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Crystal structures of quinacridones{ Erich F. Paulus, a Frank J. J. Leusen b and Martin U. Schmidt* c Received 8th September 2006, Accepted 24th November 2006 First published as an Advance Article on the web 7th December 2006 DOI: 10.1039/b613059c The crystal structure of the a I -phase of quinacridone was determined from non-indexed X-ray powder data by means of crystal structure prediction and subsequent Rietveld refinement. This a I -phase is another polymorph than the a-phase reported by Lincke [G. Lincke and H.-U. Finzel, Cryst. Res. Technol. 1996, 31, 441–452.]. The crystal structures of the b and c polymorphs were determined from single crystal data. The knowledge of the crystal structures can be used for crystal engineering, i.e., for targeted syntheses of pigments having desired properties, especially for the syntheses of new red pigments. Introduction Quinacridone (Pigment Violet 19, formula 1) is the most important pigment for red–violet shades. The annual produc- tion totals several 1000 tons with a sales volume of more than 100 million euros per year. The b-phase is reddish violet whereas the c-phase is red. Both phases are used for the colouration of laquers and paints, plastics and printing inks. 1 In solution quinacridone is yellow (see Fig. 1). As visible from the different colours of the individual polymorphs, the crystal structure has a large impact on the pigment properties. For any structure–property relationship, as well as for crystal engineering, knowledge of the crystal structures is required. In earlier publications, the crystal structures of a and c quinacridone have been published, 2,3 but the structure of the a-phase may be questionable. The structures of the a I and b phases were published only on conferences 4,5 and in a survey article. 6 Here we report the crystal structures of the a I , b, and c phases. 1. Polymorphs of quinacridone: A real chaos Various polymorphs have been described in patents and journals, including the phases a, b,B I , c, c9, c I , c II , c III , c IV , d, D, e, and f. 7–18 All phases were characterised by X-ray powder diffraction. The phases a, b, and c were found already in 1955. 7,8 A closer look at the powder diagrams of the individual phases reveals that in fact b and B I 9 describe identical phases, and all c-phases 3,10–16 belong to only one polymorphic form (c).{ Furthermore, the ‘‘d-phases’’ are either equal to the c-phase 14 or they consist of a mixture of c with a trace of b-phase. 17 Also the D-phase 18 is the same polymorph as c. To complete the chaos, two e-polymorphs have been described, stating that they clearly differ from each a Institut fu ¨r Geowissenschaften Facheinheit Mineralogie/ Kristallographie, Johann Wolfgang Goethe-Universita ¨t Frankfurt am Main, Senckenberganlage 30, D-60054, Frankfurt am Main, Germany b Institute of Pharmaceutical Innovation, University of Bradford, Bradford, UK BD7 1DP c Institut fu ¨r Anorganische und Analytische Chemie, Johann Wolfgang Goethe-Universita ¨t Frankfurt am Main, Max-von-Laue-Str. 7, D-60438, Frankfurt am Main, Germany. E-mail: [email protected] { Electronic supplementary information (ESI) available: Additional crystallographic data for the c phase (Tables S1 and S2, Fig. S1). See DOI: 10.1039/b613059c Fig. 1 Colours of quinacridone polymorphs (industrial samples) (a): From left to right: a I , a II , b, and c phases. Far right: dichloro- quinacridone (Pigment Red 209). (b) Quinacridone in solution (small amounts of quinacridone dissolved in 500 ml boiling DMSO at 189 uC; photo taken at about 185 uC). { The differences seen in the X-ray powder diagrams of the various c phases result probably from (i) differences in crystal size, morphology, and lattice defects, all resulting in isotropic or anisotropic peak broadening, thus affecting the peak heights and the overlapping of neighbouring peaks; (ii) inadequate measurement conditions, e.g. measurements in reflection mode resulting in preferred orientation effects; (iii) contamination with other polymorphic phases (e.g. c III contains a); (iv) contamination with additives, starting materials or byproducts, which are incorporated in the crystal lattice, causing lattice distortions and thereby shifts in the peak positions (an investigation on the lattice distortions in 21 different c-quinacridones is given by Lincke. 16 ). Furthermore, peak positions listed in patents can be affected by zero point errors or by admixture of K a2 radiation. Additionally the peak positions depend on the algorithm for extracting the positions from the diagram. PAPER www.rsc.org/crystengcomm | CrystEngComm This journal is ß The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 131–143 | 131

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Page 1: Reprint Chinacridon CrystEngComm

Crystal structures of quinacridones{

Erich F. Paulus,a Frank J. J. Leusenb and Martin U. Schmidt*c

Received 8th September 2006, Accepted 24th November 2006

First published as an Advance Article on the web 7th December 2006

DOI: 10.1039/b613059c

The crystal structure of the aI-phase of quinacridone was determined from non-indexed X-ray

powder data by means of crystal structure prediction and subsequent Rietveld refinement. This

aI-phase is another polymorph than the a-phase reported by Lincke [G. Lincke and H.-U. Finzel,

Cryst. Res. Technol. 1996, 31, 441–452.]. The crystal structures of the b and c polymorphs were

determined from single crystal data. The knowledge of the crystal structures can be used for

crystal engineering, i.e., for targeted syntheses of pigments having desired properties, especially

for the syntheses of new red pigments.

Introduction

Quinacridone (Pigment Violet 19, formula 1) is the most

important pigment for red–violet shades. The annual produc-

tion totals several 1000 tons with a sales volume of more than

100 million euros per year. The b-phase is reddish violet

whereas the c-phase is red. Both phases are used for the

colouration of laquers and paints, plastics and printing inks.1

In solution quinacridone is yellow (see Fig. 1).

As visible from the different colours of the individual

polymorphs, the crystal structure has a large impact on the

pigment properties. For any structure–property relationship,

as well as for crystal engineering, knowledge of the crystal

structures is required. In earlier publications, the crystal

structures of a and c quinacridone have been published,2,3

but the structure of the a-phase may be questionable. The

structures of the aI and b phases were published only on

conferences4,5 and in a survey article.6 Here we report the

crystal structures of the aI, b, and c phases.

