Polypyridine‐Based Ru(II) Molecular Switch for Cyanide Detection and CO ₂ ‐Induced Reversibility in Aqueous Medium: Experimental and DFT Insights

ABSTRACT This work involves a detailed exploration of the synthesis, photophysical, and redox behavior of a Ru(II) complex bearing mixed terpyridine–bipyridine ligands, formulated as [Ru(L)(4,4 / ‐dmbpy)(pz)], where L = 4 / ‐(3,5‐dimethylphenyl)‐2,2 / :6 / ,2 // ‐terpyridine, pz = pyrazole, and 4,4 / ‐dmbpy = 4,4 / ‐dimethyl‐2,2 / ‐bipyridine. The metal complex was characterized through various methods, including single‐crystal X‐ray diffraction, FT‐IR, 1 H NMR, UV‐vis spectroscopy and mass spectrometry. Under ambient conditions, the complex exhibits notable luminescence and demonstrates selective recognition of cyanide ion in water medium through deprotonation of the acidic pyrazole proton, resulting in luminescence quenching, an “off‐state” response. Upon exposure to CO 2 gas, the emission resumes, indicating an “on‐state.” Similarly, lowering the temperature enhances emission intensity (on‐state), whereas heating quenches emission (off‐state), thereby exhibiting characteristics of a reversible molecular switch. These emission responses, modulated by cyanide, CO 2 and temperature, were further employed to construct two‐input binary logic gates. Additionally, density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations were performed to elucidate the nature of the electronic transitions and support the interpretation of the experimental spectral features.