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.