1. Polymorphs of quinacridone: A real chaos

Various polymorphs have been described in patents and

journals, including the phases a, b, BI, c, c9, cI, cII, cIII, cIV,

d, D, e, and f.7–18 All phases were characterised by X-ray

powder diffraction. The phases a, b, and c were found already

in 1955.7,8 A closer look at the powder diagrams of the

individual phases reveals that in fact b and BI9 describe

identical phases, and all c-phases3,10–16 belong to only one

polymorphic form (c).{ Furthermore, the ‘‘d-phases’’ are

either equal to the c-phase14 or they consist of a mixture of

c with a trace of b-phase.17 Also the D-phase18 is the same

polymorph as c. To complete the chaos, two e-polymorphs

have been described, stating that they clearly differ from each

aInstitut fur Geowissenschaften Facheinheit Mineralogie/Kristallographie, Johann Wolfgang Goethe-Universitat Frankfurt amMain, Senckenberganlage 30, D-60054, Frankfurt am Main, GermanybInstitute of Pharmaceutical Innovation, University of Bradford,Bradford, UK BD7 1DPcInstitut fur Anorganische und Analytische Chemie, Johann WolfgangGoethe-Universitat Frankfurt am Main, Max-von-Laue-Str. 7, D-60438,Frankfurt am Main, Germany.E-mail: [email protected]{ Electronic supplementary information (ESI) available: Additionalcrystallographic data for the c phase (Tables S1 and S2, Fig. S1). SeeDOI: 10.1039/b613059c

Fig. 1 Colours of quinacridone polymorphs (industrial samples)

(a): From left to right: aI, aII, b, and c phases. Far right: dichloro-

quinacridone (Pigment Red 209). (b) Quinacridone in solution (small

amounts of quinacridone dissolved in 500 ml boiling DMSO at 189 uC;

photo taken at about 185 uC).

{ The differences seen in the X-ray powder diagrams of the various cphases result probably from (i) differences in crystal size, morphology,and lattice defects, all resulting in isotropic or anisotropic peakbroadening, thus affecting the peak heights and the overlapping ofneighbouring peaks; (ii) inadequate measurement conditions, e.g.measurements in reflection mode resulting in preferred orientationeffects; (iii) contamination with other polymorphic phases (e.g. cIII

contains a); (iv) contamination with additives, starting materials orbyproducts, which are incorporated in the crystal lattice, causinglattice distortions and thereby shifts in the peak positions (aninvestigation on the lattice distortions in 21 different c-quinacridonesis given by Lincke.16). Furthermore, peak positions listed in patentscan be affected by zero point errors or by admixture of Ka2 radiation.Additionally the peak positions depend on the algorithm for extractingthe positions from the diagram.

PAPER www.rsc.org/crystengcomm | CrystEngComm

This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 131–143 | 131

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other,14,18 but in fact both are c-phases. The f-phase seems to

be a mixture of at least three different phases (a, b, and c).

Hence, are there only those 3 polymorphs left, which have

already been described in 1955, namely a, b, and c?

No, there are four polymorphs! Because, what is described

as ‘‘a-phase’’, are indeed two different phases, which we will

denote as aI and aII:

— aI is the phase described by W. S. Struve,8 Labana and

Labana19 and others,14,18 which is formed during synthesis or

by grinding with NaCl.

— aII is the phase investigated by Lincke and Finzel.2

This phase is formed upon recrystallisation in H2SO4 (see

chapter 7).

The colours of aI and aII are considerably different: whereas

aI has a dull reddish-violet shade, aII is red (only slightly more

bluish than c-quinacridone), see Fig. 1.

Also the powder diagrams of aI and aII are clearly different,

as can be seen in Fig. 2.

Both a-phases are stable, also at elevated temperatures:

e.g. Ogawa et al. obtained the aI-phase from sublimation

at 140–170 uC,20 and Lincke treated aII crystals in solvents

at 70–80u for 4 weeks;2 at room temperature the a-phases

are stable for many years.21 Harsh conditions are required

to transform the a-phases to the b- or c-phase (see following

chapter).

2. Industrial syntheses and applications

Quinacridone has been known since 193522 and industrially

produced since 1958.23 There are several synthetic routes; the

most important one is shown in Scheme 1.

The crystal structures of the intermediates sodium aniloate

(2), anilic acid (3) and its calcium salt were recently

determined.24 The final ring closures are achieved by a

treatment in polyphosphoric acid (!) at 120 to 140u, followed

by hydrolysis with water (warning: vivid exothermic

reaction). The resulting quinacridone precipitates as a fine,

insoluble powder. Depending on the synthetic conditions,

the syntheses can give the aI, aII , b or c phases, or a mixture

of phases. The b and c phases are produced industrially, either directly,

or via the a-phases.25–28 Tradenames are e.g. ‘‘1Hostaperm

Red Violet ER02’’ for the b-phase and ‘‘1Hostaperm Red

E5B02’’ for the c-phase. b and c phases do not interconvert;

both are stable up to high temperatures. The b and c phases

have high photostabilities and high fastness to weathering.

Therefore they are used for automotive finishes, powder

coatings, paints, plastics and high-grade printing inks. The

a-phases are not commercially used (except as intermediates in

the syntheses of b and c phases) due to their less than optimal

application properties, and because they may convert to the b

or c phases during their application in a coating or plastics at

elevated temperatures.

Quinacridone is internationally registered in the Colour

Index as ‘‘C.I. Pigment Violet 19’’, independently of the

phase and of the producer. The German name for quinacri-

done, ‘‘Chinacridon’’ implies an etymological connection

with ‘‘China’’, but in fact quinacridone is not a ‘‘China-

cridon’’ but a ‘‘Chin-acridon’’, namely the 5,12-dihydro-

quino[2,3-b]acridine-7,14-dione.

Fig. 2 X-Ray powder diagrams of quinacridone: From the top: aI,

aII, b, and c phases. The diagrams were measured in transmission on a

STOE-Stadi-P diffractometer with curved Ge[111] monochromator,

using Cu Ka1 radiation and a linear position-sensitive detector.

Scheme 1 Industrial synthesis of quinacridone.

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3. Crystallisation of quinacridones

All polymorphs of quinacridone are insoluble or nearly

insoluble in water and all other solvents, even at elevated

temperatures. This is a typical behaviour for organic pigments,

caused by the combination of hydrogen bridges and very dense

van der Waals packing, resulting in high lattice energies.29

There are only two ways to recrystallise quinacridone: by

vacuum sublimation, and by protonation, e.g. using concen-

trated sulfuric acid, with subsequent dilution or evaporation. If

a solution of quinacridone in concentrated sulfuric acid is

placed on a glass slide, the solution absorbs moisture from the

air, and the growth of quinacridone crystals can be observed

under the microscope within a few minutes.30 By this

procedure, the aII-phase is formed as radial bundles of needles,

which are not suitable for single crystal X-ray analysis, and

even do not give a good X-ray powder diagram.

Generally, crystals of quinacridone show many lattice

defects and a strong mosaicity; sometimes they are strongly

bent (see Lincke31 for impressive photos of such bad crystals).

