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?

CrystEngComm Pub Date: 2022-07-21 DOI: 10.1039/D2CE00490A

Abstract

Two polymorphic forms of the ternary adduct (2-amino-5-chloropyridine)·(9-anthracenecarboxylic acid)·(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.

Graphical abstract: 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
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