Tautomerization in 2,7,12,17-tetraphenylporphycene and 9-amino-2,7,12,17- tetraphenylporphycene: Influence of asymmetry on the direction of the transition moment

Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17-tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S ₀-S ₁ transition-moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S ₀ and S ₁ states (1.4×10 ¹² s ^-1 and 2.7×10 ¹¹ s ^-1, respectively) have been determined by performing time-resolved anisotropy studies. In the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S ₁ state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9-amino-2,7,12,17-tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition-moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the "locking" of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers. Hydrogen transfer and transition moments: Intramolecular double hydrogen exchange in asymmetrically substituted porphycene occurs with a similar rate to that in analogous symmetrical derivatives but is not accompanied by a large change in the direction of the transition moment. This result is explained by an analysis of the molecular orbitals that are involved in two lowest-energy transitions (see figure).