4. Crystal structure of b-quinacridone

Crystals of b-quinacridone were obtained from Prof. Lincke.32

A crystal with dimensions 0.45 6 0.17 6 0.05 mm3 was fixed

in a Mark tube using a tiny amount of grease and placed on a

4-circle diffractometer (Nicolet) equipped with a scintillation

counter. We used the omega scan to improve the resolution of

neighboured reflections (very high mosaicity, relatively large c

axis). The speed of measuring was varied between 2 and

20u min21, depending on the weakness of a reflection. Every 68

reflections the standard reflection (112), the mutual deviations

of which were less than 1.7%, was measured again. The phase

problem was solved by direct methods using the program

SHELXTL.33 The non-hydrogen atoms could be found in the

phased Fourier map, whereas the hydrogen atoms had to be

included in calculated positions. Despite the low crystal

quality, it was possible to refine the non-hydrogen atoms

anisotropically with reasonable results. Hydrogen atom posi-

tions were calculated with a C–H distance of 0.93 A.

Crystallographic and refinement data are given in Tables 1

and 2. Atomic coordinates of the non-hydrogen atoms are

shown in Table 3.

CCDC reference numbers 620257–620259. For crystallo-

graphic data in CIF or other electronic format see DOI:

10.1039/b613059c

The molecular structure of quinacridone in the b-phase is

shown in Fig. 3. The angles between the different rings of the

molecule are smaller than 1.7u, i.e. the molecule is planar. The

crystallographic site symmetry is 1.

In all quinacridone polymorphs, the molecules are con-

nected with their neighbours by 4 hydrogen bonds of the type

N–H…OLC. In b-quinacridone, each molecule is bonded to

two neighbouring molecules via two hydrogen bonds each

(Fig. 4, SCHAKAL plot34). The resulting chains are not

parallel, but half of the chains run in the [110] direction, the

other half in the [110] direction (Fig. 5). Nevertheless all chains

are symmetrically equivalent. In the b direction, the molecules

form stacks. The normal vector of the molecular plane is tilted

by 32.0u with respect to the b axis. These stacked chains form

layers parallel to (001), which are held together by van der

Waals interactions only.

The chains themselves are not exactly planar, but exhibit

small steps between the molecules. The height of the steps is

0.35 A. In order to investigate if these steps are caused by a

packing effect, we performed energy minimisations on the

crystal structure of b-quinacridone, and on a single molecular

chain. We used the Dreiding force field35 with atomic charges

calculated by the Gasteiger method.36 Upon optimisation, the

isolated molecular chain is exactly planar, whereas in the

optimised b-quinacridone packing the chains continue to

Table 1 Crystal data for aI, b and c quinacridones (standarddeviations in brackets)

Crystal phase aI b c

Space group, Z P1, Z = 1 P21/c, Z = 2 P21/c, Z = 2Unit cell dimensionsa/A 3.802(2) 5.692(1) 13.697(9)b/A 6.612(3) 3.975(1) 3.881(3)c/A 14.485(6) 30.02(4) 13.4020(10)a/u 100.68(8) 90. 90.b/u 94.40(6) 96.76(6)u 100.44(1)uc/u 102.11(5) 90. 90.Volume V/A3 346.7(1) 674.5(9) 700.6(7)Temperature T/K 293(2) 293(2) 293(2)

Table 3 Atomic coordinates and equivalent isotropic displacementparameters (in 1023 A2) for b-quinacridone. Ueq is defined as one thirdof the trace of the orthogonalized Uij tensor

x y z Ueq

O(1) 0.2030(8) 0.4159(15) 0.0873(2) 59(2)N(1) 0.7900(9) 20.1576(17) 0.0778(2) 47(2)C(01) 0.4916(13) 0.221(2) 0.1677(2) 57(2)C(02) 0.6420(13) 0.125(2) 0.2043(3) 59(2)C(03) 0.8501(12) 20.050(2) 0.1986(2) 56(2)C(04) 0.9001(12) 20.143(2) 0.1572(2) 53(2)C(05) 0.7430(10) 20.062(2) 0.1197(2) 46(2)C(06) 0.5353(11) 0.133(2) 0.1242(2) 46(2)C(07) 0.6497(10) 20.076(2) 0.0385(2) 43(2)C(08) 0.4406(11) 0.110(2) 0.0414(2) 41(2)C(09) 0.3805(12) 0.228(2) 0.0847(2) 43(2)C(10) 0.2955(11) 0.181(2) 0.0021(2) 43(2)

Table 2 Refinement data for b-quinacridone

Empirical formula C20 H12 N2 O2

Formula weight 312.32Temperature/K 293(2)Wavelength/A 0.71070Calculated density/Mg m23 1.538Absorption coefficient/mm21 0.101F(000) 324Crystal size/mm3 0.45 6 0.17 6 0.05h range for data collection 2.73 to 24.00uLimiting indices 26 ¡ h ¡ 6, 0 ¡ k ¡ 4,

234 ¡ l ¡ 34Reflections collected/unique 2124/1064 [R(int) = 0.1302]Completeness 99.9% to h = 24.00uRefinement method Full-matrix least-squares on F2

Data/restraints/parameters 1064/0/109Goodness-of-fit on F2 0.908Final R indices [I . 2s(I)] R1 = 0.0898, wR2 = 0.2057R indices (all data) R1 = 0.1997, wR2 = 0.2728Largest diff. peak and hole 0.276 and 20.300 e A23

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exhibit steps. This finding shows that the steps are not caused

by interactions within the chain, but by the stacking of the

chains: the formation of the steps decreases the empty volume

between the edges of the molecules in neighbouring stacks.

b-Quinacridone is an ideal test case for quantum mechanical

calculations in the solid state: in the a direction, the molecules

are connected by hydrogen bridges, in the b direction the

molecules are held together by van der Waals and electrostatic

interactions (dispersion, induction, polarisation and Coulomb

energies etc.), whereas in the c direction, there are only van der

Waals interactions between C and H atoms. Hence from the

lattice parameters a, b, and c of an optimised structure one can

easily observe whether the applied calculation methods are

suitable to describe the various intermolecular interactions.

Furthermore, one can try to reproduce the UV/vis solid state

spectra37 of b and c quinacridones.

