Curvature-induced vitrification and polymorphism in corannulene

Polymorphism in polyarenes significantly impacts their applications, yet remains underexplored in curved species like corannulene. To this end, we utilised fast scanning calorimetry and X-ray diffraction to explore its stability regimes. Unlike its flat counterpart perylene, corannulene exhibits a glass transition at Tg = 323 K when cooled from the melt at rates exceeding - 500 K s-1. Upon heating, three thermal events at ca. 380, 430, and 500 K reveal hitherto unknown polymorphs. Diffraction data link these transitions to well-defined and thermally activated rotations within the crystal, inducing a sharp density decrease matching Tg. These changes are accompanied by abnormal atomic displacement parameters. Unlike flat polyarenes of a similar molecular size, the diffraction data also provide strong evidence that this solid ought to be regarded as two distinctly different materials below and above Tg. Altogether, our findings bring to the fore the unanticipated complexity underpinning the structure and thermodynamic response of solid corannulene, the simplest geodesic polyarene.