Functional group introduction and aromatic unit variation in a set of π-conjugated macrocycles: revealing the central role of local and global aromaticity.

Martina Rimmele,Maxie M Roessler,Florian Glöcklhofer,Martin Heeney,Maryam Seif-Eddine,Felix Plasser,Wojciech Nogala

doi:10.1039/d1qo00901j

Abstract

π-Conjugated macrocycles are molecules with unique properties that are increasingly exploited for applications and the question of whether they can sustain global aromatic or antiaromatic ring currents is particularly intriguing. However, there are only a small number of experimental studies that investigate how the properties of π-conjugated macrocycles evolve with systematic structural changes. Here, we present such a systematic experimental study of a set of [2.2.2.2]cyclophanetetraenes, all with formally Hückel antiaromatic ground states, and combine it with an in-depth computational analysis. The study reveals the central role of local and global aromaticity for rationalizing the observed optoelectronic properties, ranging from extremely large Stokes shifts of up to 1.6 eV to reversible fourfold reduction, a highly useful feature for charge storage/accumulation applications. A recently developed method for the visualization of chemical shielding tensors (VIST) is applied to provide unique insight into local and global ring currents occurring in different planes along the macrocycle. Conformational changes as a result of the structural variations can further explain some of the observations. The study contributes to the development of structure–property relationships and molecular design guidelines and will help to understand, rationalize, and predict the properties of other π-conjugated macrocycles. It will also assist in the design of macrocycle-based supramolecular elements with defined properties.

Highlights

Introduction πConjugated macrocycles provide an exciting playing field for the discovery of effects and properties that cannot usually be attained with organic compounds
Synthesis In contrast to the synthesis of the unsubstituted macrocycle BCyc (Scheme 1), which can be achieved by a Wittig reaction of terephthalaldehyde with p-xylylenebis(triphenylphosphonium bromide),[16] the synthesis of the π-conjugated macrocycles with ester groups at the vinylene bridges requires α,α′-dioxoarylenediacetic acids and arylenediacetic acids as the precursors.[29,30]
The study advances our fundamental understanding of such systems, helping to develop structure–property relationships and molecular design guidelines for π-conjugated macrocycles, paving the way to the development of macrocycles for new applications and assisting in the design of macrocycle-based supramolecular elements

Summary

Introduction

Conjugated macrocycles provide an exciting playing field for the discovery of effects and properties that cannot usually be attained with organic compounds. Their study began in the early 1960s and many π-conjugated macrocycles have been synthesized since.[1] only recently the field has moved. Paracyclophane-1,9,17,25-tetraene, a [2.2.2.2]cyclophanetetraene with benzene units, denoted BCyc here, to switch between a locally aromatic neutral state and a globally aromatic doubly reduced state (Scheme 1).[16,17] This switching enabled excellent redox properties and application as high-performance organic sodium-ion battery anode material. The macrocyclic geometry was further found to result in voids in the solid-state packing that can facilitate ion diffusion. Further applications of π-conjugated macrocycles are possible if the molecular structure is adapted

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