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.
(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 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.