Synthesis of a Luminescent Azaphosphole

A carbazole-based azaphosphole (compound 3) has been prepared by acid-catalyzed dehydrocyclization of 1-phosphino-9H-carbazole (2) with benzoyl chloride. Unlike other benzazaphospholes having sigma(2),lambda(3)-multiply bonded phosphorus atoms, compound 3 displays significant fluorescence in solution, as does primary phosphine 2. Compound 3, as well as the materials on route to its synthesis, were characterized by multinuclear NMR, UV/Vis, and fluorescence spectroscopy. The optical data of 2 and 3 are consistent with results of DFT and TDDFT [6-311G+(d,p) CAMB3LYP] calculations. The computed structure of compound 3' (R = Ph) is also consistent with the experimental X-ray structural determination, which reveals a planar heterocyclic system and slight rotation of the p-tolyl group out of plane with respect to the heterocyclic core.