Spectral and Acid-Base Properties of Tetra-Meso-Alkyl-and Tetra-Meso-Phenyl Substituted Derivatives of Porphyrin

Abstract
he basic and spectral properties of a series of meso-substituted porphyrins with alkyl and aryl substituents were investigated. Quantum-chemical calculations of meso-substituted porphyrins were performed in the DFT approximation (hybrid functional B3LYP). It is shown that the introduction of four bulk substituents (tert-butyl groups) causes significant out-of-plane deformations of the porphyrin macrocycle and has a decisive influence on spectral properties of the compound. The basic properties of porphyrins were studied by spectrophotometric titration using acetonitrile – HClO₄ system. Spectral parameters of the molecular and protonated forms of 10 meso-substituted porphyrins were determined. It has been established that for porphyrins with a planar macrocycle structure, the nature of substituent has a decisive influence on the basicity of the ligand. Dissociation constants of cationic forms of meso-substituted porphin derivatives were measured. It is shown that pKb values obtained are in good agreement with the classical ideas about the nature of substituents, even if they are introduced into the macrocycle indirectly, but through the phenyl “buffer”.

Keywords: porphyrins, basic properties, protonated forms, coordination properties