Synthesis and Structure of an S‐Shaped Double Expanded Helicene With Seven Fused Anthracene Units

A long double expanded helicene consisting of seven fused anthracene units was synthesized. After an oligoarene consisting of four ethynyl groups was prepared from a 1,5‐diborylanthracene, four‐fold intramolecular alkyne cycloisomerization of the precursor with PtCl 2 catalyst yielded the desired compound as an orange solid in 13% yield. X‐ray analysis revealed that this compound has an S‐shaped nonplanar framework of approximately C i symmetry with two helical sites, P , P , P / M , M , M form, where the two terminal anthracene units are stacked to the central anthracene unit on opposite sides via π–π interactions. DFT calculations indicated the existence of multiple conformers that differ in the helicity of one or two [4]helicene moieties. In the UV/vis and fluorescence spectra, the absorption and emission bands were broad and red‐shifted relative to those of the short double expanded helicenes. The structural and spectroscopic features are discussed on the basis of intramolecular interactions and molecular strain.