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 T_g = 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 T_g. 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 T_g. Altogether, our findings bring to the fore the unanticipated complexity underpinning the structure and thermodynamic response of solid corannulene, the simplest geodesic polyarene.