High Performance Pigments || Imidazolone-Annellated Triphenedioxazine Pigments

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20Imidazolone-Annellated Triphenedioxazine PigmentsMartin U. Schmidt

20.1Introduction

Imidazolone-annellated triphenedioxazine pigments, also called “benzimidazo-lone-dioxazine” pigments, are a relatively new class of violet pigments with verygood fastness properties and extremely high color strengths. The structural for-mula is shown in Figure 20.1.

This chapter starts with some remarks on the structure of Pigment Violet 23(Figure 20.2), which is at present the most important triphenedioxazine pigmentand served as a starting point for the development of the imidazolone-annellatedcompounds. Subsequently, the chapter focuses on syntheses, properties, and crys-tal engineering of imidazolone-annellated triphenedioxazines including PigmentBlue 80 and Pigment Violet 57.

High Performance Pigments. Edited by Edwin B. Faulkner and Russell J. SchwartzCopyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31405-8

O

N

N

OX

XNH

NNH

NOO

R'

R

1

X = H, ClR,R' = H, CH3, C2H5, Ph, subst. Ph, ...Pigment Blue 80: X = Cl, R = R' = C2H5Pigment Violet 57: X = Cl, R,R' = CH3/C2H5

Figure 20.1 Imidazolone-annellated triphenedioxazine pigments,also called “benzimidazolone-dioxazine” pigments.

O

N

N

OCl

Cl

N

N

2

Figure 20.2 Pigment Violet 23.

20.2On the Structure of Pigment Violet 23

When Pigment Violet 23 (P.V. 23) was discovered, it was assumed to have a linearstructure (3 in Figure 20.3). However, in 1987 a single-crystal X-ray analysisproved the actual S-shape of the molecule (Figure 20.4) [1]. Nevertheless, the oldwrong linear structure model persists in the literature.

The linear isomer 3 can be synthesized in a different approach (Figure 20.3). Itslinear molecular structure was confirmed by single-crystal X-ray structure analy-ses (Figure 20.5) [1, 2]. The compound 3 has a blue shade and is not producedindustrially.

20 Imidazolone-Annellated Triphenedioxazine Pigments342

NH2

N O

OCl

ClCl

Cl

NH2

N+ +

NH

O

OCl

ClNH

N

N

O

N

N

OCl

Cl

N

N

Oxidation

P.V. 23(correct structure)

O

N

N

OCl

Cl

N

N

NH2

N OCH3

O

OCl

ClCl

Cl

NH2

NOCH3

+ +

NH

O

OCl

ClNH

N

N

OCH3

OCH3

- 2 CH3OH

3Noncommercial

linear isomer

2

- 2 HCl - 2 HCl

Figure 20.3 Synthesis of P.V. 23 and of the linear isomer 3.

(a)

34320.2 On the Structure of Pigment Violet 23

(b)

(c)

Figure 20.4 Crystal structure of P.V. 23 (single-crystal X-ray analysis [1]) (a) Molecular structure.(b) Crystal structure, view along the b-axis, i.e., along the columns. (c) Crystal structure, viewdirection [–1 0 1], i.e., perpendicular to the columns; a- and c-axis horizontal, b-axis vertical [3].

20 Imidazolone-Annellated Triphenedioxazine Pigments

Pigment Violet 23 crystallizes in the monoclinic space group P21/c with latticeparameters of a = 15.717 Å, b = 4.666 Å, c = 18.12 Å, a = c = 90°, b = 104.76°. Theunit cell contains two molecules. The molecules are stacked in columns parallel tothe b-axis (Figures 20.4b and c). Inside the columns the molecules are connectedby van der Waals interactions. (A special ”p–p“ interaction does not seem reason-able for any commercial organic pigment; in all cases there are only van der Waalsinteractions, enhanced by some Coulomb interactions between the partial chargeson the atoms).

Pigment Violet 23 is used in coatings, paints, plastics, printing inks, and otherspecial-purpose media. P.V. 23 is often used as a shading component to turn cop-per-phthalocyanine blue pigments more reddish. Generally, the fastness proper-ties of P.V. 23 are satisfactory, but in paints, it does not fully reach the weatherfastness of copper-phthalocyanine blue pigments. In plastics, P.V. 23 is slightlysoluble, resulting in decreasing light and weather fastness when the amount ofP.V. 23 is below the limiting concentration. For this reason there is an ongoingneed for new violet high-performance pigments. This led to the development ofnew triphenedioxazine pigments with annellated imidazolone moieties.

