A spontaneous single-crystal-to-single-crystal polymorphic transition involving major packing changes

Area of Science:

• Crystallography
• Solid-state chemistry
• Carbohydrate chemistry

Background:

• Polymorphism, the ability of a solid material to exist in multiple crystalline forms, is crucial in materials science and pharmaceuticals.
• Understanding the conditions and mechanisms of polymorphic transitions is essential for controlling material properties.

Purpose of the Study:

• To investigate the crystallization behavior and polymorphism of 4,6-O-Benzylidene-α-d-galactosyl azide.
• To elucidate the mechanism and conditions of polymorphic transitions between different crystal forms.

Main Methods:

• Single-crystal X-ray diffraction (SCXRD) for structural analysis.
• Differential scanning calorimetry (DSC) for thermal analysis.
• Variable temperature powder X-ray diffraction (PXVT) for in-situ transition monitoring.
• Polarized light microscopy (PLM) for observing transition dynamics.

Main Results:

• Two distinct polymorphs (α and β) of 4,6-O-Benzylidene-α-d-galactosyl azide were identified, differing in crystal habit and space group (monoclinic P21 vs. triclinic P1).
• Polymorph α undergoes an irreversible endothermic transition to polymorph β before melting, explaining their similar observed melting points.
• The transition occurs via a single-crystal-to-single-crystal (SCSC) mechanism, both thermally induced and spontaneous at room temperature, with nucleation and growth observed via PLM.

Conclusions:

• The solvent-dependent crystallization leads to distinct polymorphs with significant structural differences.
• The observed SCSC transition mechanism, despite structural dissimilarities, highlights unique solid-state transformation pathways.
• The findings provide insights into controlling crystal forms and understanding solid-state reactions in carbohydrate derivatives.