5. Crystal structure of c-quinacridone

The crystal structure of c-quinacridone was first investigated

by Koyama et al. in 1966.38 They determined the correct unit

cell and space group, however, due to limitations in their data

they found that the molecule is not planar, but adopts an S

shape: the pyridone ring was strongly bent along the N…CLO

axis, resulting in interplanar angles of about 40u between the

terminal phenyl and the central benzene rings. Later this

turned out to be wrong; in fact the molecule is planar. Crystal

data and figures of this structure analysis are also included in a

Japanese paper by Nagai and Nishi in 1968.39 In 1971, the

crystal structure was investigated by Chung and Scott, but the

structure could not be solved.40

For our X-ray structure determination we used crystals of

c-quinacridone, which were grown by sublimation in vacuum

at 300 uC.41 The best crystal was a thin, bent plate with

dimensions of 0.65 6 0.3 6 0.01 mm3. Details of the crystal

structure solution and refinement are given in the ESI.{ The

crystal structures of b- and c-quinacridone were published at

conferences by Paulus in 1989 and Dietz in 1991.4,5 The

structure of c-quinacridone was confirmed (with better R

values) by Potts et al. in 1994 and Mizuguchi et al. in 2002.3

Potts et al. grew single crystals by sublimation at 420 uC at

about 1023 mbar, yielding small, red plate-like crystals. One of

these crystals, with dimensions 0.35 6 0.075 6 0.015 mm3,

was measured using synchrotron rays and an area detector.

Mizuguchi et al. achieved to grow a single crystal with

dimensions 0.33 6 0.10 6 0.04 mm3 from DMF solution

after having purified the compound twice by sublimation at

about 430 uC.

The crystal packing of the b and c phases are remarkably

different: In b-quinacridone, each molecule is connected to

two neighbouring molecules by two hydrogen bonds each,

but in c-quinacridone, each molecule is connected by single

hydrogen bonds to four neighbouring molecules, see Fig. 6.

Consequently, in the c-phase, the molecules are not arranged

in chains, but they form a criss-cross pattern, see Fig. 7 and 8.

Along the b axis, the molecules are stacked; the normal vector

Fig. 4 b-Quinacridone, view direction [110].

Fig. 5 b-Quinacridone, view direction approx. [735]. The chains

running in the [110] direction are drawn darker than the chains running

in the [110] direction.

Fig. 3 Molecular structure of quinacridone in the b-phase. Ellipsoids are drawn with 50% probability.

134 | CrystEngComm, 2007, 9, 131–143 This journal is � The Royal Society of Chemistry 2007

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of the molecules is tilted with respect to the b axis by 37.1u. The

packing differences are the reason for the different colours of

b- and c-quinacridone (see section 8).

b and c quinacridone crystallise in the same space group:

P21/c, Z = 2, with molecules on inversion centres. But in the

c-phase, the c axis is doubled and the a axis is halved. Despite

the considerably different lattice parameters and the comple-

tely different packings, the X-ray powder diagrams show some

common features (see Fig. 2).

The b phase is about 3 percent more dense than the c phase,

but according to experimental observations the c phase seems

to be the more stable one, at least at high temperatures. Also in

the lattice energy calculations (see below) the c phase is calcu-

lated to be thermodynamically more stable than the b phase.

6. Crystal structure of aI-quinacridone

6.1. X-Ray powder diagrams and solid state NMR

Single crystals of this phase cannot be grown. The powder

diagrams typically show 8–9 broad peaks; hence indexing is

not possible (later the compound turned out to be triclinic; but

with 6 lattice parameters every set of 8–9 broad lines could be

indexed).

Solid state 13C- and 15N-NMR measurements were carried

out under cross-polarisation magic-angle-spinning conditions.

Under these conditions crystallographically equivalent atoms

are magnetically equivalent; if the compound contains more

than one molecule per asymmetric unit, the NMR peaks start

to split. The spectra of aI, b, and c quinacridone are different

from each other, but in all cases it is clearly visible that the

crystals contain only half a molecule per asymmetric unit.

6.2. Crystal structure prediction of quinacridone polymorphs

The crystal structures of quinacridone were predicted by one

of the authors (FJJL) in 1995.6 The b and c phases were

reproduced well, and the unknown structure of the aI-phase

was solved, and subsequently refined by Rietveld methods.

The results were presented42,43 on various occasions in order to

demonstrate the power of the newly developed ‘‘polymorph

predictor’’ software package, but the atomic coordinates have

not yet been published. Recently, Panina repeated the calcula-

tions, without distinguishing between aI and aII phases.44 Here

we report the original work of Leusen and Paulus,45 and add a

careful Rietveld refinement of the aI-phase; additionally we

made calculations on the possible disorder in aI-quinacridone.

A crystal structure prediction is the determination of the

energetically favourable packings of a molecule with a given

molecular geometry.46 The molecular structure of quinacri-

done was optimised with the 6–31 G** basis set in the ab initio

quantum mechanics package Gaussian92,47 and atomic

charges were fitted to the electrostatic potential.48 These

charges were used in combination with the Dreiding 2.21 force

field,35 and Ewald summation.49 For the prediction of possible

polymorphs, the program ‘‘polymorph predictor’’ within the

Cerius2 molecular modelling environment50 was used. The

algorithm is based on the method of Gdanitz and Karfunkel.51

Firstly, a rigid body Monte Carlo search procedure ran in the

top 17 crystallographic space groups in order of occurrence

(together accounting for more than 95% of the known

molecular crystals52). A total of 29 365 trial structures were

generated. Secondly, a clustering algorithm was applied to

Fig. 6 c-Quinacridone; one molecule with 4 neighbours. View

direction [130]. c axis horizontal, a and b axes vertical.

Fig. 7 c-Quinacridone, view direction [010].

Fig. 8 c-Quinacridone, view direction [001]. a axis horizontal, b axis

vertical.

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remove duplicate structures. Finally, the 408 most promising

and distinct structures were subjected to a lattice energy

minimisation with respect to all degrees of freedom (lattice

parameters, position and orientation of molecules, intramole-

cular flexibility). The resulting 13 lowest energy crystal

structures are listed in Table 4. The calculations were made

with one molecule per asymmetric unit, situated on a general

position. Since the molecule has 2/m symmetry, higher crystal

symmetries could occur during the minimisations.

In order to verify the polymorph prediction results,

simulated powder X-ray patterns of the predicted crystal

structures were compared to the experimental powder pat-

terns. The most stable predicted structure (No. 1) was found in

calculations made in space group P21, Z = 2. The final

structure has P21/c symmetry. Its simulated powder pattern

was in good agreement with the experimental pattern of the c

polymorph. Rietveld refinement53 was applied to refine the

structure to a Rp -factor of 9.6%. The result is in excellent

agreement with the c-quinacridone structure determined from

single crystal data.

Structure 8 (as numbered in Table 4) showed a good fit with

the powder pattern of the b polymorph (Rp -factor of 12.4%

after Rietveld refinement), and indeed resembles the structure

of the b polymorph. Also here the space group changed from

P21 to P21/c upon optimisation.

