Non-Buckling Stacks of Nanocarbon Molecules

An octagon-centered nanocarbon molecule, phenine dehydrogenated tetramethano-o-tetraphene (phenine dH-ToT), is reported. The synthesis employed machine-learning-augmented concentration-dependent yield analysis (ML+CYAN) to optimize a tetrameric macrocyclization while simultaneously extracting kinetic information. Crystallographic analysis revealed a unique molecular structure featuring a flattened octagon center surrounded by alternating pentagon and hexagon rims with slight buckling. In the crystal, two molecules were azimuthally offset to form an S₈-symmetric dimer stack with a pentagon-over-hexagon arrangement. Detailed variable-temperature NMR studies revealed that the S₈-dimer was assembled in an entropy-driven manner and remains frozen without buckling dynamics in solution, in contrast to the rapid buckling dynamics observed for saddle-shaped phenine [n]circulene dimers. Upon dissociation into monomers, phenine dH-ToT was liberated to undergo buckling dynamics and was observed as a time-averaged D_4h-symmetric species. These results demonstrate the key role of polygonal arrangements in controlling buckling dynamics in nanocarbon bilayer stacks.