Ground and excited state tautomerization in 9-acetoxy-2,7,12,17-tetra-n-propylporphycene

Substitution of 2,7,12,17-tetra-n-propylporphycene by an acetoxy group lifts the degeneracy in two trans tautomeric species which, by symmetry, are identical in the parent compound. Absorption spectra reveal the presence of both tautomers in comparable quantities in the ground electronic state, even at cryogenic temperatures. Analysis of the absorption pattern at low temperatures points to ground state tunneling of two hydrogens between two non-equivalent forms. On the contrary, the fluorescence occurs from only one tautomeric form, indicating the localization of the protons on two nitrogen atoms in the lowest excited singlet state, and a rapid tautomerization upon excitation of the higher energy species.