6 Molecular and Crystal Structures of Polymorphic …
233
6.2.2 QA (QNACRD) and Its N,N’-Dibutylated Derivative
(WAMFAS)
6.2.2.1 QA
QA (QA, 5,12-dihydroquinolino[2,3-b]acridine-7,14-dione) is another important dye
with applications to organic pigments intensively studied in the middle of the twentieth century (Zollinger 2003; Faulkner and Schwartz 2009; Hunger and Schmidt
2018; Lincke 2000, 2002; Labana and Labana 1967). Pc, QA and diketopyrrolopyrrole were regarded as the three major organic dye chromophores in the twentieth
century because of their impact on the realms of both industrial applications and
chemistry (Zollinger 2003). Many polymorphs of QA in the form of powders have
been reported in the literature and patents, but, currently, many can be considered as
groupable into one of the four polymorphic forms with successfully analysed structures (Paulus et al. 2007). The three polymorphic forms α, β and γ were reported in
the early stage of the development of QA pigment applications. Several structural
reports based on single crystals are available for the β- and γ-forms (Paulus et al.
1989; Nishimura et al. 2006; Potts et al. 1994; Mizuguchi et al. 2002), and many
attempts have been made towards the structural analysis of the α-form (Leusen 1994,
1996; Lincke and Finzel 1996). In 2007, Schmidt et al. showed that the reported αform includes two polymorphs, α
I and α
II , and analysed the structure of the α
I -form
using the Rietveld method based on powder diffraction data (Paulus et al. 2007).
The structure of the α
II -form was reportedly difficult to analyse because of its small
crystal size and poor crystallinity. Finally, the α
II -form structure was investigated
using 3D electron diffraction combined with X-ray diffraction measurements and
DFT calculations (Gorelik et al. 2016), although the structure is considered to require
further experimental consideration. Table 6.3 lists the crystallographic parameters of
these four polymorphs of QA. Numerous methods have been reported for the preparation of the reported QA crystal forms, particularly for pigment preparation. For
crystallographic analysis, vapour sublimation has generally been used. The α-form
is metastable and has no direct commercial importance, although it is used as an
intermediate to prepare the reddish-violet β- and red γ-forms, which are important
pigments. Some substituted QAs have also been applied as notable. For QA, the
formation of solid solutions (mixed crystals) is also an essential technical and scientific characteristic. Some solid solutions are also available on the market as organic
pigments (Hunger and Schmidt 2018). These QA pigments cover a wide range of
colours from orange to violet with good to excellent fastness.
Figure 6.6 shows the molecular structures of the four polymorphic forms of QA.
Among them, the structure of the α
II -form should be cautiously interpreted because
of the limited data accuracy arising from the measurement method and sample crystallinity, although the determined structure has been confirmed theoretically. It is
also noted that the quality of the reported analyses is diverse and depends largely
on the quality of the measured crystals, which may render it difficult to quantitatively discuss their molecular structures. Nevertheless, Fig. 6.6 shows no significant
233
6.2.2 QA (QNACRD) and Its N,N’-Dibutylated Derivative
(WAMFAS)
6.2.2.1 QA
QA (QA, 5,12-dihydroquinolino[2,3-b]acridine-7,14-dione) is another important dye
with applications to organic pigments intensively studied in the middle of the twentieth century (Zollinger 2003; Faulkner and Schwartz 2009; Hunger and Schmidt
2018; Lincke 2000, 2002; Labana and Labana 1967). Pc, QA and diketopyrrolopyrrole were regarded as the three major organic dye chromophores in the twentieth
century because of their impact on the realms of both industrial applications and
chemistry (Zollinger 2003). Many polymorphs of QA in the form of powders have
been reported in the literature and patents, but, currently, many can be considered as
groupable into one of the four polymorphic forms with successfully analysed structures (Paulus et al. 2007). The three polymorphic forms α, β and γ were reported in
the early stage of the development of QA pigment applications. Several structural
reports based on single crystals are available for the β- and γ-forms (Paulus et al.
1989; Nishimura et al. 2006; Potts et al. 1994; Mizuguchi et al. 2002), and many
attempts have been made towards the structural analysis of the α-form (Leusen 1994,
1996; Lincke and Finzel 1996). In 2007, Schmidt et al. showed that the reported αform includes two polymorphs, α
I and α
II , and analysed the structure of the α
I -form
using the Rietveld method based on powder diffraction data (Paulus et al. 2007).
The structure of the α
II -form was reportedly difficult to analyse because of its small
crystal size and poor crystallinity. Finally, the α
II -form structure was investigated
using 3D electron diffraction combined with X-ray diffraction measurements and
DFT calculations (Gorelik et al. 2016), although the structure is considered to require
further experimental consideration. Table 6.3 lists the crystallographic parameters of
these four polymorphs of QA. Numerous methods have been reported for the preparation of the reported QA crystal forms, particularly for pigment preparation. For
crystallographic analysis, vapour sublimation has generally been used. The α-form
is metastable and has no direct commercial importance, although it is used as an
intermediate to prepare the reddish-violet β- and red γ-forms, which are important
pigments. Some substituted QAs have also been applied as notable. For QA, the
formation of solid solutions (mixed crystals) is also an essential technical and scientific characteristic. Some solid solutions are also available on the market as organic
pigments (Hunger and Schmidt 2018). These QA pigments cover a wide range of
colours from orange to violet with good to excellent fastness.
Figure 6.6 shows the molecular structures of the four polymorphic forms of QA.
Among them, the structure of the α
II -form should be cautiously interpreted because
of the limited data accuracy arising from the measurement method and sample crystallinity, although the determined structure has been confirmed theoretically. It is
also noted that the quality of the reported analyses is diverse and depends largely
on the quality of the measured crystals, which may render it difficult to quantitatively discuss their molecular structures. Nevertheless, Fig. 6.6 shows no significant