Novel pH tunable fluorescent sensor with dual recognition mode

A novel pH fluorescent chemosensor based on 2-hydroxybenzaldehyde 1-naphthoyl hydrazone (SNH) has been synthesized. This sensor has the ability to respond to both low and high pH values at different wavelengths. In acidic media, the fluorescence of SNH enhanced dramatically at shorter wavelength (λem = 380 nm, ϕf = 0.08) while in basic media the fluorescence at longer wavelength (λem = 500 nm, ϕf = 0.02) increased. At intermediate pH values (5–9), is detected the appearance of a weak emission at 460 nm, likely due to a keto-phototautomer associated to an intramolecular excited-state proton transfer. A complex mechanism tunable in the range of pH 1–11 with at least three fluorescent species at λem = 380, 460 and 500 nm is proposed.