Topic 7.
PHTHALOCYANINES
INTRODUCTION
The phthalocyanines represent the most important chromophoric system developed during the
twentieth century. In 1907, von Braun and Tscherniak were engaged in a study of the chemistry of o-
cyanobenzamide, 5.1, and discovered that when this compound was heated a trace amount of a blue
substance was obtained. This compound was almost certainly metal-free phthalocyanine, 5.2. In
1927, de Diesbach and von der Weid reported that when 1,2-dibromobenzene was treated with
copper(I) cyanide in boiling quinoline for eight hours, a blue product was obtained in reasonable
yield.5 This was almost certainly the first preparation of copper phthalocyanine (CuPc). In 1928, in
the manufacture of phthalimide by Scottish Dyes (later to become ,part of ICI) from the reaction of
phthalic anhydride with ammonia in a glass-lined reactor, the formation of a blue impurity was
observed in certain production batches. This contaminant was isolated as a dark blue, insoluble
crystalline substance. Ultimately, the compound proved to be iron phthalocyanine (FePc). An
independent synthesis involving passing ammonia gas through molten phthalic anhydride in the
presence of iron filings confirmed the findings. Following this discovery, the colour manufacturing
industry was quick to recognise the unique properties of the compounds and to exploit their
commercial potential. The phthalocyanines have subsequently emerged as one of the most
extensively studied classes of compounds, because of their intense, bright colours, their high stability
and their unique molecular structure.
STRUCTURE AND PROPERTIES OF PHTHALOCYANINES
The phthalocyanine system, which may be considered as the tetraaza derivative of
tetrabenzoporphin, is planar, consisting of four isoindole units connected by four nitrogen atoms
forming an internal 16-membered ring of alternate carbon and nitrogen atoms. Most phthalocyanines
contain a central complexed metal atom, derivatives having been prepared from most of the metals in
the periodic table. The central metal atom is in a square-planar environment. The phthalocyanines
are structurally related to the natural pigments chlorophyll, 5.4, and haemin, 5.5, which are porphyrin
derivatives. However, unlike these natural colorants, which have limited stability, the phthalocyanines
exhibit exceptional stability and they are in fact probably the most stable of all synthetic organic
colorants. Copper phthalocyanine, used here as an example, is usually illustrated as structure 5.3,
which contains three benzenoid and one o-quinonoid outer rings.
However, it has been established that the molecule is centrosymmetric and this means that structure
5.3 should be regarded as only one of a large number of resonance forms contributing to the overall
molecular structure. The extensive resonance stabilisation of the phthalocyanines may well account
for their high stability. The phthalocyanines are aromatic molecules, a feature that has been attributed
to the 18 p-electrons in the perimeter of the molecules. Phthalocyanines, together with porphyrins,
are referred to in general as aza[18]annulenes, the term annulene denoting a conjugatedcyclic
system of methine groups. The metal phthalocyanines in general show brilliant, intense colours. The
colours of traditional phthalocyanine dyes and pigments are restricted to blues and greens, although
recent years have seen the development of several derivatives whose absorption isextended into the
near-infrared region of the spectrum. In particular, copper phthalocyanine, 5.3, CI Pigment Blue 15, is
by far the most important blue pigment, finding almost universal use as a colorant in a wide range of
paint, printing ink and plastics applications. In fact, there is a convincing argument that it is the most
important of all organic pigments.It owes this dominant position to its intense brilliant blue colour and
excellent technical performance. The pigment exhibits exceptional stability to light, heat, solvents,
alkalis, acids and other chemicals. In addition, copper phthalocyanine is a relatively low cost product
since. Copper phthalocyanine exhibits polymorphism, which refers to the ability of a material to adopt
different crystal structural arrangements or phases. While the industrial importance of
phthalocyanines is dominated by traditional applications as pigments and, to a lesser extent, textile
dyes, they have been extensively investigated for a wide range of other applications because of their
unique light absorption, electronic and chemical properties and their high stability. As functional
colorants, they are of interest in electrochromic systems, electrophotography, optical data storage,
organic solar cells and as photosensitisers for photodynamic therapy of cancer.
