Molecules in collective ecstasy

When fluorescent dye molecules nestle together perfectly, something completely new arises: an excited state distributed over many molecules. Such “collective stimuli” can be used in many ways – for example for organic solar panels, in sensors, for ultra-fast data transmission or in microscopy. Together with colleagues from ETH Zurich, EPFL, the Paul Scherrer Institute (PSI) and IBM Research Zurich, Empa researchers have succeeded in making such chemical light amplifiers ten times more efficient than before.

“What we see here is an energy transfer that takes place much faster than in any semiconductor,” enthuses Jakob Heier. The physicist works in Empa’s “Functional Polymers” department, and the discovery he and his team made could bring movement in many areas – such as sensor technology, optical data transmission or the manufacture of organic solar cells. We are talking about islands of dye molecules with a perfect internal structure. In the professional world, such structures are called J-aggregates. Although they have been known for more than 80 years, they have recently received special attention in research. This is due to the special electronic interior of these dye islands.
To understand what Heier and his colleagues have found, a short excursion into the world of dyes helps: if a dye is to glow, the molecule must first be activated – also with light. Optical brighteners in detergents, for example, absorb UV light and emit bluish (visible) light – this is why white clothes shine so brightly blue in the UV light of a discotheque. The light emitted is lower in energy than the light emitted, because part of the energy is converted into vibrations, i.e. heat, in the dye molecule.

Molecules as energy antennas

The J-aggregates studied by Heier and Empa PhD student Surendra Anantharaman behave differently than individual color molecules. In these molecular islands, the dye molecules are well sorted, close to one another, similar to matches in a box.

The dye molecule “does not have to” glow in this constellation, but “can” pass on its energy to a neighboring molecule. But there is one crucial difference compared to “classic” silicon semiconductors: In a silicon semiconductor, such as a solar cell, the excitation energy is transported via charge carriers, for example electrons, which to a certain extent “hop” through the material. In J-aggregates, on the other hand, the electrons only oscillate back and forth in the dye molecule and never leave it. Instead of entire electrons, only oscillations are transmitted – similar to the case of transmitting and receiving antennas in the macroscopic world. In fact, J-aggregates can “send” energy on a very small scale – extremely quickly and across hundreds of molecules.

Heavy losses for 80 years

The phenomenon of J-aggregates and their special energy transfer was discovered in 1936 by Edwin E. Jelley in the USA and Günter Scheibe in Germany, independently of one another. But so far around 95 percent of the radiated energy has been lost and could not be passed on. The fault were “construction errors” in the system. In reality, the molecules weren’t so perfectly lined up. And whenever the energy pulse hit one of these defects on its journey through the J-unit, the transport process was interrupted. An ordinary molecular oscillation stopped the transmission, a little heat was created, and it was game over.

The perfect antenna forest

With the support of researchers from ETH Zurich, EPF Lausanne, PSI and IBM Research Zurich, the Empa team has now succeeded in developing a dye system in which up to 60 percent of the incident light is emitted as light. However, this also means that up to 60 percent of the energy can be passed on without loss – compared to the previously possible five percent, this is a sensation. The key to success were perfectly built dye islands, which were created in a fine emulsion of water and hexylamine. An emulsion is a mixture of liquid droplets in another liquid – milk or mayonnaise are emulsions that everyone knows.
The Empa researchers observed that not every emulsion worked: it had to be a so-called bicontinuous emulsion, which means that the droplets floating in the outer liquid must not be separated from one another, but must have united to form streak-shaped structures. Only then does the examined dye form the desired, error-free J-aggregates and can “send” the absorbed energy over long distances without loss.

Dye molecules are lined up in a bicontinuous emulsion – similar to matches in a box. Only then does the signal transmission succeed.

Failures are part of it

The now published study also mentions – in good scientific tradition – the failed attempts and the history of the successful experiment. After all, chemists and physicists all over the world should be able to build on the experience of Empa researchers. For example, it did not achieve the goal of crystallizing the dye in the form of thin layers on a solid surface. Too many defects in the crystals destroyed the transmission. Even aqueous solutions, in which the dye collects into tiny droplets, do not work. Only the bicontinuous emulsions lead to signal transmission – and only if there are individual dye molecules left in a liquid phase that fill holes and close gaps in the J-aggregates – that is, can «repair» defects.

Which applications are conceivable?

There is definitely still a long way to go before the researchers can make technical use of what has now been achieved in an emulsion. But then the signal transmission through dyes could penetrate many areas of everyday life. For example, it is possible to capture weak infrared light with the help of these dyes and convert it into digital signals with the help of quantum dots – an advantage for sensors or for solar cells that are supposed to deliver electricity even in very weak light. Due to their unique properties, J-aggregates are also suitable for applications in quantum computers and in optical data transmission.

Finally, the signal-conducting dye aggregates could be useful in diagnostics in living tissue: Infrared light, i.e. thermal radiation, penetrates deep into human tissue without damaging the cells. J-aggregates could make this radiation visible and digitize it. This could significantly facilitate and improve high-resolution microscope images in living tissue.

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