Beta-Substituent and Metal Effect on Their Structures and Electrochemical Properties

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 beta-amino substituted porphyrins provided highly twisted porphyrin dimers in excellent yield. Sequential use of amination and oxidation of 5,10,15,20-tetramesitylporphyrins afforded highly twisted porphyrin oligomers. The structures of the dimers proved dependent on substituents at porphyrin periphery. This time, we succeeded in the synthesis of beta-substituted porphyrin oligomers by using the oxidative dimerization protocol. Introduction of more bulky substituents at meso-positions on oxidation process provided the corresponding twisted porphyrin dimers with larger twisted angles. As a result, beta-substituents induced more distortion of porphyrin oligomers. In addition, electrochemical analysis revealed the distortion caused reduction of electronic communications between each porphyrin unit. We also found beta-methyl substituted porphyrin oligomer exhibited solvent-dependent and temperature dependent optical property. Theoretical calculations as well as electrochemical measurements suggested that conformational change by solvent and temperature caused change of absorption spectrum of the tetramer. Such conformational change dependent on solvent is quite rare for porphyrin oligomers and that should lead to novel stimuli-responsive porphyrin oligomers. Secondly, we also investigated the central metal effect on the electrochemical properties and structures of the dimers. The central Ni(II) metal can be readily removed by treatment of concentrated sulfonic acid. Insertion of Zn(II) and Cu(II) metals induced planarization of porphyrin planes, resulting in smaller twisting angle of the corresponding dimers. In the case of the Cu(II) complex, the presence of magnetic interactions between two porphyrin units were revealed by temperature dependent magnetic susceptibility measurements. 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. Figure 1