Protonation of the porphyrin-ketones and their complexes: Verification of spectral forms and mechanisms

Protomeric forms of the porphyrin-ketones (or oxo-chlorins) are investigated by optical spectral methods. The main spectral forms of octaethylporphine-ketone and coproporphyrin-ketone tetramethyl ester appearing in organic solvents in the presence of trifluoroacetic acid, in SDS micellar solution and in sulfuric acid, namely, the free base, monocation, dication and trication, are identified and described in detail. Their mutual transformations are shown to proceed differently for the two dyes. The possibility of protonation of the macrocycle is also proven for the porphyrin-ketone complexes. Two new spectral forms of platinum(II) and palladium(II) complexes of the porphyrin-ketones appearing in organic solvents on addition of TFA are described. Possible mechanisms of protonation, including formation of oxonium ions and carbocations, are discussed for these dyes.