Intramolecular Alkyne Aromatization: Unexpected Synthesis of Expanded [9]Helicene and π-Extended Double [4]Helicene,...

Intramolecular alkyne aromatization is a powerful tool that enables the synthesis of nonplanar polycyclic aromatic hydrocarbons. Herein, an unexpected intramolecular alkyne aromatization via a trifluoroacetic acid-promoted cyclization is described, in which two structural isomers, expanded [9]helicene (1) and π-extended double [4]helicene (2) are obtained. A possible rearrangement mechanism is proposed to account for the formation of 1. The geometric and optoelectronic properties of these two nonplanar molecular nanocarbons are comprehensively investigated by single-crystal X-ray, UV-vis absorption, photoluminescence spectra, and cyclic voltammetry analysis. These two structural isomers exhibit wide energy gaps with similar energy levels, which further apply them as molecular floating gate in organic field-effect transistor nonvolatile memory (OFET-NVM) devices. The nonplanar geometry of 1 and 2 shows a remarkable effect on charge-trapping behaviors in OFET-NVMs; the 1-based device displays a more than threefold wider memory window (MW, 44.5 V) than that of the 2-based device (14.2 V), and a large charge-trapping density of 1.08 × 10¹³ cm^-2. The distinct different charge-trapping behavior is likely attributed to the different molecular geometries, resulting in different molecular-packing modes. It is revealed in this study that controlling the geometry of molecular nanocarbons is a new strategy for application in organic electronics.