SYNTHESIS OF PHTHALOCYANINES
The synthesis of metal phthalocyanines requires essentially the presence of three components: a
phthalic acid derivative, such as phthalic anhydride, phthalimide, phthalonitrile or o-cyanobenzamide,
source of nitrogen (in cases where the phthalic acid derivative does not itself contain sufficient
nitrogen) and an appropriate metal derivative. Commonly the reaction requires high temperatures
and may be carried out in a high boiling solvent or as a ‘dry bake’ process. In this way, using
appropriate starting materials and reaction conditions, virtually the entire range of metal
phthalocyanines may be prepared. Substituted phthalocyanines are prepared either by using an
appropriately substituted phthalic acid derivative as a starting material, or by substitution reactions
carried out on the unsubstituted derivatives. Metal-free phthalocyanines are conveniently prepared by
subjecting certain labile metal derivatives, such as those of sodium orlithium, to acidic conditions.
Although the structure of copper phthalocyanine is rather complex, its synthesis is remarkably
straightforward. It may be prepared in virtually quantitative yield from readily available, low cost
starting materials. Two chemically related methods, the phthalic anhydride and phthalonitrile routes,
are commonly used for its manufacture. Both involve simultaneous synthesis of the ligand and metal
complex formation in a template procedure.
(a) The phthalic anhydride route In the most commonly-encountered version of this method,
phthalic anhydride is heated with urea, copper(I) chloride and a catalytic amount of ammonium
molybdate in a high boiling solvent. An outline of the process is given in Scheme 5.1. Urea acts as
the source of nitrogen in the process, the carbonyl group of the urea molecule being displaced as
carbon dioxide. In essence, phthalic anhydride reacts with urea or products of its decomposition or
polymerisation, resulting in progressive replacement of the oxygen atoms by nitrogen and, ultimately,
the formation of the key intermediate 1-amino-3-iminoisoindoline, 5.8. The presence of ammonium
molybdate is essential to catalyse this part of the sequence. Subsequently, this intermediate
undergoes a tetramerisation with cyclisation aided by the presence of the copper ion to form copper
phthalocyanine. (b) The phthalonitrile route In this process, phthalonitrile, 5.9, is heated to around
200 1C with copper metal or a copper salt, with or without a solvent. A mechanism for the
phthalonitrile route to copper phthalocyanine has been proposed as illustrated in Scheme 5.2.
It is suggested that reaction is initiated by attack by a nucleophile (Y_), most likely the counteranion
associated with the Cu21 ion, at one of the cyano groups of the phthalonitrile activated by its
coordination with the Cu21 ion. Cyclisation to isoindoline derivative 5.10 then takes place. Attack by
intermediate 5.10 on a further molecule of phthalonitrile then takes place and, following a series of
similar reactions including a cyclisation step, facilitated by the coordinating role of the Cu21,
intermediate 5.11 is formed. When copper metal is the reactant, it is proposed that two electrons are
transferred from the metal, allowing elimination of Y_ to form copper phthalocyanine [route (i)].
Consequently, the Cu(0) is oxidised to Cu(II) as required to participate further in the reaction. When a
copper(II) salt is used, it is suggested that Y1 (the chloronium ion in the case of CuCl2) is eliminated
to form CuPc [route (ii)]. The product in this case, rather than copper phthalocyanine itself, is a
monochloro derivative, formed by electrophilic attack of Cl1 on the copper phthalocyanine initially
formed. Copper monochlorophthalocyanine is important as it exists exclusively in the a-crystal form,
which, unlike unsubstituted CuPc, is stable to solvents. It has been suggested that the single chlorine
atom sterically prevents conversion into the b-form. Both the phthalic anhydride and phthalonitrile
routes generally produce a crude blue product, which is of far too large a particle size to be of use as
a pigment. Scheme 5.3 shows an outline of some important substitution reactions of copper
phthalocyanine. Synthesis of the phthalocyanine green pigments involves the direct exhaustive
halogenation of crude copper phthalocyanine blue with chlorine or bromine or an appropriate mixture
of the two halogens, depending on the particular product required, at elevated temperatures in a
suitable solvent, commonly an AlCl3/NaCl melt. These reactions are examples of electrophilic
substitution, reflecting the aromatic character of the copper phthalocyanine molecule.
