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 PtCl₂ 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.