6 Molecular and Crystal Structures of Polymorphic …
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6.2.1.1 CuPc
CuPc is the most significant type of organic pigment covering blue and green hues.
The CuPc pigment is known to have 11 polymorphs (Hunger and Schmidt 2018),
although structural analyses have been completed for its α-, β-, γ- and ε-forms (Erk
and Hengelsberg 2003; Erk et al. 2004), of which the α- and β-forms are commercially
important. The industrial preparation methods of these four polymorphs are described
in detail in an excellent pigment book (Hunger and Schmidt 2018). For pigment
applications, the polymorphs of CuPc are prepared using acid pasting, milling and
solvent treatment. Vacuum sublimation is generally used to grow single crystals of
Pcs of analytical quality for X-ray single-crystal structural analysis.
The crystal structures of Pcs, including CuPc, were first reported by Robertson
et al. in the early stage of X-ray single-crystal structural analysis (Robertson 1935).
The structure of the β-form with good quality was determined by Brown (1968),
after which other analytical reports with atomic coordinates on the β-form were
available (Jiang et al. 2018). Metal-free Pc and other metal Pc compounds have
been considered isostructural (Fábián and Kálmán 2004) in their α- and β-forms, but
the α-form of CuPc varies (Erk and Hengelsberg 2003; Erk et al. 2004) from the
known α-form Pc structure (Ashida et al. 1966). The accurate crystal structure of the
α-form of CuPc was determined by single-crystal X-ray data (Erk 2004) and also
confirmed by different groups using the electron diffraction of small single crystals
formed on a KCl plate (Hoshino et al. 2003). In the latter study, the lattice constants
were directly determined by electron diffraction analysis, the crystal structure was
estimated according to the molecular packing energies and the results were confirmed
by several experimental considerations, including Rietveld analysis of the powder
sample. The crystal structures of the γ- and ε-forms have also been analysed by the
combination of Rietveld and energy minimisation methods (Erk et al. 2004).
Figures 6.2 and 6.3 present the molecular conformation and molecular arrangements of the four CuPc polymorphs, respectively, and Table 6.1 lists the corresponding crystallographic parameters. Figure 6.3 was prepared by reference to
Fig. 3.2 in Hunger and Schmidt (2018). In all forms, the Pc ring is almost planar. The
triclinic α-form (CUPOCY14 (Erk 2004)) creates simple dye-stacking columns along
the a-axis, which are arranged into a three-dimensional (3D) structure. The molecular planes are tilted towards the stacking axis by approximately 25°. The β-form
(CUPOCY10 (Brown 1968)), which is isomorphous to the β-forms of other Pc structures, shows a well-known herringbone arrangement. The γ-form (CUPOCY16 (Erk
et al. 2004)) is isomorphous to the known α-form Pc structure, where the molecules
align in a herringbone fashion with a tilt angle of approximately 25°. In the ε-form
(CUPOCY15 (Erk et al. 2004)), the molecules are also arranged in a herringbone
fashion. In the β- and ε-forms, the tilt angle of the molecular plane towards the
stacking direction is approximately 46°. In these four polymorphs, the molecules are
stacked at a small interplanar distance of approximately 3.4 Å with different degrees
of overlap between the two stacking molecules.
227
6.2.1.1 CuPc
CuPc is the most significant type of organic pigment covering blue and green hues.
The CuPc pigment is known to have 11 polymorphs (Hunger and Schmidt 2018),
although structural analyses have been completed for its α-, β-, γ- and ε-forms (Erk
and Hengelsberg 2003; Erk et al. 2004), of which the α- and β-forms are commercially
important. The industrial preparation methods of these four polymorphs are described
in detail in an excellent pigment book (Hunger and Schmidt 2018). For pigment
applications, the polymorphs of CuPc are prepared using acid pasting, milling and
solvent treatment. Vacuum sublimation is generally used to grow single crystals of
Pcs of analytical quality for X-ray single-crystal structural analysis.
The crystal structures of Pcs, including CuPc, were first reported by Robertson
et al. in the early stage of X-ray single-crystal structural analysis (Robertson 1935).
The structure of the β-form with good quality was determined by Brown (1968),
after which other analytical reports with atomic coordinates on the β-form were
available (Jiang et al. 2018). Metal-free Pc and other metal Pc compounds have
been considered isostructural (Fábián and Kálmán 2004) in their α- and β-forms, but
the α-form of CuPc varies (Erk and Hengelsberg 2003; Erk et al. 2004) from the
known α-form Pc structure (Ashida et al. 1966). The accurate crystal structure of the
α-form of CuPc was determined by single-crystal X-ray data (Erk 2004) and also
confirmed by different groups using the electron diffraction of small single crystals
formed on a KCl plate (Hoshino et al. 2003). In the latter study, the lattice constants
were directly determined by electron diffraction analysis, the crystal structure was
estimated according to the molecular packing energies and the results were confirmed
by several experimental considerations, including Rietveld analysis of the powder
sample. The crystal structures of the γ- and ε-forms have also been analysed by the
combination of Rietveld and energy minimisation methods (Erk et al. 2004).
Figures 6.2 and 6.3 present the molecular conformation and molecular arrangements of the four CuPc polymorphs, respectively, and Table 6.1 lists the corresponding crystallographic parameters. Figure 6.3 was prepared by reference to
Fig. 3.2 in Hunger and Schmidt (2018). In all forms, the Pc ring is almost planar. The
triclinic α-form (CUPOCY14 (Erk 2004)) creates simple dye-stacking columns along
the a-axis, which are arranged into a three-dimensional (3D) structure. The molecular planes are tilted towards the stacking axis by approximately 25°. The β-form
(CUPOCY10 (Brown 1968)), which is isomorphous to the β-forms of other Pc structures, shows a well-known herringbone arrangement. The γ-form (CUPOCY16 (Erk
et al. 2004)) is isomorphous to the known α-form Pc structure, where the molecules
align in a herringbone fashion with a tilt angle of approximately 25°. In the ε-form
(CUPOCY15 (Erk et al. 2004)), the molecules are also arranged in a herringbone
fashion. In the β- and ε-forms, the tilt angle of the molecular plane towards the
stacking direction is approximately 46°. In these four polymorphs, the molecules are
stacked at a small interplanar distance of approximately 3.4 Å with different degrees
of overlap between the two stacking molecules.