A Molecular Turnstile as an E‐Field‐Triggered Single‐Molecule Switch: Concept and Synthesis

A new single-molecule switching concept relying on the E-field-dependent orientation of a push-pull system (via its dipole moment) is presented. As first steps towards realizing the E-field-triggered single-molecule motion, the synthesis of turnstile 1, which is designed to bridge a gap within a carbon nanotube junction, is reported. Turnstile 1 consists of a hexakis(m-phenyleneethynylene) macrocycle decorated with phenanthrene-functionalized oligo(phenyleneethynylene) (OPE) wires at opposite ends, as well as a para-connected push-pull rod as rotator unit. The similarity of the rotor dimension with the surrounding macrocycle guarantees efficient p-stacking between both subunits when an E-field is applied. To introduce a dipole moment into the rotator, the structure is terminally functionalized with a nitrile group on one side and with a dimethylamino group on the opposite end. Synthetic protocols based on Sonogashira-Hagihara couplings were developed to build up the macrocycle scaffold. The highly functionalized open precursor A is the key building block of the sequence as it allows a twofold intramolecular palladium-catalyzed cyclization reaction to be performed to obtain the target turnstile in a very good yield of 68 %. The target structure was fully characterized by NMR spectroscopy and mass spectrometry. Furthermore, H-1-H-1 NOESY NMR experiments pointed to a pseudo-rotation of the push-pull rod within the turnstile structure on the time scale of the NMR experiment.