboratory led to the determination of the bond angles and the spatial configuration of the phthalocyanine moleFigure 1 . Metal Phthalocyanine. cule (mol. wt. 514). M = metal, such or copper. The metal atom and the four surrounding isoindole nitrogen atoms mast all lie strictly in m e plane. For the metal aat.m, being unique, must coincide with the centre of symmetry . . . (4).
Phthalocyanine compounds were the first compounds of several metals found to have planar symmetry. It was known in 1936 that 4-coordinate bivalent platinum and copper can exhibit planar symmetry, and that nickel often exhibits planar symmetry. Phthalocyanines were the first examples of planar symmetry for cobalt, iron, and manganese metals. The most remsrkahk result is provided by beryllium, for which a tetrahedral symmetry is well established by investigations of its bemoylpyruvic acid derivative and of its basic acetate . . . . The planar arrangement appean to be very unstable, for the anhydrous beryllium compound readily forms 8. dihydrate even in moist air. This behavior is not paralleled by other phthaloeyanines except the magnesium derivative (6).
Investigation of a number of cohalt compounds by X-ray analysis led Porai-Koshits to conclude that cobalt phthalocyanine is the only cohalt compound in which the cohalt valencies are planar (6). The applicability of modern X-ray methods to the determination of the structural details of complex molecules has nowhere been more beautiiully exemplified than in the researches by Robertson and co-workers on phthelocymine and its salts (7).
Several phthalocyanines hold the metal atom in the center of the ring by a covalent bond while other
phthalocyanines hold the metal atom in the center of the ring by an electrovalent bond. For example, mercury phthalocyanine is said to be an electrovalent compound because it is insoluble in organic solvents and it is easily decomposed to phthalocyanine in acidic solution. Copper phthalocyanine, however, which does not decompose in acidic solution, is covalent. The ease of formation of the phthalocyanine molecule is remarkable. Heating an intimate mixture of urea, phthalic anhydride, copper(1) chloride, and catalyst together at 200°C for several hours results in the formation of copper phthalocyanine in a yield as high as 90% of theoretical. The reader is referred to Table 3-1 on page 105 of Ref. (3) for a listing of the 46 elements known to form phthalocyanine derivatives. Details and references for preparative procedures also are given (3). Properties of Phthalocyanine
Phthalocyanine compounds may exhibit remarkable stability to heat. Copper phthalocyanine, for example, is stable up to 550°C a t which temperature it readily sublimes. Generally these compounds do not have a melting point but they decompose a t elevated temperatures. Single molecules of copper phthalocyanine on a tungsten surface have been photographed by Muller (8) using the field electron microscope a t a magnification of 2.8 X 10% Although the phthalocyanine molecule is roughly 10 X 10 A in dimensions, and the normal resolving power of the field electron microscope is 20 A, the additional magnification is attributed to a spreading of the electron orbits in the observation field because they are emitted from points of high field strength a t the edge of the molecule in a direction away from the molecule. Bright quadruplet patterns, strikingly similar in shape to the actual shape of the phthslocyanine molecule, appeared on the screen. Other bright spots formed doublets and occasionally a doughtnuhhaped pattern or some odd-shaped pattern would appear (8).
