Synthesizing Aza[n]helicenes to the Limit: Hydrogen-bond-assisted Solubility and Benzannulation Strategy

Synthetic challenges toward anomalous structures and electronic states often involve handling problems such as insolubility in common organic solvents and oxida-tive degradation under aerobic conditions. We de-signed benzo-annulated aza[n]helicenes, which bene-fit from both the suppressed elevation of HOMO ener-gies and high solubility due to hydrogen-bonding with solvent molecules to overcome these challenges. This strategy enabled the synthesis of six new aza[n]helicenes ([n]AHs) of different lengths (n = 9–19) from acyclic precursors via one-shot oxidative fu-sion reactions. The structures of all the synthesized aza[n]helicenes were determined by X-ray diffraction (XRD) analysis, and their electrochemical potentials were measured by cyclic voltammetry. Among the syn-thesized aza[n]helicenes, [17]AH and [19]AH are the first heterohelicenes with a triple-layered helix. The noncovalent interaction (NCI) plots confirm the exist-ence of an effective π-π interaction between the layers. The absorption and fluorescence spectra red-shifted as the helical lengths increased, without any distinct satu-ration points. The optical resolutions of N-butylated [9]AH, and [11]AH were accomplished and their circu-lar dichroism (CD) and circularly polarized lumines-cence (CPL) were measured. Thus, the structural, (chir)optical, and electrochemical properties of the aza[n]helicenes were comprehensively analyzed.