Chemists from the University of Würzburg have combined different dye molecules in aggregates and thereby observed surprising properties. Their discovery may help to use sunlight more efficiently for the generation of energy.
Plants use accumulations of dye molecules, so-called light-harvesting complexes, to capture sunlight and to convert water (H2O) and carbon dioxide (CO2) to energy-rich organic compounds and oxygen (O2) through photosynthesis. For this, they have to transport the energy gained by light absorption over numerous dye molecules to the photosynthetic reaction centers. Individual dye units, so-called chromophores, are kept in close proximity by the surrounding protein shell to ensure efficient energy transfer.
Two Balls Linked by a Spring
The optical absorption properties of the light-harvesting complexes differ significantly from the ones of the individual dye molecules. This is due to the so-called exciton coupling between the dye molecules. The coupling can be visualized by two balls hanging on threads linked by a spring. If one ball is moved from its position of rest, the second ball will be also influenced. On molecular level the displacement of the ball corresponds to the excitation of a molecule through light absorption.
This phenomenon is well understood for systems comprising like dye molecules (homo-aggregates) while little is known about coupling between different chromophores. New insights currently come from the group of Professor Frank Würthner, chairholder of Organische Chemie II at the University of Würzburg and director of the Center for Nanosystems Chemistry. The multidisciplinary journal Nature Communications reports on this in its online version.
Quadruple Stacks of Dye Molecules
The investigation of coupling between dye molecules requires aggregates with distinct orientation of the dye units. The coworkers of Professor Würthner succeeded in preparing appropriate aggregates in the form of stacks comprising four chromophores. For this they used merocyanine dyes which form well-defined aggregates due to their strong dipolar character. “By the chemical linkage of two equal chromophores via a naphthalene unit we could obtain a molecule that dimerizes in solution forming stacks of four equal chromophores”, explains David Bialas, PhD student in the group of Frank Würthner and author of the publication.
Afterwards, the scientist went one step further: they linked two different merocyanine chromophores exhibiting different absorption properties to obtain hetero-aggregates in form of quadruple stacks.
“The structure of the dye stacks in solution could be investigated with the help of nuclear magnetic resonance spectroscopy”, says Eva Kirchner, who is also a PhD student in the group and was involved in the project. An unambiguous proof for the existence of the quadruple stacks was obtained by X-ray structural analysis. For this, the team had to grow suitable crystals, which is a challenging task for dye aggregates.
Unexpected Absorption Properties
The observations during the spectroscopic studies of the absorption properties were unexpected. “The results indicated exciton coupling between the dye molecules not only for the homo-aggregate but also for the hetero-aggregate”, explains Bialas. Quantum mechanical calculations confirmed a strong exciton coupling between the different types of chromophores in the hetero-aggregate. “This contradicts the common belief that strong coupling is only possible between the same types of chromophores”, says the scientist.
Fast Energy Transfer
Exciton coupling not only influences the absorption properties of dye aggregates but is also an indication for fast energy transfer between the molecules. This could be used by scientists in the future to harvest sunlight more efficiently since the usage of different dye molecules allows to cover a broader absorption of the solar spectrum. Thus, more energy may be gained which can be converted to current or chemical energy.
Interfaces between different materials and their properties are of key importance for modern technology. Together with an international team, physicists of Würzburg University have developed a new method, which allows them to have an extremely precise glance at these interfaces and to model their properties.
In his Nobel Lecture on December 8, 2000, Herbert Kroemer coined the saying “the interface is the device”. Kroemer referred to the mature field of semiconductor heterostructures, which form the basis of all modern electronics.
However, now, in the advent of novel, powerful devices based on the more complex and versatile topological and correlated materials, the statement is timelier than ever. Such materials are at the focus of research in the Department of Physics and Astronomy at Würzburg University: Currently, 16 groups are working in this field, and a Collaborative Research Center (CRC 1170) was established in 2015, which is funded by the German Science Foundation (DFG) with nearly 10 Million euro.
Publication in Nature Quantum Materials
In the recent years physicists from Würzburg University and coworkers from Germany, Canada, the U.S.A. and Korea developed a new method to uncover important charge properties of correlated oxide interfaces with unprecedented atomic scale resolution. The team of Professor Vladimir Hinkov and his coworkers report about this experimental method in the current issue of the Nature Journal “NPJ Quantum Materials”.
“Conventional electronic chips are based on networks of so-called p-n junctions, interfaces between semiconductors carrying positive and negative charges, respectively,” says Vladimir Hinkov, describing the background of this research. There are several drawbacks to such a setup: First, the junctions are thick, often of the order of hundreds of interatomic spacings. Second, operating the network requires the movement of many electrons, which costs a lot of energy due to electrical resistance. Third, semiconductors do not intrinsically have magnetic properties and their electron configuration is very basic. “This dramatically limits the ways to build functional junctions and to realize magnetic applications,” Hinkov reports.
