Multiple Helicenes Featuring Synthetic Approaches and Molecular Structures

Helicenes are attractive non-planar polycyclic aromatic hydrocarbons with unique optical and electronic properties. In recent years, the chemistry of “multiple helicenes” with plural helicities in a single molecule has also grown rapidly as a research area due to intriguing properties such as highly distorted structures, multidimensional intermolecular interactions, and enhanced chiroptical properties. In this review, we summarize the synthesis and the molecular structures of multiple helicenes. Taking into consideration that the applied π-conjugated framework largely affects their synthesis, molecular structures, and physical properties, we categorize them according to the central π-conjugated core from smaller to larger aromatic rings and not the criterion of the original classification of multiple helicenes (multiplicity and the number of fused rings). Furthermore, we cover structural parameters and aromaticity of multiple helicenes, especially for the individual benzene rings, based on the definition of “twisting angle” and NICS values, respectively.