For porphycenes isolated in supersonic jets or helium nanodroplets, tunneling splittings are observed due to quantum delocalization of two inner protons. The values of the splittings are vibrational-mode-selective.

 

In condensed phases, trans-trans tautomerization occurs in femtoseconds/picoseconds, depending on the peripheral substitution pattern.

 

The trans form is dominant, but the rare cis1 form has also been detected. The cis2 tautomer is postulated to be a crucial structure in the process of radiationless depopulation of the lowest excited singlet state.

 

The main path of tautomerization in porphycene is a self-exchange process, i.e., the substrate and product are chemically identical. We have developed  methodologies for the determination of ground and excited state tautomerization rates for such a particular case of a chemical reaction.