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Tautomerism-induced Cis-Trans Isomerization of Pyridylethenyl N-Confused Porphyrin.

Research paper by Ryuichi R Sakashita, Yasutaka Y Oka, Hisanori H Akimaru, Praseetha E PE Kesavan, Masatoshi M Ishida, Motoki M Toganoh, Tomoya T Ishizuka, Shigeki S Mori, Hiroyuki H Furuta

Indexed on: 27 Jul '17Published on: 27 Jul '17Published in: Journal of Organic Chemistry



Abstract

Pyridylethenyl-substituted N-confused porphyrins (NCPs) were synthesized and their cis-trans isomerization was studied. Among 4 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 the reaction was second order and the activation energy of the thermal isomerization from cis to trans isomer was DeltaGo‡cistrans = 35.7 kcal/mol at 298 K, which is significantly smaller than that of Ni complex, 42.3 kcal/mol. Intermolecular proton transfer induced cis-trans isomerization mechanism was proposed.