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

4,6-O-Benzylidene-α-d-galactosyl
azide crystallizes into two morphologically distinct polymorphs depending
on the solvent. While the α form appeared as thick rods and
crystallized in P21 space group (monoclinic)
with a single molecule in the asymmetric unit, the β 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 α irreversibly undergoes endothermic
transition to polymorph β 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 α to β 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.