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Molecular Electronics
The guiding principle of this research is that biological systems can
provide useful paradigms for developing electronic and computational devices
at the molecular level. For example, natural photosynthetic reaction centers
are photovoltaic devices of molecular dimensions, and the principles dictating
the operation of reaction centers may be useful in the design of synthetic
optoelectronic switches. In this project, several classes of molecular
photovoltaic species are being synthesized and studied. These include porphyrin-fullerene
dyads, carotenoid-fullerene dyads, a carotenoid-porphyrin-fullerene triad,
carotene-porphyrin-imide triads, and molecular dyads and triads containing
two porphyrin moieties.
The approach involves the design and synthesis of dyads, triads and
other supermolecular species using the techniques of organic chemistry.
The newly-prepared molecules are then studied by a variety of physical
methods, including time-resolved laser spectroscopy, nmr spectroscopy,
and cyclic voltammetry in order to determine how, and how well they functioned
as molecular electronic elements. The information gained can then be used
to design new generations of these molecules.
Once functional molecular photovoltaics, logic gates, or other elements
have been prepared, ways must be developed for interfacing these with electronic
circuits. Possibilities are being investigated in a collaborative project
with Professor Michael Kozicki, in the Department of Electrical Engineering.
The triad shown above is an example of a molecule that may be useful
in molecular electronic applications. Buckminsterfullerene (C60)
and its relatives have generated considerable excitement in recent years
due to their status as new and unusual forms of carbon which are completely
unrelated to the many carbon compounds synthesized by living organisms.
In spite of their non-biological origin, it turns out that fullerenes are
nearly ideal as components of molecules that mimic natural photosynthetic
energy and electron transfer.
This molecular "triad" consists of a synthetic porphyrin (P) covalently
linked to both a fullerene (C60) and a carotenoid polyene (C)
(J. Am. Chem. Soc. 1997, 119, 1400-1405). When the
porphyrin absorbs light, it donates an electron to the fullerene, yielding
C-P· +
-C60· -.
The carotenoid then transfers an electron to the porphyrin to give a final
C· +
-P-C60· -
charge-separated state. This state has a relatively long lifetime,
and stores a considerable fraction of the light energy as electrochemical
potential energy. This conversion of light energy to electrochemical potential
is analogous to the way plants carry out solar energy harvesting during
photosynthesis.
The charge-separated state is formed even at 8 degrees Kelvin in a frozen
environment, and ultimately decays by charge recombination to yield the
carotenoid triplet excited state, rather than the original ground-state
molecule. The generation in the triad of a long-lived charge separated
state by photoinduced electron transfer, the low-temperature electron transfer
behavior, and the formation of a triplet state by charge recombination
are phenomena heretofore observed mostly in photosynthetic reaction centers.
The triads are molecular-scale photovoltaic cells. Their nanometer size
and their ability to generate an electrical response to light may help
point the way to the development of molecular-scale (opto)electronic devices
for communications, data processing, and sensor applications.In fact, the
triad shown above functions as a molecular-scale AND logic gate. Two inputs
(light and a weak magnetic field) are required to switch on the output
of the gate, which may be detected optically or electrically.
("Magnetic Switching of Charge Separation Lifetimes in Artificial Photosynthetic
Reaction Centers," D.Kuciaukas, P. a.Liddell, A.L.Moore, T.A.Moore and
D.Gust, J.Am.Chem. Soc., 120 ,10880-10886 (1998).)
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