Solvation interactions and photostability of tetrakis(1-methylpyridyl)porphyrin derivatives

Abstract

In this paper, a series of water-soluble meso-tetrakis(1′-methyl-pyrid-4- and -3-yl)porphyrin tetratosylate and meso-tetrakis(1′-methyl-pyrid-4- and -3-yl)porphyrin tetraiodide were synthesized with high yields. All synthesized compounds were characterized by 1Н NMR, FT-IR, UV–vis spectroscopy and elemental analysis. The enthalpy changes of meso-tetrakis(1′-methyl-pyrid-4- and 3-yl)porphyrin tetratosylates dissolution in water, alcohol and phosphate buffer (pH = 6.86) were determined by the calorimetric method at 298.15 K. The effect of peripheral substituents and anion of the porphyrin molecule on the enthalpy of solvation have been studied by calorimetry and UV–vis spectroscopy.To understand more clearly the nature of changes in the meso-substituents and anion of a porphyrin molecule, we have also synthesized N-methyl-pyridinium salts (1-methyl-pyridium tosylate and 1-methyl-pyridium iodide) model. The processes of their dissolution under the same solution conditions have been investigated. The nature of the anion and isomerism of the pyridylporphyrin substituent significantly affects the thermochemical characteristics of the compounds transition into solution. In particular, porphyrin-tosylates were found to be solvated several times better than its iodide analogs.Stability to photooxidative degradation is one of the key characteristics of chromophore and characterizes the kinetic stability of a compound in solution under UV-irradiation influence in atmospheric oxygen conditions. In this regard, the stability to photooxidative destruction of porphyrinic salt derivatives has been also measured in aqueous solutions. The results indicated that N-methyl-pyridylporphyrins were stable and slowly degrade under the influence of light radiation in aqueous solutions. It follows from the observed rate constant values of porphyrins (kobs) photooxidative destruction that 4N-substituted porphyrins are much more stable than their 3N-substituted analogs. A similar dependence was observed for the half-flow time of the studied compounds.