Non‐Alternant Non‐Benzenoid Aromatic Compounds: Past, Present, and Future

Abstract

The development of the chemistry of non-alternant non-benzenoid hydrocarbons is reviewed in connection with the International Symposium on Novel Aromatic Compounds (ISNA) and the author's personal anecdotes. Past molecules include the classics of this class of compounds, pentalene, azulene, and heptalene, and the structurally integrated congeners such as indacenes formed by benzannelation and benzinterposition, which were extensively studied during the last century. Indenofluorene isomers constructed by benzannelation to indacenes represent present molecules, which have attracted much interest in recent years in view of their potential in optoelectronic applications. The unusual properties of the most exotic isomer, indeno[2,1-b]fluorene, are specifically described to demonstrate that the benzannelation is not simply a modification for stabilization. Finally, as future molecules, those consisting of linear and cyclic arrays of indenofluorene motifs formed by further extension are proposed.

Characterization of Local Aromaticity in Polycyclic Conjugated Hydrocarbons Based on Anisotropy of π‐Electron Density

“…It is interesting to note that the increase in the aromatic character of 17 is accompanied with more stability of this compound (obtained from linear fusion) relative to 18 (obtained from angular annelation) (∼9.37 kcal/mol). This observation is in agreement with experimental findings associated with higher chemical reactivity of 18 than 17 …”

Reductive Aromatization/Dearomatization and Elimination Reactions to Access Conjugated Polycyclic Hydrocarbons, Heteroacenes, and Cumulenes

“…In addition, they enhance our fundamental understanding of chemical and physical phenomena such as the nature of the chemical bond. For a more thorough exploration of properties and applications of these materials, we direct the reader to excellent reviews on the topic by Wu, Tobe, Abe, Kubo, and Haley …”

Construction of Polycyclic π‐Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8‐Diphenyl‐9,10‐bis(phenylethynyl)phenanthrene

“…Moreover, recent advances in transition metal‐mediated functionalizations such as borylation, arylation, and silylation have provided methods to develop fascinating azulene‐based materials . Unlike alternant π‐conjugated systems, the non‐alternant structure of azulene contributes to the construction of peculiar π‐conjugated systems . In addition to the electronic perturbations of the alternant π‐conjugated systems, many three‐dimensional nanocarbon structures with curvature have been proposed by embedding pentagons or heptagons in the hexagonal honeycomb lattice .…”

“…[15][16][17][18] Unlike alternant p-conjugated systems, the non-alternant structure of azulenec ontributes to the construction of peculiar p-conjugated systems. [19][20][21][22][23][24][25][26] In addition to the electronic perturbations of the alternant p-conjugated systems, many three-dimensional nanocarbon structuresw ith curvature have been proposed by embedding pentagons or heptagonsi nt he hexagonal honeycomb lattice. [5,[27][28][29][30][31] Thus, the embedding of an azulenec ore into alternant PAHs could change both the molecular geometry and the electronicp roperties of the originalP AHs.…”