2‐Aryl‐3H‐1,3‐Benzazaphosphole Oxides: Synthesis, Optical Properties, and Excited State Intramolecular Proton Transfer

Inclusion of a heteroatom to the phosphole ring is a promising strategy to intrinsically modulate the optical properties of phosphole derivatives. We report on a series of 2‐aryl‐3H‐1,3‐benzazaphosphole oxides that were efficiently prepared via sequential C−P cross‐coupling, dehydrative [3+2] cycloaddition, and ring‐oxidation reactions. The inclusion of one nitrogen atom into the benzophosphole framework caused red shifting of the absorption and emission maxima, reflecting the greater stabilization of the LUMO level. 2‐(2‐Hydroxyphenyl)benzazaphosphole oxide underwent excited state intramolecular proton transfer and emitted a weak fluorescence from the excited state of the N−H tautomer. We have established a convenient method for the synthesis of 2‐aryl‐3H‐1,3‐benzazaphosphole oxides, which exhibit the characteristic optical properties reflecting the electronic effects of the β nitrogen atom. The ortho‐hydroxyphenyl derivative underwent ESIPT in solution to emit considerably Stokes‐shifted fluorescence in the range of 600–800 nm.