Constructing ESIPT-Capable α-Cyanostilbene Luminogens: Influence of Different Aromatic Substitutions Tethered to H-Acceptor (CH = N) on Photophysical Properties, Cu²⁺ and Fe³⁺ Detection

Herein, two excited-state intramolecular proton transfer (ESIPT)-capable α-cyanostilbene luminogens were synthesized by Schiff base reaction of salicylaldehyde-like α-cyanostilbene candidate with 1-naphthylamine and 3-biphenylamine, respectively. We systematically analyzed their photophysical properties compared with their analogue, and demonstrated that their fluorescence behaviors could be elaborately modulated by different aromatic substitutions tethered to H-acceptor (CH = N). In virtue of the outstanding solid fluorescence, the 3-biphenylamine-decorated fluorophore was applied for monitoring Cu²⁺/Fe³⁺ qualitatively on the TLC-based test strip in real time and sensing Cu²⁺/Fe³⁺ quantitatively in the THF/H₂O medium (f_w = 90%, pH = 7.4). When the probe chelated with Cu²⁺/Fe³⁺, similar “turn-off” fluorescence signal outputs were triggered. From the fluorescence titration experiments, the detection limits were evaluated as 7.97 × 10^− 8 M for Cu²⁺ and 8.24 × 10^− 8 M for Fe³⁺, and the binding constant (K_α) values of the complexes were found to be 7.80 × 10⁴ M^-1 for Cu²⁺ and 9.06 × 10⁴ M^-1 for Fe³⁺. Job’s plots indicated that probe complexed with Cu²⁺/Fe³⁺ in a 2:1 binding stoichiometry ratio. Furthermore, the probe was used to accurately quantify the Fe³⁺ spiked in real water specimens. This study offered a new perspective to construct ESIPT-capable α-cyanostilbene luminogen as the potential luminescent probe.