The development of optical chemosensors for anion detection has grown rapidly in recent years; however, dual-anion sensors remain scarce, and many reported systems rely on UV excitation, limiting their applicability under environmental conditions. In this work, we present a comprehensive theoretical protocol to elucidate the photophysical and transduction mechanisms of a reported dual-anion sensor based on 1,2-dihydroquinoline-3-carbaldehyde functionalized with benzothiazole-2-acetonitrile (D2-CH3). Ground-state (S0) and excited-state (singlet and triplet) properties were investigated using time-dependent density functional theory (TD-DFT), complemented by multiconfigurational post-Hartree–Fock calculations at the CASSCF/NEVPT2 level. The parent sensor exhibits colorimetric and fluorometric responses toward AsO2− and CN−, which are rationalized in terms of metal-to-ligand charge transfer (MLCT) and chelation-enhanced fluorescence (CHEF) mechanisms, respectively. Building on this mechanistic understanding, eleven new candidate sensors were theoretically designed and analyzed through three functionalization strategies: substitution at the dihydroquinoline unit (D2-R), the benzothiazole unit (B2-R), or both (DB2-R), using R = –CH3, –NH2, –SH, and –OH. The free sensors and their interactions with AsO2−, CN-, and OH- were systematically examined. Simulated absorption and emission spectra, oscillator strengths, electron-transfer and radiative kinetic parameters, interaction energy analyses, and structural stability assessments collectively reveal the relationships between molecular structure, transduction mechanisms, and optical response. The results demonstrate that moderately electron-donating substituents effectively shift absorption into the visible region without compromising dual-anion selectivity. All proposed systems are predicted to selectively detect CN− via a CHEF mechanism, while four candidates (D2-OH, D2-SH, B2-SH, and B2–NH2) are identified as promising UV-free dual-anion sensors. Overall, this work establishes a robust theoretical framework to guide the rational design of next-generation dual-anion optical sensors with improved visible-light performance.
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