Fine-tuning of single-molecule conductance by tweaking both electronic structure and conformation of side substituents.

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.

A chain extension method for apocarotenoids; lycopene and lycophyll syntheses.

The novel C5 benzothiazolyl (BT) sulfone containing an acetal group was prepared as a building block for the chain-extension of apocarotenoids. The double Julia-Kocienski olefination of the BT-sulfone with C10 2,7-dimethyl-2,4,6-octatrienedial and deprotection of the resulting acetal groups efficiently produced C20 crocetin dial. The higher homologues of C30 and C40 apocarotenoids were prepared from C20 crocetin dial by the repeated application of the Julia-Kocienski olefination of the C5 BT-sulfone and hydrolysis. The scopes of the Julia-Kocienski olefination in the total synthesis of carotenoid natural products were evaluated using the C10+C20+C10 coupling protocol. The olefination was sensitive to the steric factor and bulky C10 ß-cyclogeranyl BT-sulfone was not able to react with C20 crocetin dial, however, lycopene and lycophyll were efficiently produced by the Julia-Kocienski olefination of C10 geranyl BT-sulfone and hydroxygeranyl BT-sulfone, in which protection of the hydroxyl group was not necessary.