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