Artificial photosynthesis is, for the moment, what-if.
What if you could shine light into seawater, move some electrons around, and crank out a clean-burning hydrogen fuel? What if the same sort of process would also yield new materials, or nutrients, or compounds for treating disease?

These aren't just wild-eyed dreams in search of funding. They are on the agenda. For more than two decades, the intellectual quest for a basic understanding that could lead to artificial photosynthesis has made Tom Meyer, Kenan professor of chemistry, one of the best-funded and most-cited chemists in the world.

First, the idea.

"Think about a tree," Meyer says. "Sunlight comes into the leaves, and the tree uses it to make sugars and oxygen out of water and carbon dioxide. If we want to use the energy from this tree, we cut it down and burn it. Or the tree dies and a hundred million years later we are burning the oil and the coal."

But the burning, he says, makes a mess.

"Every time you burn a fossil fuel, you are releasing more CO₂ into the atmosphere, more greenhouse gases. Even if you use catalytic systems and control the burning, it's an inefficient, clumsy process that involves rudimentary chemistry, crude smokestacks, boilers, and catalytic converters, and lots of other big, expensive stuff. The idea of artificial photosynthesis is to capture the energy now, in a neat little closed system using much more sophisticated chemistry."

For Tom Meyer, there's something offensive about a crude, messy system. So it's not at all surprising to find that the model for his life's work is so elegant and clean: a green leaf, lit with sunlight. "The fascination," he says, "is how to do what happens in that green leaf, to understand the basic principles. How do you tap the energy in a molecule?"

It was a simple question that opened a wilderness of new territory. With his research teams of students and post-docs, he has proceeded step-by-step through the tangles and thickets, mapping the terrain.

The first step was to control electron transfer (see the illustration, "Electron transfer"), using light to excite electrons and then learning how to move them from one molecule to the next. Meyer and his team found molecules that could absorb light efficiently, converting its energy to molecular energy. The next step was to store this energy, to keep the electrons moving in a useful and orderly way.

To get an idea how tough this is to manage, imagine a bunch of excitable children playing musical chairs. When the music plays, the children hop up and go looking for a new place to land. One kid lands on somebody's lap.

"So the problem, unfortunately, is not solved yet," Meyer says. "If you let this sit, all that's going to happen is that the electron is just going to slowly drift back to where it came from. So you've got to fight against this back-transfer of electrons."

On this topic, Meyer's research group has become a world leader, both in theory and experimentation. Using work on electron-transfer rates by Rudolph A. Marcus, who won the 1992 Nobel Prize in Chemistry, Meyer's team is designing molecules that can avoid the back-transfer.

To do this, he says, you've got to make the molecule fancier, and add a catalyst or two. He uses the example of ethylene, derived from petroleum. Adding oxygen to ethylene is one step in making plastics. The trouble is, oxygen and ethylene won't get together on their own. They need a catalyst—a molecule that binds to both oxygen and ethylene.

Finding or making the right catalysts takes a lot of experimentation, and many of the catalysts Meyer employs are artificial—designer molecules called "transition metal complexes." Because these molecules are so useful in electron transfer, Meyer's research group has worked a great deal with some fairly exotic metallic chemistry. In fact, each new step toward artificial photosynthesis has demanded a new branch of expertise, a new base of information. Rather than drop everything each time they hit a blind spot, the team has for years taken a "modular approach," pushing ahead in those parts of the system for which they had good information, plugging in components as they could.

With each new set of students and post-docs, each new round of funding and research publications, the shape of artificial photosynthesis grows more and more distinct. Meyer is close, tantalizingly close, to revealing the whole. But he knows that it will probably be many years before the chemistry will be ready for industries and engineers. The most dramatic application, of course, would be energy production. But other uses, such as technologies for manufacturing new chemicals using sunlight, might be first in line.

Meanwhile, the intensive work with chemical reactions to light already has led to several breakthroughs. A Swiss scientist, Michael Graetzel, created a new medium for photovoltaics—using sunlight to generate electricity, as in solar heaters or solar-powered instruments and machines. Graetzel has developed a thin film coated with chemicals that Meyer and his group helped to invent.

As dramatic as such applications might be, they are not what drives Tom Meyer. For him, the attraction is that missing piece of knowledge, that basic insight into how something works. These pieces aren't easy to come by. They resist. And that's how he knows when he's onto something—when he feels the resistance.

"On my team, when we reach a kind of comfort level, that's when I worry," he says. "When the science gets hard, and we're really struggling, I know we're onto something good."

Article by Neil Caudle. Originally published in the Winter 1998 issue of Endeavors magazine.
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