Controlling the helicity of π-conjugated oligomers by tuning the aromatic backbone twist

The properties of pi-conjugated oligomers and polymers are commonly controlled by side group engineering, main chain engineering, or conformational engineering. The last approach is typically limited to controlling the dihedral angle around the interring single bonds to prevent loss of pi-conjugation. Here we propose a different approach to conformational engineering that involves controlling the twist of the aromatic units comprising the backbone by using a tether of varying lengths. We demonstrate this approach by synthesizing an inherently twisted building unit comprised of helically locked tethered acenes, bearing acetylene end-groups to enable backbone extension, which was applied in a series of nine helical oligomers with varying backbone length and twist. We find that the optical and electronic properties of pi-conjugated systems may be determined by the additive, antagonistic, or independent effects of backbone length and twist angle. The twisted oligomers display chiral amplification, arising from the formation of secondary helical structures. One approach to altering the properties of pi-conjugated oligomers is conformational engineering, in which the degree of rotation around the bonds linking monomers is restricted. Here the authors apply the conformational engineering approach on individual monomers using tethers of varying lengths to twist the aromatic units, and study the effects of varying the angles.