Phthalocyanine compounds behave as catalysts under certain circumstances. The first mention of phthalocyanine compounds as catalysts is the work of Calvin, Cockbain, and Polanyi (9) who studied the effect of the presence of crystals of phthalocyanine and copper phthalocyanine on the activation of n~olecular hydrogen. Their initial results indicated that crystals of phthalocyanine and copper phthalocyanine catalyze atomic interchange between molecular hydrogen and water vapor and catalyze formation of water from hydrogen and oxygen. Since then the phthalocyanines have been used as Volume 41: Number 5, Moy 1964 / 245
catalysts in the conversion of para-hydrogen, the reaction between hydrogen and deuterium oxide, the decomposition of hydrogen peroxide and other peroxides, a great variety of organic oxidations, the isomerization of dimethyl maleate to dimethyl fumarate, and the polymerization of methyl methacrylate. The remarkable stability of phthalocyanine compounds includes the resistance of some of them to atmospheric oxidation a t 100°C or higher. In aqueous acid solution, strong oxidizing agents oxidize phthalocyanines to phthalic residues, whereas in nonaqueons solution an oxidation product that can be reduced readily to the original compound is usually formed. Reduction in the phthalocyanine molecule can take place a t the central metal atom or at any of the 16 peripheral carhon atoms on the fonr phenylene rings. The extent of reduction in the center of the molecule is limited by the number of valency states attainable by a given metal atom and by the phthalocyanine ligand. The highest valency state that has been attained a t the center of the phthalocyanine molecule is fonr, as illustrated by dichlorostannic phthalocyanines Complete reduction of the central metal atom to a valency of zero has been attained in the case of copper phthalocyanine with potassium in liquid ammonia. The simplest reduction of the peripheral carbon atom of phthalocyanine componnds has been accomplished by the addition of a hydrogen atom a t each of the available 16 carbon atom sites, by synthesis from 3,4,5,6-tetrahydrophthalonitrile giving hexadecahydrophthalocyanine. Therefore, the minimum and maximum limits of reduction of the peripheral carbon atoms have been accomplished. It would appear possible to make phthalocyanine compounds of the intermediate stages of reduction as well. Eley and Vartanyad were the first to observe semiconductivity in phthalocyanine compounds. Probably the earliest studies of organic substances as intrinsic semiconductors mere started in 1948 when Eley and Vartanyan discovered the unusual temperrtture dependence of the resistivities of the phthdocyanines and their metal derivatives ( 1 0 ) .
Electrical conductivity in the phthalocyanines can he induced by the impingement of light as well as by application of an electrical field. Phthalocyanine was shown to exhibit photoelectric sensitivity by Putseiko in 1949 (11). I t has been shown that phthalocyanine has a greater photoeffect than magnesium, zinc, and copper phthalocyanines (12). Kearns and Calvin (13) have found that magnesium phthalocyanine discs coated with a thin film of air-oxidized tetramethyl-pphenylene diamioe are organic systems which show the photovoltaic effect. a suggestion has been msde that the primary quantum conversion process in photosynthetic tissues involves the creation and separation of charge to opposite sides of an asymmetrically constructed lamina. followed bv the t r a.. ~ ~ i-nof e :both the electrons and the boles which then lead to their respective chemical processes; namely, reduction of carbon dioxide and oxidation of the water to oxygen. This has led us to study model systems as semiconductors with a view to creating sn organic photovoltaic junb tion ( 1 3 ) .
Phthalocyanine compounds also participate in photochemical reactions, act as photosensitizers, and may luminesce or fluoresce. 246
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Knowledge of the photochemical properties of chlorophyll has stimulated interest in the photochemical properties of the phthalocyanines and of magnesium phthalocyanine in particular (14-18). The possibility of obtaining identical reaction products of chlorophyll, magnesium phthalocyanine, and their analogs by dark and photochemical methods has been demonstrated. As a result of the initial photochemical change there is formed a reactive form of pigment that may be a negatively charged free radical. The primary rednced ionized form is then converted to a secondary slightly ionized or nonionized form. The conversion from primary reduced form to secondary reduced form probably proceeds by a dark reaction. The changes in electrical conductivity of M solution of chlorophyll and magnesium phthalocyanine during reduction demonstrates this hypothesis (15). After cessation of illumination there is a rapid drop in conductivity. Chlorophyll and magnesium phthalocyanine are oxidized photochemically or in the dark more rapidly than pheophytin and phthalocyanine, whereas pheophytin and phthalocyanine are rednced faster than chlorophyll or magnesium phthalocyanine (14, 18). Reduction in the dark is slower than the photochemical reduction (18). It is concluded that the presence of magnesium in chlorophyll or in magnesium phthalocyanine increases the capacity for photo-oxidation, while capacity for photo-reduction is more pronounced in the absence of magnesium. Photochemical reduction of riboflavin and safranin T. in lo-' M solution. in ethvl alcohol and in uvridine. hf. 10-'10-3 M solutions "of ascorbic and pyruvic acids can be sensitized by chlorophyll or magnesium phthalocyanine in the red region of the spectrum. Magnesium phthalocyanine and chlorophyll exert photosensitizing action in the oxidation of oleic acid as well as of ascorbic acid (17). The process appears to he a chain reaction with no temperature dependence, indicating that the photochemical stage is the ratedetermining step. Chlorophyll and magnesium phthalocyanine are photosensitizers in the reduction of methyl red by ascorbic acid in pyridine (16). A
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A reciprocal relation between photochemical and chemiluminescent reactions is to be expected. In one case, absorbed light leads to chemical action, and in the other, chemical processes result in light emission (18).
