Construction of a 2,2′-biazulene framework via Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans

Abstract

Dibenzofurans featuring a 2,2′-biazulene framework were prepared in good yields by Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans in 100% H₃PO₄. NMR, UV-Vis, and fluorescence spectroscopies were used to investigate the structural and optical properties of the products prepared. Remarkably, the annulated products exhibited fluorescence, with the longest wavelength of azulene derivatives reported to date, which extended into the near-infrared region under acidic conditions.