Attosecond Spectroscopy Captures Electron Transfer in Organic Molecules

The redistribution of electronic density in molecules after they absorb light is an ultrafast phenomenon that involves quantum effects and molecular dynamics. The ability to measure the electron and charge transfer dynamics of this phenomenon with extreme temporal resolution could provide insight into the mechanics behind these processes, making it possible to engineer the properties of molecules for optimal control.

One way to study the early stages of coupled electron-nuclear dynamics is to expose the molecules to attosecond (as) extreme-UV (XUV) pulses. Scientists can use ultrashort UV pulses from high-order harmonic sources or free electron laser facilities to initiate and observe the response of molecules to photoionization, on timescales ranging from the femtosecond (fs) to the as.

A team comprising researchers at Politecnico di Milano, Madrid Institute for Advanced Studies in Nanoscience, Autonomous University of Madrid, and Complutense University of Madrid investigated the first steps of charge-transfer processes, initiated by ionization in nitroanilines. To capture time-resolved measurements of the processes, the researchers used XUV pump/few-fs IR-probe spectroscopy, combined with many-body quantum chemistry calculations.

By exposing the nitroaniline molecules to as pulses, the researchers were able to observe and analyze the earliest stages of charge transfer with extraordinary precision.

Based on experimental evidence and detailed numerical simulations, the researchers found that electron transfer from the electron donor amino group occurred within less than 10 fs and was driven by a synchronized movement of nuclei and electrons. They determined that this was the amount of time needed by the nitrogen atom to change its hybridization so that an electron could be transferred to the rest of the molecule.

This activity was followed by a relaxation process that occurred over a sub 30 fs timescale as the nuclear wave packet spread in the excited electronic states of the molecular cation. By monitoring the production of ionic fragments and computing the temporal evolution of the electronic density in different regions of the molecular structure, the team was able to time-resolve the coupling between electronic and nuclear motion and reveal the joint nature of the electron transfer process occurring at the few-fs timescale.

The ultrafast redistribution of energy and electronic charge in molecules after photoexcitation is pertinent to physics, chemistry, and materials science. Photoinduced electron transfer and charge transfer govern photosynthesis in plants. Electron and charge transfer also drive many of the processes that fall somewhere between purely quantum effects and molecular dynamics.

So far, scientists have not been able to describe in detail the initial steps of electron and charge transfer and the ultrafast processes directed by coupled electron-nuclear motion in molecules after photoionization. Consequently, precise temporal information on the steps of the electron and charge transfer processes has never been fully addressed.

By measuring the first steps of the charge transfer process in nitroanilines after photoionization and determining the time required for an electron to be transferred from an atom to an adjacent chemical bond, along with the associated structural changes, the researchers have identified how long an electron takes to initiate charge migration in molecules — answering a fundamental question in chemistry. The findings set the stage for future advancements in the theoretical understanding and practical applications of as science.

The work is part of TomATTO (Timescale in Organic Molecular optoelectronics, the ATTOsecond), a project supported by the European Research Council with €12 million ($13.1 million) in funding. TomATTO aims to improve the conversion efficiency of solar energy through a better understanding of how to control the excitation of molecules in solar cells. TomATTO has three primary goals: to record the first electronic processes initiated by light absorption, to design new organic materials to control electronic dynamics, and to develop computational methods to understand the results.

The research was published in Nature Chemistry (www.doi.org/10.1038/s41557-024-01620-y).