Brønsted acid-mediated double annulation for the synthesis of difurocarbazoles

Difuro[3,2-c:2',3'-g]carbazoles represent a previously inaccessible class of angularly fused heteroaromatic systems. Herein, we report a Brønsted acid-mediated double-annulation strategy that enables the direct construction of these highly fused frameworks from readily available carbazoles and 1,2-diones. The one-pot transformation rapidly increases molecular complexity through the sequential formation of two fused furan rings in a single operation, affording difurocarbazoles in yields of up to 70%. The methodology is compatible with a range of substrates and provides efficient access to symmetric derivatives, while a complementary stepwise protocol enables the synthesis of structurally diverse unsymmetric analogues. Beyond establishing the first synthetic entry to difuro[3,2-c:2',3'-g]carbazoles, this work demonstrates how progressive annulation can be used to modulate the electronic structure of carbazole-based π-systems. Photophysical investigations reveal that the extended π-conjugation resulting from the second annulation leads to bathochromic shifts in both absorption and emission compared with the corresponding mono-furocarbazole, accompanied by a moderate decrease in fluorescence quantum yield. Electrochemical and theoretical investigations further provide insights into the electronic properties of the extended π-framework. This double-annulation strategy provides a versatile platform for the development of new angularly fused carbazole-based materials.