Structures 12 and 13 are in fact identical. In both cases the

optimised structures have additional inversion centres, and the

resulting crystal symmetry is P1, Z = 1 with the molecule on

the inversion centre. The simulated X-ray powder diagram

was similar to the experimental diagram of the aI-phase. A

preliminary Rietveld refinement with DBWS converged with a

Rp -factor of 9.7%.

These Rp factors for the aI, b, and c phases prove beyond

any doubt that the crystal structures of the three quinacridone

polymorphs, including the previously undetermined aI

form, have been successfully predicted—in the correct

stability order.

What are the other structures listed in Table 4? Structure 2

is similar to c (structure 1), but in the wrong space group

(Pbca, Z = 8). Apparently, the c packing is most favourable

in this space group, although the symmetry does not

allow reproduction of the c structure. Also 4,11-dichloro-

quinacridone crystallises in Pbca with a criss-cross lattice, but

with Z = 4.4,40

Structures 3, 5 and 7 are termed pseudo-c1; their packing

within the layers of hydrogen bonded molecules is identical to

c, but the space group symmetry does not allow the efficient

inter-lacing packing at one side of each layer. The van der

Waals energy penalty is 1.2 kcal mol21 in comparison to c, and

the density decrease is 0.05 g cm23. For structures 9 and 11,

termed pseudo c2, the space group symmetry prohibits the

inter-lacing packing at both sides of each layer. Consequently,

the van der Waals energy penalty and density decrease are

about twice the values observed for pseudo-c1: 2.3 kcal mol21

and 0.10 g cm23, respectively. A similar analysis applies to

structure 10 (pseudo-b), when compared to b (structure 8).

These eight structures can therefore be discarded.

Finally, structure 4, which is reproduced by structure 6 in a

different space group, shows a hydrogen bonding pattern

identical to c. However, the orientation of the molecules with

respect to each other is different, as if the stacks of molecules

are squashed. Despite a higher density than c, the Coulomb

contribution to the lattice energy is about 1 kcal mol21 less

favourable. Since this polymorph is not observed experimen-

tally, a crystal dynamics simulation was performed to assess its

thermodynamic stability at room temperature (using exactly

the same force field and charges as applied in the prediction

sequence). During the simulation, which was performed with a

constant number of molecules in the lattice, constant pressure

and temperature (300 K), both the unit cell and its contents

were fully flexible. After about 30 ps, the structure decayed to

the c polymorph, which shows that the energetic path leading

from this polymorph (predicted at 0 K but not stable at room

temperature) to the stable c form has a low barrier due to the

identical hydrogen bonding pattern.

6.3. Rietveld refinement of the aI-phase

In order to determine the crystal structure of aI-quinacridone

as accurately as possible, the powder diagram was carefully

measured in transmission geometry on a STOE-Stadi-P

diffractometer equipped with a curved Ge[111] mono-

chromator and a linear position sensitive detector. Cu Ka1

Table 4 Predicted polymorphs of quinacridone

Lattice parameters Lattice energy/kcal mol21

PhaseNo. Space groupa Z a/A b/A c/A a/u b/u c/u Density/g cm23 Total v. d. Waals Coulomb H-bond

1 P21 2 13.86 3.96 13.45 90.0 78.2 90.0 1.436 2366.5 24.7 2392.3 27.2 c2 Pbca 8 13.50 55.25 3.95 90.0 90.0 90.0 1.408 2366.1 25.4 2392.2 27.2 pseudo-c3 C2/c 8 56.52 3.98 13.47 90.0 80.4 90.0 1.390 2365.4 25.9 2392.1 27.2 pseudo-c1

4 P21 2 14.93 6.38 8.51 90.0 60.6 90.0 1.468 2365.2 24.6 2391.4 26.9 new5 Pnma 8 13.55 57.22 3.90 90.0 90.0 90.0 1.371 2365.1 26.1 2391.9 27.2 pseudo-c1

6 Pbca 8 13.04 35.04 6.34 90.0 90.0 90.0 1.432 2365.0 26.0 2392.3 27.0 new7 Pbcn 8 58.61 3.82 13.62 90.0 90.0 90.0 1.361 2364.8 26.4 2392.0 27.2 pseudo-c1

8 P21 2 4.12 5.62 33.50 90.0 117.4 90.0 1.507 2364.5 22.2 2389.1 25.8 b9 Pna21 4 13.58 3.88 29.36 90.0 90.0 90.0 1.341 2364.0 27.0 2391.8 27.2 pseudo-c2

10 Pca21 4 61.44 5.73 3.96 90.0 90.0 90.0 1.487 2364.0 22.3 2388.9 25.8 pseudo-b11 C2 4 29.32 3.91 13.54 90.0 89.8 90.0 1.337 2363.9 27.1 2391.8 27.2 pseudo-c2

12 P1 2 4.03 32.19 6.91 85.6 64.9 60.8 1.484 2363.8 22.3 2389.3 25.6 a13 P1 1 3.94 16.15 6.90 80.4 64.7 62.4 1.477 2363.8 22.0 2389.3 25.4 aa Space group used in the prediction (Z9 = 1, molecule on general position).

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radiation was used. The sample was spinning during the mea-

surement. The powder diagram was of very low quality, which

was caused by the low crystallinity, not by the measurement

conditions.

The Rietveld refinement was carried out with the program

GSAS.54 Since the powder diagram showed just some humps

in the range 2h . 35u, only the 2h range 3–35u was taken into

account for the refinement. The profile was described by a

pseudo-Voigt function55 with the asymmetry correction of

Finger, Cox and Jephcoat.56

In the first step only the scaling factor was refined.

Subsequently restraints for bond distances, bond angles and

planar groups were introduced. The crystallographic inversion

centre of the molecule was used: the Rietveld refinement was

carried out with half a molecule, which was fixed to the

inversion centre using a dummy atom at (0,K,0). Hydrogen

atoms were included throughout the whole refinement.

Generally a Le Bail fit is carried out before starting the

Rietveld refinement. For aI-quinacridone, the Le Bail fit

looked promising (R = 2.3%, Rwp = 3.0%, red. x2 = 5.356), but

using the profile parameters from the Le Bail fit in subsequent

Rietveld refinement did not result in a reliable refinement.

This may be caused by the peak broadening, which did not

allow a reliable determination of peak profile parameters in the

Le Bail step.

Therefore we started directly with the Rietveld refinement;

atomic coordinates, peak profile parameters, and lattice

parameters were refined alternately.

The Rietveld refinement converged with R = 3.7%, Rwp =

4.8%, (Rexp = 1.4%, RF2 = 4.5%), red. x2 = 11.90 for 92

reflections in the 2h range 3.0 to 35.0u. Although the applied

restraints were weak, the resulting molecular structure was

close to the molecular structure found in the single crystal

structure determinations. The final Rietveld plot is shown in

Fig. 9. Crystallographic data are included in Table 1. Atomic

coordinates are given in Table 5.