In 1938 Helberger observed the luminescence of magnesium and zinc phthalocyanines in boiling tetralin containing small amounts of benzoyl peroxide, hydrogen peroxide, or air (20). A certain amount of the phthalocyanine is destroyed with intense red luminescence which persists several minutes. Soon after boiling begins, ammonia is formed. Apparently the magnesium atom is removed and converted to magnesium oxide; phthalocyanine decomposes to phthalic residues and ammonia; and tetralin is converted t o a-tetralone. Evstigneev and Krasnovskii (21) found that magnesium phthalocyanine fluoresces with emission as a narrow red band a t 670-675 mp only about 5 mp beyond the main fluorescence band of chlorophyll. The life of the excited state of chlorophyll and phthalocyanine pigments, of the order of 1 0 P see, has beeumeasnred directly by the extinguishing of fluorescence by the phase flnorometer which has a resolving power of 2 X lo-" sec (22).
A numher of phthalocyanine compounds have been examined for use in the Szilard-Chalmers process for the preparation of radioactive isotopes of the central atom. Payne, Scargill, and Cook (23) irradiated cobalt phthalocyanine in the Harwell BEPO reactor with a neutron flux of 1012n/cm2 sec obtaining separation of radioactive product from the bombarded compound in yields as high as 98%. They showed that the Szilard-Chalmers process in phthalocyanines is useful for obtaining high specific activity radioisotopes of short half life. Phthalocyanine compounds m y play a significant role in the development of radioisotope applications where short-lived radioisotopes are useful, such as in medical therapy. Numerous "phthalocyanine-type" compounds have been reported. Phthalocyanine-type compounds are defined here as porphin derivatives with one, two, three, or four aza groups joining the pyrrole nuclei, with one or more inorganic-organic groups replacing the @-hydrogenatoms, and with any suitable metal replacing the two hydrogen atoms in the center of the molecule. From one to eight of the 8-hydrogen atoms may he replaced. Phthalocyanine-type compounds, like the phthalocyanines, are pigments which are insoluble in aqueous solutions and the common organic solvents. Unlike the phthalocyanines, substitution on the &carbon atoms of the pyrrole nuclei may be unsymmetrical. Phthalocyauine and phthalocyanine-type compounds possess remarkable color properties. All phthalocyanine and phthalocyanine-type compounds reflect light in the blue-green portion of the spectrum. Copper phthalocyanine has been the building block of phthalocyanine color technology because of its outstanding color properties, retention of the copper cation, and ease and cost of manufacture. Copper phthalocyanine has three polymorphic forms: or-, 6-, and y-. The pigmentary properties of copper phthalocyanine change with time in organic media due to change in crystal structure. The tendency of copper phthalocyanine to crystallize and to flocculate has led to the development of technology to minimize change in crystal form during industrial use. This technology makes use of the reaction of copper phthalocyanine with other compounds such as chlorine and sulfonic acid to introduce one or two chlorine atoms or sulfonic acid groups onto the phthalocyanine ring. Several thousand phthalocyanine dyes have been synthesized by the attachment of sulfuric acid groups, ternary and quaternary salt groups, sulfur containing groups, azo groups, vat dye groups, leuco groups, chrome groups, precursor groups, triazine compounds, and other groups to from 1 to 1G of the peripheral carbon atoms of the phthalocyanine molecule. Before the advent of phthalocyanine compounds, ultramarine hlue and iron hlue were the leading hlue pigments. Copper phthalocyanine has 40 times greater tinting strength than ultramarine blue and 4 times greater tinting strength than iron blue. It sublimes without deco~npositiona t 500-600°C, and it is extremely inert to alkali and acid attack. Although ionic copper has a deleterious effect on the aging of rubber, copper phthalocyanine may be added to rubber with no effect on the aging properties of the rubber. As a result of these outstanding color properties, copper phthalocyanine pigments and dyes which had never
