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.

本研究合成了吡啶乙烯基取代的氮错位卟啉（N-confused porphyrins, NCPs），并系统研究了其顺反异构化行为。在四种可能的异构体中，芳香性与吸收、发射性能差异显著的trans-3H与cis-2H型结构被成功分离。顺式异构体可通过邻近吡咯型-NH与吡啶型氮原子间的分子内氢键实现显著稳定。该热致顺反异构化反应即使在30 ℃下即可发生，向体系中添加吡啶可显著加速该反应进程。动力学研究表明，该异构化反应为二级反应；在298 K下，顺式至反式异构化的标准活化吉布斯自由能ΔG0‡_cis→trans = 35.7 kcal/mol，远低于对应镍配合物的对应值（42.3 kcal/mol）。本研究提出了一种由分子间质子转移诱导的顺反异构化反应机制。