Construction Of Helical Structures With Multiple Fused Anthracenes: Structures And Properties Of Long Expanded Helicenes

Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl2-catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the terminal anthracene moieties at 0.33 nm interlayer distance. In the UV-vis and fluorescence spectra, the absorption and emission bands were red-shifted as the number of fused anthracene units was increased. The characteristic broad and long-lived emission bands of the long analogues are explained by the excimer-like stabilization of the excited state. These photophysical data as well as their cyclic voltammetric data are discussed on the basis of the pi-conjugation and interlayer pi...pi interactions in the molecular structures and the molecular orbitals. The barrier and mechanism of helical inversion are also reported.