Vertically π‑Expanded Coumarins: The Synthesis and Optical Properties

A regioselective synthesis of naphtho­[2,1,8-<i>def</i>]­coumarins has been realized through a concise route that involves the intramolecular Friedel–Crafts reaction of benzo­[<i>f</i>]­coumarins. Tetracyclic, planar products were prepared starting from assembly of the suitably substituted coumarin via the Pechmann reaction of 2-naphthols with acetone-1,3-dicarboxylates, followed by an intramolecular Friedel–Crafts reaction. In contrast to earlier report, the main product of the condensation reaction performed at 130 °C was the corresponding sulfonic acid and not the phenol itself. The one-pot process afforded the desired phenol in 39% yield. This reaction has been extended to some naphthalenediols. The model 5-hydroxy-naphtho­[2,1,8-<i>def</i>]­coumarin was transformed into corresponding dimer using various pathways including intermolecular oxidative aromatic coupling. Photophysical studies revealed that 5-hydroxy-naphtho­[2,1,8-<i>def</i>]­coumarin has the most bathochromically shifted both absorption and emission among all π-expanded coumarins bearing one OH functionality. In general, all prepared coumarin-phenols as well as their hexyl ethers displayed moderate to strong greenish-yellow fluorescence, except of dimer that emits at 552 nm. The fluorescence of these dyes was strongly dependent on polarity of the solvent. Computational studies supported interpretation of optical properties for the selected compounds.