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Relationship between the optical properties of coloured pigments and their morphology and crystalline structure
Progress in Organic 0 Elsevier Sequoia
Coatings, 6’(1978) S-A., Lausanne -
31
310. 48 Printed in the Netherlands
RELATIONSHIP BETWEEN THE OPTICAL PROPERTIES OF COLOURED PIGhlENTS AND THEIR MORPHOLOGY AND CRYSTALLINE STRUCTURE
L. CHROMY Silesian
University,
Katowice
(Poland)
E. KAMINSKA Paint
Research
1nsfitut.z.
Gliwice
(Poland)
Contents Introhuction, 31 Relationship .between colour and chemical constitution, 32 Relationship between colour and morphology of pigment particles, 34 Relationship between crystal structure of organic pigments and their colour 36 Perylene pigments, 38 5.1 Relationship between colour and particle morphology, 38 5.2 Conditions of crystalline transformation of perylene diimide, 41 5.3 Phase transition mechanism, 43 Concltision, 47 References, 46
properties,
1. Introduction At the beginning of the twentieth century, the organic pigment industry was closely connected with the dyestuff industry, utilizing its intermediates and insoluble dyes. However, the development of the new and more complex pigments such as phthalocyanines,quinacridones and others has led to the separation of the pigment industry. It is characteristic that the number of chemical compounds used as pigments has not changed considerably during the last thirty years. However, the great progress in this field involves manufacturing a number of pigments varying in colour, application and resistance properties by modification of the crystal structure and morphology of a single chemical compoulrd. Initially, the work was purely experimental. Then the structural investigations were undertaken, aimed at the determination of the relationships between substituent type, particle morphology, crystal structure and colour properties of the pigment. A pigment in a paint forms a suspension of particles of crystalline solid dispersed in a liquid medium; i.e. a paint binder. The primary pigment par-
32 titles, neglecting aggregates and agglomerates; are built of crystals and crystallites, and their size, size distribution and shape influence the brightness, intensity, shade, purity and colour depth of the pigment. A very important factor is also the crystal structure of the pigment, which is now being widely investigated for both inorganic and organic pigments.
2. Relationship
between
colour
and chemical
constitution
The theory relating the colour of a compound to its chemical constitution originated during the course of investigating the structure of molecules of coloured organic compounds_ It was found that the position and form of the absorption bands of organic compounds in the visible and ultraviolet part of the.absorption spectrum are closely related to the structure of the molecules [l] _ Molecules comprising chromophoric groups and having similar electronic structures have similar absorption spectra, i.e. the shape of the extinction curve, the position of the maxima, and the intensity of the bands. The presence of other groups near the chromophores changes the spectrum shape. The changes may result in the displacement of the absorption bands in the long-wave region (bathochromic effect), to the short-wave region (hypsochromic effect), or the increase or decrease of intensity of the absorption bands (hyperchromic and hypochromic effects, respectively). Good agreement between theoretical and experimental data was not achieved until the absorption spectra of the organic compounds were interpreted in terms of classical vibration theory_ More accurate interpretation of the spectrum was attained by applying the assumptions of quantum mechanics [ 1, 21 _ Absorption of visible or ultraviolet light by the molecules causes an electron shift from a lower to an upper energy level. The energy increase of 4E is determined by the Bohr equation AE
(kcal/mol)
2.860 - lo5 = --___ x (A)
The energy of the electron shift depends on the wavelength h of the light being absorbed_ If a given compound has a single absorption band at a specified wavelength, then it is possible to determine the colour of the light absorbed and at the same time the colour of the compound_ This statement is illustrated in Table 1. From the graph in Fig. 1 it can be stated that the relationship between energy and wave number is linear, while that between energy and wavelength is non-linear_ In quantum chemistry, the relationship between a compound constitution and its spectrum is described by the valence bond (VB) and molecular orbital (MO) methods. Both models are particular cases of the LCAO method. The MO and VB theories have to be greatly simplified for application to organic substances exhibiting strong absorption of visible light, i.e. pigments.
33
TABLE
1
Coloursofsubstancesshowingsingle
absorptionbandsinthe
Wavelength (A)
Wavenumber -1 (cm )
E (kcal mole-')
2000-3000 3000-4000 4000-4350 4350-4800 4800-4900 4900-5000 5000-5600 5600-5800 5800-5950 5950-6050 6050-7500
50,000 33,333 25,000 23,000 20,840 20,410 20,000 17,860 17,240 16,610 16,530
143.0 -95.4 95.4 - 71.5 71.5 - 65.7 65.7 - 59.6 59.6 -58.4 58.4 - 57.2 57.2 - 51.1 51-l-49.3 49.3 -48-I. 48.1 -47.8 47.3 - 38.1
-33,333 -25,000 - 23,000 -20,840 - 20,410 - 20,000 - 17,860 -17,240 - 16,810 -18,530 - 13,330
visibleregion ofthespectrum Colourof light. absorbed
Colour of compound
violet blue green-blue blue-green green
yellow-green yellow orange red purple violet blue green-blue blue-green
yellow-green yellow
orange red
i60
4110 420
400 7 9.I -6 E = y"
so
60
50
'10
lo-'
Fig. l-Relation
between
20
JO
v ( cd)
excitation energy and wavelength.
