Theoretical Chemistry, Practical Solutions

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All scientific discoveries, large and small, start with one common goal—to advance knowledge, be it about our bodies and health, our environment, or even our universe. But the most exciting discoveries, built on years of research and collaboration, fundamentally change how we understand our world.

In a special summer series, we'll will talk with six Northwestern scientists whose work is already changing their fields, and could potentially change our lives.

Mark Ratner, professor of chemistry and co-director of the Initiative for Sustainability and Energy at Northwestern (ISEN), has been an innovator in his field since the early 1970’s. Now, he and his collaborators are working on how chemistry might be used to solve our world’s energy and sustainability crisis. We spoke with Ratner about his past, present, and future research, and what theoretical chemistry is all about.

Tell me about the focus of your work.
I’m a theoretical chemist. What I care about is the dynamic evolution of molecular systems. What this means is for a given molecule in a given environment, if you pass current through it, if you shine light or sound on it, if you try to twist it or distort it, what does it do? How does it respond to that? And how can it store information, energy, and charge?

Some of these molecules are molecules that nature has made— light-absorbing molecules for photosynthesis, oxygen-carrying molecules in your blood, or molecules from the visual cortex. But the great thing about chemistry is that you can make new molecules. So the question is, can you make a new molecule that has a new function? Or do something that nature does, but do it much better?

Mark Ratner (photo courtesy of the Initiative for Sustainability and Energy at Northwestern)Mark Ratner (photo courtesy of the Initiative for Sustainability and Energy at Northwestern)What’s the difference between an experimental chemist and a theoretical chemist?
Theoretical chemists write equations, they analyze data, they do computations. They don’t have a laboratory, they don’t have chemicals, they don’t have specialized equipment. It’s actually a very new part of chemistry. If you look at universities 70 years ago, there were very few theoretical chemists. When I first came to Northwestern, there was one.

Now there are more. It’s kind of like theoretical physics, in that it’s become a crucial part of the discipline. It helps us to understand how molecules behave in time and what their properties are. That’s really what it’s about.

Some people say that doing theory in chemistry is like playing tennis with the nets down, which is what Robert Frost said about blank verse, because you don’t ever do experiments. But you do -  computer experiments. We collaborate a lot with experimental people. Very often my students will work with my group theoretically, and with someone else’s group doing experiments.

What are some of the most interesting things your lab has found over the years?
My very first student, around 1973 or 1974, suggested that could you use a single molecule to act as a circuit element. For instance, in computers there are electrical circuits that do all the jobs. They remember things—memory circuits—or do logic—logic circuits. And he proposed that, instead of using little bits of silicon, you could use individual molecules.

That turned out to be a pretty important idea because, at the time, nobody had seen a single molecule. Nobody had figured out how to address a single molecule. Nobody thought that single molecule function could be measured, and certainly nobody thought that a single molecule could act as a logic or memory device.

Since that time, things have changed a lot. Now we can make single molecule structures, we can see them, we can measure them, we can look at electric current going through them, and people have thought about them as exactly what my student suggested – logic and memory devices. So that’s a long-term thing that’s been going since 1974.

Most of what is more recent than that has to do energy in one way or another. We’re looking for molecules that could act as photodetectors, or photoconversion devices, so that you could shine light on them and they could help you create electricity. These are called organic photovoltaics.

What aspect of your work are you most excited about now?
That’s easy – trying to make artificial photovoltaics, artificial photofuels. In other words, using chemistry to solve the energy and sustainability problem. There’s a long way to go, but progress is coming pretty fast. Northwestern is one of the premier places in the world to do this, and we’re closely linked with Argonne, which also has very strong people in this area, so it’s spectacular.

It’s something that everyone in society really cares about, for good reason. It’s also my other job at Northwestern, because I’m the co-director of the energy and sustainability effort. It’s exciting, because it’s a real-world problem that can be addressed by chemistry.

You said there is a long way to go. Do you have a rough estimate as to when we might see optimized artificial photovoltaics?
Optimized is hard to say, because you can always tinker with things. We’re still tinkering with light bulbs many, many years after they were invented. I think the question is, can we come up with something that is viable under a reasonable economic scenario? We’re not there, but I think we will be, certainly within ten years, and I hope faster than that.

How did you become interested in theoretical chemistry?
I started graduate school in 1964. This means that when Sputnik went up in 1957, I was in junior high school. That was a time when American society was very frightened, because we thought that, since the Russians had developed space travel and we hadn’t, that they would bomb us from the sky. So there was a lot of interest in science in secondary schools all across the country. I got involved in a bunch of programs that were intended to train high school kids and junior high school kids to be interested in science.

Then, as an undergraduate, I started majoring in math, and I didn’t like it. Then I majored in English and didn’t like it, so I tried physics and didn’t like it either. Then I went back to English, and still didn’t like it. Finally, I went to chemistry, and I liked that.

And I liked it in graduate school. In graduate school, I was converted from experiments to theory because my advisor and his wife, who were both theoretical chemists, invited me for dinner. She was a wonderful cook, and I was sold.

So she swayed you with her cooking?
Absolutely. I was only 23 – food was a big deal [laughing].

What advice to you have for young people when choosing a career?
There are different things that people require in a career – for one thing, it has to provide enough income to keep your life going. So it has to be remunerative, but it also has to be remunerative on an intellectual basis and on an emotional basis.

Science is a very new profession, in a sense. The word “scientist” only goes back to the 19th century. Before that, they were called natural philosophers, and there weren’t very many of them. Theoretical chemistry in particular is a very new sub-discipline, but all of the modern sciences are new. All of them, except perhaps mathematics and physics, don’t go back more than a few centuries. So there’s a lot of new stuff out there to discover.

But I don’t think science is for everybody. You need to be curious about the world around you. You need to be driven to go after hard problems that you can work on for a week, or a month, or a year, or five years, or maybe longer, before you find the answer. And occasionally, there isn’t an answer.

I think chemistry has an advantage over some of the other disciplines in that you make things. I don’t make things – experimental chemists make things. Theoretical chemists contribute to making things by suggesting what the properties of the thing should be.

Theoretical chemistry is a good place to be if you’re interested in explanations, as well as making experiments and doing experiments. If you have an opportunity to be at a place like Northwestern, where you have fantastic students to work with all the time, and pretty wonderful facilities, that’s sort of like winning the lottery.

But, I think young people should go into what they like. If you don’t like it, it may remunerate you very well, it may provide status, but I don’t know that it will get you out of bed in February when it’s three below zero. But if it’s something you love, it will.

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