Insights into the turn-on mechanism of SNN fluorescent probe for Al3+ and Zn2+ detection

The rapid detection of metal ions in organisms and the environment is crucial. A comprehensive understanding of sensing mechanisms is essential for developing efficient probes. We systematically investigate the dynamical process and luminescence properties of the SNN probe and its complexes using the time-dependent density functional theory (TD-DFT) methods. By analyzing the potential energy surface and Born-Oppenheimer molecular dynamics simulations, we found that the twisted intramolecular charge transfer (TICT) process in the excited state of the SNN molecule inhibits the excited-state intramolecular proton transfer (ESIPT) process, leading to fluorescence quenching. However, upon binding with Al³⁺ or Zn²⁺, the molecular structural torsion is restricted, significantly enhancing fluorescence emission. Consequently, the complexes SNN-Zn²⁺ and SNN-Al³⁺ emit bright fluorescence. The computational results help elucidate how metal ions activate the luminescent properties of the probe and its response mechanism, providing valuable theoretical guidance and reference for the design and development of novel fluorescent probes.