Bullvalene-Containing Molecular Glasses

Organic molecular glasses are attractive matrices to disperse active ingredients in pharmaceuticals or electronic devices. Typically, they i) have lower glass transition temperatures than inorganic or polymeric glasses, making them easier to process, and ii) are less prone to phase segregation from other organic active materials. However, there is a dearth of functional groups that are known to induce glass formation in preference to crystallization. We have investigated the relationship between the shapeshifting isomerism of heterodisubstituted bullvalenes (BVs) and their properties as amorphous molecular glasses. Substituting a constitutionally dynamic BV unit in place of the 1,4-phenylene motif in the molecular structures of two well-known liquid crystal mesogens, 4-cyano-4'-pentylbiphenyl and 4-cyano-4'-butylbiphenyl, produces materials that readily form glasses. The properties of the two glasses are compared to analogous glasses with fixed constitutions. Using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), we show that, unlike the fixed-structure glasses, the BV-containing molecular glasses fracture at low temperatures, which is indicative of them having larger thermal expansion coefficients. This article highlights the capability of shapeshifting building blocks to induce glass formation and to alter the physical properties of the resulting molecular materials, producing macroscopic effects that are observable by eye.