Experimental and theoretical investigations of the optical and photoluminescence behaviour of a tetramorphic ternary molecular salt cocrystal – a quantum crystallography perspective†

Four polymorphs of the ternary molecular salt cocrystal complex (3-hydroxypyridinium)·(9-anthracenecarboxylate)·(trinitrobenzene) were isolated. In all four polymorphs proton transfer occurred from the carboxylic acid to the nitrogen atom of the pyridine ring to form ternary molecular salts. Form I crystallizes as orange needles/rods, form II crystallizes as red blocks, and forms III and IV both crystallize as orangey-yellow needles. Differential scanning calorimetry indicates that form II is the thermodynamically most stable form, further supported by energy lattices calculated within periodic boundary conditions. Quantum theory of atoms in molecules (QTAIM) analysis revealed strong hydrogen bonds and charge-transfer interactions, with notable variations in the strength of these interactions across the polymorphs. Polymorph I showed the strongest charge-assisted O–H⋯O hydrogen bonds, while polymorph II displayed the most significant π⋯π interactions. Photoluminescence and UV-vis studies showed that the polymorphs exhibit differing band gaps, correlating with their observed colours and electronic structures. These results emphasize the role of polymorphism in modulating the physical and chemical properties of multi-component molecular crystals.