What is molecular energy transport

Fast energy transport between unequal partners

Chemists at the University of Würzburg have combined different dye molecules into aggregates and discovered surprising properties in the process. Their discovery could help us use sunlight even more effectively for energy generation.

Plants use collections of dye molecules, so-called light-harvesting complexes, to capture sunlight and to generate energy-rich organic compounds and oxygen (O2) through photosynthesis from water (H2O) and carbon dioxide (CO2). To do this, they have to transport the energy gained through light absorption to the photosynthetic reaction centers via numerous dye molecules. Individual dye units, so-called chromophores, are supposed to ensure efficient energy transfer and are kept in spatial proximity by the surrounding protein shell.

Two balls connected by a spring

The absorption properties of these light-harvesting complexes differ significantly from the properties of the individual dye molecules. The so-called excitonic coupling between the dye molecules is responsible for this. The coupling can be illustrated with two balls hanging on a thread, which are connected to each other by a spring. If a ball is deflected from its rest position, this also has an effect on the second ball. At the molecular level, the displacement of the sphere corresponds to the excitation of a molecule through the absorption of light.

This phenomenon has been well researched for systems with the same dye molecules (homo-aggregates). In contrast, little is known about the coupling between different chromophores. New findings are now coming from the working group headed by Professor Frank Würthner, holder of the Chair for Organic Chemistry II at the University of Würzburg and head of the Center for Nanosystem Chemistry.

Stack of four made of dye molecules

Investigating the coupling between dye molecules requires aggregates in which the exact orientation of the dye units is known. Professor Würthner's employees have now succeeded in representing corresponding aggregates in the form of stacks of four chromophores. To do this, they used the merocyanine class of dyes, which, due to their strong dipolar properties, form well-defined aggregates. "By chemically linking two identical merocyanine chromophores via a naphthalene unit, we were able to generate a molecule that dimerizes in solution and thus forms stacks of four identical chromophores," explains David Bialas, doctoral student at Frank Würthner's chair and author of the Study.

The researchers then went one step further: They linked two different merocyanine chromophores, each with different absorption properties, in order to obtain corresponding hetero-aggregates, i.e. stacks of four with different chromophores.

"We were able to elucidate the structure of the dye stacks in solution with the help of nuclear magnetic resonance spectroscopy," reports Eva Kirchner, who also did her doctorate at the chair and was involved in the project. The X-ray structure analysis finally provided unequivocal proof of the existence of the four-pack. To do this, the team had to “grow” suitable crystals, which is rarely possible for dye aggregates.

Unexpected absorption properties

The spectroscopic examination of the absorption properties revealed the unexpected. “The results indicate an excitonic coupling between the dye molecules not only for the homoaggregate, but also for the heteroaggregate,” explains Bialas. Quantum mechanical calculations finally confirmed a strong excitonic coupling also between the different dye molecules in the hetero-aggregate. “This contradicts the widespread opinion that strong couplings are only possible between the same chromophores,” says the scientist.

Fast energy transfer

Excitonic coupling not only influences the absorption behavior of the dye aggregates, but also indicates a rapid energy transfer between the molecules. Researchers could take advantage of this in the future to use sunlight effectively. The use of different dye molecules makes it possible to cover a broad spectrum of absorption of sunlight and thus capture as much energy as possible and convert it into electricity or chemical energy, for example.