Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes

Abstract

Invited for the cover of this issue is Shinji Toyota and co‐workers at Tokyo Institute of Technology and Okayama University of Science. The image depicts a spirally rising dragon to represent the helical molecular structures in the manuscript. Read the full text of the article at 10.1002/chem.202004720.

Consecutively fused single‐, double‐, and triple‐expanded helicenes

“…This order is likely relevant to the rigidity of the expanded helicene backbones, resulting from restriction of inversion of the helicities as well as internal rotation of the phenyl pendants, thus leading to a suppression of the nonradiative decay. The observed tendency was opposite to those for the classical 75,76 and expanded 49 helicenes with a spiral geometry, in which more π-extended single helicenes tended to show lower Φ F values probably due to more favorable intersystem crossing. However, this tendency was consistent with the classical S-shaped double helicenes, which exhibited higher Φ F values than the corresponding C-shaped single counterparts.…”

“…50 For the synthesis of expanded helicenes with a larger helical diameter than the corresponding classical helicenes, a substantial number of aromatic rings are required to be fused by intramolecular multifold cyclizations, which often afford a complicated product mixture due to incomplete cyclizations and/or undesired side reactions, resulting in low yields after a time-consuming isolation process. 43,[45][46][47][48][49][50] Therefore, the development of a novel synthetic approach for quantitatively producing expanded helicenes through a complete annulative π-extension of aromatization precursors is highly demanded for exploring an unexplored domain of helically twisted PAHs, which will provide a novel class of expanded helicenes with specific chiroptical, optical, electrochemical, and physical properties [43][44][45][46][47][48][49][50] inaccessible from the angularly fused typical single 17,18 and multiple [19][20][21][22][23] helicenes.…”

“…41,42 In 2017, Tilley and coworkers reported a series of spiral-shaped PAHs with a helical cavity consisting of alternating angularly and linearly fused aromatic units arranged in a helical manner and classified them as "expanded helicenes" (I in Figure 1a). [43][44][45] Since then, several expanded helicenes with unique spiral frameworks have been designed and synthesized, including expanded helicene-based macrocycles (II), 46 a helical analogue of kekulene (III), 47 helical anthracenes (IV), 48,49 and in-fjord-substituted expanded helicenes (V) (Figure 1a). 50 For the synthesis of expanded helicenes with a larger helical diameter than the corresponding classical helicenes, a substantial number of aromatic rings are required to be fused by intramolecular multifold cyclizations, which often afford a complicated product mixture due to incomplete cyclizations and/or undesired side reactions, resulting in low yields after a time-consuming isolation process.…”

“…In 2017, Tilley and coworkers reported a series of spiral‐shaped PAHs with a helical cavity consisting of alternating angularly and linearly fused aromatic units arranged in a helical manner and classified them as “expanded helicenes” ( I in Figure 1a). 43–45 Since then, several expanded helicenes with unique spiral frameworks have been designed and synthesized, including expanded helicene‐based macrocycles ( II ), 46 a helical analogue of kekulene ( III ), 47 helical anthracenes ( IV ), 48,49 and in‐fjord‐substituted expanded helicenes ( V ) (Figure 1a). 50 …”