Polymorphism and photoluminescence seen in (2-amino-5-chloropyridine)•(9-anthracenecarboxylic acid)•(trinitrobenzene): a further example of the salt-cocrystal continuum observed by virtue of isolating multiple crystal forms

Two polymorphic forms of the ternary adduct (2-amino-5-chloropyridine)center dot(9-anthracenecarboxylic acid)center dot(trinitrobenzene) were isolated. In both forms 9-anthracenecarboxylic acid displays a charge-transfer interaction with trinitrobenzene. Form I exhibits proton transfer from the carboxylic acid to the pyridine and forms a ternary molecular salt, while form II forms a ternary co-crystal. Form I crystallises out in acetonitrile, whilst form II crystalizes out in methanol. The two forms occur as concomitant pairs when crystallised from a variety of solvents. Form I crystallises as orange needles and form II crystallises as red blocks. Differential scanning calorimetry indicates that form II is the thermodynamically most stable form, and form I is the kinetically favoured form. Form I, being a molecular salt, luminesces whilst form II, being a cocrystal, does not luminesce. The two polymorphs are a further illustration of the salt/cocrystal continuum.

Automated summary

This article describes two polymorphic forms of the ternary adduct, their crystallization behavior, and luminescence properties. Experimental results indicate that one form is kinetically favored, while the other form is thermodynamically more stable.