Synthesis, Structural, and Optical Properties of Azuleno[1,2‑c]pyran-1-ones: Bro̷nsted Acid-Mediated Cyclization of 2‑Azulenylalkynes

Pyrones and their aromatic ring-fused derivatives have gained significant attention due to their diverse biological activities and potential as foundational frameworks for advanced materials. In this paper, we describe a proficient approach for the preparation of azuleno­[1,2-c]­pyran-1-ones, which are difficult to produce by using conventional methods. The synthesis was achieved through Bro̷nsted acid-mediated cyclization of 2-azulenylalkynes. The structural and optical properties of azuleno­[1,2-c]­pyran-1-ones were characterized by single-crystal X-ray analysis, NMR, UV/vis, and fluorescence spectroscopies. Under acidic conditions, these compounds displayed notable spectral alterations and emission, distinct from their spectra in neutral medium. These results suggest that azuleno­[1,2-c]­pyran-1-ones hold great potential for applications in organic electronic materials and fluorescent pH sensors.