Intramolecular exciplex formation and complexing behavior of 1-(2-naphthalenecarboxy)-n-(p-substituted benzenecarboxy)oxaalkanes as fluorescent chemosensors for calcium and barium ions

1-(2-Naphthalenecarboxy)-n-(p-substituted benzenecarboxy)oxaalkanes (2NPnX, X=H, Cl, CF3, CN; n=5, 6) as fluorescent sensors by exciplex emission for metal ions were synthesized. We investigated the relationship between the substitution effects and the metal ion recognition. Before the metal salts were added, the substitution effect was found. The fluorescence quantum yields (Φf) decrease as the electronegativity of the p-substituent of the acceptor benzoate increases. 2NPnCN (n=5, 6) with the most negative free energy of electron transfer (ΔGET) showed the most efficient quenching and exciplex emission. When Li+, Na+, K+ and Mg2+ were added to the acetonitrile solution of 2NPnX (X=H, Cl, CF3, CN; n=5, 6), the shape and intensity of the fluorescence spectra did not change. However, the spectra of all the 2NPnXs significantly changed with the addition of Ca2+ and Ba2+. Especially, due to the large fluorescent enhancement factor of 2NPnCN (n=5, 6) for Ca2+, they are well-suited for use as a fluorescent chemosensor for Ca2+.