Date Published: June 01, 2019
Publisher: International Union of Crystallography
Author(s): Hee-Soo So, Shinya Matsumoto.
A very rare example of a diketopyrrolopyrrole derivative showing polymorphism with completely different molecular conformations and a different arrangement of neighbouring molecules is reported. The thermodynamic stability relationships between the three polymorphic forms are also interpreted.
Polymorphism refers to the phenomenon where a compound has two or more crystal structures for the same chemical substance (Bernstein, 2002 ▸). Different polymorphs play an important role in the pharmaceutical, food and pigment industries because they exhibit different physicochemical properties in the crystalline state (Borka & Haleblian, 1990 ▸; Threlfall, 1995 ▸; Zollinger, 2001 ▸; Inabe & Tajima, 2004 ▸; Le Révérend et al., 2010 ▸; Hunger & Schmidt, 2018 ▸). Therefore, a method for controlling the occurrence of polymorphs is required, and considerable efforts have been made to characterize different polymorphs (Threlfall, 1995 ▸; Miller et al., 2005 ▸; Pellizzeri et al., 2014 ▸).
The crystal structures of three polymorphic forms of chlorinated DPP with propyl substituents were characterized, and lattice energy calculation and Hirshfeld surface analysis were carried out. All polymorphic forms showed different molecular conformations, as well as crystal structures. In the analysed crystal structures, PR3R and PR3Y showed intermolecular interactions with directional Cl⋯Cl interactions, whereas PR3O showed a herringbone packing structure without any specific intermolecular interactions. This result suggests that the packing effect by different arrangements of the surrounding molecules seems to play an important role in controlling polymorphism in the DPP chromophore. Although we have not discussed the differences in colour between the three polymorphs in this paper, as a result of preliminary molecular orbital calculations based on time-dependent density functional theory using the coordinates of the molecule, there was no significant difference in the absorption characteristics in the three polymorphs. Therefore, the difference in their colours needs to be characterized proceeding with examination of the intermolecular interactions including fluorescence properties, and this is now in progress.