High frequency molecular vibrations trigger an electron movement

Whether in solar cells or in the human eye, whenever certain molecules absorb light, electrons inside move from their fundamental state in a higher and excited state. This results in the transport of energy and load, leading to the separation of charges and possibly to the production of electricity.

An international team of scientists led by Dr. Antonietta de Sio and Professor Dr Christoph Lienau of the research group in ultra-fast nano-optics at the University of Oldenburg, Germany, has now observed the first stages of this process in a complex dye molecule. As researchers report in Nature chemistryHigh frequency vibrations of atomic nuclei in the molecule play a central role in this transfer of load induced by light.

Their experiences have shown that the forces that these vibrations exercise on electrons incorporate load transport, while the processes in the surrounding solvent, which were previously supposed to initiate the transfer of load, only begin a later stage.

“Our results provide new information for a better understanding of transport transport, for example in organic solar cells, and could contribute to the development of more effective materials,” said Sio.

The coloring coloring was synthesized by a group of researchers led by Professor Peter Bäuerle of the University of Ulm, also in Germany. This type of color molecule is the basic component of a plastic used in organic solar cells to convert sunlight into electricity.

“The molecules each consist of three units – a central unit linked to two identical groups, one on the right and one on the left side,” explains Sio.

The central unit of the molecule is an electron donor – a material that easily abandons electrons. The two external groups, on the other hand, can accept excited electrons. They are known as electron acceptors. During light excitement, the electrons can therefore, in theory, move to one of the two accepting units, the one on the right or the one on the left.

This process, known as the termination of symmetry in the excited state, produces a displacement characteristic of the color of the light emitted by the molecule – an effect called solvatochrism – the transforming from blue to red. However, the microscopic mechanism that triggers the initial rupture of symmetry was largely unknown so far.

The Oldenburg team has decided to take a closer look at the process of breaking symmetry. Doctoral students Katrin Winte and Somayeh Souri have used ultra-old laser spectroscopy techniques with a temporal resolution less than 10-10-femtosecond (a femtosecond is equal to a millionth of a billionth of a second) to excite color molecules. With this method, they were able to follow the movements of electrons and nuclei in the first thousand femtoseconds after light excitation.

Their experiences have shown that laser pulses trigger high frequency vibrations between the atoms of the coloring molecule during the first 50 femtoseconds after photoexcultation.

“Carbon atoms in the molecule begin to vibrate,” clarifies Sio.

These vibrations modify the energy states in the molecule, creating a favorite movement of movement for excited electrons. On the other hand, the molecules of the surrounding solvent environment seem to be “frozen” on this time scale – only on a slower time scale of several hundred femtoseconds do not reorganize and also stabilize the process of evacuation of symmetry so that the molecule settles in a new state, which produces the characteristic shift in the spectrum of colors emitted.

To confirm these unexpected results, the researchers repeated the experience with another solvent in which the solvatochromism – the interaction between the dye and the solvent – does not occur. Nevertheless, here too, the initial intramolecular vibrations have been observed.

Quantum chemical simulations carried out in collaboration with researchers from LOS Alamos National Laboratory in the United States and the University of Bremen in Germany supported experimental results.

“Our results provide convincing evidence of the dominant role of vibonic coupling to high-frequency molecular vibrations, and not to the fluctuations of solvents, such as the main engine of the rupture of ultra-fast symmetry in quadrupolar dyes”, explains Lienau, and adds that this mechanism can also apply to materials and nanostructures in the solid state.

“Control of load interaction with molecular vibrations and with the surrounding environment is essential for the technological applications of these materials,” said Sio. “As such, our results can have significant implications for the design of effective materials sensitive to light, as well as to advance our understanding of load transport induced by light in systems on a nanometric scale.”