6.4. Description of the crystal structure of aI-quinacridone

In aI-quinacridone, the molecules show the same hydrogen

bonding pattern as in the b polymorph, i.e. each molecule is

connected to two neighbouring molecules, thus forming a

molecular chain (Fig. 10). The main difference between the

structures of the aI- and the b-phases is that all chains are

parallel in the aI-phase, whereas there are two different chain

orientations in the b-phase. In aI- as well as in b-quinacridone,

the chains are not completely planar, but there are small

steps between the molecules (Fig. 11). Lattice energy calcula-

tions again show that these steps must be considered as

a packing effect caused by the stacking of the molecules in the

a direction.

aI-Quinacridone is isostructural to 2,9-dimethylquinacri-

done; both compounds form a continuous series of mixed

crystals (solid solutions).

In principle, aI-quinacridone can also be used as a test struc-

ture for quantum mechanical calculations in the solid state.

The calculations may even be easier than for b-quinacridone,

since in aI-quinacridone there is only one molecule per unit

cell. On the other hand, the accuracy of the structural data for

aI-quinacridone is limited; thus, the isostructural compound

2,9-dimethylquinacridone would probably be a better choice.

6.5. Calculation of disorder in aI-quinacridone

The molecular packing of aI-quinacridone would not change,

if all quinacridone molecules were rotated by 180u around the

Fig. 9 Rietveld plot of aI-quinacridone. Measured diagram black; simulated diagram red; background green; difference plot blue.

Table 5 Atomic coordinates for aI-quinacridone from Rietveldrefinement (standard deviations in brackets)

x y z

C1 20.1801(9) 0.2899(8) 0.0039(2)C2 20.0950(6) 0.9384(3) 0.3294(1)C3 20.2214(7) 0.8813(3) 0.4013(1)C4 20.4174(7) 0.6726(3) 0.3955(1)C5 20.4314(6) 0.5204(3) 0.3149(1)C6 20.2718(7) 0.5766(3) 0.2449(1)C7 20.0973(5) 0.7822(3) 0.2491(1)C8 0.0479(9) 0.8326(3) 0.1582(1)C9 0.0258(9) 0.6712(7) 0.0767(2)C10 20.1523(8) 0.4601(3) 0.0806(2)H12 0.0137(9) 1.080(2) 0.3320(2)H13 20.2016(22) 0.9878(6) 0.4616(3)H14 20.5250(19) 0.6310(4) 0.4523(8)H15 20.5273(9) 0.3772(13) 0.3166(2)H16 20.4150(9) 0.2832(13) 0.1618(2)H17 20.3004(13) 0.1535(15) 0.0079(3)N18 20.3104(6) 0.4163(3) 0.1616(1)O19 0.2160(13) 1.0388(14) 0.1601(3)

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long axis of the molecule, i.e. by exchanging the N–H and CLO

groups. The hydrogen bond pattern would also be maintained.

Lattice energy minimisations show that this alternative

structure is worse in energy by only 0.21 kcal mol21, which

is not a significant value and it depends on the charge model

used. Hence, from lattice energy calculations we cannot decide

which orientation is the correct one. We also calculated the

energy when only one chain is rotated (using a larger

superstructure with 9 molecules per unit cell): After optimisa-

tion the energy is only 0.24 kcal mol21 higher than the original

Fig. 10 aI-Quinacridone, view direction [100].

Fig. 11 aI-Quinacridone, view direction approx. [111].

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structure. The small energy differences between all these

models indicate that the real structure of aI-quinacridone may

be disordered, i.e. it may contain chains, which are rotated by

180u along their chain axis.

This disorder of a few single chains would also be possible

for the b-phase.

If only a single molecule is rotated, the energy gets very high

(increase of at least 20 kcal mol21), and the structure in the

neighbourhood of the rotated molecule gets distorted, because

energetically unfavourable CLO…OLC and N–H…H–N con-

tacts are formed. This indicates that although the whole chains

may be disordered, the local ordering of molecules within the

chains is very high.

The solid state NMR experiments did not indicate disorders,

but it is questionable whether a rotation of a whole chain

would be detectable.

In the Rietveld refinement, the rotation of all quinacridone

molecules would lead to a good fit with R values similar to the

R values from the fit with the original orientation. The reason

is that a carbon atom has almost the same diffracting power as

a N–H group, thus it is only the oxygen atom which makes the

difference. But the location of a single oxygen atom is not

reliable with the present data.

Hence it cannot be ruled out that in the true structure of

aI-quinacridone, the positions of the CLO and N–H groups

have to be exchanged. In any case, the packing motif (parallel

chains of molecules) would be maintained.

6.6. Electron diffraction on aI-quinacridone

The aI-phase was investigated by electron diffraction by

Ogawa et al:20 Purified quinacridone was vacuum-deposited

on alkali halide single crystals at 140–170 uC, and quinacri-

done single crystals with sizes up to 700 6 100 6 20 nm

were grown. By tilting the sample in the transmission electron

microscope, the authors found the same d-spacings as

Labana et al.,19 which confirms that the sample contained

the aI-phase. The pattern was indexed and the intensities of

120 h0l reflections were measured. Ogawa et al. also succeeded

in getting high-resolution TEM images showing the lattice of

quinacridone in the (010) plane (Fig. 12).

The crystal structure of aI-quinacridone was solved

from a Patterson map and the HRTEM images, and refined

against the observed intensities. The lattice parameters

were a = 14.5 A, c = 6.37 A, b = 103u, assuming a = c =

90u. The parameter b was estimated to be about 4 A

leading to Z = 1. The molecule was found to be inclined

against the (010) plane. The space group was not given

explicitly, but crystallographic considerations lead to P1

as the only possibility: from the lattice parameters, the

crystal system seems to be monoclinic. There are only three

monoclinic space groups which allow for Z = 1, namely P2,

Pm, and P2/m. But all these space groups require the

molecule to be exactly parallel to the (010) plane; thus

monoclinic space groups can be ruled out and the crystal

lattice must be triclinic. Consequently, the angles a and c

(which cannot be measured from h0l reflections), may be

different from 90u. In a triclinic system with Z = 1, the space

group must be P1, since the molecule has an inversion centre.

To the best of our knowledge the atomic coordinates have not

been published yet.

Since a = c = 90u was assumed in the electron diffraction,

the lattice parameters in real space are different from those

determined from X-ray powder data. However a comparison

of the reciprocal lattice parameters shows that the electron

diffraction data correspond quite well to our result, which

confirms our structure solution (Table 6).

