Tautomerization in Single Molecules of Porphycene

the molecular symmetry of free base porphycene that dictates a rotation of transition moment directions upon tautomerization has enabled the observation of a basic chemical reaction on a single molecule level. The procedure can be extended to studies of more complicated porphyrinoids, such as sapphyrin, a pen-tapyrrolic macrocycle in which three inner hydrogen atoms can migrate within the cavity formed by five nitrogen atoms. 

The data presented in this work demonstrate large differences between the tautomerization characteristics in porphycene and its parent isomer, free base porphyrin. The origin for these dissimilarities may be due to different electronic structure or to distinct geometries. It seems at present that the latter are mainly responsible for the different patterns of tautomerism in 1 and 2. In particular, strong, nearly linear hydrogen bonds in porphycenes lead to low barriers and rapid tautomerization rates. 

The electronic structure seems to play a less important role, as may be speculated on the basis of similar values of NMR chemical shifts of peripheral hydrogen atoms, or by considering analogies in the pattern of the electronic spectra. In order to elucidate in more detail the role of electronic structure in tautomerism, we are now studying porphyrins in which peripheral substitution leads to inner cavity parameters similar to those of porphycenes. 

Korbelik, J. Moan, Q. Peng, J. Natl. Cancer Inst. 1998, 90, 889-905. 

Sessler, S. J. Weghom, Expanded, Contracted and Isomeric Porphyrins, Elsevier Science, Oxford, 1997. 

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Using somewhat more sophisticated, the so-called radial and azimuthal, modes of polarization enabled the observation of tautomerism in single molecules of porphycenes [23, 24] (Figure 3.5) and porphyrins [25]. These techniques also allowed obtaining the 3D orientation patterns for single molecules of porphycenes embedded in a polymer film [24]. 

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