A NOVEL SPECTROSCOPIC METHOD FOR THE INVESTIGATION OF STRUCTURAL PHASE-TRANSITIONS IN MOLECULAR-CRYSTALS

A simple experimental technique for the investigation of structural phase transitions in molecular crystals in the temperature range 77-300 K has been developed and applied to a wide variety of organic molecular crystals. The 'Photokinematical Approach', as the method has been termed, utilizes the responsiveness of photoluminescence of guest aromatic ketone molecules to the changes in crystalline structure of host molecular crystals as sensitive probe for the identification of structural phase transformations in host molecular crystals. In this method the prediction of the number of distinct polymorphs a given molecular crystal may exist, their corresponding optimum temperatures of existence and the transition temperatures is based on the observation of characteristic changes in the luminescence intensity of the guest ketone molecules that occur in response to host crystalline modifications. The reliability of the method has been established from the conformity and reproducibility of the results the technique yields in case of its application to a number of well characterized molecular crystals. The results reveal close correlation between the occurrence of structural phase transitions and the occurrence of noncoplanar molecular conformation in polymorphic crystals.