A Spontaneous Single-Crystal-to-Single-Crystal Polymorphic Transition Involving Major Packing Changes

4,6-O-Benzylidene-alpha-d-galactosyl azide crystallizes into two morphologically distinct polymorphs depending on the solvent. While the a form appeared as thick rods and crystallized in P2(1) space group (monoclinic) with a single molecule in the asymmetric unit, the beta form appeared as thin fibers and crystallized in P1 space group (triclinic) with six molecules in the asymmetric unit. Both the polymorphs appeared to melt at the same temperature. Differential scanning calorimetry analysis revealed that polymorph alpha irreversibly undergoes endothermic transition to polymorph beta much before its melting point, which accounts for their apparently same melting points. Variable temperature powder X-ray diffraction (PXRD) experiments provided additional proof for the polymorphic transition. Single-crystal XRD analyses revealed that alpha to beta transition occurs in a single-crystal-to-single-crystal (SCSC) fashion not only under thermal activation but also spontaneously at room temperature. The SCSC nature of this transition is surprising in light of the large structural differences between these polymorphs. Polarized light microscopy experiments not only proved the SCSC nature of the transition but also suggested nucleation and growth mechanism for the transition.