Effects of Anions, Cations and Supporting Electrolyte on the Electrochemistry and Spectroelectrochemistry of Neutral, Protonated and Deprotonated Free-Base Porphyrins in Nonaqueous Media

A series of free-base porphyrins were examined by electrochemistry and spectroelectrochemistry in nonaqueous media containing a variety of added anions or cations and different supporting electrolytes which were able to bind with the neutral and/or electrogenerated forms of the macrocycle. The investigated neutral porphyrins are represented as H₂(P) while the protonated and deprotonated porphyrins are represented as [H₄(P)]²⁺, [H₃(P)]⁺, [H(P)]^- and [(P)]^2-. The protonated porphyrins were generated by addition of an organic acid to solution, while the deprotonated porphyrins were formed by addition of base to give the compounds shown in Chart 1.

UV-vis spectra of the prophyrins were measured before, during and after the addition of different anions, cations and supporting electrolytes to solution. Cyclic voltammetry was used to measure the redox potentials for oxidation or reduction under the different solution conditions, while spectroelectrochemistry was used to monitor UV-vis spectra during each electron addition or electron abstraction. Formation constants for anion or cation binding were measured using both spectroscopic and electrochemical techniques. The measured shifts of absorption bands and shifts of reduction potentials were shown to vary with changes in the binding constants and this was markedly dependent upon both the nature of the porphyrin substituents and the porphyrin geometry, i.e., the planarity of the macrocycle. A proposed mechanism for anion and cation interaction with the free-base porphyrins will be presented.