Vibrational gating of double hydrogen tunneling in porphycene

A procedure that enables determining the reaction rate from the analysis of fluorescence anisotropy is described and applied to the investigation of double hydrogen transfer between inner-cavity nitrogen atoms in electronically excited porphycene. Tautomerization proceeds as a thermally activated synchronous double hydrogen tunneling. The barrier to the reaction is dynamically modulated by a vibration that simultaneously changes the strength of two intramolecular hydrogen bonds. Different mechanisms of tautomerization in porphycene and its parent isomer, porphyrin, can be understood by analyzing the potentials for hydrogen transfer.