PROTONATION OF PORPHOCYANINE - A NEW EXPANDED TETRAPYRROLIC MACROCYCLE

The porphocyanine nucleus contains six nitrogen atoms, four of which a re theoretically capable of accepting a proton in acidic solution. How ever, only the monoprotonated and diprotonated forms have been observe d. These two forms were characterized by their visible and fluorescenc e spectra. pK(a) values of 6.0 +/- 0.1 and 4.4 +/- 0.1 were determined by spectrophotometric titration. The monoprotonated monocationic form can be observed in methanol.