Spectroscopy of Acid-Base Interactions of Azaporphyrins and Their Metallocomplexes in Solutions

The states of azaporphyrins and their metallocomplexes in acidic media were analyzed on the basis of the visible and near-UV absorption spectra. It is shown that the electronic spectra of the acidic forms of azaporphyrins can be qualitatively interpreted by applying first-order perturbation theory to the four-orbital model of porphyrin spectra. The process of acid-base interaction (ABI) with weak conjugated multicenter bases, e.g., azaporphyrins, is rather complicated and includes, depending on the medium, the stages of formation of acid associates (incomplete ABI), H-associates, ion associates, and the protonated forms (complete ABI). Both meso-and endocyclic nitrogen atoms can be involved in the porphyrin ABIs. Analysis of the stability constants of the acidic forms indicates that the sequential meso-aza-substitution at the porphyrin macrocycle diminishes basicity of the nitrogen meso-atoms. Unlike phthalocyanines, at most two nitrogen meso-atoms of tetraazaporphyrins participate in the complete ABI.