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Planar and Nonplanar Free‐Base Tetraarylporphyrins: β‐Pyrrole Substituents and Geometric Effects on Electrochemistry, Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media
Author(s) -
Fang Yuanyuan,
Bhyrappa P.,
Ou Zhongping,
Kadish Karl M.
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303141
Subject(s) - protonation , porphyrin , chemistry , pyrrole , deprotonation , free base , photochemistry , cyclic voltammetry , electrochemistry , organic chemistry , ion , salt (chemistry) , electrode
Abstract 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 CH 2 Cl 2 . The neutral compounds are represented as H 2 (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 H 2 (P) to [H 4 (P)] 2+ in CH 2 Cl 2 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 logβ 2 for proton addition to the core nitrogen atoms of the macrocycle. Cyclic voltammetry was performed after each addition of TFA or TBAOH to CH 2 Cl 2 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 H 2 (P), whereas the nonplanar H 2 (P) derivatives are reduced to [(P)] 2− in CH 2 Cl 2 containing 0.1 M tetra‐ n ‐butylammonium perchlorate (TBAP). Thus, in both cases an electrochemically initiated deprotonation is observed.