Dibenzo-Extended <i>s</i> -Indacenodicarbazole Isomers with Tunable Antiaromatic Characteristics: Impact of Aromaticity, Syntheses, and Properties

The Clar sextet rule is powerful in predicting photophysical properties, stability, and diradical characteristics in benzenoid polycyclic aromatic hydrocarbons, while the Glidewell-Lloyd (GL) rule of aromaticity is applicable for nonbenzenoid systems. Herein, we report the syntheses of two dibenzo-extended <i>s</i>-indacenodicarbazole isomers with enhanced double-bond character of the bond-fusing carbazole and <i>s</i>-indacene units. In line with the GL rule, both isomers tend to avoid the formation of the smallest 4nπ group <i>s</i>-indacene by pushing electrons from the electron-rich nitrogen (placed in different locations) to the <i>s</i>-indacene core to restore the smallest 4n + 2 groups. In this process, isomer <b>1</b> retains its Clar sextets while also partly acquiring an additional 4n + 2 group (cyclopentadienyl anion) as it switches from the <i>para</i>-quinoidal structure <b>1</b> to one of its zwitterionic (ZW) forms. In contrast, isomer <b>2</b> cannot restore aromaticity in the <i>s</i>-indacene core in its ZW forms unless <i>one</i> Clar sextet is lost, which seems unlikely. Therefore, the driving force for <b>2</b> to switch to the ZW form that prevents the formation of <i>s</i>-indacene should be weaker, suggesting the antiaromatic character of the <i>s</i>-indacene core in <b>2</b> to be greater than that in <b>1</b>. The greater antiaromaticity of <b>2</b> is experimentally validated by single-crystal, ¹H NMR, and HOMO-LUMO gap analyses and further supported by HOMA, NICS(1)_<i>zz</i>, and ring-current computations. Both isomers exhibit a <i>p</i>-type charge transport in field-effect transistor devices, with moderate hole mobility on the order of 10^-4 cm²/(V s). Doping of <b>2</b> with F₄-TCNQ led to a 6000-fold increase in conductivity, highlighting the potential of <i>s</i>-indacene-embedded polycyclic antiaromatics in materials science.