20.3Imidazolone-Annellated Triphenedioxazine Pigments

From azo pigments it is known that the introduction of benzimidazolone moietiesresults in a considerable decrease of solubilities, yielding pigments with excellentsolvent fastness, as well as fastness to migration and blooming (Chapter 10). Thebenzimidazolone moieties form a network of intermolecular hydrogen bonds,resulting in increased lattice energies and reduced solubilities [3].

The combination of benzimidazolone moieties with the triphenedioxazine chro-mophore leads to imidazolone-annellated triphenedioxazine pigments, also called“benzimidazolone-dioxazine” pigments (4). In these pigments, the carbazole frag-ments of P.V. 23 are replaced by benzimidazolone moieties (Figure 20.6).

The parent compound (4a, R = R′ = H) can form eight intermolecular hydrogenbonds (four as H donor, four as acceptor). The pigment 4a is completely insolublein all solvents, even under very harsh conditions – a similar behavior is observed

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Figure 20.5 Molecular structure of the linear compound 3,determined by single-crystal X-ray analysis. Hydrogen atomshave been calculated in idealized positions.

20.3 Imidazolone-Annellated Triphenedioxazine Pigments

for the azo pigment P.O. 72, which also contains two benzimidazolone groups. In4a, the interaction between the molecules in the solid state is so strong that theamorphous powder coming out of the chemical syntheses cannot be finished intoa dispersable pigmentary form.

In order to get a compound which has a slightly increased molecular mobilityand is amenable to industrial finishing procedures, two ethyl groups were intro-duced, resulting in compound 4d. This compound, having a better dispersability,is registered as Pigment Blue 80.

20.3.1Syntheses

Pigment Blue 80 and other imidazolone-annellated triphenedioxazine pigmentsare synthesized in an analogous manner as P.V. 23 and its linear isomer.

Route A starts from chloranil and 5-amino-benzimidazolones (5) (Figure 20.7)[4]. In contrast to P.V. 23, the linear isomer is formed. The linear structure ofP.Bl. 80 was proven by 1H-NMR experiments [4, 5], and by X-ray structure analysis(see below).

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O

N

N

OCl

ClNH

NNH

NOO

R'

R

4

4a: R = R' = H (noncommercial)4b: R = R' = CH34c: R = CH3, R' = C2H54d: R = R' = C2H5, Pigment Blue 80Solid solution of 4b+4c+4d (1:2:1): Pigment Violet 57

Figure 20.6 Imidazolone-annellated triphenedioxazine pigments of commercial interest.

O

N

N

OCl

ClNH

NNH

NOO

R'

R

NH2NH

NO

RO

OCl

ClCl

Cl

NH2NH

NO

R'

NH

NO

R

Cl

Cl

NH

O NH

O

NH

NO

R'

O

N

N

OCl

Cl

NNH

O

R'

NNH

O

R

NH2NH

NO

ROCH3

O

OCl

ClCl

Cl

NH2NH

NO

R'OCH3

NH

NO

R

Cl

Cl

NH

O NH

O

NH

NO

R'OCH3

OCH3

+ +

Oxidation

4

5

6 1

43

2

7

+ +

- 2 CH3OH

BetuoRAetuoR

- 2 HCl- 2 HCl5 6

Figure 20.7 Syntheses of imidazolone-annellated triphenedioxazine pigments.


The oxidation step of route A is preferably carried out with MnO2 under acidicconditions [6]. Alternatively, the oxidation can be carried out under alkaline condi-tions with various oxidation agents [7]. In the presence of MnO2 under acidic con-ditions, N-acylated 5-amino-benzimidazolones generally undergo hydroxylation inthe 6-position [8]. Therefore, it can be concluded that the oxidation to triphene-dioxazines also proceeds via a hydroxylation in the 6-position of the benzimidazo-lone fragment, before the final ring closure gives the oxazine system.

The synthetic route B starts from 5-amino-6-methoxy-benzimidazolones (6) [9].For the parent compounds 4a (R = R′ = H), both routes are feasible, whereas forthe substituted derivatives (4b to 4d) the route A is preferred, because the corre-sponding starting materials 5 are better accessible than the methoxy derivatives 6.

A dechlorination is not observed on any synthetic route, which is in contrast tothe synthesis of P.V. 23 (see Chapter 12).

