Stacked antiaromatic porphyrins

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.

Rational Synthesis of Antiaromatic 5,15‐Dioxaporphyrin and Oxidation into β,β‐Linked Dimers

“…The Hückel 20p-electron antiaromatic characteristics of 5,15dioxaporphyrin were revealed by 1 HNMR spectroscopy, and its electronic structure was characterized experimentally by UV/Vis/NIR absorption and MCD spectroscopies and theoretically by TDDFT,N ICS,a nd GIMIC calculations. 5,15-Dioxaporphyrin is stable under ambient conditions, whereas it forms a b,b-linked dimer upon oxidation instead of attaining 18p-electron aromatic oxidation state.S ince it is recently reported that antiaromatic norcorrole exhibits unique conducting properties arising from the small HOMO-LUMO gap [37] and its laminated dimer attains three-dimensional aromaticity, [38] the extension of such research to 5,15-dioxaporphyrin is of significant interest, and investigations along these lines is being intensively pursued in our laboratory.…”

Theoretical Study on Open‐Shell Singlet Character and Second Hyperpolarizabilities in Cofacial π‐Stacked Dimers Composed of Weak Open‐Shell Antiaromatic Porphyrins

“…In such 1D bulk materials, tuning inter‐monomer distance alternation patterns, which may be tuned by chemical modifications of monomers and/or physical perturbations, are shown to be one of the important factors to enhance the bulk phase NLO susceptibility ,. Antiaromatic porphyrin 1B with appropriate substituent groups can also form up to trimers with interesting geometric and electronic structures by self‐assembly . Exploration of open‐shell‐character‐based molecular design strategies for tuning the aggregations of antiaromatic molecules like porphyrins is therefore expected to contribute to further progress in expression of unique functionalities in open‐shell materials.…”

Pyridine‐Fused Bis(Norcorrole) through Hantzsch‐Type Cyclization: Enhancement of Antiaromaticity by an Aromatic Bridge

“…It has been shown that norcorrolatonickel(II) 1 , a readily obtainable air‐stable antiaromatic 16‐π‐electron tetrapyrrolic porphyrinoid, can be effectively functionalized by means of diverse substitution, addition, insertion, or dimerization reactions. Amination of the macrocycle followed by oxidative annulation has been recently shown to be an effective way to fuse aromatic rings .…”