Spectroscopic study of 3-aryl-7-methoxy-coumarin, iminocoumarin and bis-iminocoumarin derivatives in solution

• 3-Aryl-coumarin derivatives were strongly fluorescent in solution.
• 3-(2-Thienyl)-7-methoxycoumarin was of particular interest, including in protic solvents.
• In contrast, fluorescence was markedly decreased for imino derivatives and dimers.

3-Phenyl-5,7-dimethoxycoumarin and 3-(2-thienyl)-7-methoxycoumarin were strongly fluorescent in organic solution, with quantum yields reaching 0.84. In contrast, the iminocoumarin analogs of these dyes were poorly emissive. This shows that the behaviour of coumarins cannot be systematically transposed to their iminocoumarin analogs. The spectroscopic properties of iminocoumarin dimers were also examined. These dimers present little spectroscopic interest in comparison with the parent compounds. Above all, this study indicates the untapped potential of fluorescent 3-aryl-7-methoxycoumarins, which have scarcely been studied. In particular, 3-(2-thienyl)-7-methoxycoumarin exhibited good emission efficiency in a wide range of solvents, and could be very useful as a fluorescent label including in protic media.