The crude form may be converted into an appropriate pigmentary form either by treatment with
suitable organic solvents or by treatment with aqueous surfactant solutions. Treatment
ofpolyhalogenated copper phthalocyanines 5.12 with thiophenols in the presence of alkali at high
temperatures in high boiling solvents gives the near-infrared absorbing polyarylthio CuPc derivatives
5.13 as illustrated in Scheme 5.3. This process provides an example of aromatic nucleophilic
substitution in the phthalocyanine system. X-Ray structural analysis of these arylthio derivatives
demonstrates that the sulfur atoms are located in the plane of the CuPc system while the aryl groups
are twisted to accommodate the steric congestion. The disruption of planarity, and hence of the
molecular packing, provides these derivatives with solubility in organic solvents.
Direct phthalocyanine dyes
The Direct Turquoise Blue Light-resistant is obtained by the sulfonation of the copper phthalocyanine
with 25% oleum and subsequent release in the form of the sodium salt. The latter is dissolved in the
water and has affinity to the cellulose fibers; applied at painting the cotton fabrics, viscose fiber, silk,
varnishes for wallpapers, as well as in the polygraphic and paper industries.
At the reaction of the copper phthalocyanine (Cu-P) with chlorosulfuric acid in the presence of thionyl
chloride, the copper phthalocyanine tetrasulfochloride - Cu-P(SO2Cl)4 is obtained. The last one is an
initial compound in the production of a range of dyes.
For instance, at the production of the Direct Turquoise Blue Light-resistant C dye used Cu-P(SO 2Cl)4
and ammonia. The resulting product is a diammonium salt of the copper phthalocyanine disulfamide -
Cu-P(SO2NH)2(SO3-NH4+)2. This compound has very durable and clear color.
As another example, the interaction of the copper phthalocyanine tetrasulfochloride and 3-amino-5-
sulfosalicylic acid with the formation of the Chromium Turquoise Blue for silk can be given:
Active phthalocyanine dyes
On the basis of the phthalocyanine dyes we can get the active dyes of greenish-blue and bright-
green tones by the addition of substituents that are able to react with a colored material forming the
covalent bonds.
At the interaction of the copper phthalocyanine tetrasulfochloride with chloroethylamine hydrochloride
the Active Turquoise Blue 23 dye is produced:
β-chloroethyl (-CH2CH2Cl) group is the active group; it reacts with amino and hydroxy groups of the
dyed fibers.
Active phthalocyanine dyes are also obtained by using the cyanuric chloride. For this purpose, the
copper phthalocyanine tetrasulfochloride is treated by the 1,3-phenylenediamine-4-sulfo acid, and
then the amino group is acylated with the cyanuric chloride or the product of substitution of one of its
chlorine atoms by the residue of aminosulfo acid, for example:
Phthalocyanogens
Phthalocyanogens are compounds which used for the production of the insoluble phthalocyanine
dyes directly on the fiber.
The most abandoned one is the Phthalocyanogen 43M:
Phthalocyanogen dyes are used for smooth dyeing and printing on the cotton fabrics; used in the
form of pastes that include metal salts – chelants (chelating agents), solvents with high boiling
temperature and different additives possessing the surface-active properties.
Vat phthalocyanine dyes
The main interest is directed to the monosulfoacid of the cobalt phthalocyanine: it is insoluble in the
water as the one sulfo group is not enough for a large molecular mass of the molecule [6].
Sulfo group eases the transfer of the monosulfoacid of the cobalt phthaocyanine into solution of the
leuco-compound [7] that is formed by the reduction of this acid with dithionite in the basic medium.
The leuco-compound has affinity to the cellulose, but at the oxidation it turns into the initial insoluble
dye, i.e. monosulfoacid has properties of the vat dye. The "vat" is an olive color.
Porphyrins
Being the analogues of phthalocyanines, porphyrins are macrocycles consisting on the pyrrole
residues joined to each other by the methine groups. In other words, the basis of the chromophore
system of the porphyrins is the tetra-azo-cyclo-hexadecene rings.The simplest representative of this
group is the porphine.
Porphyrins have an important role in the nature. As an example, the green chlorophyll magnesium
complex of one of the porphyrins, the red iron complex – blood hemoglobin of the living organisms,
can be given.
Complexes of the porphyrins with different metals are useful for dyeing plastics.
The structural analogues of the phthalocyanine are the derivatives of tetrabenzoporphine,
tetrabenzoporphyrin and their complexes with metals (Zn, Cd, Fe, Mg).
tetrabenzoporphine
