The enhancement of the D-A effect of an asymmetric Schiff base by introducing acetyl groups into diaminomaleonitrile: Synthesis, red fluorescence and crystal structure

An asymmetrical salen-type organic ligand was designed and synthesized by a new strategy developed using a precursor Ac-DMN, which is a diaminomaleonitrile (DMN) incorporated with an acetyl group. In this study, two types of asymmetrical ligands, namely Ac-DMN-salicylaldehyde (L⁰) and Ac-DMN-4-N,N-diethyl-salicylaldehyde (L¹), and their Zn(ii) coordination complexes were studied. With the electron-pushing substituent, L¹ showed interesting photoluminescence behaviour distinct from that of L⁰. In a THF solution, the maximum fluorescence emission of L¹ red-shifted to 90 nm compared with that of L⁰. Furthermore, in a THF-H₂O solution, L¹ exhibited aggregation-induced emission (AIE), but L⁰ exhibited aggregation-caused quenching (ACQ). Upon coordination with Zn(ii), the maximum emission wavelengths in THF for both the ligands were red-shifted to 100 nm and 105 nm, respectively. The crystalline solid-state photoluminescence properties were studied based on the single-crystal structural analysis.