Pr ³ ⁺ ‐Driven Dihydrophthalazine Synthesis and Its Application in Nitrophenol Sensing and Charge‐Transfer Complex Formation: Insights from UV–Vis and DFT Study

ABSTRACT Development of efficient and sustainable catalytic method has been of keen interest in the area of green synthesis. Herein, we report for the first time, Pr 3+ mediated rearrangement of phenolphthalein hydrazide to its corresponding dihydrophthalazine and its application toward sensing of nitrophenols have also been explored. The sensing performance of the dihydrophthalazine ligand was evaluated toward o‐nitrophenol, p ‐nitrophenol, and 2,4,6‐trinitrophenol that is, picric acid. The ligand exhibited marked selectivity for picric acid as evidenced by the appearance of a new charge‐transfer absorption band through π–π stacking interaction at ∼360 nm in the UV–vis spectrum. The binding stoichiometry between dihydrophthalazine and picric acid was elucidated using the Job plot method based on UV–vis spectroscopic measurement. Additionally, the synthesized dihydrophthalazine demonstrated sensitivity toward strong bases, displaying a visible color change from light pink to light purple. Furthermore, as an electron‐rich donor, dihydrophthalazine forms a charge transfer complex with the electron deficient acceptor picric acid, which is corroborated by both experimental (UV–vis spectroscopy) and theoretical (DFT) calculations.