Substitution position effects of an electron-withdrawing group on the tautomer fluorescence of Coumarin–urea derivatives with an acetate anion

Abstract

To improve the fluorescence properties of coumarin–urea derivatives having a phenyl urea group bound at the 7-position of the coumarin ring (7CU derivatives), the substitution position effects of the electron-withdrawing group, CF3, on the photochemical behaviors of 7CU derivatives were investigated in the absence and presence of an acetate anion (AcO−). Both o- and m-CF3-7CU were prepared besides p-CF3-7CU, all of which bear a CF3 group at different positions on a phenyl ring opposite to the coumarin ring (o-, m-, and p-isomers, respectively). These three CF3–7CU derivatives underwent excited-state intermolecular proton transfer (ESPT) in the presence of AcO− followed by emissive tautomer formation, resulting in dual fluorescence. To elucidate the fluorescence state of tautomer, we compared the spectroscopic results of CF3–7CU derivatives including the rate constants for ESPT with regard to the inductive and resonance effects. The results revealed that the emission yield of tautomer of o-CF3-7CU was an order of magnitude larger than those of both m- and p-CF3-7CU due to the effective inductive effect of a CF3 group; this observation indicates that o-CF3-7CU is a suitable anion sensor. Our findings will contribute to the development of new molecular designs for fluorescent probes with excellent visibility.