A theoretical study on the ICT mechanism of zinc ion fluorescent probe based on triphenylamine

The development of fluorescent probes for zinc ion (Zn²⁺) detection has significant importance in biological, environmental, and materials sciences. This study investigates the fluorescence enhancement mechanism of a novel triphenylamine-based probe (SHK) upon Zn²⁺ coordination using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The PBE0/TZVP method revealed that free SHK exhibits weak fluorescence due to intramolecular charge transfer (ICT) between the triphenylamine hydroxybenzaldehyde and 2-hydrazinyl benzothiazole moieties, characterized by a large spatial electron-hole separation (D = 6.10 Å) and low oscillator strength (f = 0.0385). Upon Zn²⁺ binding, the formation of SHK-Zn stabilizes the trans-configuration, enhances structural rigidity, and suppresses ICT by reducing spatial separation (D = 1.44 Å). Instead, Metal-ligand interaction and extended conjugation dominate, leading to localized excitation (LE) with a narrowed HOMO-LUMO gap. This transition results in a significant red shift in absorption (345 nm → 436 nm) and emission (489 nm → 536 nm), accompanied by enhanced fluorescence intensity. Frontier molecular orbital (FMO) and natural transition orbital (NTO) analyses confirmed the shift from ICT-dominated transitions in SHK to LE-dominated transitions in SHK-Zn. These findings provide a molecular-level rationale for the Zn²⁺-induced fluorescence "turn-on" behavior, emphasizing the critical role of structural rigidification and electronic redistribution in modulating photophysical properties.