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

Abstract

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‐(Me2N)Ph‐ groups on these characteristics was of consideration. For two aza‐BODIPYs studied, a near‐IR absorption band was observed at circa λabs=796 nm. Due to the pronounced intramolecular charge transfer (ICT) exerted by the presence of strongly electron‐donating p‐(Me2N)Ph‐ substituents, the compounds studied were weakly emissive with the singlet lifetimes (τS) in the picosecond range. Nanosecond laser photolysis experiments of the brominated aza‐BODIPYs revealed T1→Tn absorption spanning from ca. 350 nm to ca. 550 nm with the triplet lifetimes (τT) 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 λabs=790 nm was observed with the concomitant appearance of a blue‐shifted absorption band at λabs=652 nm, which was accompanied by a prominent emission band at λfl=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.

Near‐Infrared Photoactive Aza‐BODIPY: Thermally Robust and Photostable Photosensitizer and Efficient Electron Donor

“…This additional absorption band we attributed to the π‐π * transitions involving orbitals below the HOMO or above the LUMO. A similar spectral feature was observed for aza‐BODIPY with dimethylamino groups (Me 2 N−) in para ‐ position of the phenyl rings at 3,5‐positions of the chromophore core (Figure ) . THF solutions of aza‐dipyrromethene 5 exhibit a bright green color with λ abs equal 673 nm ( ϵ =48 000 M −1 cm −1 ).…”

“…the dihedral angles formed between the adjacent pyrrole rings, the hybridization of the nitrogen atoms of the amino group and the optical properties (λ abs ) of a set of aza‐BODIPYs bearing p ‐aminophenyl substituents at 3,5‐positions. The crystallographic data and the optical properties of two of the structures presented in Figure were recently reported by us in our ChemPhysChem paper …”

“… A comparison of the dihedral angles (top) between the five‐membered rings of the central chromophore and the hybridizations of the nitrogen atoms of the amino groups for different ADPM and aza‐BODIPYs substituted with either p ‐(Me 2 N) or p ‐(Ph 2 N) groups …”

“…Furthermore, aza‐BODIPYs were considered as fluorescent chemosensors for the detection of ammonia, cysteine, homocysteine and glutathione in living cells, Hg 2+ , F − , in vivo detection of H 2 S and NO, carbon dioxide, glucose in whole blood and pH‐sensitive sensors . Aza‐BODIPYs have also been tested as photoactive components in photovoltaic devices and nonlinear optical materials …”

“…Our interest in efficient yet stable organic triplet photosensitizes capable of light absorption from the visible and near‐IR part of the spectrum has lead us to the discovery of novel aza‐BODIPYs . Herein, we report the five‐steps synthesis and a comprehensive study on the photophysical and electrochemical properties of aza‐BODIPY substituted with strongly electron‐donating p ‐(diphenylamino)phenyl substituents ( p ‐Ph 2 N) at 3,5‐positions (see Scheme ).…”