Revisiting Kekulene: Synthesis and Single-Molecule Imaging

Abstract

Four decades after the first (and only) reported synthesis of kekulene, this emblematic cycloarene has been obtained again through an improved route involving the construction of a key synthetic intermediate, 5,6,8,9-tetrahydrobenzo[m]tetraphene, by means of a double Diels–Alder reaction between styrene and a versatile benzodiyne synthon. Ultra-high-resolution AFM imaging of single molecules of kekulene and computational calculations provide additional support for a molecular structure with a significant degree of bond localization in accordance with the resonance structure predicted by the Clar model.

Clar Rules the Electronic Properties of 2D π‐Conjugated Frameworks: Mind the Gap

“…KEKULENE [3] was first synthesized 40 years ago andh as been studied ever since. [25,[50][51][52][53] The molecular structure of KE-KULENE [3] shows ac haracteristicb ond alternation that matches perfectly Clar's p-sextetr ule. This rule states that the most representative structureo fabenzenoid polyaromatic hydrocarbon will correspond to the KekulØ resonance structure that possesses the largestp ossible number of disjoint aromatic psextets.…”

“…The properties of these molecules are well known, and their electronic properties can be easily rationalized by Clar's aromatic p-sextet rule. [25] In fact, this rule constitutesa ni ntuitive and powerful tool to account for many properties of polycyclic aromatic hydrocarbons, and have already been used successfully to explain the relative energetics of holey graphenes. [5,26] In this work, we report ac omputational study that shows how the electronic structure of thesem aterials can be intuitively understood by Clar's aromatic p-sextet rule.…”

Synthesis and assembly of extended quintulene

“…This indicates that the π-electrons of Kekulene are delocalized within benzenoid rings (Clar model) rather than around the entire molecule (Kekule model) 5 . The same characteristics have later been derived for Kekulene homologs, including cyclo[d,e,d,e, e,d,e,d,e,e]decakisbenzene 6 , septulene 7 , octulene 8 , and extended Kekulenes [9][10][11][12] . These cycloarenes provided valuable models to further develop theories on superaromaticity 13,14 , global aromaticity 15 , and related phenomena.…”

“…For instance, cyclo[d,e,d,e, e,d,e,d,e,e]decakisbenzene 6 represents a defective graphenic structure, which is formed by replacing two carbon atoms with four hydrogen atoms in the graphene lattice. Whereas, Kekulene and its π-extended analogs [9][10][11][12] are subunits of porous graphene 16 with six-member-ring pores. On the other hand, cycloarenes with a pentaradial, heptaradial, or octaradial inner cavity are nongraphitic and adopt a curved geometry.…”

Ultra-high Yield On-surface Synthesis and Assembly of Circumcoronene into Chiral Electronic Kagome-honeycomb Lattice

“…Such a peculiar bond localization within circumcoronene can be described based on the Clar's Aromatic Sextet rule, which predicts the formation of maximal number of aromatic sextets (seven in circumcoronene) with a C=C localization at the molecular corners. It is worth mentioning that ncAFM contrast of peripheral C-C bonds in circumcoronene associated with an increased bond localization of aromatic rings also resembles that of Kekulene molecule [25] .…”