Tautomerization in the Lowest Excited Triplet State

The exchange of hydrogens between the two tautomers was found to be slow on the EPR time scale at 100 K. The upper limit of the reaction rate was estimated at 

It may be instructive to compare the excited state behavior of 1 with that of 2. It is well known that photoexcitation of free base porphyrins leads to trans-trans conversion [86]. This process occurs even at liquid helium temperatures, with the efficiency estimated as about 1% [87]. The mechanism is still under debate. Most probably, photo induced tautomerization does not occur in Sj it has been suggested that the reactions can proceed in Tj [88] or even via higher excited triplet states [87]. It was also postulated that the cis structure in Tj can be involved [89]. However, no such structure has yet been detected. 

The differences in tautomerization in the lowest excited singlet and triplet states of 1 and 2 reflect the pattern observed for the ground state. In contrast to porphyrin, both trans and cis structures are detected in porphycenes, separated by a very small energy gap. The tautomerization barriers are also much lower in 1. Finally, at least in the case of the lowest excited singlet state, synchronous transfer of two hydrogen atoms seems to be preferred over a stepwise mechanism, even when both forms are present (la and lb). 

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