Abstract
In this paper, the conformational polymorphism of a chlorinated diketopyrrolopyrrole (DPP) dye having flexible substituents in a non-hydrogen-bonding system is reported. The propyl-substituted DPP derivative (PR3C) has three polymorphic forms, each showing a different colour (red, orange and yellow). All polymorphs could be obtained concomitantly under various crystallization conditions. The results of the crystal structure analysis indicate that PR3C adopts different conformations in each polymorph. The packing effect caused by the difference in the arrangement of neighbouring molecules was found to play an important role in the occurrence of the observed polymorphism. The thermodynamic stability relationship between the three polymorphs was identified by thermal analysis and indicates that the yellow polymorph is the thermally stable form. The results indicate that the yellow form and orange form are enantiotropically related, and the other polymorph is monotropically related to the others.
Highlights
Polymorphism refers to the phenomenon where a compound has two or more crystal structures for the same chemical substance (Bernstein, 2002)
Important factors affecting conformational polymorphism which results from the different conformations of the same molecule are the number of torsional degrees of freedom of the molecular structure and the introduction of flexible substituents (Nangia, 2008; Cruz-Cabeza & Bernstein, 2014)
Before and after the Differential scanning calorimetry (DSC) measurement, each powder sample was characterized by powder X-ray diffraction (PXRD) measurements, which were carried out using a Rigaku R-AXIS Rapid imaging-plate diffractometer with a graphite-monochromated Cu K radiation ( = 1.54187 A ) source at room temperature
Summary
Polymorphism refers to the phenomenon where a compound has two or more crystal structures for the same chemical substance (Bernstein, 2002). Important factors affecting conformational polymorphism which results from the different conformations of the same molecule are the number of torsional degrees of freedom of the molecular structure and the introduction of flexible substituents (Nangia, 2008; Cruz-Cabeza & Bernstein, 2014). In this respect, our group has shown that polymorphs of a series of 2,5-diamino-3,6-dicyanopyrazine derivatives can be induced by conformational changes arising from the flexibility of the dibenzyl groups substituted at its two amino positions (Matsumoto et al, 2006). Run No 6 resulted in two different combinations of concomitant polymorphs
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