Herein, we describe a method to fine-tune the conductivity of single-molecule wires by employing a combination of chemical composition and geometrical modifications of multiple phenyl side groups as conductance modulators embedded along the main axis of the electronic pathway. We have measured the single-molecule conductivity of a novel series of phenyl-substituted carotenoid wires whose conductivity can be tuned with high precision over an order of magnitude range by modulating both the electron-donating character of the phenyl substituent and its dihedral angle. It is demonstrated that the electronic communication between the phenyl side groups and the molecular wire is maximized when the phenyl groups are twisted closer to the plane of the conjugated molecular wire. These findings can be refined to a general technique for precisely tuning the conductivity of molecular wires.

Tunable carots: Construction of the stable carotenoid wires with a specific conductance value is possible by the attachment of phenyl groups to the polyene chain to overcome the in vitro instability of natural carotenoids, the perfect molecular wires utilized in various biological processes. Diverse electronic natures of the substituents on the phenyl groups provide the carotenoids with tunable conductance (see figure).

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A convergent synthetic method for new types of carotenoid compounds containing phenyl substituents at C(13) and C(13') has been developed by the coupling of allylic sulfone and 2,7-diphenyloct-4-enedials, followed by the double elimination strategy. These carotenoid compounds are fairly stable and legitimate candidates for nanosized molecular wires, which show diverse conductance values according to the electronic nature of the substituent group in the phenyl ring.

A general method for the construction of diphenyl‐substituted carotenoids has been developed through the stereoselective synthesis of dienyl sulfones with a phenyl substituent. Systematic synthetic pathways to the dienyl sulfones were delineated starting from readily available acetophenones with para‐substituent X of various electronic natures, which provided the carotenoids with diverse physicochemical characteristics. The sulfone olefination method together with the Ramberg–Bäcklund reaction produced a 9,9′‐cis‐10,10′‐diphenylcarotene and all‐trans‐9,9′‐diphenylcarotenes. Conductance measurements of the all‐trans carotenoids by the scanning tunnelling microscopy break‐junction method revealed a positional effect of the phenyl groups as well as a polar effect of the phenyl substituent X according to the electronic nature.