IC
34
Another theory that gave good results in the case of the substances discussed is also based on quantum mechanics and is called the free electron theory (FE). The FE model refers only to the TTelectrons occurring in the conjugated systems, and because of these, the dyes and pigments have their colour. It can be assumed that the absorption spectrum of dyes comprising large molecules depends mainly on the size, shape and symmetry of that part of the molecule encompassing the x electrons_ This theory was initially applied to polyolefins and then widened to branched compounds, e.g. cyanines, basic dyes and phthalocyanines. It can also be assumed that the modified PE model, taking into consideration the perturbation theory, is applicable to azo dyes and pigments Cl] _ 3. Relationship
between
colour
and morphology
of pigment
particles
A pigment can be defined as a mixture of solid particles comprising crystals or crystallites arranged in a crystal lattice. Owing to the pigment structure, the colour and particularly its characteristics such as brightness, intensity, shade, purity and depth are determined not only by its chemical constitution but also its physical form. Here, the physical phenomena, i.e. light scattering and absorption by the pigment particles, are of great importance. Both light absorption and diffusion depend on the size of the particles. The optics of pigment layers in paint coatings has been developed by Kubelka and Munk [3,4]. It is determined by the Kubelka-Munk equation;
K
-= s
(1 -R)2 2R
The absorption coefficient K and scattering coefficient S are dependent on the incident light wavelength. Dunken [35] showed that the K and S coefficients of the pigment mixture are additive functions of the K and S coefficients for individual components_ For two components A and B, the Kubelka-Munk equation can be written
K -= S
QKA + bKB asA
+ bSB
The theory of Kubelka-Munk sometimes does not meet practical co&i: Cons. It is applicable to ideally diffused light only, i.e. light of unoriented radiation; this refers both to the incident light and to that within the layer. Thus other theories have been developed which, while taking these deviations into account, are necessarily more complicated [5]. Organic pigments are used as solid substances. Individual particles of the coloured compounds (organic) have anisotropic properties_ Therefore the arrangement of the absorbing particles in the crystal lattice influences’the light absorption and in consequence the colour of the pigments. In contrast, dyes are used mainly in solution, for which the solid state orientation is destroyed, so the crystalline modifications of the same com1
pound used as a pigment may vary in colour properties. Hence the morpholo,gy of pigment particles can be of great importance in determining the coloured compound application. The first trials of preparing pigments from coloured organic compounds involved maximum particle comminution. In the case of vat dyes, particle comminution can be achieved by means of vat reduction followed by the osidation of the leuco form under specified conditions. The acid pasting process has become a more widely used technique for pigment preparation. It consists of solubilizing the coloured compound in concentrated acids, i.e. sulphuric, phosphoric, chlorosulphonic or other acids, or its conditioning in solutions of these acids at a specified temperature within a predetermined period of time. After diluting the solution with an excess of water the finely dispersed solid phase is obtained. In the last twenty years a number of patents [6 - 91 have been issued claiming other methods of pigment treatment, e.g. grinding with inorganic salts and p-toluenesulphonic acid salts in the presence of solvents as well as treating the pigments with organic solvents or water under pressure at elevated temperatures. Applying the appropriately selected methods, it is possible to prepare pigments comprising particles of the appropriate size and shape which influence in turn their colour and technological properties. There are many publications [ 10 - ZO] which describe investigations of the influence of pigment particle size and size distribution on the optical properties of the pigment. However, the influence of particle shape on optical properties is not yet fully explained, and literature data are rather empirical. In a number of publications [ 1, 17,211, it is stated that the hiding power of the pigment depends on the light scattered by the pigment particles. Hiding power increases as the particle diameter decreases, reaching the maximum when this diameter becomes close to the value of the incident light wavelength, and then decreases after further comminution of the particles. On the other hand, the colour.properties of the pigment, i.e. the shade, colour purity and the tinting strength, depend on the incident light absorption. The absorption magnitude determines the tinting strength, whereas the position and width of the absorption band in the visible light spectrum determines the colour and colour shade purity. Honigmann [22], based on his own investigations and others, has found some general principles concerning the influence of particle size on pigment structure. He assumes that the pigment particles are spherical, thus further considerations are restricted to pigments the particles of which are built of purely crystalline aggregates being compact or monocrystals, because this is the only case when it is possible to compare theoretical with experimental data. Furthermore, the cohesion of these particles should be sufficiently poor to make it possible to destroy their agglomerates by dispersion. If the conditions mentioned above are satisfied, then the following statements are true: (1) Organic pigments show their optimum colour properties at a particle size of 0.05 - 1 mm. (2) Blue pigments show their optimum colour properties in the lower range of particle size, while for the red ones it is in the higher range.
36
(3) Hiding power and light resistance increase as the particle size increases in the given range, whereas the tinting strength increases when the particle size decreases. (4) Pigment colour shades change continuously depending on particle size. Generally, the red and green pigments of small particle size are bluish and larger particles are yellowish. Blue pigments of larger particle size have a greenish shade and of smaller size have a reddish shade. The influence of particle size on pigment properties has been investigated by Pilpel [ 12) and Kresse [ 19]_ Pilpel presents a number of methods for determining the shape of pigment particles in relation to their size, and also descri.bes some phenomena created by the variety of shapes_ A different shape of the particles affects both brightness and colour shade of the pigment. For example, with yellow and red pigments comprising acicular particles, the increase in cross-section of the needles causes the colour shade to be more red, while with green chromium oxide a diameter increase causes the colour shade to be more blue.
4. Relationship betkzeen colour properties ,’
crystal
structure
of organic
pigments
and
their
Differences in the colour shade of a pigment having a given chemical constitution can be effected by the different polymorphic forms being present in it, assuming that the size of the polymorphic particles is uniform. The polymorphism of organic pigments has been investigated for phthalocyanines and quinacridone pigments. Phthalocyanines are particularly interesting pigments owing to the fact that their colour shade covers the range from blue to green, and in this range they are the best pigments in terms of colour brightness, tinting strength and resistance to light, elevated temperature, and chemicals_ The polymorphic properties of the phthalocyanine compounds were known to German researchers in the thirties, but the earliest information on this matter was given in the BIOS and FIAT reports. According to Smith [ 161 and others 123, 241, the cupriferrous phthalocyanine dye exists in five crystalline modifications, but of these only two, concluded that the pigment can the Q and p, are useful. Honigmann [22,25] have different crystalline as well as physical forms, i.e. the crystal structure and the size and shape of the particles. Differences in colour and other properties can result from its occurrence in various crystalline or physical forms. X-ray analysis is the only method for determination of the form of the pigment, i.e. its crystallinity or polymorphism_ The crystalline modifications result in new diffraction lines, while the various physical forms do not give new lines, though their X-ray spectra are not identical_ Based on these assumptions, Honigrnann reduced six forms of cupriferrous phthalocyanine to only three. By analogy, Grelat [33] described four crystalline modifications of the indanthrene blue, whereas Honigmann presented only two modifications_
37
