Quadruple[6]Helicene Featuring Pyrene Core: Unraveling Contorted Aromatic Core with Larger Effective Conjugation.

Abstract

Multiple helicenes display distinct aromatic cores characterized by highly twisted rings that are shared or fused with constituent helicene moieties. Diversifying these aromatic cores unlocks avenues for creating multiple helicenes with distinct properties and topologies. Herein we report the synthesis of a quadruple[6]helicene featuring pyrene as the aromatic core. The synthesis involved key steps of the annulative π-extension reaction and Scholl reaction. By extending multiple helicenes along the axial direction, the degree of contortion of the aromatic core can be controlled from nearly flat to highly twisted. Notably, quadruple[6]helicene exhibits a significant red-shift of 0.49 eV compared to quadruple[4]helicenes, of which the red-shift arises from both π-extension and augmented effective conjugation due to enhanced twisting. Quantum chemical calculations demonstrate that the degree of contortion in the pyrene core adeptly governs the energy levels of the HOMO and LUMO, which offers an alternative strategy beyond mere enlargement of the π backbone. An intriguing serendipitous finding reveals the formation of one-molecule-thick supramolecular homochiral nanosheets through self-interlocking interactions of enantiomers in single crystals, a rare packing motif for multiple helicenes.