Near-Infrared fluorescent unsymmetrical aza-BODIPYs: Synthesis, photophysics and TD-DFT calculations

In view of the ever-growing demand for efficient NIR fluorophores for biomedical applications, we herein report the synthesis and properties of four unsymmetrical aza-BODIPYs exhibiting NIR fluorescence. Highly desirable photophysical and photochemical properties were induced in these molecules due to the presence of both strongly electron-withdrawing p-nitrophenyl rings (p-NO₂Ph-) and mildly electron-donating p-methoxyphenyl rings (p-MeOPh-) within the aza-BODIPY core. In particular, upon excitation with λ_abs the unsymmetrical aza-BODIPYs studied exhibited NIR emission with λ_f ranging from 699 nm to 718 nm in toluene. The fluorescence quantum yields (Φ_f), depending on the substitution pattern, ranged from Φ_f = 0.49 to Φ_f = 0.22 and the fluorescence lifetimes ranged from τ_f = 1.90 ns to τ_f = 3.59 ns. Aza-BODIPY with electron-donating substituent at 3 position and electron-withdrawing substituent at 5 position was identified as cell permeable, NIR emitting fluorophore suitable for bioimaging.