Effect of fine-tuning of intermolecular interactions on crystallisation outcome: A case study of polymorphs of 4-hydroxybenzaldehyde

4-Hydroxybenzaldehyde (4-HOBAL) is an important organic compound widely used in the synthesis of various polymers and pharmaceuticals. It is crystallised in two polymorphic forms, namely Form-I and Form-II. Subtle differences in intermolecular interactions between 4-HOBAL molecules in Form-I and Form-II are analysed to understand the difference in the relative stability of the two polymorphs. Packing energy at room temperature is found to be slightly more stabilising for the stable polymorphic Form-I than for the metastable polymorphic Form-II. It is demonstrated that fine-tuning of the intermolecular interactions by changing the solvent polarity can regulate the outcome of the crystallisation procedure for 4-HOBAL. Crystallites obtained from solvents with a relative polarity of more than 0.654 have Form-I whereas those obtained from solvents with polarity less than 0.654 have Form-II. Colourisation of Form-I crystallites obtained from solutions with water–acid mixtures as solvent is explained based on the presence of hydrated 4-HOBAL molecules within these crystallites. These modified molecules act as local defects that absorb visible photons making the crystallites coloured.