Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Tracing Photochemical Reactions

05.04.2013
When light hits organic molecules, it triggers processes that are of considerable interest to scientists. But the individual steps of the reaction are very hard to identify. A study group at the University of Würzburg has now accomplished this task – with a sophisticated approach.

It might well be every chemist's dream: to actively control chemical reactions at a molecular level, to form or break chemical bonds at will so as to create tailored substances with special properties. As a precondition, however, this requires highly accurate knowledge of the numerous individual steps of which chemical reactions usually consist. In many areas, such knowledge is not yet available and neither is it easy to gain.


A sequence of ultrashort laser pulses (left) leads to the chemical reaction of a merocyanine dye (in the middle), which can be analyzed with multidimensional spectroscopy (right).
Graphics: Martin Kullmann, University of Würzburg

But now Stefan Rützel and some of his colleagues in the team of Professor Tobias Brixner, the head of the Department for Physical Chemistry I at the University of Würzburg, have developed a method that can be used to clearly identify at least the precursor states of chemical reactions. Their research is published in the current issue of the prestigious journal Physical Review Letters. Due to its special scientific importance, the editors even placed the Würzburg study as a research highlight in the Synopsis section of Physics (Opens external link in new windowphysics.aps.org).

Research on a femtosecond time scale

There are two requirements for unlocking the secret of chemical reactions on an atomic scale: Speed and skill. This is because photochemical reactions are inconceivably fast even though they often include the formation of several intermediary products, usually taking place within the space of only a few femtoseconds, i.e. a few millionths of one billionth of a second.

Nevertheless, it is possible for the scientists to shed "light" on the chemical processes, using ultrashort laser pulses emitted by femtosecond lasers. The molecules are sort of "scanned" with the laser light over a certain time period so that the dynamics of the reaction processes can be mapped. This widely used method is known as "pump probe spectroscopy".

A laser pulse in duplicate

"Pump probe spectroscopy uses a laser pulse to initiate a certain reaction. A second laser pulse then probes the dynamics induced by the first pulse," Tobias Brixner says, explaining how the method works. This enables you, among other things, to determine the characteristic lifetime of excited states and to identify competing reaction pathways.

However, this method still does not solve the following problem: "In a pump probe experiment, it is very difficult to identify the special state of a molecule from where the reaction starts," Brixner explains. This is because the laser pulse creates a multitude of such states.

Ingenious experimental setup

Despite these difficulties, Stefan Rützel and some of his colleagues in Brixner's study group have now succeeded in clearly identifying such precursor states of reactions, using an ingenious experimental setup. For this purpose, they combined laser pulses of various wavelengths in the visible range with each other and studied their time-resolved correlation. In this way, they obtained information as to whether certain electronic transitions in the start and end states are quantum-mechanically linked to each other. In other words: Whether a certain electronic state is the precursor of another one.

In the experiment, the study group examined the molecule merocyanine, which exists in two distinct spatial arrangements, called conformations. After excitation with light, only one configuration gives rise to the formation of a cation, i.e. an ion with a positive charge, as the scientists were able to show. The method they developed thus enabled them to identify the special precursor that needs to be excited for the desired reaction to take place.

Promising method for application in photovoltaics and data processing

The researchers hope that this method of tracing reaction paths via electronic states might also be applied to the study of many other chemical processes. Potential areas of application include photovoltaic processes or data storage and data manipulation in optical storage media.

The study was funded by the German Research Foundation (DFG) within the Research Unit "Light-Induced Dynamics in Molecular Aggregates" (FOR 1809).

Tracing the Steps of Photoinduced Chemical Reactions in Organic Molecules by Coherent Two-Dimensional Electronic Spectroscopy Using Triggered Exchange. Stefan Ruetzel, Martin Kullmann, Johannes Buback, Patrick Nuernberger, and Tobias Brixner. Physical Review Letters, DOI:10.1103/PhysRevLett.110.148305

Contact person

Prof. Dr. Tobias Brixner, T: +49 (0)931 31-86330
brixner@phys-chemie.uni-wuerzburg.de

Gunnar Bartsch | Uni Würzburg
Further information:
http://www.uni-wuerzburg.de

More articles from Life Sciences:

nachricht Topologische Quantenchemie
21.07.2017 | Max-Planck-Institut für Chemische Physik fester Stoffe

nachricht Topological Quantum Chemistry
21.07.2017 | Max-Planck-Institut für Chemische Physik fester Stoffe

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: 3-D scanning with water

3-D shape acquisition using water displacement as the shape sensor for the reconstruction of complex objects

A global team of computer scientists and engineers have developed an innovative technique that more completely reconstructs challenging 3D objects. An ancient...

Im Focus: Manipulating Electron Spins Without Loss of Information

Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.

For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...

Im Focus: The proton precisely weighted

What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.

To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...

Im Focus: On the way to a biological alternative

A bacterial enzyme enables reactions that open up alternatives to key industrial chemical processes

The research team of Prof. Dr. Oliver Einsle at the University of Freiburg's Institute of Biochemistry has long been exploring the functioning of nitrogenase....

Im Focus: The 1 trillion tonne iceberg

Larsen C Ice Shelf rift finally breaks through

A one trillion tonne iceberg - one of the biggest ever recorded -- has calved away from the Larsen C Ice Shelf in Antarctica, after a rift in the ice,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Closing the Sustainability Circle: Protection of Food with Biobased Materials

21.07.2017 | Event News

»We are bringing Additive Manufacturing to SMEs«

19.07.2017 | Event News

The technology with a feel for feelings

12.07.2017 | Event News

 
Latest News

Ultrathin device harvests electricity from human motion

24.07.2017 | Power and Electrical Engineering

Scientists announce the quest for high-index materials

24.07.2017 | Materials Sciences

ADIR Project: Lasers Recover Valuable Materials

24.07.2017 | Materials Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>