Double hydrogen transfer between four nitrogen atoms constituting the inner cavity of porphycene, structural isomer of porphyrin, has been investigated under various regimes: liquid and solid condensed environments, supersonic jets and helium nanodroplets, single chromophores probed by various microscopy techniques. The picture that emerges from these studies is that of a concerted process with an important contribution of tunneling. The values of tunneling splittings observed for porphycenes isolated in supersonic beams are electronic-state and vibrational-mode-selective, clearly pointing to a multidimensional character of the tautomerization coordinate. Vibrations that lower or increase the tautomerization barrier have been identified.In condensed phases, tautomerization rates change dramatically for differently substitued porphycenes. The reaction can be slowed down, from tens of femtoseconds to hundreds of picoseconds, by varying the strength of the two intramolecular NH���N hydrogen bonds. A spectacular finding is that the tautomerization rate in single tetraphenylporphycene molecules embedded in a polymer matrix can vary by many orders of magnitude over time periods of seconds or minutes.A rare cis1 tautomeric form has been detected in single porphycene molecules located on a copper surface. The cis1-cis1 interconversion can be controlled by changing the position of a single copper atom in the vicinity of the molecule.                      

Tautomerism in porphycenes

248th National Meeting (2014)

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