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S. Matsumoto and J. Hwang
A typical example of polymorphism in the dye industry is the application of
organic dyes as pigments (Zollinger 2003; Faulkner and Schwartz 2009; Hunger and
Schmidt 2018; Whitaker 1977, 1995). Many examples of polymorphs are applied as
organic pigments, where several polymorphs of the same dye molecule are available
on the market as different products with different colours and other physicochemical properties. Another important example is functional dye applications (Zollinger
2003), such as the use of organic dyes as photo-conducting materials (Law 1993).
Many patents and publications have been reported on the application of near-infrared
(NIR)-absorbing dyes to the charge generation layer of the Carlson process in electrophotography. Much of the research and development of potential dyes has been
performed on powder and thin-film states. These efforts have made it possible to
replace inorganic compounds with organic dyes as charge generation materials,
resulting in the availability of several phthalocyanine (Pc) compounds on the market
(Law 1993).
The importance and examples of dye polymorphs are described in books on
dyestuffs (Zollinger 2003), organic pigments (Faulkner and Schwartz 2009; Hunger
and Schmidt 2018; Whitaker 1977, 1995) and functional dyes (Freeman and Peters
2000; Würthner 2005; Kim 2006). Some reviews on dye polymorphs and their molecular and crystal structures have also been published (Engel 1996; Erk et al. 2004;
Lincke 2000, 2002; Paulus et al. 2007). Along with the development of single-crystal
X-ray analysis, the number of structural reports on dye polymorphs has markedly
increased. Similarly, the development of structural analysis should also be highlighted based on carefully measured powder diffraction data (Hunger and Schmidt
2018). This inspired us to analyse the crystal structure of target polymorphs whose
single-crystal growth is difficult under any conditions.
This chapter reviews the molecular and crystal structures of several dyes and
coloured organic compounds with four or more polymorphs whose structural data
are available from the Cambridge Crystallographic Data Centre (CCDC). The
differences in the molecular and crystal structures between polymorphs are briefly
described. The number of polymorphs was checked through the Cambridge Structural Database (CSD) (The Cambridge Structural Database. The Cambridge Crystallographic Data Centre. https://www.ccdc.cam.ac.uk/solutions/csd–system/compon
ents/csd/), and a keyword search was performed through SciFinder (SciFinder. The
American Chemical Society. https://www.cas.org/products/scifinder) using a combination of several related keywords. The results were verified from the viewpoint
of coloured compounds and atomic coordinate availability. Some candidates with
phase transitions induced by ambient conditions, such as a temperature change, were
excluded, as well as the polymorphs observed in molecular complexes, including
solvates. Further, some compounds whose polymorphs were reported to be prepared
at room temperature were selected. Figure 6.1 shows the chemical structures of the
compounds reviewed in this chapter.
The authors re-produced the molecular and crystal structure images for the target
compounds to obtain uniform viewing directions. For the molecular geometries,
two images are depicted, one viewed parallel and the other perpendicular to the
chromophore or related conjugated system. The molecular arrangements are basically
S. Matsumoto and J. Hwang
A typical example of polymorphism in the dye industry is the application of
organic dyes as pigments (Zollinger 2003; Faulkner and Schwartz 2009; Hunger and
Schmidt 2018; Whitaker 1977, 1995). Many examples of polymorphs are applied as
organic pigments, where several polymorphs of the same dye molecule are available
on the market as different products with different colours and other physicochemical properties. Another important example is functional dye applications (Zollinger
2003), such as the use of organic dyes as photo-conducting materials (Law 1993).
Many patents and publications have been reported on the application of near-infrared
(NIR)-absorbing dyes to the charge generation layer of the Carlson process in electrophotography. Much of the research and development of potential dyes has been
performed on powder and thin-film states. These efforts have made it possible to
replace inorganic compounds with organic dyes as charge generation materials,
resulting in the availability of several phthalocyanine (Pc) compounds on the market
(Law 1993).
The importance and examples of dye polymorphs are described in books on
dyestuffs (Zollinger 2003), organic pigments (Faulkner and Schwartz 2009; Hunger
and Schmidt 2018; Whitaker 1977, 1995) and functional dyes (Freeman and Peters
2000; Würthner 2005; Kim 2006). Some reviews on dye polymorphs and their molecular and crystal structures have also been published (Engel 1996; Erk et al. 2004;
Lincke 2000, 2002; Paulus et al. 2007). Along with the development of single-crystal
X-ray analysis, the number of structural reports on dye polymorphs has markedly
increased. Similarly, the development of structural analysis should also be highlighted based on carefully measured powder diffraction data (Hunger and Schmidt
2018). This inspired us to analyse the crystal structure of target polymorphs whose
single-crystal growth is difficult under any conditions.
This chapter reviews the molecular and crystal structures of several dyes and
coloured organic compounds with four or more polymorphs whose structural data
are available from the Cambridge Crystallographic Data Centre (CCDC). The
differences in the molecular and crystal structures between polymorphs are briefly
described. The number of polymorphs was checked through the Cambridge Structural Database (CSD) (The Cambridge Structural Database. The Cambridge Crystallographic Data Centre. https://www.ccdc.cam.ac.uk/solutions/csd–system/compon
ents/csd/), and a keyword search was performed through SciFinder (SciFinder. The
American Chemical Society. https://www.cas.org/products/scifinder) using a combination of several related keywords. The results were verified from the viewpoint
of coloured compounds and atomic coordinate availability. Some candidates with
phase transitions induced by ambient conditions, such as a temperature change, were
excluded, as well as the polymorphs observed in molecular complexes, including
solvates. Further, some compounds whose polymorphs were reported to be prepared
at room temperature were selected. Figure 6.1 shows the chemical structures of the
compounds reviewed in this chapter.
The authors re-produced the molecular and crystal structure images for the target
compounds to obtain uniform viewing directions. For the molecular geometries,
two images are depicted, one viewed parallel and the other perpendicular to the
chromophore or related conjugated system. The molecular arrangements are basically