Spectroscopic and theoretical studies of acid-base behaviors of n-confused porphyrins: Effects of meso -aryl substituents

The acid-base properties of a series of meso-aryl-substituted N-confused porphyrins (NCPs) were examined in aqueous sodium dodecyl sulfate (SDS) micellar solutions by both spectrophotometric methods and theoretical calculations. Reflecting the unsymmetrical structure of NCP having an outward-pointing pyrrolic nitrogen atom, the first and second protonations were distinguishable in the absorption and ¹H NMR spectra, unlike for porphyrins, and the pK₃ and pK₄ values were determined discretely. The individual basicities of the NCPs were directly related to the inductive effect of para substituents on the meso-phenyl groups: A linear relationship between the pK₃ (pK₄) and Hammett σ_para parameters was revealed. In the case of deprotonation, the structure of monoanionic NCP species was similarly characterized by the absorption and ¹H NMR spectra. For the second deprotonation, the pK₁ value was determined to be 11.39 for the NCP derivative with pentafluorophenyl groups. DFT calculations support the changes in electronic structures and aromaticity of the cationic and anionic species. It is demonstrated that NCPs are easily protonated and deprotonated compared to the corresponding regular congeners.