Synthesis and liquid-crystal properties of T-shaped quinolines via formal aza-Diels-Alder reaction

T-shaped quinoline cycloadducts were efficiently synthesized via a Yb3+-mediated three-component reaction of aldehydes, aromatic alkynes, and isoxazolamines. Their structures were confirmed by 1H and 13C NMR spectroscopy and high-resolution mass spectrometry (HRMS). Mechanistically, the transformation proceeds through arylpropargylamine intermediates and can be considered a formal [4 + 2]-cycloaddition between imine-derived aza-dienes and alkynes. DFT calculations support a stepwise pathway involving 2-azadiene and alkyne interactions, accounting for the observed T-shaped regiochemistry. Investigation of their liquid crystal properties revealed glassy nematic behavior, as evidenced by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), with schlieren textures consistently observed across all samples. Photophysical studies showed absorption maxima at 267–300 nm and emission at 400-418 nm, large Stokes shift (102-151 nm), and quantum yields of ∼20 % for phenyl- and ∼40 % for naphthyl-substituted derivatives, with solution fluorescence spanning purple to pale cyan.