Antiaromaticity in Nonbenzenoid Oligoarenes and Ladder Polymers

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and fully-conjugated ladder polymers are leading candidates for organics electronics, as their inherent conformational rigidity encourages electron delocalization. Many of these systems consist of fused benzenoid or heterocyclic aromatic rings. Less frequently, however, PAHs are reported with character that alternates between the aromaticity of benzene fragments and the antiaromaticity of a nonbenzenoid moiety. This chapter will focus on recent work published on the theory, synthesis, and properties of two such systems: [N]phenylenes containing 4π-electron cyclobutadienoid character, and diaryl[a,e]pentalenes containing 8π-electron pentalenoid character.

Modulating Paratropicity Strength in Diareno-Fused Antiaromatics

“…NICS-XY scans were performed on a series of diarenopentalenes and diarenocyclobutadienes (Figure ), and a summary of the computational data for the three sets of compounds is presented in Table . Diarenopentalenes were chosen because they have received significant investigation in recent years as promising organic semiconductors. ,,, In addition to materials applications, , diarenocyclobutadienes are derivatives of biphenylene, which has long been of interest to organic chemists as a stable example of a cyclobutadiene ring. ,− The NICS-XY plots of both the pentalene and cyclobutadiene series show a similar shape to those of the diarenoindacenes, with peak positive NICS πZZ values over the core and outer rings that have similar negative NICS πZZ values to those of the diarenoindacenes (Figure a,b). When the peak NICS πZZ values are plotted against the double bond character of the fusion bond and its bond length, the results again show that shorter, more double-bond-like fused bonds lead to higher NICS πZZ values in the core (Figure c,d).…”

“…c NICS value over the center of the aromatic ring fused directly to the antiaromatic core. materials applications, 74,75 diarenocyclobutadienes are derivatives of biphenylene, which has long been of interest to organic chemists as a stable example of a cyclobutadiene ring. 19,76−78 The NICS-XY plots of both the pentalene and cyclobutadiene series show a similar shape to those of the diarenoindacenes, with peak positive NICS πZZ values over the core and outer rings that have similar negative NICS πZZ values to those of the diarenoindacenes (Figure 10a,b).…”

Naphthazarin-Polycyclic Conjugated Hydrocarbons and Iptycenes

“…1,[5][6][7][8][9][10][11][12][13] Other efforts explore a redesign of the bonding architecture of the core molecule, such as through the incorporation of formally antiaromatic cyclobutadiene units, present in [N]phenylenes and phenylenecontaining oligoacenes (POAs). [14][15][16][17][18][19] Alternation of cyclobutadiene units with either benzene ([N]phenylenes) or acene units (POAs) results in localized conjugation along the molecular backbone, which can improve chromophore stability. 15,16 Key synthetic approaches to building acene or acene-like small molecules include sequential Diels-Alder reactions, 16,20,21 cross-coupling methods, 6,22 or the [2+2+2] methodology used by Vollhardt and coworkers to synthesize extended [N]phenylenes.…”

“…To this end, synthetic chemists have explored structural modifications to the acene core to achieve improved stability, processability, and electronic properties. Strategies include appending bulky or electron-withdrawing substituents, , or including heteroatoms into the core acene unit, such as in the synthesis of anthradithiophenes or azaacenes. ,− Other efforts explore a redesign of the bonding architecture of the core molecule, such as through the incorporation of formally antiaromatic cyclobutadiene units, present in [N]­phenylenes and phenylene-containing oligoacenes (POAs). − Alternation of cyclobutadiene units with either benzene ([N]­phenylenes) or acene units (POAs) results in localized conjugation along the molecular backbone, which can improve chromophore stability. , …”