Huisgen Cycloaddition of Azidoazulenes: Synthesis, Structural and Optical Properties of 2- and 6-(1,2,3-Triazol-1-yl)azulenes

We developed an efficient and modular route to 2- and 6-(1,2,3-triazol-1-yl)azulenes to expand the synthetic accessibility and functional scope of azulene-based π-systems with stimulus-responsive photophysics. Readily accessible 2- and 6-azidoazulenes, prepared in excellent yields via S_NAr reactions of haloazulenes, were subjected to Cu(I)-catalyzed Huisgen [3 + 2] cycloaddition with a broad range of terminal alkynes to afford the corresponding triazolylazulenes in good to high yields, followed by acid-mediated decarboxylation and Staudinger reduction to enable further diversification to 2-azulenyltriazoles and a 6-aminoazulene derivative. Single-crystal X-ray diffraction analysis revealed substitution-position-dependent torsional arrangements and variations in π-conjugation between the azulene and triazole units. Photophysical characterization by UV/Vis absorption and fluorescence spectroscopy showed pronounced halochromism under acidic conditions, and selected derivatives displayed substantially enhanced fluorescence quantum yields. Overall, these results establish the azulene-1,2,3-triazole motif as a versatile building block for designing optoelectronic π-systems with acid-responsive emission properties.