Pyrazoline derived new "off-on-off" fluorescent pH sensors

Pyrazolines are reported with a broad range of bioactivities, excellent fluorescence properties and easy synthesis. But unfortunately, there is no or, very limited information available about their pH sensor application. Additionally, there is a little information about "off-on-off" type of pH sensors in literature. To fulfill the gap in literature and try to find out new possible "off-on-off" pH sensor, in this study it was planned to investigate the changes in the photo-physical properties of four pyrazolines D1-D4, 4-(5-(3,4-dimethoxyphenyl)-3-(4-substitutedphenyl)-4,5-dihydro-1H-pyrazol-1-yl)benzene sulfonamide, in water and different pH (2, 4, 6, 8, 9, 10, 12) in which on phenyl was H (D1), F (D2), Cl (D3) and Br (D4). The compound Dl exhibited excellent fluorescence properties and high fluorescence quantum yields in water. When the compounds D2-D4 which have electron-withdrawing substituents F, Cl and Br in D2-D4, respectively, were considered, it was observed that fluorescence quenching occurred in the molecules in question due to increased intersystem crossing probability while D1 did not show fluorescence quenching. However, the photo-physical properties of the compounds D1-D4 displayed important changes as a function of pH. It follows from these important changes that the compounds D1-D4 have "off-on-off" type of fluorescent pH sensor properties based on intramolecular charge transfer (ICT). These results indicated that these pyrazoline derivatives would be able to act as an efficient "off-on-off" fluorescent pH sensor in biological, environmental and medical etc. areas.