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 lift s the degeneracy in two trans tautomeric species which, by symmetry, are id entical in the parent compound. Absorption spectra reveal the presence of b oth tautomers in comparable quantities in the ground electronic state, even at cryogenic temperatures. Analysis of the absorption pattern at low tempe ratures points to ground state tunneling of two hydrogens between two non-e quivalent forms. On the contrary, the fluorescence occurs from only one tau tomeric form, indicating the localization of the protons on two nitrogen at oms in the lowest excited singlet state, and a rapid tautomerization upon e xcitation of the higher energy species. (C) 2000 Elsevier Science B.V. All rights reserved.