Tautomerism-Induced Cis–Trans Isomerization of Pyridylethenyl N‑Confused Porphyrin

Pyridylethenyl-substituted
N-confused porphyrins (NCPs) were synthesized,
and their cis–trans isomerization was studied. Among four possible
isomers, trans-3H and cis-2H types
of structures, of which aromaticity and absorption/emission properties
differ largely, were isolated. The cis-isomer was largely stabilized
by the intramolecular hydrogen bonding between the pyrrolic-NH and
the pyridinic-N in the vicinity. The thermal cis–trans isomerization
proceeded even at 30 °C, which was significantly accelerated
by the pyridine added to the system. The kinetic studies revealed
that the isomerization reaction was second-order and the activation
energy of the thermal isomerization from cis to trans isomer was ΔG0⧧cis→trans = 35.7 kcal/mol at 298 K, which is significantly smaller
than that of Ni complex (42.3 kcal/mol). An intermolecular proton
transfer induced cis–trans isomerization mechanism was proposed.