Generation of pyridinylethylidenemalononitrile cyclic dimer: A facile route to multi-stimuli-responsive far-red/NIR luminogens

A simple synthetic strategy was successfully employed to develop NIR luminophores with switchable color-contrasting luminescence properties. D-A molecular motifs with dicyanovinylene appended pyridinyl moiety as the electron-withdrawing unit was not reported among luminescent molecules so far, owing to its unstable precursor compound pyridinylethylidenemalononitrile. By modifying the Knoevenagel condensation reaction pathways, the present work succeeded for the first time in identifying and crystallizing a novel cyclic dimerization product of the precursor compound. It further reports a new, simple, efficient synthetic route to achieve push-pull far-red/NIR emissive luminophores with the dicyanovinylene appended pyridinyl skeleton as the potent electron withdrawing unit. The enhanced charge transfer and the acidochromic characteristics make them suitable candidates in multi-stimuli-responsive smart materials with anti-counterfeiting applications. A new insight into the origin of pyridinylethylidenemalononitrile dimer derived far-red/NIR luminogens was introduced by a novel synthetic strategy following an unconventional mechanistic trajectory. The donor-acceptor molecular skeleton renders multi-stimuli-responsive luminescence switching properties to the synthesized molecules. [Display omitted] •A novel cyclic dimerization product was prepared by the Knoevenagel reaction of 4-acetyl pyridine with malononitrile.•A New synthetic strategy was employed to develop D-A molecular skeleton with dicyanovinylene appended pyridinyl moiety as the electron-withdrawing unit.•N⋯H and C-H … π supramolecular interactions assisted structural rigidity render charge transfer properties with far-red/NIR solid state emission.•High contrast acidofluorochromic properties by pyridine to pyridinium ion transformation.