Tuning the Electrochemistry of Free‐Base Porphyrins in Acidic Nonaqueous Media: Influence of Solvent, Supporting Electrolyte, and Ring Substituents

Abstract

International audienceA detailed study of reduction potentials, electroreduction mechanisms, and acid-base chemistry was carried out on two series of free-base porphyrins in nonaqueous media. The first series is represented by four -pyrrole-substituted tetraphenylporphyrin (TPP) derivatives, two of which are planar and two of which are nonplanar in their nonprotonated form. The second comprises porphyrins with 0-4 meso-phenyl groups on the macrocycle. Equilibrium constants for the conversion of each neutral porphyrin to its diprotic [H4P](2+) form were determined and the electrochemistry was then elucidated as a function of: 1)type of nonaqueous solvent, 2)anion of supporting electrolyte, 3)porphyrin ring substituents, and 4)concentration of acid added to solution. Spectroelectrochemistry was used to characterize absorption spectra of each electroreduced species and, when combined with results of the above studies, significantly improves our ability to tune redox reactivity of these types of compounds

A Comprehensive Study on Tetraaryltetrabenzoporphyrins

“…For some of the TBPs, irreversible signals are observed as either anodic or cathodic currents. These signals are believed to originate from side products that are formed during oxidation/reduction of the TBP …”

“…For some of the TBPs, irreversible signals are observed as either anodic or cathodic currents.T hese signals are believed to originate from side products that are formedd uring oxidation/reduction of the TBP. [69] Ac omparison of electrochemical properties of 2 with its "smaller brother" H 2 TPP shows that the oxidations of 2 occur at much lower potentialt han for H 2 TPP (see Figure 7a and Ta ble 2). This behavior is consistent with the literature, where the narrowing of the HOMO-LUMO gap is described by destabilizationo ft he porphyrins HOMO upon p-extension.…”

Protonation and Electrochemical Properties of Pyridyl‐ and Sulfonatophenyl‐Substituted Porphyrins in Nonaqueous Media

“…There is no evidence for phlorin formation in DMF solutions with low TFA concentrations, but in the presence of excess TFA, the current‐voltage curves resemble those described for TPPH 2 and other related porphyrins in nonaqueous solvents containing added acid. This is shown by the cyclic voltammograms in Figure c, where three well‐defined redox processes are observed for each porphyrin in DMF containing excess acid.…”

“…Pyridyl and sulfonato porphyrins and their analogues have attracted considerable attention for use in a broad range of applications . Our own interest in these types of compounds has been in elucidating their acid‐base and redox properties in nonaqueous media and then analyzing the data as a function of changes in the porphyrin macrocycle and ring substitu‐ ents, planarity of the porphyrin macrocycle, and solvent properties …”

“…In two recent studies, we reported how the redox properties and central nitrogen protonation constants of free‐base tetraarylporphyrins could be substantially altered in non‐aqueous media by changes in the solvent, supporting electrolyte and/or porphyrin ring substituents. The current manuscript follows, in part, from our initial investigations in that we have now characterized the acid‐base and redox properties for a series of free‐base and metalated mono‐ and bis‐pyridyl porphyrins in up to five different non‐aqueous solvents.…”