Deep supercooling, crystallisation & dynamically tunable disorder in the simplest geodesic polyarene

Abstract

Corannulene is a fascinating organic molecule known to sustain an abnormally resilient supercooled liquid state relative to other polycyclic aromatic hydrocarbons, yet little is known about the mechanisms underpinning this (rather unique) thermophysical behaviour. To bridge this gap, fast scanning calorimetry has been deployed for the first time to explore this deeply supercooled regime and subsequent crystallization pathways at temperatures as low as 200 K below the melting point (Tm = 542 K). Isothermal crystallization following crash cooling from the melt reveal the existence of a distinct crossover region at temperatures ~0.7Tm, associated with the formation of distinct polymorphs. These solid phases are starkly different from each other, both in terms of thermodynamic stability and kinetics of crystallization. The range 0.7–0.9Tm gives rise to a thermodynamically stable crystal whose assembly is controlled by the formation of a critical nucleus at the mesoscale. Metastable crystals are invariably created for 0.6–0.7Tm, and these are characterized by additional relaxation pathways upon heating and diffusion-controlled crystal-growth kinetics. At the molecular level, the dynamical disorder associated with well-defined rotations of corannulene into metastable basins translates into a fully fledged admixture of physically distinct corannulene-tetramer supramolecular clusters. The overall stoichiometry and amorphous character of such an admixture can be tuned by adjusting the temperature from which the supercooled liquid transforms into the solid. Our results position corannulene as a novel and exciting playground to induce and control dynamically driven disorder in molecular solids.

Felix Fernandez-Alonso

Ikerbasque Research Professor

Ikerbasque Research Professor at the Materials Physics Center, Donostia - San Sebastián.