Ground- and excited-state tautomerism in porphycenes

Owing to short distances between the inner nitrogen atoms and, therefore, strong intramolecular hydrogen bonds, porphycenes display completely different tautomeric properties than porphyrins, their constitutional isomers. Tunneling splitting caused by delocalization of two inner hydrogen atoms is observed for porphycenes isolated in supersonic jets. The barrier to tautomerization is higher in the lowest excited singlet state than in the ground state. Still, rapid exchange of inner hydrogen atoms is detected in S-1. The mechanism of tautomerization involves synchronous double hydrogen tunneling, activated by excitation of a low-frequency mode, which modulates the NH center dot center dot center dot N separation. This separation can also be strongly altered by peripheral substitution. In porphycenes with alkyl substituents on the ethylene bridges, the NH center dot center dot center dot N distances become extremely small. For these derivatives, both trans and cis tautomeric forms are detected. Tautomerism in porphycenes was also monitored on a single-molecule level.