A boron dipyrrin (BODIPY) dye was designed as a molecular single-component electrophore for redox flow batteries. All positions of the BODIPY core were assessed on the basis of literature data, in particular cyclic voltammetry and density functional calculations, and a minimum required substitution pattern was designed to provide solubility, aggregation, radical cation and anion stabilities, a large potential window, and synthetic accessibility. In-depth electrochemical and physical studies of this electrophore revealed suitable cathodic behavior and stability of the radical anion but rapid anodic decomposition of the radical cation. The three products that formed under the conditions of controlled oxidative electrolysis were isolated, and their structures were determined by spectroscopy and comparison with a synthetic model compound. From these structures, a benzylic radical reactivity, initiated by one-electron oxidation, was concluded to play the major role in this unexpected decomposition.
The formation of meso‐aryl‐BODIPYs (boron dipyrrins) through the acidic condensation of 3,4‐dialkylpyrroles with aromatic aldehydes, followed by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) oxidation and BF2 complexation, has been reviewed. Surprisingly, it was found that the major products from these reactions were not the anticipated symmetric BODIPYs, but non‐symmetric derivatives carrying one benzyl substituent at the BODIPY α position. The best yields and simple purification conditions were be achieved if the oxidant was not employed in the one‐pot reaction sequence. Electron‐rich benzaldehydes provided the best results, whereas precursors with electron‐withdrawing substituents gave significant amounts of the symmetric BODIPYs as side‐products. This unexpected result can be rationalized mechanistically on the basis of two reaction pathways that diverge from a common intermediate at an early step in the condensation sequence. Preliminary reactivity investigations showed chlorination to give unexpected results, but a typical substitution and coupling chemistry.
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