The CYcupriferrous phthalocyanine modification has a redder shade, in very small particles, and exhibits a tendency to transformation in the p modification under the influence of temperature and organic solvents. The p modification is thermally less stable and has a more green shade. The particles vary from very small in size, suitable as pigments, to needles of 2 mm in length. Mann and van Norman [26] investigated the microcrystals of the cupriferrous phthalocyanine and indanthrene blue by electron diffraction and found two cryklline forms, one of them being formed in the course of the spontaneous recrystallization occurring under electron bombardment in an electron microscope_ In both cases it is characteristic of this transformation that the formation of the large long crystals occurs after the decay of the grainy ones. Suito and Uyeda [ 271 investigated the phase transition of the cupriferrous phthalocyanine in a colloidal suspension where an aromatic solvent was used as a dispersing medium. This phase transition, leading also to the formation of the stable long needles-shaped crystals, is a serious problem in the paint industry, because such a process occurs in every paint. Hitherto, not many results of basic research work concerning the phase transition mechanism have been published. Authors have stated that the phase transformation proceeds in successive steps. At first, the growth of the unstable crystals occurs -without noticeable changes in crystal structure_ The appearance ,of the unstable crystals is similar to the stable ones and the crystal lattice is not changed. The influence of the dispersing medium on the transformation process was considerable, and the crystals formed differed each from other, depending on the applied solvent in spite of an extremely low solubility of the cupriferrous phthalocyanine in the organic solvents used in the dyestuff industry. The phase transition rate changes with temperature as well as with the use of the other solvent. This proves that the growth of the unstable crystals is effected by the recrystallization resulting from the differences in solubility of the larger or smaller pigment particles in suspension. The authors determined the inter-planar distances for three main lattice planes as follows: 12.9, 12.1 and 3.79 i%. Two planes of the largest distances - 12.9 and 12.1 rsi - are usually parallel to the longitudinal axis of the long crystals, while the planes of 3.79 ,4 distance lie within the axis. The distance of 3.79 i$ was assumed as one of the periods of the elementary space cell based on both the data obtained and the fact that this value is very often cited for a great number of other compounds, e.g. condensed multicyclic aromatic compounds such as indanthrene, flavanthrone and violanthrone being susceptible to the formation of the individual long crystals, particularly under vacuum condensation_ The distances along the longitudinal axis of these crystals are always in the range 3 -76 - 3.79 8,, being very close to the interplanar distance of graphite, 3.56 a. Lincke [28] gives the value of this distance for many pigments comprising large particles to be equal to 3-22 - 3.56 a _ It was found that the crystal structure of a metastable form of cupriferrous phthalocyanine is isomorphous to platinum phthalocyanine, which was determined by Robertson [29] to be monoclinic. This fact was also established independently by Brown 1301, who used the X-ray diffraction method. He stated
38 that the elementary cells of platinum phthalocyanine were twice as large as the cupriferrous phthalocyanine cells. The cupriferrous phthalocyanine molecules have the planar configuration comprising four identical isoindole rings and a central cupriferrous atom. The molecules has a shape similar to a tetrapetalous flower with the diagonal distance being above 13 A _ Two molecules in the crystal are closely packed within the columns parallel to the longitudinal axis of the long crystals forming the lattice planes of large distances: 12.9 A and 12.1 .& for the unstable form and 12.6 and 9.6 A for the stable one. But the intermolecular distances in a particular column are close to that distance in graphite, and the changes of inclination angle and direction of the molecule planes to the column axis probably cause changes in the lattice period along the axis distance from 3.79 to 4.79 a during transformation of the unstable form to the stable one. Small crystals of the unstable form show a tendency to attain the shape of the stable crystals_ Similarities were found both in crystal appearance and in their lattice orientation. After growth, the metastable crystals form thin flakes. It is supposed that the displacement of the molecules takes place mainly at the comers of the crystal lattice, even the particular crystals, where the solvent “loosens” in cases of extremely low solubility, and makes it possible to obtain the stable nucleus by formation of sufficient space for rotation of the particular molecules. Finally, the molecules reach a stable position in the crystal. The enthalpy measurements carried out for both crystalline forms by Benyon and Humphries 131) prove that the stable form is in an energy state 2.46 kcal/mol lower than the unstable form. The displacement of the molecules will occur spontaneously. The nucleation can be retarded if the initial particles are very small, and then in the initial transformation phase the nucleation will occur simultaneously with the recrystallization effect. 5. Perylene
pigments
In the scientific literature there is not much information on the relationships between morphology, crystalline structure and colour properties of perylene pigments. Therefore it is advisable to present the results of our investigations 1343 in this field, as they confirm and widen the earlier findings. The colour and fastness of the tested diimides of perylene-3,4,9 JO-tetracarboxylic acid depend on the substituent type pendant to the imide group; e.g., methyldiimide is amaranth whereas methoxyphenyldiimide is bright red. The presence of the methyl group causes the pigment to be darker. Its solubility decreases while its decomposition temperature increases in the following order: methyl, methoxyphenyl and ethoxyphenyl-di&nid&. _, - -. __ “. 5.1. Relationship between colour and particle morphology :\ The perylene diimides p-urified by the vat process method h_Tve particles in the form of aggregates and agglomerates resulting in large spec&ic surfaces and irregular lamella_r form. Their colour shade is rather dir&&d the tinting strength is low in comparison with their large specific surfaces.
(a)
a
A
-
_
0
im
(cl Fig.
2 (a)
- (e).
(For
caption
see over-leaf.)
._.
_
=‘~
b
(d)
Fig. 2. Electron micrographs of various forms of perylene diimides (X 10,000): (a) after the vat process; (b, c), after treatment in acid medium; (d), after treatment in toluene; (e), after treatment in nitrobenzene; (f) after treztment in quinoline.
41
The pigments of the bluish shade have acicular particles of O-08 - 0.35 pm in length. The larger and longer the crystals and crystallites, the more bluish the colour shade. Decreasing the particle size and increasing the cross-section of crystals and crystallites cause the colour shade to become more yellowish. The colour purity increases with decreasing size of particles. In the range 0.05 - 0.1 pm mean particle size, the tinting strength is high and almost independent of the particle size. Above O-1 pm, the tinting strength decreases gradually when the size of particle increases and then it decreases sharply when the particle diameter exceeds 0.2 pm (Table 2). In general, the bluish pigments have a lower tinting strength compared with the yellowish ones. The pigments prepared by treatment with organic solvents have an acicular crystalline structure, and the magnitude of the particles is proportional to the treatment temperature_ The pigments recovered from aqueous solutions of sulphuric acid have crystals of oval habit and the sizes are more uniform than in the case of acicular crystals_ X-ray phase analysis proved the existence of different crystalline structures of the pigments discussed. These structures are not associated with the shape and particle size of the pigments. 5.2
Conditions of crystalline transformation of peryleize diimide Depending on the medium, two polymorphic modifications are formed. The cx modification is formed in solutions of sulphuric acid, acetic acid and formamide. Under the influence of organic solvents, the Q modification undergoes a transformation into the stable fl modification. The polymorphic transformation depending on the medium occurs in accordance with the scheme shown in Fig. 3. The polymorphic transformation leads to particles of the same size which differ in colour shade, i.e. the Q modification is yellowish-red and the (3 modification is bluish-red. The basic pigment purified by the vat process is slightly crystalline_ This pigment, independently of the medium used for its treatment, underwent recrystallization in solvents associated with a decrease of specific surface and the growth of crystals, while their habit did not always undergo the transformation. The recrystallization process is conditioned by the solution saturation and the occurrence of chemical potential differences on the surface of the pigment particle. Recrystallization changes the physical properties of the pigments if the size of their particles does not exceed 0.1 pm. The recrystallization process can result from molecular surface diffusion or transfer of the molecules through the saturated solution_ In the case of a substance of solubility lower than 1 mg per litre, i.e. perylene diimide, the molecular transfer resulting in the recrystallization occurs mainly through surface diffusion, and through the solution only to a limited extent. Equilibrium is attained when the different sites on the surface reach the same chemical potential, or the potential difference is lower than the activation energy. During the recrysallization process, the crystalline aggregates become more compact and the growth of crystals occurs. In many cases their shape is changed, and some-
_
-?
-
-
-.
-
0.3992 0.3849 0.3896 0.3377 0.3328
a
“,
0.3503 0.3864 0.3710 0.3571 0.3658 0.3500
.I
,,,
CIE 1931
P
modification
Polymorpllic
,,,
_
,,>
-
29.00 27.88 25.60 50.70 46.41
0.3188 0.3090 0.3004 0.3136 0.3080
^
39.76 30.65 35.70 36.43 33.31 38.44
0.3130 0.3172 0.3155 0.3072 0.3090 0.3077
Y
Y
-
_
7.1719 3.044 1 2.4804 4.8967 0.96GO
-.
c1
8.6184 9.6318 11.7819 -7.2847 -4.5664
-
AL
Id
8.5520 5.4531 4.2059 5.7960 1.8553
_
L.,,
-1183692 -10.3833 -14 a2062 3.8531 3.7964
-
Aa
L (I b coordinates
Colour properties and particle size of perylcne diimidcs
TABLE 2
-
_
-7.5286 -2.8955 -1.0764 1.320.1 3.7811
-5.5405 -3.3899 -0.8426 -0.7230 1.6063
Ab
L, ,,
- _
16.1312 14.4550 18.4875 8.3460 7.0400
_
_
,,,
.>
-
0.07 0.08 0.10 0.32 0.35
137.5 142.5 155.0 7883 85.8
”
0.04 0.08 0.05 0.16 0.09 0.17
_
(nm)
size
.
