π-System Bistability Determines the Circularly Polarized Luminescence in Helicene para-Phenylenes

We resolve the origin of the anomalous circularly polarized luminescence (CPL) observed in helicene para-phenylenes (HPPs), a new class of chiral macrocyclic nanocarbons reported recently. We show by synthesis of HPPs that embed [5]helicene unit that the presence of two emission bands in the CPL, each with the opposite handedness, relates to the topological bistability of their π-electron system. This bistability allows the HPPs to adopt Möbius and Hückel conformations that differ in the orientability of their π-system. We demonstrate that the bistability can be controlled by molecular strain and it can be effectively used to turn off one of the emissions. Our work is thus the first to report that the topological bistability of π-electron system can dramatically impact the chiroptical properties in single-stranded Möbius molecular nanocarbons. In addition, we demonstrate that the parity of the number of π-electrons in the delocalization paths in the doubly-oxidized HPPs with [5]helicenes permits induction of global ring currents, i.e., these compounds may display global (anti)aromaticity.