Novel Y-shaped multi-stimuli responsive pyrene substituted quinoxaline and pyridopyrazine-based push-pull molecules showing solvatochromism, aggregation induced excimer emission, acidofluorochromism and moisture detection

The design, synthesis, and characterization of a series of novel Y-shaped push-pull compounds based on the quinoxaline and pyrido[2,3-b]pyrazine moieties as electron acceptors, 1,4-phenylene rings as spacers, and pyrene units as electron donors was carried out. Their structural, photophysical, solvatochromic, aggregation, acidofluorochromic, moisture sensing and thermal characteristics were investigated. It was observed that the substitution of an electron withdrawing group (EWG) on the quinoxaline moiety led to red shifts in the absorption and emission spectra due to enhanced intramolecular charge transfer (ICT). Crystal structure of 3a showed twisted structure with herringbone packing arrangement and intermolecular π-π stacking of pyrene units. Compounds 3a-c displayed aggregation induced excimer emission in aggregated states in THF-water mixtures due to intermolecular π-π interactions among pyrene units. The concentration dependent studies of 3a-d showed monomer/excimer conversion at higher concentrations in THF solvent. The excited lifetime analysis revealed a biexponential decay at lower concentrations with a major contribution from monomeric species while at higher concentrations, a triexponential decay was observed with a major contribution from the excimeric species. The lifetimes for the monomeric species (2.9–3.3 ns) were found to be higher than the excimeric species (1.3–2.6 ns). These compounds displayed reversible acidofluorochromism, in solution and in solid state, with TFA followed by subsequent treatment with TEA. Compounds were also able to detect up to 0.005 v/v % moisture in THF solvent through TICT mechanism. Filter paper strips coated with these compounds were able to detect TFA vapors and moisture, which shows the practical application of these compounds.