.
-
Mean particle
100.0 129.9 111.0 109,o 118.8 103.5
(%I
AL:
12.4607 7.1059 4.9549 7.6219 2.6374
strength
Tinting
difference
Colour
Ip N
43
Rmorphic
I
Solwd
a-modlf~rolion
Fig.
3. The
structure
treotbng B- modifirotinn
scheme
of crystalline
transformation
of perylene
diimide.
times their crystalline structure is also transformed_ The crystalline transformation is influenced by the chemical potential differences resulting from the lattice energy differences in both stable and unstable modifications. After the vat process, the perylene diimide is in the p modification. In _ all the organic solvents used, except acetic acid and formamide, only growth of crystals takes place, and sometimes the crystal habit.transformation, defined as the “physical form” transformation according to Honigrnann’s nomenclature. Treatment in acetic acid, formamide and sulphuric acid solutions also causes crystalline structure transformations_ The transformation of the p modification into the (Y form occurs rapidly, not only in the case of the slightly crystalline basic .pigment but also with the highly crystalline pigment prepared by the solvent treatment technique. 5.3 Phase transition mechanism To determine the factors influencing the polymorphism of the perylene pigments is a very difficult matter, due to various interactions, mainly those between molecules in a crystalline state. The pigment molecule cannot be considered only as the composition of the atoms, and thus the interactions occurring in the atom cannot be transferred to the molecule, but it should be considered as the completely separate formation_ From this point of view, the crystal is not the arranged set of the molecules, ions or atoms, but it is the single large molecule in which the electrons, partic-ularly those delocated, belong to the whole crystal and not just to the specific molecules or atomic groups or atoms. Thus every hypothesis that tries to explain the arrangement of the molecules in the crystal in terms of quantum mechanics is only an approximation of the real state, causing some possible effects to be determined experitientally _ The perylerie pigments considered are cyclic imides in which the hydrogen atom is substituted by the methoxyphenyl group. The resonant structures of
44
the cyclic imide can be described by analogy with the succinic imide resonant structures, owing to their similarity_ The electron pair at the nitrogen atom is located near two carbonyl groups_ If, in the medium comprising proton donors, proton addition to the nitrogen atom occurs forming the conjugated acid, then the molecule loses its initial stability, thus it can be espected that proton addition to the carbony1 oxygen mostly probable. Recently, the mechanism of the reactions occurring in the paint systems as well as their properties have been explained through the formation of hydrogen bonds between particular paint components. Perylene pigments are basic diimides (proton acceptors)_ It was found experimentally that in solutions of sulphuric acid, acetic acid and formamide, being proton donors for perylene diimides, the phase transition of the p modification to the Q’ modification occurs_ The formation of the polymorphous CYmodification can be explained through the formation of transient complex compounds with the solvent. This is proved to some extent through the appearance of the i.r. spectra of the perylene diimides with appropriate solvents (Fig. 4). The charact.eristic vibrations of the I amide band for the perylene diimide in nujol occur at frequencies of 1,660 cm-’ and 1,700 - 1,705 cm- ‘. Bellamy [ 321, for the compound of the formula
in the solid state, gives frequencies of 1,660 and 1,690 cm- 1 _ These bands for the perylene diimides in acetic acid and formamide are broadened and displaced, while with other solvents, e.g. nitrobenzene, such effects do not occur. It may therefore be concluded that the polymorphism of the perylene pigments is caused by the different arrangement of the diimide molecules in the crystal lattice, effected in turn by the formation of complexes in the proton-forming medium ofsulphuric acid, acetic acid and formamide (Fig. 5). The packing of the perylene diimide molecules in the crystal lattice is presented, taking into consideration the earlier works describing the crystal structure of phthalocyanines and other pigments which consist of large planar molecules comprising condensed rings analogous to the graphite structure. The distance between the slip planes in graphite is 3.36 A. The X-ray photographs of all the perylene diimides tested have a strong reflection at angles corresponding to the interplanar distance characteristic for the rr polymorphic modification obtained in a proton-forming medium, which is larger (3.49 A) than for the p modification prepared in other media (3.06 A). The Q’ modification is unstable in organic solvents owing to its lower decomposition temperature, and transformation into the p modification occurs during treatment in solvents. The solvents cause the intermolecular forces to be weakened, and also cause a change of the molecule arrangement due to the
.,
. .- _
1)
, > ,_ ..,
. _ - . . ,,, _. -
_
XNJell!WSUeJ~
5
FJ3Uf2ll&llStl~J~ llJil3J+j
llKI3Jad
Cl3UR~l!WSlEJ~ lUCI3JCid
KNJell]llJSUCJ~
1UXlJad
46
on
,;_~$z
‘,
&N-
0 Fig.
Fig.
0
5. Complex
d-
6.
so9
of perylene
diimide
in a proton-forming
medium.
mociif~caticn
S-ray
diffraction
of perylene
diimide.
fact that the molecule planes become close to each other. Thus the thermodynamically more stable polymorphic modification of the shorter interplanar distances between the diimide molecule surface is being formed. The packing of the diimide molecules in the crystal lattice columns for both polymorphic modifications is illustrated in Fig. 7. The large molecules of the pigments can lie out of the plane due to steric hindrance as shown in the figure. Further arguments supporting the metihanism of the phase transitions should be obtained from investigations on the wetting heat of pigments in solvents, making it’possible in turn to determine the hydrogen bonding power.
6 _ Conclusion The production of pigments has been known for centuries, and in many cases the procedures developed have been based on experience rather than on scientific knowledge_ Improvements in the quality of pigments achieved during the last twenty years are quite impressive. This is a result of the great
47
j3 - modification
5 - modification Fig.
7. Hypothetical
packing
of
the diimide
molecules
in the crystal
attention paid to the scientific background of pigment paper, we have tried to review some important aspects
of perylene
preparation_ thereof.
diimide.
In this
References 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 16
D. Patterson, Pigments: ti Introduction to their Physical Chemistry, Elsevier, London, 1965. K. Guminski, Chemia teoretyczna, PWN, Warsaw, 1969. P. Kubelka and K. Munk. Z. Tech. Phys., 12 (1931) 593. P. Kubelka, J. Opt. Sot. Am., 3s (1949) 4-19; 44 (1954) 330. H. G. VSlz, Prog. Org. Coat., 1 (19’73) 1. U. S. Pat. 3032299. Osterreichisches Pat. 244462. Swiss. Pat. 372163. Br. Pat. 1056299. W. Gerstner, J. Oil Colour Chem. Assoc., 49 (1966) 954. W. Gerstner, J. Oil Colour Chern. &soc., 49 (1966) 457. N. Pilpel. Paint. Manuf., 39 (7) (1969) 23 - 28. A. R. Hanke, J. Opt. Sot. Am., 56 (5) (1966) 7 - 13. D. Patterson, Paint’Oil Colour J., 140 (1964) 175. J. D. Easton and F. M. Smith, J. Oil Colour Chem. Assoc., 47 (1964) 250. F:‘M. Smith, J. Oil Colour Chem. Assoc., 49 (1966) 614. R. A. Bolomey and L. M. Greenstein, J. Paint. Technol., 44 (566) (1972) 39. 545 - 550. P. Hanser, M. Herman and B. Honigmann, Farbe Lack, 76 (6) (1970)
