240
S. Matsumoto and J. Hwang
optical properties of a series of benzyl-substituted derivatives indicated that their
fluorescent properties are also correlated with the molecular conformation of each
polymorph (Hirosawa et al. 2017). The pentamorphic derivative has been reported
as an example of conformational polymorphs with colour differences (Cruz-Cabeza
and Bernstein 2014).
Two 2,5-diamino3,6-dicyanopyrazine dyes (KELFOX and KELGEO) were found
to exhibit more than four differently coloured polymorphs. The p-Cl derivative (2,5bis(bis(4-chlorobenzyl)amino)-3,6-dicyanopyrazine, KELFOX) has been reported
to have five polymorphs (Matsumoto et al. 2006; Akune et al. 2015, 2017a), as well
as an amorphous solid (Akune et al. 2017a) and benzene solvate (Akune et al. 2016),
and the o-Br derivative (2,5-bis(bis(2-bromobenzyl)amino)-3,6-dicyanopyrazine,
KELGEO) has been reported to have four polymorphs (Matsumoto et al. 2006).
Table 6.5 summarizes the crystallographic parameters for these nine polymorphic
forms. Figures 6.10 and 6.11 depict the molecular conformations of the five forms
of the p-Cl derivative and four forms of the o-Br derivative, respectively, where Y,
YO, DO, RO, R, and RV are the abbreviations of the crystal colours of yellow,
yellowish-orange, dark-orange, reddish-orange, red and reddish-violet, respectively.
The chromophore is composed of a pyrazine ring with amino nitrogens and cyano
groups and adopts a planar geometry in all forms.
The structural similarity in terms of molecular conformation for the crystal forms
of the 12 analogous 2,5-diamino3,6-dicyanopyrazine dyes with benzyl groups was
examined (Akune 2017) using the RMSD tool in the Mercury software according to
the methods described in a review on conformational polymorphism (Cruz-Cabeza
and Bernstein 2014). The molecular structures of the p-Cl Y and p-Cl DO forms
were grouped similarly as conformation type 1. This conformational similarity has
also been observed in the yellow crystals of other p-substituted benzyl derivatives
(Akune et al. 2015). The red and reddish forms of the p-Cl and o-Br derivatives
(p-Cl R, p-Cl RV, o-B RO and o-Br R) were categorised as conformation type 2. This
structural type has also been confirmed for the molecular structures of orange- and
red-coloured forms of other p- and o-derivatives. The YO forms of both derivatives
are categorised as structural type 3, in which four benzyl groups spread upwards and
downwards from the pyrazine skeleton. This group also applies to the orange form of
the p-F derivative and the red form of the o-I derivative. The molecular shape of the
o-Br Y form is unique from the other forms (type 4). These four structural types are
considered to correlate with the electronic states of the amino nitrogen atoms. In the
type 1 conformation, the amino groups form a trigonal planar geometry, suggesting
sp
2 -like hybridisation of the amino nitrogen atom. Meanwhile, a tetragonal geometry
occurs in the type 2 conformation, indicating that the amino nitrogen atoms adopt
sp
3 -like hybridisation. In type 3, the p- and o-derivatives have different geometries at
the amino position, where the former adopts a trigonal geometry and the latter adopts
a tetragonal geometry. The yellow phase of the o-Br derivative also has trigonal planar
amino groups, but the phenyl ring conformations are different from those of types
2 and 3. These structural differences in the electronic states of the amino nitrogen
atom have also been reported to be correlated with the optical properties of these
crystal forms (Matsumoto et al. 2006; Hirosawa et al. 2017).
S. Matsumoto and J. Hwang
optical properties of a series of benzyl-substituted derivatives indicated that their
fluorescent properties are also correlated with the molecular conformation of each
polymorph (Hirosawa et al. 2017). The pentamorphic derivative has been reported
as an example of conformational polymorphs with colour differences (Cruz-Cabeza
and Bernstein 2014).
Two 2,5-diamino3,6-dicyanopyrazine dyes (KELFOX and KELGEO) were found
to exhibit more than four differently coloured polymorphs. The p-Cl derivative (2,5bis(bis(4-chlorobenzyl)amino)-3,6-dicyanopyrazine, KELFOX) has been reported
to have five polymorphs (Matsumoto et al. 2006; Akune et al. 2015, 2017a), as well
as an amorphous solid (Akune et al. 2017a) and benzene solvate (Akune et al. 2016),
and the o-Br derivative (2,5-bis(bis(2-bromobenzyl)amino)-3,6-dicyanopyrazine,
KELGEO) has been reported to have four polymorphs (Matsumoto et al. 2006).
Table 6.5 summarizes the crystallographic parameters for these nine polymorphic
forms. Figures 6.10 and 6.11 depict the molecular conformations of the five forms
of the p-Cl derivative and four forms of the o-Br derivative, respectively, where Y,
YO, DO, RO, R, and RV are the abbreviations of the crystal colours of yellow,
yellowish-orange, dark-orange, reddish-orange, red and reddish-violet, respectively.
The chromophore is composed of a pyrazine ring with amino nitrogens and cyano
groups and adopts a planar geometry in all forms.
The structural similarity in terms of molecular conformation for the crystal forms
of the 12 analogous 2,5-diamino3,6-dicyanopyrazine dyes with benzyl groups was
examined (Akune 2017) using the RMSD tool in the Mercury software according to
the methods described in a review on conformational polymorphism (Cruz-Cabeza
and Bernstein 2014). The molecular structures of the p-Cl Y and p-Cl DO forms
were grouped similarly as conformation type 1. This conformational similarity has
also been observed in the yellow crystals of other p-substituted benzyl derivatives
(Akune et al. 2015). The red and reddish forms of the p-Cl and o-Br derivatives
(p-Cl R, p-Cl RV, o-B RO and o-Br R) were categorised as conformation type 2. This
structural type has also been confirmed for the molecular structures of orange- and
red-coloured forms of other p- and o-derivatives. The YO forms of both derivatives
are categorised as structural type 3, in which four benzyl groups spread upwards and
downwards from the pyrazine skeleton. This group also applies to the orange form of
the p-F derivative and the red form of the o-I derivative. The molecular shape of the
o-Br Y form is unique from the other forms (type 4). These four structural types are
considered to correlate with the electronic states of the amino nitrogen atoms. In the
type 1 conformation, the amino groups form a trigonal planar geometry, suggesting
sp
2 -like hybridisation of the amino nitrogen atom. Meanwhile, a tetragonal geometry
occurs in the type 2 conformation, indicating that the amino nitrogen atoms adopt
sp
3 -like hybridisation. In type 3, the p- and o-derivatives have different geometries at
the amino position, where the former adopts a trigonal geometry and the latter adopts
a tetragonal geometry. The yellow phase of the o-Br derivative also has trigonal planar
amino groups, but the phenyl ring conformations are different from those of types
2 and 3. These structural differences in the electronic states of the amino nitrogen
atom have also been reported to be correlated with the optical properties of these
crystal forms (Matsumoto et al. 2006; Hirosawa et al. 2017).