Behaviour of Regioisomeric Bithiophenes in the Oxidative Synthesis of Tetrathieno-Fused π-Expanded Fluorenes and Their Characterization

Differently-linked bithienyl-attached fluorenes have been synthesized. While the oxidative photodicyclization of 2,3′-bithiophene-containing fluorene was successful to yield tetrathieno-fused regioisomeric π-expanded fluorenes, the reaction with 2,2′-bithiophene-containing fluorene was very sluggish. Interestingly, the connectivity in bithiophenes appears to determine the (photo)reactivity. On the one hand, under mild Scholl conditions, 2,3′-bithiophene-containing fluorene yielded a highly regioselective product while 2,2′-bithiophene-containing fluorene led to polymerization. The optical and electrochemical properties of tetrathienoannulated π-expanded fluorenes were investigated, and they were found to be quite different from those of the phenanthreno-annulated π-expanded analogues. These newly obtained sulfur-edged π-expanded fluorenes may hold promise in organic electronics.