Pristine isolation of sixfold-symmetric oblate C72 fullerene validating the isolated pentagon rule and the spherical-saturation principle

Fullerenes satisfying the isolated pentagon rule (IPR) have been isolated across the C60–C96 series, with the exception of C72, whose sole IPR isomer has remained elusive. Here we report the isolation and structural characterization of pristine D6d-C72. Single-crystal X-ray diffraction and 13C NMR spectroscopy establish its sixfold symmetry and highly oblate geometry, with a minor-axis length of only 5.79 Å, making it the flattest IPR fullerene and the shortest fullertube identified. To rationalize its formation despite its predicted thermodynamic instability, we introduce a spherical-saturation principle that quantitatively compares the experimental stability of fullerene cages and their derivatives through chlorination-based energetic indices. The analysis shows that D6d-C72 is sufficiently spherically saturated to survive in carbon soot, whereas lower-energy non-IPR C72 isomers require derivatization for stabilization. Owing to its anisotropic structure, D6d-C72 forms single-molecule junctions with two distinct conductance states. Mechanical elongation over only 2.5 Å produces a conductance switching ratio of 147.9, corresponding to a switching efficiency of 59.2 Å−1. These findings fill a long-standing gap in fullerene chemistry and identify D6d-C72 as a promising molecular component for mechanically controlled single-molecule electronic devices. Fullerenes that satisfy the “isolated pentagon” rule have been isolated throughout the C60–C96 series, with the exception of C72. Here, the authors isolate pure D6d-C72 with an oblate structure and demonstrate mechanical conductivity switching.