Protonation of porphocyanine - A new expanded tetrapyrrolic macrocycle

The porphocyanine nucleus contains six nitrogen atoms, four of which are theoretically capable of accepting a proton in acidic solution. However, only the monoprotonated and diprotonated forms have been observed. These two forms were characterized by their visible and fluorescence 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.