48 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
P. J_ M. B. J. W. B. F. B. G. J. C. J. L. M. E. D.
Kresse, J. Oil Colour Chem. Assoc., 49 (1960) 808. D. Hildreth and D. Patterson, J. Sot. Dyers Colour., SO (1964) 474 - 479. Moikowski, Staub, 22 (1962) 335 - 339. Honigmann, J. Paint Technol., 38 (493) (1966) 77. Stabenow, Finish, 15 (1968) 334. Herbst and K. Merkle, Dtsch. Farben Z., 24 (1970) 365. Honigmann, Farbe Lack, 70 (1964) 787. A. Mann and E. van Norman, J. Appl. Phys., 19 (1948) 1097. Suito and N. Uyeda, Kolloid Z., 193 (2) (1963) 7. Lincke, Farbe Lack, 76 (1970) 764. M. Robertson, J. Chem. Sot. A, (615) (1935); (195) (1936) 1; (219) (1937). J. Brown, J. Chem. Sot. A, 10 (1968) 2488. H. Benyon and A. R. Humphries, Trans. Faraday Sot., 51 (1955) 1065. J. Bellamy, Advances in Infrared Group Frequencies, Methuen, London, 1969. Grelat, FATIPEC Congressbook, 1964. Kaminska, Diss.. Silesian University, Katowice, 1975. R. Duncan, J. Oil Colour Chem. Assoc., 32 (1949) 396.
Coatings, 6’(1978) S-A., Lausanne -
31
310. 48 Printed in the Netherlands
RELATIONSHIP BETWEEN THE OPTICAL PROPERTIES OF COLOURED PIGhlENTS AND THEIR MORPHOLOGY AND CRYSTALLINE STRUCTURE
L. CHROMY Silesian
University,
Katowice
(Poland)
E. KAMINSKA Paint
Research
1nsfitut.z.
Gliwice
(Poland)
Contents Introhuction, 31 Relationship .between colour and chemical constitution, 32 Relationship between colour and morphology of pigment particles, 34 Relationship between crystal structure of organic pigments and their colour 36 Perylene pigments, 38 5.1 Relationship between colour and particle morphology, 38 5.2 Conditions of crystalline transformation of perylene diimide, 41 5.3 Phase transition mechanism, 43 Concltision, 47 References, 46
properties,
1. Introduction At the beginning of the twentieth century, the organic pigment industry was closely connected with the dyestuff industry, utilizing its intermediates and insoluble dyes. However, the development of the new and more complex pigments such as phthalocyanines,quinacridones and others has led to the separation of the pigment industry. It is characteristic that the number of chemical compounds used as pigments has not changed considerably during the last thirty years. However, the great progress in this field involves manufacturing a number of pigments varying in colour, application and resistance properties by modification of the crystal structure and morphology of a single chemical compoulrd. Initially, the work was purely experimental. Then the structural investigations were undertaken, aimed at the determination of the relationships between substituent type, particle morphology, crystal structure and colour properties of the pigment. A pigment in a paint forms a suspension of particles of crystalline solid dispersed in a liquid medium; i.e. a paint binder. The primary pigment par-
32 titles, neglecting aggregates and agglomerates; are built of crystals and crystallites, and their size, size distribution and shape influence the brightness, intensity, shade, purity and colour depth of the pigment. A very important factor is also the crystal structure of the pigment, which is now being widely investigated for both inorganic and organic pigments.
2. Relationship
between
colour
and chemical
constitution
The theory relating the colour of a compound to its chemical constitution originated during the course of investigating the structure of molecules of coloured organic compounds_ It was found that the position and form of the absorption bands of organic compounds in the visible and ultraviolet part of the.absorption spectrum are closely related to the structure of the molecules [l] _ Molecules comprising chromophoric groups and having similar electronic structures have similar absorption spectra, i.e. the shape of the extinction curve, the position of the maxima, and the intensity of the bands. The presence of other groups near the chromophores changes the spectrum shape. The changes may result in the displacement of the absorption bands in the long-wave region (bathochromic effect), to the short-wave region (hypsochromic effect), or the increase or decrease of intensity of the absorption bands (hyperchromic and hypochromic effects, respectively). Good agreement between theoretical and experimental data was not achieved until the absorption spectra of the organic compounds were interpreted in terms of classical vibration theory_ More accurate interpretation of the spectrum was attained by applying the assumptions of quantum mechanics [ 1, 21 _ Absorption of visible or ultraviolet light by the molecules causes an electron shift from a lower to an upper energy level. The energy increase of 4E is determined by the Bohr equation AE
(kcal/mol)
2.860 - lo5 = --___ x (A)
The energy of the electron shift depends on the wavelength h of the light being absorbed_ If a given compound has a single absorption band at a specified wavelength, then it is possible to determine the colour of the light absorbed and at the same time the colour of the compound_ This statement is illustrated in Table 1. From the graph in Fig. 1 it can be stated that the relationship between energy and wave number is linear, while that between energy and wavelength is non-linear_ In quantum chemistry, the relationship between a compound constitution and its spectrum is described by the valence bond (VB) and molecular orbital (MO) methods. Both models are particular cases of the LCAO method. The MO and VB theories have to be greatly simplified for application to organic substances exhibiting strong absorption of visible light, i.e. pigments.
33
TABLE
1
Coloursofsubstancesshowingsingle
absorptionbandsinthe
Wavelength (A)
Wavenumber -1 (cm )
E (kcal mole-')
2000-3000 3000-4000 4000-4350 4350-4800 4800-4900 4900-5000 5000-5600 5600-5800 5800-5950 5950-6050 6050-7500
50,000 33,333 25,000 23,000 20,840 20,410 20,000 17,860 17,240 16,610 16,530
143.0 -95.4 95.4 - 71.5 71.5 - 65.7 65.7 - 59.6 59.6 -58.4 58.4 - 57.2 57.2 - 51.1 51-l-49.3 49.3 -48-I. 48.1 -47.8 47.3 - 38.1
-33,333 -25,000 - 23,000 -20,840 - 20,410 - 20,000 - 17,860 -17,240 - 16,810 -18,530 - 13,330
visibleregion ofthespectrum Colourof light. absorbed
Colour of compound
violet blue green-blue blue-green green
yellow-green yellow orange red purple violet blue green-blue blue-green
yellow-green yellow
orange red
i60
4110 420
400 7 9.I -6 E = y"
so
60
50
'10
lo-'
Fig. l-Relation
between
20
JO
v ( cd)
excitation energy and wavelength.
