Self-assembled, helically stacked anionic aggregates of 2,5,8,11-tetra-tert-butylcycloocta[1,2,3,4-def; 5,6,7,8-d′e′f′] bisbiphenylene, stabilized by electrostatic interactions

Tetraanions of alkyl-substituted derivatives of cycloocta[1,2,3,4-def, 5,6,7,8-d′e′f′]bisbiphenylene (BPD) and their counter lithium cations self-assemble to form helically stacked assemblies, including a dimer, a trimer, and a tetramer. NMR self-diffusion measurements and unprecedented magnetic shielding effects for the sandwiched lithium cations support their aggregated nature. The D₂-tetramer assembly is fully characterized by NMR spectroscopy, providing unequivocal evidence for a helix of four tetraanionic BPD layers with an estimated relative twist angle of about 45° and interlayer spacing of ca. 4 Å. The barrier for racemization through the in-plane inter-deck rotation is ΔG₂₀₀^‡ = 9.5 ± 0.2 kcal mol^-1 in the dimer compared to > 15 kcal mol^-1in the tetramer.