Water-soluble dye – ZnII octacarboxy substituted tetrapyrazinoporphyrazine: Spectral-luminescence study of behavior in aqueous solutions

ZnII complex of octacarboxy substituted tetrapyrazinoporohyrazine, ZnTPyzPA(COOH)8, was prepared by mild alkaline hydrolysis of the corresponding octaethyl ester, ZnTPyzPA(COOEt)8, which was synthesized by template cycloteramerization of 5,6-di(ethoxycarbonyl)pyrazine-2,3-dicarbonitrile 4 in the presence zinc(II) acetate in o-dichlorobenzene. Both complexes were characterized by IR, UV–vis, NMR spectroscopy (1H, 13C). Conformational analysis performed by DFT method in agreement with spectra study indicate that one of COOH group in each pyrazine ring participates in intramolecular hydrogen bonding with the adjacent pyrazine nitrogen atom. Unlike octacarboxy substituted ZnII phthalocyanine for ZnTPyzPA(COOH)8 remains non-aggregated not only in the octaanionic form present in the basic medium but in tetraanioinic form existing in acid aqueous solutions since intramolecular H-bonding prevents intermolecular interactions. However, the fluorescence is almost completely quenched for tetraanion which can be related to the proton transfer in the excited state from carboxylic group to the pyrazine nitrogen. Aggregation is observed only for non-ionized form of ZnTPyzPA(COOH)8 existing in strongly acidic medium (1 M HCl solution, pH < 1).