7. On the structure of the aII-phase

In 1996, Lincke and Finzel tried to solve the crystal structure

of aII-quinacridone.2 They dissolved c-quinacridone from

Ciba-Geigy in 96% sulfuric acid, and added this solution to

a beaker with water and ice under agitation, causing the

quinacridone to precipitate immediately. The crystal quality of

the resulting powder was subsequently improved by heating

in 3-nitrotoluene at 70–80 uC for 28 d. X-ray powder diagrams

were measured in reflection mode on a Philips PW1730

diffractometer, giving an X-ray powder diagram with 14 lines.

The crystal structure was constructed in P1 with two

Fig. 12 High resolution transmission electron micrograph of

aI-quinacridone. Image kindly provided by T. Ogawa.

Table 6 aI-Quinacridone. Comparison of the crystal structuredetermined by X-ray powder diffraction and by electron diffraction

Parameter X-Ray powder diffractiona Electron diffraction

1/a*/A 14.131 14.11/b*/A 3.690 ca. 41/c*/A 6.325 6.21a*/u 103.22 90 (assumed)b*/u 101.92 103c*/u 83.09 90 (assumed)a Structure from crystal structure prediction with subsequent Rietveldrefinement; unit cell transformed with a9 = 2c, b9 = 2a, c9 = b.

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independent molecules per unit cell, both located on inversion

centres. The molecules were assumed to form a criss-cross

pattern like in c-quinacridone. This packing was manually

fitted to the powder diagram. The final lattice parameters were

given as a = 14.934, b = 3.622, c = 12.935 A, a = 107.13, b =

92.84, c = 91.39u;2 but from the hkl values given and from the

figures shown in the paper, it becomes clear that the angles

have to be exchanged, and the correct angles are b = 107.12,

c = 92.84, a = 91.39u.Lincke noticed the differences between his structure and the

structure of Leusen and Paulus, and wrote in a letter to Paulus

on May 27, 1997, that ‘‘there are obviously two different

alpha-phases: the pigmentary a-phase and the crude a-phase’’

(i.e. aII and aI, respectively).

In the polymorph prediction, the structure proposed by

Lincke and Finzel could not have been found since it contains

two symmetrically independent molecules, which was outside

the search range of the predictions and could not be reached by

any group–supergroup transition.

The proposed aII structure looks chemically sensible and

the simulated powder diagram has some similarities with the

experimental powder diagram. But on the other hand: (i) a

powder diagram with 14 lines can always be fitted by a triclinic

unit cell (6 parameters) with two independent molecules (2 6 3

orientational parameters); (ii) structures in P1, Z = 2, with

both molecules on inversion centres are quite rare, and have

not been observed for any quinacridone derivative or any

other organic pigment;57 (iii) upon optimisation with the

Dreiding force field, the structure transforms to the c

structure (which is, strictly speaking, not definite proof

against this metastable polymorph, since it has been observed

in other cases that two experimentally observed polymorphs

collapse into the same minimum upon optimisation, e.g.

terephthalic acid43).

In our opinion the structure of the aII-phase remains

questionable. The red colour, and some similarities between

the X-ray powder diagrams of the aII and c phases (especially

of cIV) suggest that aII may exhibit a criss-cross pattern; but

much more detailed analysis is required.

8. On the colours of quinacridones

Why are quinacridones reddish to violet in the solid state, but

yellow in solution?

Quantum mechanical calculations show that the isolated

quinacridone molecule should be yellow or orange. Also very

diluted solutions of quinacridone show a yellow colour. (If one

tries to dissolve larger amounts of quinacridone, one gets an

orange or red clear liquid, which contains colloids of solid

quinacridone; within a few weeks the colloids aggregate

forming an orange–red precipitation, and the remaining

solution becomes yellow.)

The red or violet colours of solid quinacridones are a solid

state effect, which is caused by two factors:

(i) The formation of intermolecular hydrogen bonds

increases the conjugation within the p-system of the molecule

(see Scheme 2). In the isolated molecule, the p-systems of the

benzene rings are only weakly conjugated via CLO and N–H

groups. Hence the colour is similar to the yellow colour

observed for other substituted benzene compounds. In the

solid state, hydrogen bridges are formed, and consequently

the CLO and N–H bonds are weakened and the conjugation

between the benzene rings is increased. In principle, the

hydrogen atoms could completely move to the neighbouring

molecules (Scheme 2, right), resulting in hydroxy-pyridine

instead of pyridone moieties; but in the solid state the pyridone

tautomer is preferred. Nevertheless the increased conjugation

within the p system results in a smaller HOMO–LUMO

separation. The pp* absorption band shifts from violet (for an

isolated molecule) to green (for the crystal). Correspondingly,

the observed colour (which is always the complimentary colour

of the absorbed light wavelength) shifts from yellow to red.

Differences in the strength of the hydrogen bonds and in the

hydrogen bond patterns can result in different colours of the

quinacridone polymorphs.

(ii) The solid-state colour of an organic compound depends

not only on the pp* transition energy of a given molecule, but

also on the exciton coupling, i.e., on the interaction of the

transition dipole moments. This coupling can be positive or

negative, depending on the position and spatial orientation of

the neighbouring molecules. The coupling works through

space to all neighbouring molecules, not only to those con-

nected by hydrogen bonds. In quinacridone the transition

dipole is along the short molecular axis. Because of the

intermolecular hydrogen bonds, the transition dipoles of

neighbouring molecules are aligned in a head-to-tail arrange-

ment causing a large bathochromic shift.58 Since position and

orientation of the neighbouring molecules depend on the poly-

morphic form, the colours of the quinacridone polymorphs are

different. A rough estimation is that the effect of the exciton

coupling on the colour shift from yellow to red/violet is

Scheme 2 Explanation for the enhanced conjugation of the p system

of quinacridone in the solid state

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probably even larger than the effect caused by the formation

of hydrogen bonds.59

9. Crystal engineering

The knowledge of the crystal structures of aI, b and c

quinacridone can be used for crystal engineering:60 As shown

in Scheme 3, the chain motif of b-quinacridone does not allow

for substituents (except H) at the positions 1, 4, 6, 8, 11, or 13.

Any substituent at one or more of these positions would result

in negative steric interactions with the neighbouring molecules,

the chain motif becomes energetically unfavourable and the

compound forms a criss-cross pattern like in c-quinacridone.