IC
34
Another theory that gave good results in the case of the substances discussed is also based on quantum mechanics and is called the free electron theory (FE). The FE model refers only to the TTelectrons occurring in the conjugated systems, and because of these, the dyes and pigments have their colour. It can be assumed that the absorption spectrum of dyes comprising large molecules depends mainly on the size, shape and symmetry of that part of the molecule encompassing the x electrons_ This theory was initially applied to polyolefins and then widened to branched compounds, e.g. cyanines, basic dyes and phthalocyanines. It can also be assumed that the modified PE model, taking into consideration the perturbation theory, is applicable to azo dyes and pigments Cl] _ 3. Relationship
between
colour
and morphology
of pigment
particles
A pigment can be defined as a mixture of solid particles comprising crystals or crystallites arranged in a crystal lattice. Owing to the pigment structure, the colour and particularly its characteristics such as brightness, intensity, shade, purity and depth are determined not only by its chemical constitution but also its physical form. Here, the physical phenomena, i.e. light scattering and absorption by the pigment particles, are of great importance. Both light absorption and diffusion depend on the size of the particles. The optics of pigment layers in paint coatings has been developed by Kubelka and Munk [3,4]. It is determined by the Kubelka-Munk equation;
K
-= s
(1 -R)2 2R
The absorption coefficient K and scattering coefficient S are dependent on the incident light wavelength. Dunken [35] showed that the K and S coefficients of the pigment mixture are additive functions of the K and S coefficients for individual components_ For two components A and B, the Kubelka-Munk equation can be written
K -= S
QKA + bKB asA
+ bSB
The theory of Kubelka-Munk sometimes does not meet practical co&i: Cons. It is applicable to ideally diffused light only, i.e. light of unoriented radiation; this refers both to the incident light and to that within the layer. Thus other theories have been developed which, while taking these deviations into account, are necessarily more complicated [5]. Organic pigments are used as solid substances. Individual particles of the coloured compounds (organic) have anisotropic properties_ Therefore the arrangement of the absorbing particles in the crystal lattice influences’the light absorption and in consequence the colour of the pigments. In contrast, dyes are used mainly in solution, for which the solid state orientation is destroyed, so the crystalline modifications of the same com1
pound used as a pigment may vary in colour properties. Hence the morpholo,gy of pigment particles can be of great importance in determining the coloured compound application. The first trials of preparing pigments from coloured organic compounds involved maximum particle comminution. In the case of vat dyes, particle comminution can be achieved by means of vat reduction followed by the osidation of the leuco form under specified conditions. The acid pasting process has become a more widely used technique for pigment preparation. It consists of solubilizing the coloured compound in concentrated acids, i.e. sulphuric, phosphoric, chlorosulphonic or other acids, or its conditioning in solutions of these acids at a specified temperature within a predetermined period of time. After diluting the solution with an excess of water the finely dispersed solid phase is obtained. In the last twenty years a number of patents [6 - 91 have been issued claiming other methods of pigment treatment, e.g. grinding with inorganic salts and p-toluenesulphonic acid salts in the presence of solvents as well as treating the pigments with organic solvents or water under pressure at elevated temperatures. Applying the appropriately selected methods, it is possible to prepare pigments comprising particles of the appropriate size and shape which influence in turn their colour and technological properties. There are many publications [ 10 - ZO] which describe investigations of the influence of pigment particle size and size distribution on the optical properties of the pigment. However, the influence of particle shape on optical properties is not yet fully explained, and literature data are rather empirical. In a number of publications [ 1, 17,211, it is stated that the hiding power of the pigment depends on the light scattered by the pigment particles. Hiding power increases as the particle diameter decreases, reaching the maximum when this diameter becomes close to the value of the incident light wavelength, and then decreases after further comminution of the particles. On the other hand, the colour.properties of the pigment, i.e. the shade, colour purity and the tinting strength, depend on the incident light absorption. The absorption magnitude determines the tinting strength, whereas the position and width of the absorption band in the visible light spectrum determines the colour and colour shade purity. Honigmann [22], based on his own investigations and others, has found some general principles concerning the influence of particle size on pigment structure. He assumes that the pigment particles are spherical, thus further considerations are restricted to pigments the particles of which are built of purely crystalline aggregates being compact or monocrystals, because this is the only case when it is possible to compare theoretical with experimental data. Furthermore, the cohesion of these particles should be sufficiently poor to make it possible to destroy their agglomerates by dispersion. If the conditions mentioned above are satisfied, then the following statements are true: (1) Organic pigments show their optimum colour properties at a particle size of 0.05 - 1 mm. (2) Blue pigments show their optimum colour properties in the lower range of particle size, while for the red ones it is in the higher range.
36
(3) Hiding power and light resistance increase as the particle size increases in the given range, whereas the tinting strength increases when the particle size decreases. (4) Pigment colour shades change continuously depending on particle size. Generally, the red and green pigments of small particle size are bluish and larger particles are yellowish. Blue pigments of larger particle size have a greenish shade and of smaller size have a reddish shade. The influence of particle size on pigment properties has been investigated by Pilpel [ 12) and Kresse [ 19]_ Pilpel presents a number of methods for determining the shape of pigment particles in relation to their size, and also descri.bes some phenomena created by the variety of shapes_ A different shape of the particles affects both brightness and colour shade of the pigment. For example, with yellow and red pigments comprising acicular particles, the increase in cross-section of the needles causes the colour shade to be more red, while with green chromium oxide a diameter increase causes the colour shade to be more blue.
4. Relationship betkzeen colour properties ,’
crystal
structure
of organic
pigments
and
their
Differences in the colour shade of a pigment having a given chemical constitution can be effected by the different polymorphic forms being present in it, assuming that the size of the polymorphic particles is uniform. The polymorphism of organic pigments has been investigated for phthalocyanines and quinacridone pigments. Phthalocyanines are particularly interesting pigments owing to the fact that their colour shade covers the range from blue to green, and in this range they are the best pigments in terms of colour brightness, tinting strength and resistance to light, elevated temperature, and chemicals_ The polymorphic properties of the phthalocyanine compounds were known to German researchers in the thirties, but the earliest information on this matter was given in the BIOS and FIAT reports. According to Smith [ 161 and others 123, 241, the cupriferrous phthalocyanine dye exists in five crystalline modifications, but of these only two, concluded that the pigment can the Q and p, are useful. Honigmann [22,25] have different crystalline as well as physical forms, i.e. the crystal structure and the size and shape of the particles. Differences in colour and other properties can result from its occurrence in various crystalline or physical forms. X-ray analysis is the only method for determination of the form of the pigment, i.e. its crystallinity or polymorphism_ The crystalline modifications result in new diffraction lines, while the various physical forms do not give new lines, though their X-ray spectra are not identical_ Based on these assumptions, Honigrnann reduced six forms of cupriferrous phthalocyanine to only three. By analogy, Grelat [33] described four crystalline modifications of the indanthrene blue, whereas Honigmann presented only two modifications_
37
