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