Researchers at Nagoya University and RIKEN have developed a new two-step annulative π-extension (APEX) method that grants access to a far broader range of structurally diverse nanographenes than previous approaches allow - including a rare, non-planar structure built from fused five-, six-, and seven-membered rings.
Nanographenes - molecular fragments of graphene built from polycyclic aromatic hydrocarbons (PAHs) - are of interest for OLEDs, organic solar cells, and organic field-effect transistors, since their electronic and photophysical properties can be tuned through size and shape. APEX reactions, which build fused aromatic frameworks directly onto unfunctionalized PAHs via C-H functionalization, have made such molecules more accessible, but the structural diversity achievable has been limited by the small number of complex, predesigned reagents available for one-step approaches. The team's method splits the process in two. First, a palladium catalyst paired with the oxidant ortho-chloranil selectively arylates PAHs at specific sites using a broad range of naphthalene-based aryltrimethylsilanes - including sterically congested variants that are difficult to introduce through conventional cross-coupling - to build structurally complex polyarylene intermediates. Second, an oxidative cyclodehydrogenation (Scholl reaction), driven by DDQ and triflic acid, closes these intermediates into nanographenes containing 8 to 10 fused rings, spanning both planar and warped frameworks.
Among the products was an unexpected non-alternant nanographene combining five-, six-, and seven-membered rings. X-ray crystallography confirmed a highly curved, "negatively curved" structure, with two partial planes within the molecule tilted 34.8 degrees relative to each other - a more pronounced distortion than previously reported for similarly ring-fused nanographenes. Photophysical measurements showed this warped isomer emits a broad, red-shifted, weak orange fluorescence with a Stokes shift roughly ten times larger than its planar structural counterpart, consistent with greater structural relaxation in its excited state.
The method also proved notably step-economical: one of the synthesized nanographenes, first made by Clar and coworkers in 1960 via a multistep route in 18% overall yield, was reached directly from unfunctionalized phenanthrene in a comparable 17% overall yield using just this two-step sequence. The researchers say they intend to extend the approach toward molecular nanocarbons with an even wider range of shapes and electronic structures, including planar, curved, twisted, and porous frameworks.
