Photophysical properties of a D–π-A Schiff base and its applications in the detection of metal ions

Abstract

A D–π-A Schiff-base compound, 2-amino-3({[4-(diethylamino)-2-hydroxylphenyl]methylene}amino)-but-2-enedinitrile (H₂L), was prepared using diaminomaleonitrile and 4-(N,N-diethylamino)salicylaldehyde. Compared with H₂L at 293 K, a low temperature of 100 K makes the parallel aromatics in H₂L come closer and fluorescence emission becomes weaker because of π–π interaction-caused quenching. After grinding the crystals of H₂L, the colour changed from dark brown-red to yellow under room light and the fluorescence emission enhanced about 9-fold due to the damage of the intermolecular hydrogen bonds, leading to a decrease of non-radiative transition. H₂L showed aggregation-induced emission enhancement (AIEE) characteristics in THF/H₂O, whose mechanism is attributed to the restriction of intramolecular rotation (RIR). The UV-Vis spectra of H₂L with Cu²⁺ in THF/H₂O showed that at first a CuL complex was formed and subsequently a CuL′ complex (H₂L′ = N′,N′-bis(4-N′,N′-diethylsalicylidene) ethylenediamine) was obtained. The CuL complex turned into the CuL′ complex as time prolonged. H₂L acted as a dual channel chemosensor for Cu²⁺ ions in THF/HEPES (v/v: 2 : 8, pH = 8.0) and the CuL complex was stable in this medium. H₂L is also a naked-eye probe for Mn²⁺ ions in CH₃CN. The limits of detection are much lower than the allowable level of copper(II) and manganese(II) in drinking water set by the World Health Organization (WHO).