Synthesis of Closed‐Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms

Abstract

Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed‐heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed‐[7]helicene dimers connected at the β‐position of one of the pyrrole units with the remarkably elongated C–C bonds about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place upon UV irradiation in DMSO to give the corresponding monomers. Thus, interconversion between the closed‐helicene monomer and dimer was achieved. The optically pure dimer was photo‐dissociated into the monomers associated with circularly polarized luminescence (CPL) turn‐ON.