Chlorine-free imidazolone-annellated triphenedioxazine pigments (1, X = H)can be synthesized in a similar way as route A by starting from 2,5-dihydroxy-ben-zoquinone and using MnO2 in H2SO4 in the oxidation step [10].

20.3.2Properties

Imidazolone-annellated triphendioxazines exhibit reddish violet to reddish blueshades. The pigments have extremely high color strengths – up to 50% higherthan P.V. 23, which to our knowledge is the pigment with the highest colorstrength hitherto. The imidazolone-annellated triphenedioxazine pigments showexcellent fastness properties, making them suitable for all major applications likepaints, lacquers, coatings, plastics, printing inks, and other special fields.

20.3.2.1 Pigment Blue 80Pigment Blue 80 (4d) offers a reddish blue shade, between P.V. 23 and P.Bl. 60.Pigment Blue 80 lends itself for automotive paints. Due to its high degree oftransparency, it is particularly useful in metallic shades or effect coatings. Twogrades were introduced onto the market: one for solvent-based paints under thetrade name “Hostaperm® Blue R5R”, the other for water-based paints (“Hosta-perm® Blue R5R-WD”). Both grades were produced by Clariant. The colorstrength is extremely high: in paints, the compound can be diluted with 30 partsof TiO2 to get 1/3 standard depth, and with 300 parts of TiO2 to get a shade of1/25 standard depth. The light fastness is at the highest grade (grade 8), even in1/25 standard depth. The pigment passes the 3-year Florida exposure test evenbetter than P.Bl. 60. Because of its weather fastness, P.Bl. 80 can even be used forshading silver-metallic coatings. The fastness to solvents, acids, and bases is verygood, too [11].

In PVC, the color strength of P.Bl. 80 exceeds that of P.V. 23 by about 30%.P.Bl. 80 is also being developed for color filters and for styling applications.

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20.3.2.2 Other Imidazolone-Annellated Triphenedioxazine PigmentsThe dimethyl-substituted compound 4b shows violet to reddish violet shades, fill-ing the gap between P.V. 23 and b-quinacridone (P.V. 19, b-phase). The colorstrength of 4b in paints is about 50% higher than that of P.V. 23, and about threetimes the color strength of P.V. 19. The fastness properties of 4b are excellent, likethose of Pigment Blue 80. In contrast to Pigment Blue 80, which exists only in asingle crystalline phase, compound 4b exhibits six different polymorphic forms[12]: Upon synthesis in H2SO4 the dull d-phase is formed. The bright violeta-phase is obtained by solvent finish, e.g., in isobutanol or N-methylpyrrolidone.Under high shear conditions, e.g., by salt kneading in the present of a solvent, theg-phase emerges. This phase is of special interest because of its clear, very reddishshade. Three additional phases can be obtained via protonation with CF3COOH:The b-phase is formed with CF3COOH and subsequent “dilution” (deprotonation)with glacial acetic acid. The e-phase is formed by protonation with CF3COOH andsubsequent slow evaporation. Analoguously the f-phase precipitates from a mix-ture of CF3COOH and o-dichlorobenzene by evaporation. The thermodynamicallymost stable phases are the a and g polymorphs.

Other imidazolone-annellated triphenedioxazines show various shades rangingfrom reddish violet to reddish blue. This holds for the chlorinated compounds aswell as for the chlorine-free compounds (1, X = H). The pigments of both classesgenerally exhibit very high color strength and excellent fastness properties. Thenonchlorinated compounds (1, X = H) shows a high potential as chlorine-freealternative to P.V. 23.

Most compounds are polymorphic; e.g., the chlorine-free compound withR = R′ = Ph exists in four polymorphs, with different shades of violet.

20.3.3 Crystal Engineering on Imidazolone-Annellated Triphenedioxazine PigmentsWhy does the diethyl compound 4d (Pigment Blue 80) show a blue shade,whereas the corresponding dimethyl compound 4b exhibits a reddish violetshade? Quantum-mechanical calculations clearly show that the color of an individ-ual molecule does not change if the ethyl groups are replaced by methyl groups.Hence, the color change must be caused by differences in the crystal structures:(1) Differences in the hydrogen bond patterns may have an influence on the chro-mophoric system. (2) Solid-state colors are generally affected by exciton coupling,i.e., interactions of the transition dipole moments between the excited moleculeand all surrounding molecules. This dipole–dipole interaction works throughspace and includes all neighboring molecules, even if they do not touch theexcited molecule. Exciton coupling has proven to explain color differences, e.g., inperylene pigments (see Chapter 8) [13] or DPP pigments [14]. The exciton cou-plings depend on the crystal structures and result in color differences betweenpolymorphic forms, as it is visible, for example, for quinacridones [15–17].

