H-bonded acid-base type associates of sulfoheteryl substituted porphyrins and their influence on chromophoric properties

Abstract

Directed synthesis of novel water-soluble asymmetric porphyrins containing in a molecule three sulfo-phenyl fragments and residues of small heterocyclic molecules (benzothiazole (S-spor), benzoxazole (O-spor) and benzo-N-methylimidazole (N-spor)) was performed using metal-complex catalysis method. The state of the synthesised porphyrins in water media was examined by spectrophotometric and potentiometric titration. It was shown that in the case of porphyrins with benzothiazole and benzoxazole residues two interconnected processes - protonation of the reaction centre and formation of self-associates from protonated O-spor and S-spor forms - occur in water. For the porphyrin with benzo-N-methylimidazole residual, the protonation processes dominate and revealed association was less evident compared to O-spor and S-spor. It was proved that formation of associates from protonated forms of anionic porphyrins does not result from π-π-interaction; the porphyrins under study form the H-bonded acid-base type associates’ stabilisation of which is due to specific bindings through direct participation of sulfo groups as well as monoheteryl substitutes in the case of O-spor and S-spor. Potentiometric and spectrophotopotentiometric titrations were conducted, the pKa values which reflect the protonation process of the porphyrin reaction centre were determined. Acid strength increased in the order O-spor < N-spor < S-spor and correlated with electronegativity of heteroatom in the peripheral porphyrin’s substitute.