Versatile properties require sophisticated methods
Transition-metal oxides, on the other hand, exhibit many different properties: Some of them are ferromagnetic, others are antiferromagnetic, and others in turn are high-temperature superconductors with very unconventional properties. Forming interfaces between such materials yields a plethora of phenomena, which hold promise for novel applications such as different sensors, lossless computer memory and ultrafast processors. The price one has to pay is that more sophisticated tools are necessary to study them: This is due to the variety of phenomena and due to the much shorter length scale, over which the properties of oxides change at such heterointerfaces, which is often just a few atomic spacings.
Of crucial importance is the behavior of electrons at the interface: Do they tend to accumulate? Which orbitals do they occupy, i.e. how do the electron clouds arrange around the atoms? Is there magnetic order, i.e. do the tiny magnetic moments of the electrons called spins align relative to each other, establishing magnetic order? Physicists around the world are seeking for answers to these questions.
Measurements on an atomic scale
Hinkov and coworkers developed a new method and analysis software, and it provides answers. It is based on “resonant x-ray reflectometry”, a technique exploiting x-ray light created at a synchrotron, with the atomic-scale resolution of less than one nanometer. The physicists apply the technique on thin films of lanthanum cobalt oxide, a material that has interesting magnetic properties.
In their present work, however, the scientists have concentrated on another aspect: “Before we can delve in the rich magnetic phenomena of this material, we first have to solve a fundamental, very wide spread problem,” says Professor Hinkov. “Like many other materials, such as simple table salt and many semiconductors, lanthanum cobalt oxide consists of charged particles. These so-called ions form a sequence of positively and negatively charged atomic layers, stacked to a 15 nanometer thin film. “One can show that enormous electrostatic fields form between the layers, which is a problem, since they cost a lot of energy,” as Vladimir Hinkov explains.
“Nature is economical and avoids these field energy costs: It brings positive and negative charges to the opposite faces of the film, respectively, just like between the plates of a capacitor. A new field is formed, which is opposite to the original one and which cancels it.”
Corrugated interfaces constitute a problem
This accumulation of pure electronic charge at the film faces is called “electronic reconstruction”. According to the physicists, this is a very elegant solution, since it preserves the film face smoothness. For materials, in which electronic reconstruction is not possible, the compensating charge is provided by comparatively large ions, which results in corrugated film faces. As Hinkov explains, such corrugations are detrimental for devices based on film interfaces, especially when, like in transition-metal oxides, the material properties change on an atomic scale at the interface.
Exploiting the new method, the present work shows microscopic evidence that electronic reconstruction is indeed realized at transition-metal oxide interfaces. The method also provides a possibility to study the microscopic properties of such interfaces, which are not limited to electronic reconstruction, but encompass the arrangement of chemical elements, the electronic occupation of atomic orbitals and the spin orientation.
Successful by close, international collaboration
The special “Würzburg environment” and the close international collaboration enabled this successful work. “Such a scientific endeavor is only possible when experts from many different fields work closely together,” says Professor Hinkov. One needs excellent samples, high-precision x-ray scattering instruments, which are operated at modern synchrotron light sources, a dedicated software, and last but not least “colleagues who are willing to spend day and night at the synchrotron to perform the measurements.”
A clean, climate-friendly energy source that is virtually inexhaustible: This is the promise artificial photosynthesis holds. Chemists from the University of Würzburg have now got one step closer to reaching this goal. The scientists present their work in the journal Nature Chemistry.
Nature shows how to do it: Photosynthesis is a process used by plants to create energy-rich organic compounds, usually in the form of carbohydrates, and oxygen (O2) from carbon dioxide (CO2) and water (H2O) driven by light. If we succeeded in mimicking this process on a large scale, numerous problems of humanity would probably be solved. Artificial photosynthesis could supply the Earth with fuels of high energy density such as hydrogen, methane or methanol while reducing the amount of carbon dioxide in our atmosphere and slowing down climate change.
Developing the necessary efficient catalysts and associated dyes is a focal area of research at the Chair of Professor Frank Würthner at the University of Würzburg’s Institute of Organic Chemistry. Two of Professor Würthner’s doctoral students, Marcus Schulze and Valentin Kunz, have recently reported a partial success in this regard. They present the results of their research work in the current issue of the journal Nature Chemistry.
The Julius Maximilians University of Würzburg (also referred to as the University of Wurzburg, in German Julius-Maximilians-Universität Würzburg) is a public research university in Würzburg, Germany.
The University of Wurzburg is one of the oldest institutions of higher learning in Germany having been founded in 1402. The University initially had a brief foundation and was closed in 1415, until it was permanently reopened in 1582 under the initiative of Julius Echter von Mespelbrunn. Today, the University is named for Julius Echter von Mespelbrunn and Maximilian Joseph.
The University of Wurzburg is one of the leading universities in Germany being part of the U15 group of research-intensive German universities. The university is also a member of the distinguished Coimbra Group.