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

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 beta-position of one of the pyrrole units with remarkably elongated C-C bonds of about 1.60 angstrom. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light 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 a turn-on of circularly polarized luminescence (CPL). The oxidation of closed-[7]helicenes with [bis(trifluoroacetoxy)iodo]benzene gave the corresponding dimers connected at the beta-positions of the pyrrole units through a remarkably elongated C-C bond of about 1.60 angstrom. Homolytic bond dissociation took place upon UV irradiation to reform its monomeric form. The interconversion between monomeric and dimeric forms is associated with a turn-on of circularly polarized luminescence.image