Triflic acid‐Mediated Expedient Synthesis of Benzo[ a ]fluorenes and Fluorescent Benzo[ a ]fluorenones

Fluorene‐based polyaromatic hydrocarbons are renowned compounds for materials applications. Herein, a straightforward route via in situ acetal formation has been presented to access benzo[ a ]fluorenes by a triflic acid promoted cationic cycloisomerization of enynones in presence of trimethyl orthoformate under metal‐free conditions. In the absence of trimethyl orthoformate, the same reaction results in benzo[ a ]fluorenones. All the synthesized benzo[ a ]fluorenones are highly fluorescent in solution phase with high Stokes shift while the corresponding benzo[ a ]fluorenes are not fluorescent.