# Bullvalene‐Containing Molecular Glasses

## TL;DR

Bullvalene-containing molecular glasses can form amorphous materials with unique properties, such as increased thermal expansion, making them useful for pharmaceuticals and electronics.

## Contribution

The study demonstrates that bullvalene isomerism can induce glass formation and alter physical properties of molecular materials.

## Key findings

- Bullvalene-containing glasses fracture at low temperatures, indicating higher thermal expansion coefficients.
- Replacing phenylene motifs with bullvalenes in liquid crystal mesogens facilitates glass formation.
- Bullvalene inclusion increases thermal expansion coefficients of molecular glasses.

## Abstract

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.

Bullvalenes (BVs) undergo rapid pericyclic rearrangements between a network of constitutional isomers with a variety of different shapes. Their inherent covalent disorder lends them to form molecular glasses. We find that their inclusion in the structures of glass‐forming materials results in an increase in the thermal expansion coefficient.