Fluorescence switching by O-dearylation of 7-aryloxycoumarins. Development of novel fluorescence probes to detect reactive oxygen species with high selectivity

Abstract

Coumarins exhibit fluorescence that is dependent on the nature of their substituents. We have found that the strong fluorescence of 7-hydroxycoumarin in aqueous media is completely lost in 7-aryloxycoumarins. Judging from the relation between the electron-donating character of the substituent at the 7-position and the relative quantum efficiency of fluorescence, we considered that the fluorescence properties of 7-hydroxycoumarin derivatives can be explained in terms of the intramolecular charge transfer (ICT) mechanism. On this basis, we designed and synthesized 7-(4′-hydroxyphenoxy)coumarin, 7-(2′-hydroxyphenoxy)coumarin, 7-(4′-aminophenoxy)coumarin and 7-(2′-aminophenoxy)coumarin, which we expected would be dearylated by highly reactive oxygen species. We found that these nonfluorescent aryloxycoumarins were dearylated to afford highly fluorescent 7-hydroxycoumarin upon reaction specifically with hydroxyl radical (˙OH), but not superoxide (O₂⁻), hydrogen peroxide (H₂O₂), or singlet oxygen (¹O₂). We suggest that these compounds may be applicable as specific fluorescence probes for hydroxyl radicals in an aqueous environment.