Towards Stable Helical Structures with Enhanced Molecular Conductance by Strengthening Through-Space Conjugation.

Developing long-chain molecules with stable helical structures is of significant importance for understanding and modulating the properties and functions of helical biological macromolecules, but challenging. In this work, an effective and facile approach to stabilize folded helical structures by strengthening through-space conjugation is proposed, using new ortho-hexaphenylene (o-HP) derivatives as models. The structure-activity relationship between the through-space conjugation and charge-transport behavior of the prepared folded helical o-HP derivatives is experimentally and theoretically investigated. It is demonstrated that the through-space conjugation within o-HP derivatives can be strengthened by introducing electron-withdrawing pyridine and pyrazine rings, which can effectively stabilize the helical structures of o-HP derivatives. Moreover, scanning tunneling microscopy-break junction measurements reveal that the stable regular helical structures of o-HP derivatives open-up dominant through-space charge-transport pathways, and the single-molecule conductance is enhanced by more than 70 % by strengthening through-space conjugation with pyridine and pyrazine. However, the through-bond charge transport pathways contribute much less to the conductance of o-HP derivatives. These results not only provide a new method for exploring stable helical molecules, but also provide a stepping stone for deciphering and modulating the charge-transport behavior of helical systems at the single-molecule level.