Direct synthesis of benzo[a]carbazoles by palladium-catalyzed domino reactions: synthesis and photophysical properties of diverse benzo[a]carbazoles

A direct, concise, and atom-economical synthetic method for the generation of benzo[a]carbazoles, using a palladium-catalyzed reaction has been developed. The reaction produces gave various benzo[a]carbazoles in good to excellent yields, through Knoevenagel condensation followed by intramolecular ortho-arylation. The synthetically attractive feature of the procedure is reflected by its applicability to a wide range of indole and bromobenzaldehyde derivatives. Mechanistic aspects of the reaction involves the formation of a single isomer of two possible products which leads to the formation of benzo[a]carbazoles via a palladium-catalyzed C-H bond functionalization reaction. The development of the annulation reaction with a wide substrate scope provided a unique opportunity to evaluate photophysical properties of a series of benzo[a]carbazoles. Almost all the compounds evaluated in this study were found to emit purple to blue light in the visible region. Some interesting structure-property correlations are also described.