New Approach Facilitates Spectroscopy on Individual Molecules

A new spectroscopic measurement method, developed by scientists at the Technical University of Munich (TUM), determines the spectral properties of individual molecules, providing precise information about the interaction of single molecules with their environment.

The technique uses a compact instrument developed by the Munich scientists in collaboration with colleagues at the Politecnico di Milano to generate a double laser pulse with a controlled delay in between. The second pulse modulates the emission spectrum in a specific manner, which in turn provides information about the absorption spectrum. This information is then evaluated using a Fourier transformation.

Professor Juergen Hauer (left) and first author Erling Thyrhaug with their measuring instrument. On the screen are spectra taken with it. Courtesy of Andreas Battenberg.

The researchers demonstrated simultaneous collection of fluorescence emission and excitation spectra. They acquired the absorption and emission spectra of the investigated molecules over a broad spectral range in a single measurement to accurately determine how the molecules interact with their environment, capturing and releasing energy.

Typically, these kinds of measurements are averaged over thousands, even millions, of molecules, sacrificing important detail information. “Previously, emission spectra could be routinely acquired, but absorption measurements on individual molecules were extremely expensive,” professor Juergen Hauer said.

“The primary advantage is that we can, with little effort, transform a conventional measurement setup for acquiring emission spectra into a device for measuring emission and absorption spectra,” Hauer said. The measurement itself is relatively easy. “At 9 o’clock in the morning, we installed the apparatus into the setup at the University of Copenhagen. At half past 11, already, we had our first useful measurement data.”

Using the new spectroscopy method, the scientists plan to study individual molecules to better understand phenomena such as the energy flow in metal-organic compounds and the physical effects in molecules when they come into contact with water and other solvents. The researchers also want to display the flow of energy in a time-resolved manner to understand why energy flows faster and more efficiently in some molecules than in others. “Specifically, we are interested in the transfer of energy in biological systems in which photosynthesis takes place,” Hauer said.

The researchers will use their new method to study the light collection complex LH2 for future applications. “Once we understand the natural light-harvesting complexes, we can start thinking about artificial systems for deployment in photovoltaics,” Hauer said. The findings could form the basis for future technologies in photovoltaics. The ultimate goal, said the researchers, is the development of a novel organic solar cell.

The research was published in Proceedings of the National Academy of Sciences (https://doi.org/10.1073/pnas.1808290116).