been produced commercially until 1935 are now produced in the U. S. in the amount of more than 6,000,000 pounds per year. The development of motors that can operate at high temperatures and the development of guided missles and jet aircraft have spurred the search for high temperature lubricants and greases. Metal phthalocyanines have been used as thickeners for high temperature greases. These greases consist of a liquid such as a silicone and copper polychlorophthalocyanine. Nomenclature
Principal reactive sites in the phthalocyanine mclecule are a t the center between the four pyrrole or inner isoindole nitrogen atoms and a t the four available methine sites on each of the four phenylene nuclei. Subsidiary reactive sites include the four outer isoindole nitrogen atoms and the eight pyrrole cu-carbon atoms. Nomenclature, therefore, relates to the center of the phthalocyanine molecule, the sixteen peripheral carbon atoms, the four outer isoindole nitrogen atoms, and the eight pyrrole cu-carbon atoms. Since the center of the phthalocyanine ring is anionic and essentially bivalent in character, two univalent metal atoms or one multivalent metal atom may enter it. If the central atom has a valency greater than two, the valency is satisfied by the attachment to it of one or more anions. situated on either or both sides of the great ring. Disodium, dilithium, and dipotassium phthalocyanines are known. Dihydrogen phthalocyanine is referred to as ~hthalocvanineor as metal-free phthalocyad phthalocyanines, such as nine. ~ i x e hydrogen-metal lithium hydrogen phthalocyanine, are also possible. Often the di-metal compound is referred to without the "din prefix, as, for example, sodium phthalocyanine instead of disodium phthalocyanine. Bivalent metal phthalocyanines are described simply as, for example, copper, barium, calcium, zinc, or nickel phthalocyanine. Trivalent metal phthalocyanines are referred to with the substituent anionic group or element as, for example, chlorogallium, iodoindium, bromoaluminum, and hydroxyaluminum phthalocyanines. In the quadrivalent metal phthalocyanines, appropriate nomenclature is, for example, tin diphthalocyanine. It is not uncommon for a metal to occupy the center of the phthalocyanine ring in more than one valence state. For instance, three valence conditions for copper phthalocyanine have been identified. The valence state may then be written unambiguously as copper(O), copper(I), and copper(I1) phthalocyanines. Also, there are tin(I1) and tin(1V) phthalocyanines. Nomenclature of groups 4 5 that attach themselves to any of the sixteen peripheral carbon atoms is simplified N=C\~C- N 6111 1 N 11 3, by use of the 3,4,5,6-num' N-M-NI her system with primes, as 4 \y, shown in Figure 2. On one 1 N 11 N=C'*C-N phthalocyanine molecule it is theoretically possible to have as many as sixteen different groups connected Figure 2. 3, 4, 5, 6-number for the phthotocy.niner to the peripheral carbon
a,>, {Q::
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atoms. Also, the exact location of a group may not be known. The substituents may be inorganic elements or groups or they may be organic groups. If the position of the peripheral substituent is known, it may be, for example, mono-3-chlorophthalocyanine or mono-4chlorophthalocyanine. For two chlorine atoms there may be di-3,4-chlorophthalocyanine, di-3,3'-chlorophthalocyanine, di-3,3"-chlorophthalocyanine,etc. I n the case of mixed groups, positive identification of their location may he made with the 3, 4, 5, 6-number system such as for example tri-3-phenyl-tetra-5-cyanophthalocyanine. If the substituent group contains a noun ending, the noun is placed, as in the case of oxides mentioned above, at the end. For example, copper phthalocyanine tetrasulfonamide, phthalocyanine polycarboxylic acid, aluminum phthalocyanine trisulfonamide monosulfonic acid, and copper phthalocyanine tetra-4-sulfomonomethylamide, are described. Also mentioned are copper dichlorophthalocyanine disulfonic acid, copper tetrachlorophenylphthalocyanine tetracarboxylic acid, copper dichlorophthalocyanine disulfonic acid, and copper tetra-o-carboxyhenzamidomethylphthalocyanine monosulfonic acid.
