Substituent Effect on Oxidative Dimerization of Porphyrins

Three-dimensional π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics. In particular, twisted π-conjugated systems have been focused on for potential applications to solution processible organic materials. To date, numerous twisted organic molecules such as twistacenes have been synthesized. Under these situations, we found that oxidation of aromatic amines facilitates construction of twisted π-conjugated molecules. For example, oxidation of beta-amino substituted porphyrins provided three-dimensional porphyrins in excellent yield. Sequential use of amination and oxidation toward 5,10,15,20-tetramesitylporphyrin afforded highly twisted porphyrin oligomers. For this reaction, structures of the dimers proved dependent on the meso-substituents. This time, we investigated the substituent effect on the dimerization reactions. Introduction of more bulky substituents at meso-positions on oxidation process provided the corresponding twisted porphyrin dimers with larger twisted angles. In the case of p-tolyl and other aryl substituents without ortho substituents afforded porphyrin dimers with double spirocyclic linkages. The electrochemical analysis revealed the presence of electronic communications between two porphyrin units for the obtained dimers even through the spirocyclic linkages. The central metal also affects the conformations of the dimers. The central Ni(II) metal can be easily removed by treatment of sulfonic acid. Insertion of Zn(II) and Cu(II) metals reduced the twisted angle of the corresponding dimers mainly due to the planar features of the porphyrin units. The presence of magnetic interactions between two porphyrin units were revealed by temperature dependent magnetic susceptibility measurements on SQUID with Cu(II) analogues. In this presentation, we will disclose the structural characterizations of these dimers and would like to show the relationship between electrochemical properties and the conformations of porphyrin dimers.