Salicylaldehyde-diphenyl-azine skeleton-based ESIPT-coupled AIEgens with tunable emission and applicable as highly selective and sensitive Cu2+ ion sensor

Herein, we are reporting four novel salicylaldehyde-diphenyl-azine (SDPA) skeleton-based luminogens with tunable emission synthesized using the microwave. The photophysical studies in different solvents and acetonitrile/water fractions (fw = 0–90%) elucidate that they feature ESIPT and AIE characteristics. Further, they demonstrate colorimetric and fluorometric sensing for Cu2+ ions selectively and sensitively. The AIEgens responds TURN-OFF emission in the presence of Cu2+, having LOD values in the nanomolar range (4.4–12.1 nM), much lesser than the limit defined by US-EPA. The high values of binding constants (Ka) show strong binding, which is attributed to the paramagnetic nature of Cu2+ ions. The binding stoichiometry of the compounds and the Cu2+ ions were determined to be 2:1 using Job's plot and DFT analysis. The practical utilities of the proposed AIEgens have been analyzed by spiking Cu2+ in water samples from different sources. In conclusion, our study demonstrates that changing the substituents (electron-withdrawing in BSDPA and DBSDPA & electron-donating in DTSDPA and MSDPA) on the salicylaldehyde does not affect the selectivity and sensitivity towards Cu2+ ions. Instead, it significantly tunes emission properties towards the longer wavelength region (up to 50 nm in MSDPA compared to the unsubstituted SDPA). Overall, this study presents an environmentally benign and convenient approach, which opens up new avenues in synthesizing highly selective and sensitive red/NIR emissive AIEgens for Cu2+ ions by modifying appropriate substituents.