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 D1 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.