Dibenzoheterole-Fused s-Indacenes

Heterole (pyrrole, thiophene, furan, thiophene-S,S-dioxide)-fused s-indacenes are known for their enhanced paratropic ring-current strength. However, the outcome of the antiaromatic properties for dibenzoheterole-fused s-indacene antiaromatics remained underexplored. Carbazole-, dibenzothiophene-, dibenzofuran-, and dibenzo[b,d]thiophene-5,5-dioxide-fused s-indacenes 1-4, 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 4 showed the weakest paratropic ring-current strength for the s-indacene unit, while 1-3 showed a relatively greater paratropicity for the s-indacene unit, as evidenced by the changes in ¹H NMR chemical shifts of s-indacene protons. Such observation was explained by the electron-withdrawing effect of the sulfone group and loss of 4n + 2 aromaticity of the heterole unit for 4 reducing its s-indacene paratropicity strength as the nonaromaticity of the heterole unit reduces the π-bond character at the dibenzo[b,d]thiophene-5,5-dioxide/s-indacene fusion site to avoid antiaromatic s-indacene ring formation. The modulation of the paratropic ring-current strength of s-indacene for 1-4 was further supported by the NICS(1)_zz and ring-current (ACID) calculations.