Organic salt versus salt cocrystal: thermal behavior, structural and photoluminescence investigations

Abstract

Two new multicomponent crystal forms based on 4-nitrobenzoic acid-diethanolamine system, 1:1 molecular salt and 1:1:1 salt cocrystal were synthesized in the same conditions but different solvents and analyzed by complementary experimental techniques such as single-crystal X-ray diffraction, infrared absorption spectroscopy, thermogravimetric and kinetic analyses. The compounds differ from each other by the inclusion of 4-nitrobenzoic acid molecule in salt cocrystal and by the way of packaging of the components in crystals, as well as by the mode of connecting of organic cations with anions. Thermogravimetric analysis and kinetic study complete the structural study of the new obtained multicomponent crystals, providing additional information on their thermal stability and degradation process. The packing and inter-/intramolecular differences justify the melting points and enthalpy of fusions for the compounds studied. A good correlation of the activation energy values obtained by four different isothermal and non-isothermal isoconversional methods was observed. In both cases, the additional acid molecule in salt cocrystal decreases the activation energy values compared to salt, which can be related to a slower thermal degradation. Comparative investigation on photoluminescence properties was also discussed.