Emile R. Engel

Reversible single-crystal to single-crystal polymorphic phase transformation of an organic crystal

Two polymorphs of a substituted dihydroanthracene derivative were prepared by crystallization from different solvents. Each polymorph is reversibly interconvertible to the other by means of a single-crystal to single-crystal phase transformation.

Activation-dependent breathing in a flexible metal-organic framework and the effects of repeated sorption/desorption cycling

A non-interpenetrated metal-organic framework with a paddle-wheel secondary building unit has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO2 drying. Conventional thermal activation yields a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent prior to vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO2 over N2 and CH4 , and substantial hysteresis. Supercritical drying can be used to access a guest-free open phase. Pressure-resolved differential scanning calorimetry was used to confirm and investigate a systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption. A corresponding loss of sample integrity was not detectable by powder X-ray diffraction analysis. This may be an important factor to consider in cases where flexible MOFs are earmarked for industrial applications.