Planar and Nonplanar Free‐Base Tetraarylporphyrins: β‐Pyrrole Substituents and Geometric Effects on Electrochemistry,Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media

A series of planar and nonplanar free‐base β‐pyrrole substituted meso‐tetraarylporphyrins were characterized by electrochemistry, spectroelectrochemistry, and protonation or deprotonation reactions in neutral, acidic, and basic solutions of CH₂Cl₂. The neutral compounds are represented as H₂(P), in which P represents a porphyrin dianion with one of several different sets of electron‐withdrawing or ‐donating substituents at the messo and/or β‐pyrrole positions of the macrocycle. The conversion of H₂(P) to H₄(P)]²⁺ in CH₂Cl₂ was accomplished by titration of the neutral porphyrin with trifluoroacetic acid (TFA) while the progress of the protonation was monitored by UV/Vis spectroscopy, which was also used to calculate logβ₂ for proton addition to the core nitrogen atoms of the macrocycle. Cyclic voltammetry was performed after each addition of TFA or TBAOH to CH₂Cl₂ solutions of the porphyrin and half‐wave potentials for reduction were evaluated as a function of the added acid or base concentration. Thin‐layer spectroelectrochemistry was used to obtain UV/Vis spectra of the neutral and protonated or deprotonated porphyrins under the application of an applied reducing potential. The magnitude of the protonation constants, the positions of λ_max in the UV/Vis spectra and the half‐wave or peak potentials for reduction are then related to the electronic properties of the porphyrin and the data evaluated as a function of the planarity or nonplanarity of the porphyrin macrocycle. Surprisingly, the electroreduction of the diprotonated nonplanar porphyrins in acid media leads to H₂(P), whereas the nonplanar H₂(P) derivatives are reduced to (P)]^2? in CH₂Cl₂ containing 0.1 M tetra‐n‐butylammonium perchlorate (TBAP). Thus, in both cases an electrochemically initiated deprotonation is observed.