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.
Arizona State University
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01 February 2006
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