QnAs with Abraham Nitzan

Biology involves many reactions of oxidation and reduction that move electrons from one site to another in electron-transfer steps. At a fundamental level, organisms use electron transfer to shuttle energy. Photosynthesis, corrosion, and electronic devices operate through basic electron-transfer processes. Abraham Nitzan, a chemist at the University of Pennsylvania and a recently elected member of the National Academy of Sciences, studies electron transfer along with other chemical dynamics processes in an effort to understand how atoms interact during chemical reactions. Thanks to advances in nanoscale research, Nitzan and colleague Galen Craven have investigated electron transfer between sites of different temperatures, and found that electron transfer and heat transfer are coupled processes. Nitzan recently spoke to PNAS about his findings.

Abraham Nitzan. Image courtesy of Abraham Nitzan.

PNAS: Why are you interested in electron-transfer processes?

Nitzan: Electron-transfer processes are at the core of all oxidation-reduction (redox) reactions, including electrochemistry and corrosion reactions. Photoelectrochemistry, solar energy conversion, organic light-emitting diodes, and molecular electronic devices are presently subjects of intensive research at the interface of science and technology and are all dominated by electron transfer and electron transmission in molecular systems. Similarly, electron-transfer processes constitute fundamental steps in important biological phenomena, such as photosynthesis and vision. Electron transfer is therefore one of the most important process underlying many important chemical transformations.

PNAS: Some systems, such as photovoltaic or thermoelectric devices, convert one form of energy, such as light or heat, into electricity. What is the role of electron transfer in such systems?

Nitzan: Electron transfer and transport is central to the operation of both thermoelectric and photovoltaic devices. In thermoelectric systems electronic motion is driven by a difference in voltage formed by a temperature difference between two metal electrodes. In photovoltaic devices it is caused by optical excitation followed by charge separation and subsequent motion of the particles that carry electric charge. In both systems, motion of electrons is usually assumed to proceed at a uniform local temperature, an assumption that can fail in nanoscale devices. Our theory set the stage for going beyond this assumption.

PNAS: What does the work described in your Inaugural Article (1) reveal about the effect of a temperature gradient on electron transfer?

Nitzan: It should first be emphasized that such a question would not be asked until recently. Electron transfer is a relatively short-range process, and it was hard to imagine such transfer taking place between an electron donor and an acceptor situated at locations of different temperatures. With advances in research on nanoscale thermal transport, experiments of the kind envisioned in this paper can be done. The electron-transfer rate was found to be affected by both temperatures, with their relative importance determined by the so-called reorganization energies, or the energy associated with response of the surrounding environment to a change in a local charge, associated with the two sites.

PNAS: In addition to describing coupling between heat and electron transfer, your article (1) characterizes a phenomenon in which heat transfer continues even when the net electron flux between the two sites is zero. What causes the ongoing heat transfer?

Nitzan: The heat transfer is proportional to the thermal gradient between the two sites. The coefficient of this relationship, the thermal conductivity, can reflect different underlying physical phenomena. In metals, for example, there is a contribution to the heat conductivity from the vibrations of the metal atoms, and there is an electronic contribution by which heat is carried with the moving electrons. Here we identify a different kind of electronic contribution to the thermal conductivity in a system where electrons move not as essentially free particles as in a metal, but hop between sites, as in many organic semiconductors.

PNAS: What new applications and technologies could arise from your findings?

Nitzan: Electronic and thermal conduction are significant players in many kinds of devices, from molecular conduction junctions to photovoltaic cells and thermoelectric devices. It is not so much that this work will lead to new devices, as lead to understanding the mechanisms of these transport phenomena in such devices en route to developing ways to control them.

PNAS: How could reactions be controlled using thermal gradients?

Nitzan: Some researchers have considered temperature as a control device, but this may be premature since temperature is not easily controlled on such small-length scales. Still, temperature gradients do occur in nonequilibrium nanosystems and should be taken into account in analyzing rate processes in such systems.

PNAS: How might temperature gradients affect electron-transport processes we may already be familiar with?

Nitzan: A well-known relation that characterizes electrical and thermal transport in metals in the Wiedemann–Franz law, which is a relationship between the electronic contribution to the thermal conductivity of a metal and the metal’s electrical conduction. This work is expected to lead to an equivalent relationship between electric and thermal transport in systems where electronic motion is not essentially ballistic, like the movement of free particles, but takes place by hopping between sites.

PNAS: In which systems do you next plan to observe coupled heat and electron transfer in action?

Nitzan: It will be interesting to think of biological applications, but this is not my line of research. Rather, I expect our work to continue in the direction of transport in the context of energy-conversion processes. The Wiedemann–Franz relationship is derived from the common origin of these phenomena: the motion of quasi-free charge-carrying particles. Our work should be able to provide an equivalent relationship for systems where charge carriers are moving not as free particles but hop between redox sites. This may open a way to analyze conduction in such systems, with possible applications to heat and electron transport in thermoelectric and photovoltaic devices based on such systems.