Synthesis, Structures, and Properties of Tropone-Fused Coumarin Dyes

Abstract A nonbenzenoid aromatic ring, tropone-fused 7-methoxycoumarin dye (1a) and its chlorinated product (1b) were designed and successfully synthesized via transannulation of a coumarin derivative and dehydrobromination of the dibromo- and tribromoprecursors. X-ray crystallography revealed that 1a and 1b are distorted at the annulated tropone skeleton with bond alternation, unlike typical 3,4-annulated π-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 (ΦF = 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 ΦF = 19% owing to the formation of a charge-separated structure with the tropylium cation.