High-frequency molecular vibrations initiate electron movement

Whether in solar cells or in the human eye: whenever certain molecules absorb light, the electrons within them shift from their ground state into a higher energy, excited state. This results in the transport of energy and charge, leading to charge separation and eventually to the generation of electricity. An international team of scientists led by Dr Antonietta De Sio and Prof. Dr. Christoph Lienau from the Ultrafast Nano-Optics research group at the University of Oldenburg, Germany, has now observed the earliest steps of this process in a complex dye molecule. As the researchers report in the scientific journal Nature Chemistry, high-frequency vibrations of atomic nuclei within the molecule play a central role in this light-induced charge transfer. Their experiments showed that the forces that these vibrations exert on electrons initiate charge transport, whereas processes in the surrounding solvent, which were previously assumed to initiate charge transfer, begin only at a later stage. "Our findings provide new insights for a better understanding of charge transport, for example in organic solar cells, and could contribute to the development of more efficient materials," De Sio underlines.

The dye under investigation was synthesised by a group of researchers led by Prof. Dr Peter Bäuerle from the University of Ulm, also in Germany. This type of dye 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 core unit connected to two identical groups, one on the right and one on the left side," De Sio explains. The core unit of the molecule is an electron donor – a material that readily gives up electrons. The two outer groups in contrast can accept excited electrons. They are known as electron acceptors. Upon light excitation, electrons can therefore, in theory, move to either of the two accepting units, the one on the right or the one on the left. This process, known as excited-state symmetry breaking, produces a characteristic shift in the colour of the light emitted by the molecule – an effect called solvatochromism – turning it from blue to red. However, the microscopic mechanism that triggers the initial symmetry breaking was largely unknown up to now.

The Oldenburg team decided to take a closer look at the symmetry-breaking process. Doctoral students Katrin Winte and Somayeh Souri used ultrafast laser spectroscopy techniques with sub-10-femtosecond time resolution (one femtosecond is equal to one millionth of a billionth of a second) to excite the dye molecules. With this method, they were able to track the movements of the electrons and nuclei in the first thousand femtoseconds after light excitation.

Their experiments showed that laser pulses trigger high-frequency vibrations between the atoms of the dye molecule during the first 50 femtoseconds after photoexcitation. "The carbon atoms within the molecule begin to vibrate," De Sio clarifies. These vibrations change the energy states within the molecule, creating a preferred direction of movement for the excited electrons. By contrast, the molecules of the surrounding solvent environment appear to be "frozen" on this timescale – only on a slower timescale of several hundred femtoseconds do they also reorganise and stabilise the symmetry-breaking process so that the molecule settles into a new state, which produces the characteristic shift in the emitted colour spectrum.

To confirm these unexpected results, the researchers repeated the experiment with another solvent in which solvatochromism – the interaction between dye and solvent – does not occur. Nevertheless, here too, the initial intramolecular vibrations were observed. Quantum chemical simulations carried out in collaboration with researchers from the Los Alamos National Laboratory in the US and the University of Bremen in Germany supported the experimental results.

"Our findings provide compelling evidence for the dominant role of vibronic coupling to high-frequency molecular vibrations, and not solvent fluctuations, as the primary driver of ultrafast symmetry breaking in quadrupolar dyes," Lienau explains, and adds that this mechanism may also apply to solid-state materials and nanostructures. "Controlling the interaction of charges with molecular vibrations and with the surrounding environment is critical for technological applications of these materials," De Sio points out. "As such, our results may have significant implications for the design of efficient light-responsive materials, as well as for advancing our understanding of light-induced charge transport in nanoscale systems."

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