Polar Parallel Substituents Trigger High Conductance Paths in Carotenoid Wires

Polar phenyl substituents on the carotenoid molecular wires act as conductance modulators, depending on their dihedral angles relative to the polyene chain. In particular, planar electron-releasing phenylacetylene substituents in carotenoids 3 provide a conductance amplifying effect (up to × 10 for X = OMe) compared to carotene 2 with the native methyl substitution pattern at 13,13'-positions. By contrast, orthogonal phenyl substituents in carotenoids 1 provide ohmic tunability (up to × 1/20 for X = Br). Two electronic pathways are observed in carotenoids 3 using the STM break-junction technique: a "High" conductance pathway (10^-2-10^-3 G₀) defined by electrical contacts through the lateral phenylacetylene group and the terminal methyl sulfide group, and a "Low" conductance pathway (10^-3-10^-4 G₀), defined by the contacts between the two terminal methyl sulfide groups. These two electron pathways are confirmed by using carotenoids 4 and 5, which are designed to suppress one of the two conductance pathways. Additionally, the flexible arms of carotene 5 enable a novel electron pathway (10^-5 G₀) through flexible intramolecular π-stacking interactions with the polyene chain, suggesting new design strategies for light-harvesting molecular wires. This work demonstrates that carotenoid backbones can serve as tunable molecular wires for the construction of electronic circuits in various molecular devices.