Highly curved bowl-shaped fragments of fullerenes: Synthesis, structural analysis, and physical properties

Highly curved buckybowls 3, 4, and 5 were synthesized from planar precursors, fluoranthenes 8, benzo[k]- fluoranthenes 10 and naphtho[1,2-k]- cyclopenta[cd]fluoranthenes 12, respectively, using straightforward palladium-catalyzed cyclization reactions. These fluoranthene-based starting materials were easily prepared from 1,8-bis(arylethynyl)- naphthalenes 6. Both buckybowls 3 and 4 are fragments of C₆₀, whereas 5 is a unique subunit of C₇₀. The curved structures were identified by X-ray crystallography, and they are deep bowls. The maximum p-orbital axis vector (POAV) pyramidalization angle in both 3 and 4 is 12.88. Such a high curvature is very rarely obtained. Buckybowls 5 are less curved than the others because they have a lower density of fivemembered rings, analogous to the tube portion of C₇₀. Cyclopentaannulation increases the bowl depths of 3 and 4, but not the maximum POAV pyramidalization angle. Among the eight buckybowls studied herein, five form polar crystals. The bowl-to-bowl inversion dynamics of these buckybowls can be classified into two types; one has a planar transition structure, whereas the other has an S-shaped transition structure. A larger longitudinal length of these buckybowls corresponds to a stronger preference for the latter. The photophysical properties of these buckybowls were examined and compared with those of C₆₀ and C₇₀. Buckybowls 5 have absorption bands at wavelengths greater than 450 nm, which are similar to those of C₇₀. The chiral resolution of the mono-substituted buckybowl 4ac was also studied by using HPLC with a chiral column.