Synthesis and Spectroscopic Properties of Fused‐Ring‐Expanded Aza‐Boradiazaindacenes

A series of fused-ring-expanded aza-boradiazaindacene (aza-BODIPY) dyes have been synthesized by reacting arylmagnesium bromides with phthalonitriles or naphthalenedicarbonitriles. An analysis of the structure-property relationships has been carried out based on X-ray crystallography, optical spectroscopy, and theoretical calculations. Benzo and 1,2-naphtho-fused 3,5-diaryl aza-BODIPY dyes display markedly red shifted absorption and emission bands in the near-IR region (>700 nm) due to changes in the energies of the frontier MOs relative to those of 1,3,5,7-tetraaryl aza-BODIPYs. Only one 1,2-naphtho-fused aza-BODIPY of the three possible isomers is formed due to steric effects, and 2,3-naphtho-fused compounds could not be characterized because the final BF(2) complexes are unstable in solution. The incorporation of a -N(CH(3))(2) group at the para-positions of a benzo-fused 3,5-diaryl aza-BODIPY quenches the fluorescence in polar solvents and results in a ratiometric pH response, which could be used in future practical applications as an NIR "turn-on" fluorescence sensor.