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‐(Me2N)Ph‐ groups on these characteristics was of consideration. For two aza‐BODIPYs studied, a near‐IR ab...

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Bibliographic Details
Published in:Chemphyschem 2019-10, Vol.20 (19), p.2482-2497
Main Authors: Obłoza, Magdalena, Łapok, Łukasz, Pędziński, Tomasz, Stadnicka, Katarzyna M., Nowakowska, Maria
Format: Article
Language:English
Subjects:
Absorption spectra
aza-BODIPY
Bromination
Charge transfer
Dyes
Electrochemical analysis
electron donors
Emissivity
excited states
Optical properties
Oxidation
Photolysis
Protonation
Thermal stability
Toluene
triplet states
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Summary: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. Hybridization matters: Depending on the position of Br substituents in the structure of aza‐BODIPY with p‐Me2N groups, the molecule can exhibit either visible‐light absorption or NIR absorption. Unexpectedly, the bromine atom in close vicinity to the Me2N group forms a hydrogen bond with the H atom of the CH3 group exerting the sp3 hybridization on the nitrogen atom. That phenomenon hinders the electron‐donating effect of the Me2N group.
ISSN:1439-4235
1439-7641
DOI:10.1002/cphc.201900689