The determination of crystal structures of imidazolone-annellated triphenediox-azine pigments is challenging, since the insolubility of the compounds preventsthe growth of single crystals suitable for X-ray structure analyses. Recrystallization

347


from the melt is impossible, too, since the compounds melt far above 300 °Cunder decomposition. Sublimation is possible, e.g., 4b could be sublimed at 10–3

mbar at temperatures of about 350 °C, but single crystals have never beenobtained.

Therefore, the crystal structures of 4b (dimethyl derivative) and 4d (PigmentBlue 80) were determined from X-ray powder diagrams.

20.3.3.1 Structure Determination from X-Ray Powder DataCrystal structure determination from X-ray powder diagrams generally consists ofthree steps:

(1) indexing the X-ray powder diagram, i.e., determining thelattice parameters from the peak positions. In this step,the crystal symmetry (space group) is determined, too.

(2) structure solution, i.e., finding the approximate crystalstructure.

(3) Rietveld refinement, i.e., fitting the approximate structure tothe X-ray powder diagram.

For step (2) (structure solution) all classical approaches require the lattice para-meters as input. If step (1) failed, i.e., if the powder diagram cannot be indexed,only two methods remain:

(1) intuition or model building,(2) prediction of possible crystal structures by global lattice ener-

gy minimization.

The powder diagrams of 4b consist of some broad peaks only, reflecting the lowcrystallinity. All attempts to improve the crystallinity and to get better powder dia-grams were unsuccessful. None of the powder diagrams could be indexed. There-fore, the crystal structure of 4b was solved by global lattice energy minimizationusing the program CRYSCA [18, 19]. CRYSCA searches for the energetically mostfavorable arrangements of molecules in the solid state. The approximate molecu-lar structure is taken as input. Starting from a randomly generated crystal struc-ture, the lattice parameters, positions, and orientations of the molecules withinthe unit cell are optimized by force-field methods until a minimum is reached.This procedure is repeated 10 000 to 100 000 times from different starting values.If the molecule is flexible, the corresponding degrees of freedom, especially rota-tions around single bonds, are optimized simultaneously with the molecular pack-ing.

The molecular structure of 4b was constructed from single-crystal data of thelinear isomer of P.V. 23 (3), and of benzimidazolone azo pigments. Calculationswere carried out in the statistically most frequent space groups. The resultingcrystal structures were sorted according to energy. For all low-energy structures,X-ray powder diagrams were simulated and compared with the experimental pow-der diagrams in order to select, which of the calculated possible crystal structures

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correspond to the experimentally observed polymorphs. At energy rank no. 5, theb-phase of 4b could be found with good accuracy (see Figure 20.8). For other crys-tal phases the low information content of the X-ray powder diagrams did not allowto identify which of the predicted structures correspond to the experimental X-raypowder diagrams.

The crystal structure of Pigment Blue 80 (4d) was determined analogously, butsince the powder diagram could be indexed, the lattice parameters and the spacegroup could be used as input.

Finally, the crystal structures of b-4b and Pigment Blue 80 were fitted to theexperimental powder diagrams by Rietveld refinements. For Pigment Blue 80high-resolution powder data from synchrotron measurements were used. (Forb-4b synchrotron measurements do not make sense, since the low quality of thepowder diagram is caused by the low crystallinity of the samples, not by the dif-fractometer.) Details on the lattice energy minimizations and Rietveld refinementof 4b are given in Ref. [20].

20.3.3.2 Crystal Structures of Pigment Blue 80 and the Dimethyl Derivative (4b)The crystal structure of Pigment Blue 80 is shown in Figure 20.9. The structuredetermination confirms the linear structure of the molecule. The benzimidazo-lone groups of two neighboring molecules combine via two N–H...O=C hydrogenbonds to form eight-membered rings. By these hydrogen bonds, the moleculesare connected to chains which arrange in layers.

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Figure 20.8 Comparison of the experimental X-ray powderdiagram of b-4b (top) with the simulated diagram of astructure predicted by lattice energy minimization (bottom).At this step, no fit to the experimental data has been made yet.


