Unveiling the competitive reaction pathways and photophysical evolution of a naphthalene-benzothiazole fluorescent probe for sulfite: A combined experimental and DFT/TD-DFT study

A naphthalene-benzothiazole probe (Q) for dual-channel sulfite detection was developed. • DFT calculations confirmed a kinetically dominant Michael addition at the C11 site. • Activation energy for the C11 pathway (16.11kcal/mol) is lower than alternative sites. • Hole-electron analysis revealed an ICT-to-LE transition during fluorescence quenching. The design and synthesis of a D–π–A type fluorescent probe based on a naphthalene-benzothiazole scaffold, for the colorimetric and fluorometric detection of sulfite (SO₃²⁻), are reported. Probe Q exhibits a selective response toward sulfite in DMSO H₂O (7:3, v/v) solution, with a detection limit of 0.42 μM. The reaction mechanism, evaluated through DFT and TD-DFT calculations using a benchmarked PBE0 functional, identifies several potential nucleophilic attack sites (C11, C14, and C16). Transition state and intrinsic reaction coordinate analyses confirm that the Michael addition at the C11 site is the kinetically dominant pathway, with an activation energy of 16.11 kcal/mol. Furthermore, combined hole-electron and excited-state analyses, based on vertical excitation calculations and oscillator strengths, reveal that the fluorescence quenching originates from a transition of the S₁ state from a bright intramolecular charge transfer (ICT) state to a non-radiative state upon sulfite addition. This study provides a molecular-level, synergistic experimental and theoretical framework for the rational design of reaction-based fluorescent probes.