To a chemist, a plant leaf is an automated factory that captures light from the sun - using chlorophyll - and instantly puts this energy to work to create a host of complex substances.

Chemist Mark E. Thompson hopes someday to copy this trick. In the April 18 issue of Nature, he reported the first step: the creation of a "chemophyll" designed to capture solar energy for instant molecule making.

Like solid-state silicon solar cells, the prototype substance absorbs light and converts it into electrical potential. Unlike solar cells, but like chlorophyll, the new substance delivers this electrical potential in a form that's immediately usable for chemical reactions.

"This material would be inefficient at powering a device, like a radio or a hair dryer, that runs on electric current," Thompson said. "But it could be an extremely effective power source for chemical processes like breaking down water into oxygen and hydrogen or making methane [gas] out of carbon dioxide to create clean-burning fuels."

In plant photosynthesis, Thompson explains, the chlorophyll molecule instantly hands off the energy it captures to drive a series of chemical reactions synthesizing new plant tissue. This 'hand-off' mechanism also keeps the reaction going one-way, preventing loss of the captured energy.

"Materials like the one we have synthesized have a similar capability," said Thompson, an associate professor of chemistry in the College of Letters, Arts and Sciences. "Indeed, they might be called 'chemophylls,' because they could form the power source of man-made devices that, like green plants, would continuously make useful products as long as they are exposed to light."×

Thompson's new material possesses three characteristics that make it useful for this kind of reaction:

* It is produced in extraordinarily uniform layers and can deliver energy evenly over large surfaces. It can also be applied as a coating to an irregular surface.

* Like chlorophyll - and unlike silicon cells - its photovoltaic action is a wet process, working in an electrolyte solution conducive to chemical reactions. A layer of chemophyll might even float on the surface of the raw material that fuels a chemical reaction.

* The new substance is thermally stable - that is, it does not break down when heated to energy-producing temperatures.

Compared to silicon devices, Thompson's material is not yet very efficient at converting solar energy. But the researchers are confident they're on the right track. "Materials of similar structure with much higher efficiencies can be made," Thompson said. "In fact, we're working on them now."

The prototype chemophyll is a stable preparation that binds a layer of electron-donor substance, called PAPD, to a layer of an electron-acceptor substance, called PV.

When illuminated with visible or ultraviolet light, electrons move from PAPD to PV, creating an electrical potential. A receptor material accepts the electrons on the other side of the surface, in a continuous, one-way flow.

Thompson has demonstrated that he can stack the PAPD-PV pairs with great precision, producing virtually uniform films of two, four, six or more pairs.

The prototype device, however, works best at single-pair thickness, because the electron flow tends to stall at the borders between pairs. The scientists are now developing bridges that can ferry the electrons across borders to catch and use more incoming light.

Other researchers have stacked similar uniform devices, but the layers weren't solidly bound together - they were simply placed one on top of the other. Such multilayers had photo-reactive potential but were unstable. When light hit them and they began to work, the resulting molecular motion jarred the devices out of the structural order necessary to generate consistent and continuing electron flow.×

It will take a lot more research to make chemophylls live up to their potential, Thompson admits. "Nature's chlorophyll forms part of an extremely efficient system," he said. "Virtually every photon it absorbs creates a working electron that's efficiently taken up into chemical processes.

"Our chemophyll, by contrast, must be regarded as no more than a first try. It's far less efficient in capturing light, and we're only learning how to make the electrons it does generate perform useful chemical work," he said. "Still, we see some distinct possibilities."

Likewise, Thompson is confident that more research will lead to new chemical systems that can exploit the electrons generated by chemophyll. One possibility is to create catalysts to generate hydrogen peroxide. The chemist's long-term dream is to reduce water to oxygen and free hydrogen, which can be used as a clean-burning fuel.

Another catalyst might photochemically convert carbon dioxide to methane - a reaction that would really resemble photosynthesis, which converts carbon dioxide to plant tissues.

"The advantage of this [conversion] is that methane is much easier to store and transport than hydrogen. You might visualize giant, flat structures floating in the ocean in calm, sunny areas like the Persian Gulf or the Gulf of California and carrying out these conversions."

Thompson's scientific team includes chemist Houston Byrd, of Montavallo University in Alabama; USC chemistry postdoctoral student Elena P. Suponeva; and Princeton University chemist Andrew B. Bocarsley. The research was supported by the National Science Foundation and the American Biomimetics Corp.