Conformationally restricted and ring-fused aza-BODIPYs as promising near infrared absorbing and emitting dyes

• We summarize the synthetic strategies toward ring-fused aza-BODIPYs. • Comparison is made on aza-BODIPY skeletons with different types of fused rings. • Important photophysical properties and applications of ring-fused aza-BODIPYs are described. Aza-BODIPY is an important class of heteroatom-containing BODIPY analogue with strongly electron-withdrawing nitrogen atom instead of methine carbon atom at the meso -position. Despite the maximum absorption and emission wavelengths of conventional tetraphenyl-based aza-BODIPY dyes are less than 700 nm, which are non-optimal for their potential applications, aza-BODIPY is an ideal framework to develop near infrared (NIR) chromophores and fluorophores. Several elegant modification methods have been developed to adjust the structure of aza-BODIPY scaffold in order to extend the polyaromatic π-conjugation and further push the absorption and emission wavelength as far as possible. Among these structural modifications, the approaches to obtain conformationally restricted and ring-fused aza-BODIPY dyes by preventing the free rotation of the peripherally aryl substituents or fusing aromatic rings to the BODIPY skeleton are especially promising, which lead to a substantial bathochromic-shift of absorption and emission spectra of dyes in the NIR region beyond 700 nm. Herein, we comprehensively summarize the different synthetic strategies developed in recent literatures to access conformationally restricted and ring-fused aza-BODIPYs, and describes the important spectroscopic and/or photophysical properties to provide meaningful inspiration for further development of functional organic compounds with excellent properties as promising near infrared absorbing and emitting dyes.