This was proven experimentally on a series of substituted

quinacridones,4 as well as on quinacridone-quinone having

CLO groups in positions 6 and 13.

c-Quinacridone, forming a criss-cross pattern, is red,

whereas aI- and b-quinacridone, having a chain motif, exhibit

a dark reddish violet colour. Hence, adding a substituent at

the positions 1, 4, 6, 8, 11, or 13 is the best way to find new,

red pigments. This is especially true if the electronic effect

of the substituent is small in comparison to the effect caused

by the packing, e.g., for alkyl or chloro substituents. Even

if the H atoms are only partially substituted by other groups,

the resulting mixed crystal (solid solution) will form a criss-

cross pattern. This can be proven experimentally. Examples

include:

N Pigment Red 209, which is a mixture of 1,10-dichloro-

quinacridone with its 1,8- and 3,10-isomers, shows a bright red

shade (see photo of commercial 1Hostaperm Red EG, Fig. 1).

The powder diagram of this pigment is similar to that of

c-quinacridone. Also a solid solution containing 10% of this

mixture and 90% of unsubstituted quinacridone is isostructural

to c-quinacridone.61 In contrast, pure 3,10-dichloro-quinacri-

done,62 having no substituents at the relevant positions, forms

a chain structure, which is isostructural to aI-quinacridone.

N 4,11-Dichloro-quinacridone shows a criss-cross lattice as

expected. The structure is not fully isostructural to c-quina-

cridone, the crystal symmetry being Pbca instead of P21/c.4,40

As a result of the criss-cross pattern, 4,11-dichloro-quinacri-

done exhibits a bright orange-red shade, even as solid solution

with unsubstituted quinacridone (P.R. 207).1

On the other hand substituents at the positions 2, 3, 9, or 10

allow for both the chain and the criss-cross packing motifs. If

the chain motif is formed, the colour will be considerably more

violet than in the case of a criss-cross packing, for example:

N 2,9-Dimethyl-quinacridone (P.R. 122) forms molecular

chains4,63 and is isostructural to aI-quinacridone; consequently

its colour is considerably more violet than c-quinacridone.64 In

addition there exists a second phase of 2,9-dimethyl-quinacri-

done, which is isostructural to c-quinacridone.65

N 2,9-Dichloro-quinacridone (P.R. 202)4,66 is also isostruc-

tural to aI-quinacridone and exhibits a bluish red to violet

shade.1,19 Like most quinacridone compounds, also 2,9-

dichloro-quinacridone is polymorphic. There is a second

phase crystallising in P21/c (like b- and c-quinacridone)

which, surprisingly, does not exhibit any N–H…OLC hydro-

gen bond.67

Conclusions

This work is another example that the combination of crystal

structure prediction and Rietveld refinement is a valuable

tool to determine crystal structures from low-resolution

X-ray powder data.68 The knowledge of the crystal structures

is used to perform crystal engineering, i.e., to design new

molecular materials having targeted properties—in the case of

quinacridones e.g. to synthesise new, red pigments of industrial

importance.

Acknowledgements

The authors thank C. Buchsbaum (Univ. Frankfurt am Main)

for the Rietveld refinement of aI quinacridone. We are grateful

to T. Ogawa (Kyoto University) for the TEM micrograph of

aI-quinacridone. D. Schnaitmann, W. Schwab and T.

Schmiermund (all Clariant, Frankfurt am Main) are acknowl-

edged for their cooperation. Powder diagrams were measured

by U. Conrad (Hoechst AG, Frankfurt am Main) in

cooperation with B. Muller (Hoechst AG, now Sanofi

Aventis, Frankfurt am Main), M. Ermrich (X-ray laboratory

Dr. Ermrich, Reinheim), and E. Alig (Univ. Frankfurt am

Main). Solid state NMR experiments were performed by N.

Egger (Hoechst AG, Frankfurt am Main, now Sanofi-Aventis,

India). We thank G. Lincke (FH Niederrhein, Krefeld), F.

Prokschy (Hoechst AG, now Clariant, Frankfurt am Main)

and A. Kroh (Hoechst AG, Frankfurt am Main) for crystal-

lisations. Single crystal diffraction measurements were made

by H. Schweitzer, and Rietveld refinements were performed by

W. Heyse (both Hoechst AG, now Sanofi-Aventis, Frankfurt

am Main)—we thank both for their kind cooperation. The

authors thank M. R. S. Pinches, N. E. Austin, S. J. Maginn,

R. Lovell (all Molecular Simulations Ltd, Cambridge), and

H. R. Karfunkel (Ciba-Geigy, Basel) for their contributions to

the original crystal structure prediction. Photo images of

Scheme 3 Crystal engineering on substituted quinacridones: the

chain structure (b-phase) is only stable for X1 = X4 = H. For any

other substituent X, the compound must adopt a criss-cross packing

like in the c-phase. Correspondingly the colour switches from violet to

red shades. Substituents on the positions Y allow for both packing

motifs.

This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 131–143 | 141

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quinacridone samples were made by L. Fink and E. Alig (both

Univ. Frankfurt am Main). Financial support of Clariant

GmbH is gratefully acknowledged.

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57 Cambridge Structural Database CSD, Cambridge CrystallographicData Centre, Cambridge, UK, 2006.

58 J. Mizuguchi, A. Endo and S. Matsumoto, Nippon Gazo Gakkaishi,2000, 39, 94–102.

59 P. Erk, personal communication.60 M. U. Schmidt, Adv. Colour Sci. Technol., 2003, 6, 59–61.

61 M. Urban, M. Bohmer, J. Weber, D. Schnaitmann andM. Haberlick, Eur. Pat., 1020497, 2000.

62 T. Senju, T. Hoki and J. Mizuguchi, Acta Crystallogr., Sect. E,2006, 62, o261–263.

63 G. Lincke, Chem. Zeit., 1985, 109, 89–96; J. Mizuguchi, T. Senjuand M. Sakai, Z. Kristallogr., 2002, 217, 525–526.

64 Clariant GmbH, Organic pigments for the paint industry, Technicalinformation brochure, Frankfurt am Main, 2001.

65 Y. Otaka, Nippon Kagaku Kaishi, 1975, 1838.66 T. Senju, N. Nishimura, T. Hoki and J. Mizuguchi, Acta

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J. Appl. Crystallogr., 1999, 32, 178–186; M. U. Schmidt, in CrystalEngineering: From Molecules and Crystals to Materials, ed.D. Braga, F. Grepioni and A. G. Orpen, Kluwer AcademicPublishers, Dordrecht, 1999, 331–348; M. U. Schmidt, M. Ermrichand R. E. Dinnebier, Acta Crystallogr., Sect. B, 2005, 61, 37–45;M. U. Schmidt, D. W. M. Hofmann, C. Buchsbaum and H. J. Metz,Angew. Chem., 2006, 118, 1335–1340; M. U. Schmidt,D. W. M. Hofmann, C. Buchsbaum and H. J. Metz, Angew.Chem., Int. Ed., 2006, 45, 1313–1317.

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