Expanding Chemistry of Expanded Helicenes

Expanded helicenes are interesting compounds created by modifying the original helicene structure through the incorporation of linearly fused benzene rings, enlarging the helical diameter. Motivated by Tilley et al.'s report of a key expanded helicene structure in 2017, several research groups have synthesized such nonplanar aromatic compounds, aiming to explore their impressive structures, properties, and chiroptical performance. This review highlights recent advances in the expanded helicene chemistry through experimental and theoretical studies. The shape and length of helical structures depend on the number and combination of angularly and linearly fused benzene rings. Helical structures are classified using notations, and specific compounds corresponding to each structural form, namely, hexagonal, triangular, rhombic, or others, are introduced herein. As an extension of the molecular design, examples of nonhexagonal and heteroaromatic ring-embedded expanded helicenes are presented. Specifically, this review focuses on how the diameters, lengths, and turn numbers of helical structures depend on dynamic processes involving helical inversion and chiroptical properties (circular dichroism (CD) and circularly polarized luminescence (CPL)). The characteristics and perspectives of this molecular design are also discussed.