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

Abstract

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.

Rapid and reversible photoinduced switching of a rotaxane crystal

“…Phase transitions in the crystals of molecular compounds have been extended to spin-crossover transition-metal complexes12, mixed valence compounds3 and stimulus-responsive organic materials45. A recent application of crystalline phase transition involves positive or negative thermal expansion based on an anisotropic change of the crystal shape caused by a change in temperature678.…”

“…A recent application of crystalline phase transition involves positive or negative thermal expansion based on an anisotropic change of the crystal shape caused by a change in temperature678. Most previous studies have been concerned with full phase transition, where the structure, electronic state and/or conformation of all of the molecules within the crystals vary under crystallographic control145891011. Reversible processes for the repeated performance of crystals require a combination of a double stimulus for the forward and backward phase transition, that is, photoirradiation, temperature change, irradiation at different wavelengths and the backward process is generally 1–2 orders of magnitude slower than the forward process1213141516.…”

Detailed Investigation of the Structural, Thermal, and Electronic Properties of Gold Isocyanide Complexes with Mechano‐Triggered Single‐Crystal‐to‐Single‐Crystal Phase Transitions

“…Molecular crystals with martensitic phase transitions, in which long‐range atomic and molecular movement is negligible upon phase transition, can fall into this class. Compared with the minor crystal structure changes, some organic molecular crystals have recently been reported to show major packing changes upon thermal SCSC phase transition, in which nucleation and elongation crystal growth are caused by the molecular rearrangement . It is expected that face index measurements of these crystals would afford different results before and after the phase transition owing to the lack of a rigorous relationship between mother and daughter phases.…”

Pharmaceutical solvates, hydrates and amorphous forms: A special emphasis on cocrystals

“…Synchrotron radiation also plays an important role in energy-dispersive X-ray diffraction based experiments. Other advancements in X-ray diffractometers, like variable temperature, high-flux sources and the variability in detectors, have dramatically reduced data collection time and orientation errors [109,174,175].…”