A nonbenzenoid aromatic ring, tropone-fused 7-methoxy-coumarin dye (1a) and its chlorinated product (1b) were designed and successfully synthesized via transannulation of a coumarin derivative and dehydrobromination of the dibromoand tribromoprecursors. X-ray crystallography revealed that 1a and 1b are distorted at the annulated tropone skeleton with bond alternation, unlike typical 3,4-annulated pi-extended coumarin dyes. These tropone-fused coumarin dyes exhibited redshifted absorption and emission bands compared with that of planar 3,4-benzo-7-methoxycoumarin. These compounds demonstrated relatively efficient fluorescence emission (Phi(F) = 2-10%) despite their curved skeletons, and solvent dependency owing to the intramolecular charge transfer in the excited state. A computational study clarified that compound 1 changes to a flat structure including the tropylium unit in the excited state. Furthermore, in the fluorescence spectra of 1a with trifluoroacetic acid, the emission intensity increased dramatically to Phi(F) = 19% owing to the formation of a charge-separated structure with the tropylium cation.
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