Fluorescent Pyranoindole Congeners: Synthesis and Photophysical Properties of Pyrano[3,2- f ], [2,3- g ], [2,3- f ], and [2,3- e ]Indoles.

This paper reports the synthesis of four types of annulated pyranoindole congeners: pyrano[3,2-]indole, pyrano[2,3-]indole, pyrano[2,3-]indole, and pyrano[2,3-]indole and photophysical studies in this series. The synthesis of pyrano[3,2-], [2,3-], and [2,3-]indoles involve a tandem of Bischler-Möhlau reaction of 3-aminophenol with benzoin to form 6-hydroxy- or 4-hydroxyindole followed by Pechmann condensation of these hydroxyindoles with β-ketoesters. Pyrano[2,3-]indoles were synthesized through the Nenitzescu reaction of -benzoquinone and ethyl aminocrotonates and subsequent Pechmann condensation of the obtained 5-hydroxyindole derivatives. Among the pyranoindoles studied, the most promising were pyrano[3,2-] and [2,3-]indoles. These compounds were characterized by moderate to high quantum yields (30-89%) and a large (9000-15,000 cm) Stokes shift. More detailed photophysical studies were carried out for a series of the most promising derivatives of pyrano[3,2-] and [2,3-]indoles to demonstrate their positive solvatochromism, and the data collected was analyzed using Lippert-Mataga equation. Quantum chemical calculations were performed to deepen the knowledge of the absorption and emission properties of pyrano[3,2-] and [2,3-]indoles as well as to explain their unusual geometries and electronic structures.