The dimethyl derivative 4b turned out to be isostructural to Pigment Blue 80,although the X-ray powder diagrams are not visually similar. The structure of 4bis shown in Figure 20.10.

350

Figure 20.9 Crystal structure of Pigment Blue 80 (4d),determined from X-ray powder data. View perpendicular tothe molecular planes.

Figure 20.10 Crystal structure of the dimethyl compound 4b(b-phase), determined from X-ray powder data. View perpen-dicular to the molecular planes. At the position X there is asmall void.


20.3.3.3 Crystal Engineering: Pigment Violet 57A close investigation of the crystal structure of the dimethyl compound 4b revealsthat there is a small void between the methyl groups of nonbonded neighboringmolecules (in Figure 20.10 marked with “X”). Lattice energy calculations showedthat the density and the lattice energy increase considerably, if this void is filled byan additional methyl group, i.e., by replacing one of the existing methyl groups byan ethyl group. Calculations reveal the void to be too small for two ethyl groups:on substituting both methyl groups by ethyl groups, the ethyl groups must stickout of the plane; this is indeed observed in the crystal structure of Pigment Blue80 (Figure 20.9).

Hence, according to crystal engineering the ideal molecule would be the mixedmethyl–ethyl compound 4c (R = CH3, R′ = C2H5). However, this unsymmeticalcompound is difficult to synthesize in pure form: performing the synthesis with a1:1 mixture of 1-methyl- and 1-ethyl-benzimidazolones results in a mixture of di-methyl (4b), methyl–ethyl (4c), and diethyl (4d) compounds in a ratio of 4b:4c:4d= 1:2:1. This ternary mixture forms a solid solution (mixed crystal). Upon synthe-ses the solid solution emerges as a dull crude material, which is isostructural tothe d-phase of 4b; a subsequent finish in N-methyl-pyrrolidone or isobutanolcauses a phase transition to the desired b-phase [21]. For organic compounds,such phase transitions from one ternary solid solutions to another without separa-tion into the individual compounds have rarely been reported.

The crystal structure of the solid solution (4b+4c+4d) was determined from ahigh-resolution X-ray powder diagram measured with synchrotron radiation. Thestructure is similar to the structures of 4b and 4d, but the CH3 and C2H5 groupsare superimposed with occupancies of 50% each. The C2H5 groups are situated inthe molecular plane, as predicted. According to lattice energy calculations, the sol-id solution has a local ordering: Locally, there must be exactly one CH3 neighbor-ing one C2H5 group to fill the void; but it is without importance, if there are CH3

351

Figure 20.11 Crystal structure of the ternary solid solutionof 4b+4c+4d (1:2:1), determined from X-ray powder data.View perpendicular to the molecular planes.


or C2H5 groups at the other end of the molecules. Thus, the three compounds 4b,4c, and 4d arrange well together in a common crystal lattice (see Figure 20.11).

The density of the solid solution (q = 1.754 g cm3) is higher than the densitiesof the dimethyl (1.704) or diethyl (1.676) derivatives, the value represents a quitehigh density for an organic compound. Also the lattice energy of the solid solutionis more favorable than the averaged energy of the individual compounds.

This solid solution of 4b+4c+4d is registered as Pigment Violet 57 (P.V. 57), thecommercial name being “Hostaperm® Red Violet R5B”. P.V. 57 exhibits a brightreddish violet shade. The fastness properties in paints are as high as for PigmentBlue 80 and the dimethyl derivative 4b, which makes the pigment suitable forautomotive coatings and paints. The synthesis of P.V. 57 is similar to the synthe-ses of Pigment Blue 80 and 4b; no special effort is required for obtaining the solidsolution.

The solid solution (P.V. 57) is isostructural to Pigment Blue 80. The main differ-ence is the angle between the molecular plane and the stacking direction (see Fig-ure 20.12), which results in a shift in the stacking of neighboring molecules. Itmust be these small packing differences which are responsible for the color shiftfrom reddish violet (P.V. 57) to reddish blue (P.Bl. 80).

P.V. 57 as well as the dimethyl derivative 4b show a high potential as industrialhigh-performance pigments. It remains to be seen if they find their places be-tween P.V. 23 and b-P.V. 19.

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(a) (b)Figure 20.12 Molecular packing in the crystal structures of (a) Pigment Blue80 (b) Pigment Violet 57 (solid solution of 4b+4c+4d, b-phase). View alongthe b-axis.

