Stoddart Mechanostereochemistry Group

 

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Powering a Supramolecular Machine with a Nanoscale Power Supply

Machines need power supplies to generate mechanical movement. Just as macroscopic machines require macroscopic energy sources, natural and artificial molecular machinery require nanoscale power supplies to effect precise molecular movement. While macroscopic machines do not move or work until energy is supplied to them for a specific function, nanoscale machines down at the molecular level are in perpetual Brownian motion at ambient temperature. For machine-like functions to be performed by molecular systems, they should be able to execute stimuli-induced specific and directional mechanical movements that are otherwise restricted, except for Brownian motion. The Stoddart and Zink have developed donor–chromophore–acceptor-based molecular triads that can transduce light into electrical energy by mimicking the photosynthetic pathway. In a collaborative effort between the two research groups, a nanoscale power supply (Box) in the form of a light-harvesting molecular Triad has been developed.

The ultimate goal of this project is to develop a solar cell. Donor–chromophore–acceptor-based molecular triads that can convert light to electrical energy are some of the most effective molecules that can serve the purpose of energy transduction. Furthermore, it has been demonstrated (Small 2005, 1, 87–90, Chem. Eur. J. 2005, 11, ASAP) that the photocurrent generated by the Triad can be utilized (Box) to drive a supramolecular machine in the form of a pseudorotaxane.

A tetrathiafulvalene–porphyrin–fullerene (TTF–P–C60) molecular triad, which generates electrical current by harnessing light energy when selfassembled onto gold electrodes, has been developed. The Triad, comprised of three unique electroactive components, namely, i) an electrondonating TTF unit, ii) a chromophoric porphyrin unit, and iii) an electronaccepting C60 unit, has been synthesized in a modular fashion. A disulfide-based anchoring group was tagged to the TTF end of the molecule in order to allow its self-assembly onto gold surfaces. In a closed electrical circuit, the Triad-functionalized workingelectrode generates a switchable photocurrent of ~1.5 µA/cm2, when irradiated with a 413 nm Kr-ion laser. The electrical energy generated by the Triad at the expense of the light energy is ultimately exploited to drive (Box) a supramolecular machine in the form of a [2]pseudorotaxane comprised of a π-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) cyclophane and a π-electron-rich 1,5-bis[(2-hydroxy-ethoxy)-ethoxy]naphthalene (BHEEN) thread. The dethreading of CBPQT4+ cyclophane from the BHEEN thread has been monitored by measuring the increase in the fluorescence intensity of the BHEEN unit. A gradual increase in the fluorescence intensity of the BHEEN unit concomitant with the photocurrent generation, even at a potential (0 V) much lower than that required (–300 mV) for the direct reduction of the CBPQT4+ unit, confirmed that the dethreading process is driven by the photocurrent generated by the Triad.

 

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