Structure and Morphology of Indole Analogue Crystals

Indole and six simple analogues were crystallized in different environments to study the crystal habit changes. All crystal structures were determined by X-ray diffraction experiments. Lattice energies based on DFT-D3 periodic calculations and framework analysis were used to define the most important intermolecular interactions in the crystal structures: N-H···π (-28 kJ/mol), hydrogen bonds (-34 kJ/mol), π···π stacking interactions (-18 kJ/mol), and dipole-dipole (-18 kJ/mol). As morphology is an important feature in many industrial applications, such as photovoltaic cells, electronic devices, and drug discovery, we predicted the crystal morphology of selected crystals using the BFDH and AE models. Facet character depends on the orientation of the molecules at the surface and is therefore sensitive to the variation of crystallization conditions such as solvent, method, and temperature. All indole derivatives tend to form plate crystals with the largest {002} facet. We showed that the morphological importance of the {002} facet increases, whereas the {011} facet decreases with solvent polarity for 5-nitroindole and 4-cyanindole crystals, resulting in a change of crystal habit from needle to plate and from plate to prism, respectively.