References

Acknowledgments

The author thanks Dr. Hans Joachim Metz, Dr. Carsten Plüg, and Dr. PeterKempter (all from Clariant) for information on syntheses and properties of imid-azolone-annellated triphenedioxazine pigments, and Prof. Dr. Erich F. Paulus(former Hoechst AG) for providing single-crystal data of P.V. 23 and its isomers.

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References

1 E. Dietz, A. Kroh, E. F. Paulus,F. Prokschy, G. Lincke: “Chinacridoneund Dioxazine – Neues über bekanntePigmente”, 11. Internationales Farben-symposium, Montreux, 23–26 Septem-ber 1991, published in Chimia 45(10),p. 13 (1991).

2 E.F. Paulus, M. Kappert (Hoechst AG),unpublished results.

3 J. van de Streek, J. Brüning,S.N. Ivashevskaya, M. Ermrich,E.F. Paulus, M. Bolte, M.U. Schmidt:“The crystal structures of six industrialbenzimidazolone pigments from labora-tory powder diffraction data”, sub-mitted.

4 Patrick Boeglin, “Synthèse de nouveauxpigments et colorants heterocycliquesde structures triphenodioxazine, phtalo-perinone et perimidophtalone”, Ph.D.Thesis, Université de Haute Alsace,1998.

5 R. Born, Presentation on 9th Interna-tional Conference COLORCHEM 2002,Špindleruv Mlýn, Czech Republic,12–16 May 2002.

6 P. Boeglin, B.L. Kaul, P. Kempter: “Newtriphendioxazine compounds”,EP 0911337 (1998).

7 B.L. Kaul, C. Plüg: “Triphenodioxazinepigments, their production and theiruse”, WO 0246315 (2002).

8 C. Plüg: “Hetero-annellated ortho-ami-nophenols”, WO 0238549 (2002).

9 B.L. Kaul, P. Kempter: “6,13-Dichlorotri-phenodioxazine compounds, their prep-aration and their use as pigments”,DE 4442291 (1993).

10 B.L. Kaul, B. Piastra, P. Steffanut: “Tri-phenodioxazine pigments, their produc-tion and their use”, EP 1132432 (2001).

11 P. Kempter, G. Wilker: “Blaues bleibtBlau. Dioxazin-Blau: ein neues löse-mittelechtes Pigment”, Farbe & Lack107, 29–31 (2001).

12 M.U. Schmidt, P. Kempter, C. Plüg,R. Born: “Mixed crystals of benzimid-azolone dioxazine compounds”, EP1201718 (2002).

13 J. Mizuguchi, K. Hino, K. Tojo:“Strikingly different electronic spectraof structurally similar perylene imidecompounds”, Dyes Pigm. 70, 126–135(2006).

14 J. Mizuguchi: “Correlation betweencrystal and electronic structures in dike-topyrrolopyrrole pigments as viewedfrom exciton coupling effects”, J. Phys.Chem. A 104, 1817–1821 (2000).

15 P. Erk (BASF), personal communica-tion.

16 E.F. Paulus, F.J. J. Leusen,M.U. Schmidt: “Crystal structures ofquinacridones”, Cryst. Eng. Comm. 9,131–143 (2007).

17 J. Mizuguchi, T. Senju: “Solution andsolid-state spectra of quinacridone deriv-atives as viewed from the intermolecu-lar hydrogen bond”, J. Phys. Chem. B110, 19154–19161 (2006).

18 M.U. Schmidt, U. Englert: “Predictionof Crystal Structures”, J. Chem. Soc.,Dalton Trans. 1996, 2077–82 (1996).

19 M.U. Schmidt, H. Kalkhof: CRYSCA.Program for crystal structure calcula-tions of flexible molecules, Frankfurtam Main, 1997–2002.

20 Imidazolone-Annellated Triphenedioxazine Pigments354

29 M.U. Schmidt, M. Ermrich,R.E. Dinnebier: “Determination of thecrystal structure of the violet pigmentC22H12Cl2N6O4 from a non-indexedX-ray powder diagram”, Acta Crystallogr.B61, 37–45 (2005).

21 M.U. Schmidt, P. Kempter, R. Born:“Process for preparing new crystalpolymorphs of a methyl-substitutedbenzimidazolone-dioxazine pigment”,EP 1199309 (2002).

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