7-Pyridylindoles: synthesis, structure, and properties

A series of three isomeric 7-pyridylindoles (7-PIs) are prepared where the pyridine attachment is through C2, C3, or C4. These systems are prepared by a combination of the Bartoli reaction and the Stille coupling with an appropriate pyridyl stannane. By treatment with CH(3)I, the 7-PIs can be converted to their pyridinium salts. Deprotonation at the NH of these salts leads to a zwitterion which, in the 4-pyridyl system, also exists as a neutral isomer. The photophysical and NMR properties of these systems are discussed. All three pyridylindoles are analyzed by X-ray crystallography and shown to exist in different states of aggregation dictated by the formation of intra- and intermolecular H-bonds.