Stacked antiaromatic porphyrins

Ryo Nozawa,Soji Shimizu,Yongseok Hong,Ichiro Hisaki,Stephan Irle,Tim Kowalczyk,Hiroshi Shinokubo,Ji-Young Shin,Hiroko Tanaka,Won-Young Cha,Dongho Kim

doi:10.1038/ncomms13620

Abstract

Aromaticity is a key concept in organic chemistry. Even though this concept has already been theoretically extrapolated to three dimensions, it usually still remains restricted to planar molecules in organic chemistry textbooks. Stacking of antiaromatic π-systems has been proposed to induce three-dimensional aromaticity as a result of strong frontier orbital interactions. However, experimental evidence to support this prediction still remains elusive so far. Here we report that close stacking of antiaromatic porphyrins diminishes their inherent antiaromaticity in the solid state as well as in solution. The antiaromatic stacking furthermore allows a delocalization of the π-electrons, which enhances the two-photon absorption cross-section values of the antiaromatic porphyrins. This feature enables the dynamic switching of the non-linear optical properties by controlling the arrangement of antiaromatic π-systems on the basis of intermolecular orbital interactions.

Highlights

Aromaticity is a key concept in organic chemistry
We have recently developed an efficient synthesis of stable antiaromatic porphyrins such as dimesitylnorcorrole Ni(II) (3a), which can be obtained on a gram scale[11]
We attempted the synthesis of norcorroles with less bulky peripheral substituents in order to facilitate p–p stacking in the solid state

Summary

Introduction

Aromaticity is a key concept in organic chemistry. Even though this concept has already been theoretically extrapolated to three dimensions, it usually still remains restricted to planar molecules in organic chemistry textbooks. Corminboeuf et al.[8] have proposed that stacking of two antiaromatic systems in methano-bridged superphanes such as cyclobutadiene dimer 1 (Fig. 1) can eliminate their antiaromaticity due to the resulting threedimensional aromaticity, which results from mutual interactions between frontier orbitals of each p-system. This intriguing proposal was further supported by a theoretical study of Bean and Fowler[9] on the ring current effect in antiaromatic cyclophanes with a very short interplanar distance between the two p-systems (B2.2 Å). These stacked norcorroles represent the experimental evidence for the emergence of aromaticity in antiaromatic p-systems upon stacking

Methods

Results

Conclusion