Ground‐ and Excited‐State Tautomerization Rates in Porphycenes

Reaction rates from polarized spectroscopy : A new method based on ultrafast pump–probe polarization spectroscopy enables the measurement of the ground‐ and excited‐state reaction rates for self‐exchange processes. The technique was used to study double hydrogen transfer in porphycene and its derivatives (see figure). magnified image The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump–probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NH ⋅⋅⋅ N distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and 1 H chemical shifts of the internal protons.