Synthesis and Photophysical Study of 3-(Isoxazol-5-yl)Coumarins

Herein, efficient synthesis and photophysical study of three novel dyes based on 3-(isoxazol-5-yl)coumarins were developed, and their structures were confirmed by X-ray crystallography. The access to these fluorophores implied two condensation reaction steps under microwave conditions, starting from 3-acetylcoumarins and ending with the cyclization of the respective β-enaminone intermediate with hydroxylamine hydrochloride. By investigating solvatofluorochromism, acidochromism, and properties in the solid state, it was possible to establish that the optical properties of these dyes are coumarin ring-dependent, which was supported by TD-DFT calculations. In addition, fluorescence quantum yields (Φ_F) up to 36 % with short lifetimes decay (1.40–4.60 ns) were accompanied by a redshift behavior with electro-donating coumarins. Remarkably, solid-state studies showed a higher emission of the unsubstituted coumarin (Φ_F=64 %) due to significant supramolecular interactions; in addition, acidochromism experiments using TFA were performed, finding detection limits of up to 1.03 μM.