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S. Matsumoto and J. Hwang
completely different from the other three. One molecule forms hydrogen bonds with
four different neighbouring molecules to create a criss-cross arrangement along the
c-axis. The molecules stack along the b-axis to form a 1D stacking column; thus, the
1D columns are considered to arrange in a criss-cross pattern by hydrogen bonding.
These 1D columns also arrange along the a-axis to form a 2D molecular layer, in
which the molecules also tend to form an almost planar chain-like arrangement.
These structural differences in hydrogen bonding patterns and molecular arrangements are considered reasons for the different colours of these polymorphic forms
(Hunger and Schmidt 2018).
6.2.2.2 N,N’-Dibutylated QA
The polymorph occurrence of QA derivatives with more than four polymorphs has
also been reported for N,N’-dibutylated QA (Ye et al. 2005; Fan et al. 2009). Four
polymorphic forms (i.e. A, B, C and D) were prepared by different crystallisation
conditions, and two solvated forms were also obtained (Fan et al. 2009). The B-form
was obtained from its dichloromethane solution by slow solvent evaporation. The Cand D-forms were grown from a chloroform solution by diffusing petroleum ether and
methanol vapour, respectively, and the A-form was grown by vacuum sublimation.
Figure 6.8 depicts the A-, B-, C- and D-form molecular structures reproduced from
the related figures in Fan et al. (2009) using their CIF files, and Table 6.4 lists their
crystallographic parameters. The molecular structural differences mainly arise from
the conformation of the butyl groups. The QA core has been reported to be slightly
distorted in the A-, B- and C-forms. The average distances from the mean plane
consisting of the QA chromophore have been estimated as 0.074 Å for the A-form,
0.080 Å for the B-form, 0.098 Å for the C-form, and 0.059 and 0.044 Å for the two
independent molecules of the D-form. These values indicate that the plane of the QA
core of the two independent D-form units was also slightly distorted. No significant
differences were observed in the bond lengths of these four polymorphic forms.
Among them, only in the A-form are both butyl groups projected onto the same side
against the QA core. The butyl groups also adopted a simple zig-zag conformation in
this form. In the other forms, both butyl groups were projected onto the opposite side
of the QA core. In the D-form, there are two independent half-molecular units, one
with a simple zig-zag conformation of the butyl groups, as observed in the A-form,
and the other with butyl groups projected out of the QA core, almost perpendicular,
with a slightly elongated zig-zag conformation. The molecules in the B- and C-forms
show similar butyl group zig-zag conformations with bent terminal methyl groups.
The molecular arrangements of the four polymorphic forms of N,N’-dibutylated
QA are different, as shown in Fig. 6.9. The B- and C-forms, which have similar
conformations, also exhibit 2D structural similarity in terms of their molecular
arrangement. In the A-form, two molecules stack to form a dimer-like structure with
a distance of 3.492 Å between QA rings. The crystal structure of this form can be
interpreted according to the arrangement of this dimer-like unit, such as in anthracene
(Kitaigorodskii 1973). The molecules also form six weak hydrogen bonds with four
S. Matsumoto and J. Hwang
completely different from the other three. One molecule forms hydrogen bonds with
four different neighbouring molecules to create a criss-cross arrangement along the
c-axis. The molecules stack along the b-axis to form a 1D stacking column; thus, the
1D columns are considered to arrange in a criss-cross pattern by hydrogen bonding.
These 1D columns also arrange along the a-axis to form a 2D molecular layer, in
which the molecules also tend to form an almost planar chain-like arrangement.
These structural differences in hydrogen bonding patterns and molecular arrangements are considered reasons for the different colours of these polymorphic forms
(Hunger and Schmidt 2018).
6.2.2.2 N,N’-Dibutylated QA
The polymorph occurrence of QA derivatives with more than four polymorphs has
also been reported for N,N’-dibutylated QA (Ye et al. 2005; Fan et al. 2009). Four
polymorphic forms (i.e. A, B, C and D) were prepared by different crystallisation
conditions, and two solvated forms were also obtained (Fan et al. 2009). The B-form
was obtained from its dichloromethane solution by slow solvent evaporation. The Cand D-forms were grown from a chloroform solution by diffusing petroleum ether and
methanol vapour, respectively, and the A-form was grown by vacuum sublimation.
Figure 6.8 depicts the A-, B-, C- and D-form molecular structures reproduced from
the related figures in Fan et al. (2009) using their CIF files, and Table 6.4 lists their
crystallographic parameters. The molecular structural differences mainly arise from
the conformation of the butyl groups. The QA core has been reported to be slightly
distorted in the A-, B- and C-forms. The average distances from the mean plane
consisting of the QA chromophore have been estimated as 0.074 Å for the A-form,
0.080 Å for the B-form, 0.098 Å for the C-form, and 0.059 and 0.044 Å for the two
independent molecules of the D-form. These values indicate that the plane of the QA
core of the two independent D-form units was also slightly distorted. No significant
differences were observed in the bond lengths of these four polymorphic forms.
Among them, only in the A-form are both butyl groups projected onto the same side
against the QA core. The butyl groups also adopted a simple zig-zag conformation in
this form. In the other forms, both butyl groups were projected onto the opposite side
of the QA core. In the D-form, there are two independent half-molecular units, one
with a simple zig-zag conformation of the butyl groups, as observed in the A-form,
and the other with butyl groups projected out of the QA core, almost perpendicular,
with a slightly elongated zig-zag conformation. The molecules in the B- and C-forms
show similar butyl group zig-zag conformations with bent terminal methyl groups.
The molecular arrangements of the four polymorphic forms of N,N’-dibutylated
QA are different, as shown in Fig. 6.9. The B- and C-forms, which have similar
conformations, also exhibit 2D structural similarity in terms of their molecular
arrangement. In the A-form, two molecules stack to form a dimer-like structure with
a distance of 3.492 Å between QA rings. The crystal structure of this form can be
interpreted according to the arrangement of this dimer-like unit, such as in anthracene
(Kitaigorodskii 1973). The molecules also form six weak hydrogen bonds with four