The CYcupriferrous phthalocyanine modification has a redder shade, in very small particles, and exhibits a tendency to transformation in the p modification under the influence of temperature and organic solvents. The p modification is thermally less stable and has a more green shade. The particles vary from very small in size, suitable as pigments, to needles of 2 mm in length. Mann and van Norman [26] investigated the microcrystals of the cupriferrous phthalocyanine and indanthrene blue by electron diffraction and found two cryklline forms, one of them being formed in the course of the spontaneous recrystallization occurring under electron bombardment in an electron microscope_ In both cases it is characteristic of this transformation that the formation of the large long crystals occurs after the decay of the grainy ones. Suito and Uyeda [ 271 investigated the phase transition of the cupriferrous phthalocyanine in a colloidal suspension where an aromatic solvent was used as a dispersing medium. This phase transition, leading also to the formation of the stable long needles-shaped crystals, is a serious problem in the paint industry, because such a process occurs in every paint. Hitherto, not many results of basic research work concerning the phase transition mechanism have been published. Authors have stated that the phase transformation proceeds in successive steps. At first, the growth of the unstable crystals occurs -without noticeable changes in crystal structure_ The appearance ,of the unstable crystals is similar to the stable ones and the crystal lattice is not changed. The influence of the dispersing medium on the transformation process was considerable, and the crystals formed differed each from other, depending on the applied solvent in spite of an extremely low solubility of the cupriferrous phthalocyanine in the organic solvents used in the dyestuff industry. The phase transition rate changes with temperature as well as with the use of the other solvent. This proves that the growth of the unstable crystals is effected by the recrystallization resulting from the differences in solubility of the larger or smaller pigment particles in suspension. The authors determined the inter-planar distances for three main lattice planes as follows: 12.9, 12.1 and 3.79 i%. Two planes of the largest distances - 12.9 and 12.1 rsi - are usually parallel to the longitudinal axis of the long crystals, while the planes of 3.79 ,4 distance lie within the axis. The distance of 3.79 i$ was assumed as one of the periods of the elementary space cell based on both the data obtained and the fact that this value is very often cited for a great number of other compounds, e.g. condensed multicyclic aromatic compounds such as indanthrene, flavanthrone and violanthrone being susceptible to the formation of the individual long crystals, particularly under vacuum condensation_ The distances along the longitudinal axis of these crystals are always in the range 3 -76 - 3.79 8,, being very close to the interplanar distance of graphite, 3.56 a. Lincke [28] gives the value of this distance for many pigments comprising large particles to be equal to 3-22 - 3.56 a _ It was found that the crystal structure of a metastable form of cupriferrous phthalocyanine is isomorphous to platinum phthalocyanine, which was determined by Robertson [29] to be monoclinic. This fact was also established independently by Brown 1301, who used the X-ray diffraction method. He stated
38 that the elementary cells of platinum phthalocyanine were twice as large as the cupriferrous phthalocyanine cells. The cupriferrous phthalocyanine molecules have the planar configuration comprising four identical isoindole rings and a central cupriferrous atom. The molecules has a shape similar to a tetrapetalous flower with the diagonal distance being above 13 A _ Two molecules in the crystal are closely packed within the columns parallel to the longitudinal axis of the long crystals forming the lattice planes of large distances: 12.9 A and 12.1 .& for the unstable form and 12.6 and 9.6 A for the stable one. But the intermolecular distances in a particular column are close to that distance in graphite, and the changes of inclination angle and direction of the molecule planes to the column axis probably cause changes in the lattice period along the axis distance from 3.79 to 4.79 a during transformation of the unstable form to the stable one. Small crystals of the unstable form show a tendency to attain the shape of the stable crystals_ Similarities were found both in crystal appearance and in their lattice orientation. After growth, the metastable crystals form thin flakes. It is supposed that the displacement of the molecules takes place mainly at the comers of the crystal lattice, even the particular crystals, where the solvent “loosens” in cases of extremely low solubility, and makes it possible to obtain the stable nucleus by formation of sufficient space for rotation of the particular molecules. Finally, the molecules reach a stable position in the crystal. The enthalpy measurements carried out for both crystalline forms by Benyon and Humphries 131) prove that the stable form is in an energy state 2.46 kcal/mol lower than the unstable form. The displacement of the molecules will occur spontaneously. The nucleation can be retarded if the initial particles are very small, and then in the initial transformation phase the nucleation will occur simultaneously with the recrystallization effect. 5. Perylene
pigments
In the scientific literature there is not much information on the relationships between morphology, crystalline structure and colour properties of perylene pigments. Therefore it is advisable to present the results of our investigations 1343 in this field, as they confirm and widen the earlier findings. The colour and fastness of the tested diimides of perylene-3,4,9 JO-tetracarboxylic acid depend on the substituent type pendant to the imide group; e.g., methyldiimide is amaranth whereas methoxyphenyldiimide is bright red. The presence of the methyl group causes the pigment to be darker. Its solubility decreases while its decomposition temperature increases in the following order: methyl, methoxyphenyl and ethoxyphenyl-di&nid&. _, - -. __ “. 5.1. Relationship between colour and particle morphology :\ The perylene diimides p-urified by the vat process method h_Tve particles in the form of aggregates and agglomerates resulting in large spec&ic surfaces and irregular lamella_r form. Their colour shade is rather dir&&d the tinting strength is low in comparison with their large specific surfaces.
(a)
a
A
-
_
0
im
(cl Fig.
2 (a)
- (e).
(For
caption
see over-leaf.)
._.
_
=‘~
b
(d)
Fig. 2. Electron micrographs of various forms of perylene diimides (X 10,000): (a) after the vat process; (b, c), after treatment in acid medium; (d), after treatment in toluene; (e), after treatment in nitrobenzene; (f) after treztment in quinoline.
41
The pigments of the bluish shade have acicular particles of O-08 - 0.35 pm in length. The larger and longer the crystals and crystallites, the more bluish the colour shade. Decreasing the particle size and increasing the cross-section of crystals and crystallites cause the colour shade to become more yellowish. The colour purity increases with decreasing size of particles. In the range 0.05 - 0.1 pm mean particle size, the tinting strength is high and almost independent of the particle size. Above O-1 pm, the tinting strength decreases gradually when the size of particle increases and then it decreases sharply when the particle diameter exceeds 0.2 pm (Table 2). In general, the bluish pigments have a lower tinting strength compared with the yellowish ones. The pigments prepared by treatment with organic solvents have an acicular crystalline structure, and the magnitude of the particles is proportional to the treatment temperature_ The pigments recovered from aqueous solutions of sulphuric acid have crystals of oval habit and the sizes are more uniform than in the case of acicular crystals_ X-ray phase analysis proved the existence of different crystalline structures of the pigments discussed. These structures are not associated with the shape and particle size of the pigments. 5.2
Conditions of crystalline transformation of peryleize diimide Depending on the medium, two polymorphic modifications are formed. The cx modification is formed in solutions of sulphuric acid, acetic acid and formamide. Under the influence of organic solvents, the Q modification undergoes a transformation into the stable fl modification. The polymorphic transformation depending on the medium occurs in accordance with the scheme shown in Fig. 3. The polymorphic transformation leads to particles of the same size which differ in colour shade, i.e. the Q modification is yellowish-red and the (3 modification is bluish-red. The basic pigment purified by the vat process is slightly crystalline_ This pigment, independently of the medium used for its treatment, underwent recrystallization in solvents associated with a decrease of specific surface and the growth of crystals, while their habit did not always undergo the transformation. The recrystallization process is conditioned by the solution saturation and the occurrence of chemical potential differences on the surface of the pigment particle. Recrystallization changes the physical properties of the pigments if the size of their particles does not exceed 0.1 pm. The recrystallization process can result from molecular surface diffusion or transfer of the molecules through the saturated solution_ In the case of a substance of solubility lower than 1 mg per litre, i.e. perylene diimide, the molecular transfer resulting in the recrystallization occurs mainly through surface diffusion, and through the solution only to a limited extent. Equilibrium is attained when the different sites on the surface reach the same chemical potential, or the potential difference is lower than the activation energy. During the recrysallization process, the crystalline aggregates become more compact and the growth of crystals occurs. In many cases their shape is changed, and some-
_
-?
-
-
-.
