DIFFERENCES OF PROTON TAUTOMERISM IN SUBSTITUTED PORPHYRINS

The stability of the photochemical hole-burning process due to a back reaction of the inner protons of porphyrins is investigated as a function of various substituents of the porphyrin skeleton. For that purpose thermal cycling experiments were performed and the dynamics of the observed spectral changes were measured at different temperatures on a timescale of minutes. The four porphyrins which were investigated exhibit in the range from 90 to 170 K rather large differences in the mean rates describing the tautomerization reactions. We attribute these differences to sterical distortions of the porphyrin ring and to the different electronic properties of the involved sidegroups.