Synthesis, Photophysics and Redox Properties of Aza-BODIPY Dyes with Electron-Donating Groups

A series of novel aza-BODIPY dyes substituted with p-(dimethylamino)phenyl groups were synthesized and their spectral and electrochemical properties were compared. In particular, the impact of p-(Me₂ N)Ph- groups on these characteristics was of consideration. For two aza-BODIPYs studied, a near-IR absorption band was observed at circa λ_abs =796 nm. Due to the pronounced intramolecular charge transfer (ICT) exerted by the presence of strongly electron-donating p-(Me₂ N)Ph- substituents, the compounds studied were weakly emissive with the singlet lifetimes (τ_S ) in the picosecond range. Nanosecond laser photolysis experiments of the brominated aza-BODIPYs revealed T₁ →T_n absorption spanning from ca. 350 nm to ca. 550 nm with the triplet lifetimes (τ_T ) ranged between 6.0 μs and 8.5 μs. The optical properties of the aza-BODIPYs studied were pH-sensitive. Upon protonation of the dimethylamino groups with trifluoroacetic acid in toluene, a stepwise disappearance of the NIR absorption band at λ_abs =790 nm was observed with the concomitant appearance of a blue-shifted absorption band at λ_abs =652 nm, which was accompanied by a prominent emission band at λ_fl =680 nm. The transformation from a non-emissive to an emissive compound is associated with the inhibition of the ICT. As estimated by CV/DPV measurements, all aza-BODIPYs studied exhibited two irreversible oxidation and two quasi-reversible reduction processes. All compounds studied exhibit extremely high photostability and thermal stability.