Metal-free, visible-light-mediated phototransformations of ortho-biaryl appended chalcones: access to oxathia[5]helicenes and 1,2,3,4-tetrasubstituted cyclobutanes†

Visible-light-mediated metal-free irreversible phototransformations of ortho-bihetaryl appended chalcones are reported. Isosteric replacement of one of the aryl residues with a chalcone fragment in diarylethene (terarylene) molecules leads to a significant expansion of the synthetic capabilities of these compounds, thereby opening up access to a wide range of new functional materials. It was found for the first time that protonation of the carbonyl group in the chalcone moiety of ortho-bihetarylchalcones exclusively promotes 6π-photocyclization leading to the formation of oxathia[5]helicenes, whereas 1,2,3,4-tetrasubstituted cyclobutanes are preferably formed upon photolysis in toluene in the presence of triethylamine. The formation of the helicene framework is due to visible light-induced 6π-electrocyclization of the hexatriene system of bihetaryl chalcones, and [2 + 2]-photocycloaddition is facilitated by the aggregation of chalcone molecules, which is a consequence of the formation of a head-to-head excimer due to π–π-stacking interactions of naphthofuran polyaromatic systems. These results will provide valuable information on the photocyclization of new diarylethene analogues, which can be used to develop new efficient protocols for the synthesis of fused heteroaromatic frameworks and promising materials with wide applications in material science, molecular electronics and medicine.