Molecules in collective ecstasy

"What we see here is energy transfer that is much faster than in any semiconductor," enthuses Jakob Heier. The physicist works in Empa's Functional Polymers lab, and the discovery he has made with his team could cause a stir in many areas – such as sensor technology, optical data transmission or the fabrication of organic solar cells. We are talking about islands of dye molecules with a perfect, internal structure. Among experts, such structures are called J-aggregates. Although they have been known for more than 80 years, they recently attracted renewed attention in research. This is due to the special electronic inner life of these dye islands.

To understand what Heier and his colleagues have found, a short excursion into the world of dyes is helpful: 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 – which is why white garments shine so brightly in the UV light of a club. The emitted light is lower in energy than the light used to activate the dye, because part of the energy is converted into vibrations, i.e. heat, in the dye molecule.

The J-aggregates studied by Heier and Empa PhD student Surendra Anantharaman behave differently from individual dye molecules. In these molecular islands, the dye molecules are well ordered and very close together, much like matches in a box. In this constellation, the dye molecule does not "have" to glow, but "can" pass on its energy to a neighboring molecule.

Compared to classic semiconductors made of silicon, there is a crucial difference, though: In a silicon semiconductor, such as a solar cell, the excitation energy is transported via charge carriers, for example electrons, which "hop" through the material, so to speak. In J-aggregates, on the other hand, the electrons only oscillate back and forth in the dye molecule and never leave it. Instead of electrons, only oscillations are transmitted – similar to transmitting and receiving antennas in the macroscopic world. In fact, J-aggregates can "transmit" energy on the smallest scale – extremely fast and across hundreds of molecules.

The phenomenon of J-aggregates and their special energy transmission was first discovered as early as 1936 by Edwin E. Jelley in the US and Günter Scheibe in Germany. But up to now, about 95 percent of the radiated energy was lost and could not be transmitted. "Construction errors" in the system were to blame. In reality, the molecules were not so perfectly aligned. And whenever the energy pulse encountered one of these defects during its journey through the J-aggregate, the energy transport was interrupted. An ordinary molecular vibration terminated the transfer, a bit of heat was generated, and the game was over.

The study that has now been published 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 the Empa team. For example, it was not possible to crystallize the dye in the form of thin films on a solid surface. Too many defects in the crystals ruined the transfer. Aqueous solutions, in which the dye aggregates into tiny droplets, likewise do not work. Only bicontinuous emulsions lead to signal transmission – and only if there are individual dye molecules left in a liquid phase that can fill holes and close gaps in the J-aggregates – in other words, "repair" defects.