-
0.3992 0.3849 0.3896 0.3377 0.3328
a
“,
0.3503 0.3864 0.3710 0.3571 0.3658 0.3500
.I
,,,
CIE 1931
P
modification
Polymorpllic
,,,
_
,,>
-
29.00 27.88 25.60 50.70 46.41
0.3188 0.3090 0.3004 0.3136 0.3080
^
39.76 30.65 35.70 36.43 33.31 38.44
0.3130 0.3172 0.3155 0.3072 0.3090 0.3077
Y
Y
-
_
7.1719 3.044 1 2.4804 4.8967 0.96GO
-.
c1
8.6184 9.6318 11.7819 -7.2847 -4.5664
-
AL
Id
8.5520 5.4531 4.2059 5.7960 1.8553
_
L.,,
-1183692 -10.3833 -14 a2062 3.8531 3.7964
-
Aa
L (I b coordinates
Colour properties and particle size of perylcne diimidcs
TABLE 2
-
_
-7.5286 -2.8955 -1.0764 1.320.1 3.7811
-5.5405 -3.3899 -0.8426 -0.7230 1.6063
Ab
L, ,,
- _
16.1312 14.4550 18.4875 8.3460 7.0400
_
_
,,,
.>
-
0.07 0.08 0.10 0.32 0.35
137.5 142.5 155.0 7883 85.8
”
0.04 0.08 0.05 0.16 0.09 0.17
_
(nm)
size
.
.
-
Mean particle
100.0 129.9 111.0 109,o 118.8 103.5
(%I
AL:
12.4607 7.1059 4.9549 7.6219 2.6374
strength
Tinting
difference
Colour
Ip N
43
Rmorphic
I
Solwd
a-modlf~rolion
Fig.
3. The
structure
treotbng B- modifirotinn
scheme
of crystalline
transformation
of perylene
diimide.
times their crystalline structure is also transformed_ The crystalline transformation is influenced by the chemical potential differences resulting from the lattice energy differences in both stable and unstable modifications. After the vat process, the perylene diimide is in the p modification. In _ all the organic solvents used, except acetic acid and formamide, only growth of crystals takes place, and sometimes the crystal habit.transformation, defined as the “physical form” transformation according to Honigrnann’s nomenclature. Treatment in acetic acid, formamide and sulphuric acid solutions also causes crystalline structure transformations_ The transformation of the p modification into the (Y form occurs rapidly, not only in the case of the slightly crystalline basic .pigment but also with the highly crystalline pigment prepared by the solvent treatment technique. 5.3 Phase transition mechanism To determine the factors influencing the polymorphism of the perylene pigments is a very difficult matter, due to various interactions, mainly those between molecules in a crystalline state. The pigment molecule cannot be considered only as the composition of the atoms, and thus the interactions occurring in the atom cannot be transferred to the molecule, but it should be considered as the completely separate formation_ From this point of view, the crystal is not the arranged set of the molecules, ions or atoms, but it is the single large molecule in which the electrons, partic-ularly those delocated, belong to the whole crystal and not just to the specific molecules or atomic groups or atoms. Thus every hypothesis that tries to explain the arrangement of the molecules in the crystal in terms of quantum mechanics is only an approximation of the real state, causing some possible effects to be determined experitientally _ The perylerie pigments considered are cyclic imides in which the hydrogen atom is substituted by the methoxyphenyl group. The resonant structures of
44
the cyclic imide can be described by analogy with the succinic imide resonant structures, owing to their similarity_ The electron pair at the nitrogen atom is located near two carbonyl groups_ If, in the medium comprising proton donors, proton addition to the nitrogen atom occurs forming the conjugated acid, then the molecule loses its initial stability, thus it can be espected that proton addition to the carbony1 oxygen mostly probable. Recently, the mechanism of the reactions occurring in the paint systems as well as their properties have been explained through the formation of hydrogen bonds between particular paint components. Perylene pigments are basic diimides (proton acceptors)_ It was found experimentally that in solutions of sulphuric acid, acetic acid and formamide, being proton donors for perylene diimides, the phase transition of the p modification to the Q’ modification occurs_ The formation of the polymorphous CYmodification can be explained through the formation of transient complex compounds with the solvent. This is proved to some extent through the appearance of the i.r. spectra of the perylene diimides with appropriate solvents (Fig. 4). The charact.eristic vibrations of the I amide band for the perylene diimide in nujol occur at frequencies of 1,660 cm-’ and 1,700 - 1,705 cm- ‘. Bellamy [ 321, for the compound of the formula
in the solid state, gives frequencies of 1,660 and 1,690 cm- 1 _ These bands for the perylene diimides in acetic acid and formamide are broadened and displaced, while with other solvents, e.g. nitrobenzene, such effects do not occur. It may therefore be concluded that the polymorphism of the perylene pigments is caused by the different arrangement of the diimide molecules in the crystal lattice, effected in turn by the formation of complexes in the proton-forming medium ofsulphuric acid, acetic acid and formamide (Fig. 5). The packing of the perylene diimide molecules in the crystal lattice is presented, taking into consideration the earlier works describing the crystal structure of phthalocyanines and other pigments which consist of large planar molecules comprising condensed rings analogous to the graphite structure. The distance between the slip planes in graphite is 3.36 A. The X-ray photographs of all the perylene diimides tested have a strong reflection at angles corresponding to the interplanar distance characteristic for the rr polymorphic modification obtained in a proton-forming medium, which is larger (3.49 A) than for the p modification prepared in other media (3.06 A). The Q’ modification is unstable in organic solvents owing to its lower decomposition temperature, and transformation into the p modification occurs during treatment in solvents. The solvents cause the intermolecular forces to be weakened, and also cause a change of the molecule arrangement due to the
.,
. .- _
1)
, > ,_ ..,
. _ - . . ,,, _. -
_
XNJell!WSUeJ~
5
FJ3Uf2ll&llStl~J~ llJil3J+j
llKI3Jad
Cl3UR~l!WSlEJ~ lUCI3JCid
KNJell]llJSUCJ~
1UXlJad
46
on
,;_~$z
‘,
&N-
0 Fig.
Fig.
0
5. Complex
d-
6.
so9
of perylene
diimide
in a proton-forming
medium.
mociif~caticn
S-ray
diffraction
of perylene
diimide.
fact that the molecule planes become close to each other. Thus the thermodynamically more stable polymorphic modification of the shorter interplanar distances between the diimide molecule surface is being formed. The packing of the diimide molecules in the crystal lattice columns for both polymorphic modifications is illustrated in Fig. 7. The large molecules of the pigments can lie out of the plane due to steric hindrance as shown in the figure. Further arguments supporting the metihanism of the phase transitions should be obtained from investigations on the wetting heat of pigments in solvents, making it’possible in turn to determine the hydrogen bonding power.
6 _ Conclusion The production of pigments has been known for centuries, and in many cases the procedures developed have been based on experience rather than on scientific knowledge_ Improvements in the quality of pigments achieved during the last twenty years are quite impressive. This is a result of the great
47
j3 - modification
5 - modification Fig.
7. Hypothetical
packing
of
the diimide
molecules
in the crystal
attention paid to the scientific background of pigment paper, we have tried to review some important aspects
of perylene
preparation_ thereof.
diimide.
In this
References 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 16
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