Dibenzoheterole-Fused -Indacenes.

Heterole (pyrrole, thiophene, furan, thiophene-,-dioxide)-fused -indacenes are known for their enhanced paratropic ring-current strength. However, the outcome of the antiaromatic properties for dibenzoheterole-fused -indacene antiaromatics remained underexplored. Carbazole-, dibenzothiophene-, dibenzofuran-, and dibenzo[,]thiophene-5,5-dioxide-fused -indacenes -, respectively, were synthesized and characterized by experimental (NMR, single-crystal, UV-vis, CV) and computational (DFT) approaches to study the ground-state antiaromatic properties. Sulfone-containing showed the weakest paratropic ring-current strength for the -indacene unit, while - showed a relatively greater paratropicity for the -indacene unit, as evidenced by the changes in H NMR chemical shifts of -indacene protons. Such observation was explained by the electron-withdrawing effect of the sulfone group and loss of 4 + 2 aromaticity of the heterole unit for reducing its -indacene paratropicity strength as the nonaromaticity of the heterole unit reduces the π-bond character at the dibenzo[,]thiophene-5,5-dioxide/-indacene fusion site to avoid antiaromatic -indacene ring formation. The modulation of the paratropic ring-current strength of -indacene for - was further supported by the NICS(1) and ring-current (ACID) calculations.