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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.
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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
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).
136 | CrystEngComm, 2007, 9, 131–143 This journal is � The Royal Society of Chemistry 2007
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)
This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 131–143 | 137
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].
138 | CrystEngComm, 2007, 9, 131–143 This journal is � The Royal Society of Chemistry 2007
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.
This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 131–143 | 139
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
140 | CrystEngComm, 2007, 9, 131–143 This journal is � The Royal Society of Chemistry 2007
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
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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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.
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