Ground-and Excited-State Tautomerization Rates in Porphycenes

Reaction ratesfrom polarized spectroscopy: A new method based on ultrafast pump–probe polarization spectroscopy enables the measurement of the ground- and excited-state reaction ratesfor self-exchange processes. The technique was used to study double hydrogen transfer in porphycene and its derivatives (see figure). 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 excitedstate. A correlation was found between the tautomerization rates and 1H chemical shifts of the internal protons.