off the fifth and sixth isoindole groups by reduction with, for example, ascorbic acid. In traditional vat dye chemistry, the original pigment of desirable color properties is made soluble by reduction to a colorless or leuco compound which has affinity for the cloth to be colored. The cloth impregnated with a solution of the leuco compound is then placed in a medium that oxidizes the leuco compound to the original pigment. Historically, a leuco compound is not only colorless but it is also a reduced form of the parent compound.
Outer lsoindole Nitrogen Atoms
Figure 4. Levco metal phthalocyanine Skiler (291.
Certain of the metal phthalocyanines may he reduced to a water soluble intermediate which, upon oxidation, reverts to the original water insoluble metal phthalocyanine. It is believed that the water solubilizing effect is due to addition of a hydrogen atom a t each of two adjacent outer isoindole nitrogen atoms. Reduction may take place, for example, in a sodium hydroxidehydrosulfite vat (24). The supposed oxidation-reduction equilibrium is shown in Figure 3.
Figure 3.
Cobol1 phtholocyonine and reduced cobalt phtholocyanine in o .odium hydroxide-hydrowlRte vat (241.
The phthalocyanine compounds with cationic atoms presumably attached t o the outer isoindole nitrogen atoms as shown in Figure 3 are known as "phthalocyanine vat dyes." a-Carbon Atoms of Pyrrole Nuclei
It is postulated that phthalocyanine molecules containing five or six isoindole groups may be prepared, as depicted in Figure 4. These phthalocyanine compounds, unlike the monoplanar phthalocyanines that contain four isoindole groups, probably exist in two or three planes. They have been called leuco phthalocyanines since they are tinctorially quite weak, they are soluble in aqueous alcoholic solutions, and the original, tinctorially strong color of the parent phthalocyanine containing four isoindole groups is obtained by splitting 248
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described by Barnhort and
Polymers
I n general, there are four types of phthalocyanine polymers: (I) phthalocyanines in which the phenylene rings of 'adjacent monomers are connected in the manner of a diphenyl bond, (11) phthalocyanines in which the monomers are joined together by substituents attached to the phenylene rings, (111) phthalocyanines which share phenylene rings in common, and (IV) phthalocyauiues connected a t the central atom. It is proposed that the four types of polymers he distinguished one from the other by the nomenclature: Type I, Type 11, Type 111, and Type IV. Thus the expected polymer from the condensation of tetra-3,43',4'-cyanodiphenyl (26) is Type I ; the expected polymer from the condensation of di-3,4-chlorostyrene and copper(1) cyanide is Type I1 (27); the expected polymer from the condensation of pyromellitic dianhydride, cuprous chloride, and urea is Type I11 (25); and the polymer formed by the dehydration of silicon phthalocyanine hydroxide, PCS~(OH)~, viz., HO(PcSiO),H where x
