Design and Evaluation of a Boron Dipyrrin Electrophore for Redox Flow Batteries

Abstract

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.

Redox‐Switchable Cyan Fluorescence of a BODIPY Analog Inspired by Propentdyopent Pigments

“…The redox chemistry of BODIPY dyes is critical to their potential use in electrofluorochromic sensors, photosynthetic antenna systems, and other reactivity‐based applications , . In this context, we have employed the redox‐active dipyrrindione scaffold of several heme metabolites to synthesize a new BODIPY analog.…”

“…Structural modifications of the basic BODIPY core have a significant impact on the photophysical and electrochemical properties of these compounds, which have been synthesized using a large variety of dipyrrinato platforms , . In particular, ligand effects on the redox chemistry of BODIPY dyes have been examined for applications ranging from photoredox switches to redox flow batteries to photovoltaics , . In this context, we report the synthesis and characterization of a BODIPY complex featuring the redox‐active dipyrrindione motif (Hpdp, Scheme ), which characterizes the propentdyopent family of heme metabolites and urinary pigments …”

Redox Chemistry of BODIPY Dyes

“…Although no unpaired spin density resides on the boron or fluorine atoms in the reduced BODIPY or azaBODIPY dyes, substitution at the boron center also influences the reduction potential of a BODIPY core. Substituting one or both fluorine atoms with fused benzene rings, catecholate groups, or alkynyl moieties does not affect the stability of the radical anion or cation [5]. However, substitution at the boron center introduces more electron density into the system as the electron-withdrawing fluorine's are removed, disfavoring reduction of the BODIPY core.…”

“…Electrochemical analysis of substitution of the parent-BODIPY dye with a phenyl, chloride, methanesulfide, amide, and diethylamide substituent in the 8-position resulted in an ability to shift the reduction potential by 600 mV (Figure 4). Substituted electron acceptors in the 8-position resulted in the greatest stabilization of the radical anion from the reduction of a BODIPY or azaBODIPY core (more reversible reduction waves) [5]. Bard and coworkers have shown that the inductive electron-withdrawing effect of a cyanide substituent in the 8-position results in an easier reduction by about 300 mV, when compared to a methylated-BODIPY.…”

“…However, introduction of a more electron-donating group, such as diethylamine, results in the opposite effect [6]. Lack of the substitution in the 8-position causes for a reduced BODIPY core to be a more reactive/less stable radical anion [5]. Substitution at the 8-position of a BODIPY core can stabilize a radical anion, but substituents bearing an acidic proton residing near the 8-position, such as carboxylic acids, can result in electrophilic attack on the radical center by the acidic proton upon reduction [5].…”