Helicenes with Four Helical Turns: Dimerization of [13]Helicenes to [27]Helicenoids

Abstract

The promise of enhanced circularly polarized luminescence (CPL) from organic molecules has inspired synthetic efforts to prepare elongated multi-layer helicenes. It has proven particularly challenging to synthesize and isolate enantiomerically pure multi-turn helicenes, and consequently, clear guidelines for the molecular design of helicenes to achieve large CPL output remain elusive. We explore the oxidative dimerization of a 2-naphthol-annulated hetero[13]helicene, and observe the formation of two types of structurally distinct hetero[27]helicenes, both with four helical turns, form. With Cu^IICl(OH)-TMEDA as the oxidant, a symmetric dimer (bi[13]) formed, joining two helicenes with the same helicity in their keto tautomeric forms. Using Cu^II(OTf)₂ as the oxidant led to an unsymmetrical [27]helicenoid composed by an (M)- and a (P)-enantiomer of the [13]helicene through an unusual coupling between the 1- and 3-positions of the 2-naphthol units. Structural characterization was achieved by NMR spectroscopy and single-crystal (X-ray or electron) diffraction analysis. The enantiomers of this [27]helicenoid were analyzed by electronic circular dichroism (ECD) and CPL measurements. The spectroscopic data were corroborated by DFT calculations, and the intense CPL output is preserved despite the presence of an (M)- and a (P)-helicene in the same molecule. These [27]helicenoids are the first isolated structures that feature helical π-systems with four helical turns.