AGGREGATION OF COPPER(II) DERIVATIVES OF MESOSUBSTITUTED PORPHYRINS IN FROZEN AQUEOUS-MEDIA

The degree of aggregation of three meso-substituted Cu(II) porphyrin derivatives has been studied in frozen aqueous solutions to which polar organic molecules were progressively added. Electron paramagnetic resonance spectra show that the water-soluble porphyrins give rise to solute-solute interactions in their solutions. The relative amounts of each species present depend on the composition of the solutions, with organic solvents such as glycerol shifting the equilibria markedly towards the monomeric species. No specific dimeric species were detected for the copper(II) chelates of tetrakis(4-N-methylpyridyl)porphine (Cu(II)TMpyP-4) and tetrakis(2-N-methylpyridyl)porphine (Cu(II)TMpyP-2) under any of the solution conditions reported, whilst there was evidence to suggest the presence of a relatively low concentration of the dimeric species of the copper(II) chelate of tetrakis(4-sulfonatophenyl)porphine (Cu(II)TPPS). The tendency to aggregate is found to be Cu(II)TPPS > Cu(II)TMpyP-4 > Cu(II)TMpyP-2. The spectra of Cu(II)TMpyP-2 are particularly interesting in that two distinct species - presumed to reflect the presence of atropisomers - are progressively distinguishable on addition of a polar organic solvent.