Curvature-induced vitrification & polymorphism in corannulene, the simplest geodesic polyarene

Abstract

Fast scanning calorimetry and single-crystal synchrotron diffraction have been used to explore the regimes of stability and metastability of the curved bucky-bowl corannulene. Unlike their flat counterparts, cooling rates above 500 K/s from the melt are effective at preventing crystallisation, leading to a glassy state below ambient conditions. Upon heating from the glass, we observe a series of three sharp thermal events at ca. 380, 430, and 500 K associated with hitherto unknown polymorphs of this carbon-based solid. The single-crystal diffraction data provide a framework to understand this rich phenomenology in terms of the activation of well-defined molecular rotations that increase the occupancy of interstitial sites and trigger a sharp decrease in density near the observed glass-transition temperature. These changes are also accompanied by abnormally high atomic displacement parameters as the temperature where the first (and strongest) cold-crystallisation event is observed. The diffraction data also provide strong evidence that solid CA ought to be regarded as two distinctly different materials below and above the aforementioned glass-transition temperature. Altogether, our findings bring to the fore the unanticipated complexity underpinning the structure and thermodynamic response of solid corannulene, the simplest geodesic polyarene.

Mattia Gaboardi

Post doctoral researcher

Postdoc at the Materials Physics Center, Donostia.

Balthasar Braunewell

PhD candidate

PhD candidate at the Materials Physics Center, Donostia - San Sebastián.

Felix Fernandez-Alonso

Ikerbasque Research Professor

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