CHEMICAL SCIENCE

Photoisomerization of stilbene is highly coherent in both gas-liquid phases


On August 11, 2022, the team of Professor Hans Jakob Wörner of the Swiss Federal Polytechnic University in Zurich and the team of Professor Petr Slavícek of the University of Chemical Engineering in Prague, Czech Republic, published an article at Nature Chemistry entitled “Different timescales during ultrafast stilbene isomerization in the gas and liquid.” phases revealed using time-resolved photoelectron spectroscopy” research results.

This result reports the comparative study of the photoisomerization process of gas phase and liquid phase distyrene (Stilbene) molecules, finds that isomerization is a highly coherent process in the case of gas and liquid phases, and supports that the “friction effect” between liquid phase molecules is part of the reason for the significantly longer time scale of the isomerization process, which provides a new idea for understanding the photochemical reaction process in the liquid environment in the future.

The correspondence is Pengju Zhang, Petr Slavícek, Hans Jakob Wörner; The first authors are Wang Chuncheng, Max Water, and Zhang Pengju.

The photoisomerization process around the carbon double bond is an important mechanism for the human eye to sense light. Vision is one of the main human perceptions, so it is required that this isomerization process must be reversible and capable of rapid and repeatable throughout a human lifetime. These factors mean that such chemical reactions are highly directional and coherent, with coordinated molecular motion driving excited molecules back to ground state along predictable reaction paths. In order to be able to efficiently de-excite molecules from the electron excited state to the electron ground state, a strong coupling of electron motion and nuclear motion is required, thereby converting the potential energy introduced during light absorption into kinetic energy, which is detected experimentally. This strong coupling leads to the formation of conical intersections between different electronic states, so that the layout of the excited state can be coherent and fast transmission between different electronic states.

For the photoisomerization process of styrene, a lot of research work has been carried out, and the reaction path of gas phase molecules has been clearly understood. However, we know that most chemical reactions occur in the liquid phase environment, so it becomes necessary to discuss the effects of the liquid environment on photochemical reactions by comparing the molecular isomerization processes of the gas-liquid phases based on the same observable measurements.

Figure 1: Schematic of the protocol.

In this work, Professor Wörner’s team generated extreme ultraviolet laser pulses based on table-top high-order harmonic generation techniques, and for gas experiments, excitated stilbene molecules using 266 nm of ultraviolet light (UV), combined with a photoelectron velocity imaging spectrometer, and recorded their time-resolved photoelectron spectra (Figure 2); For liquid experiments, combined with liquid microbeam target transport technology, liquid stilbene molecules were excited by 266 nm of ultraviolet light, and their time-resolved photoelectron spectra were recorded by high-resolution photoelectron spectrometer (Figure 3). Professor Petr Slavícek of the University of Chemical Technology in Bragg used non-adiabatic molecular dynamics methods to simulate the time-resolved photoelectron spectra of gases and liquids respectively, which provided a strong theoretical support for the experimental results.

Figure 2: Time-resolved photoelectron spectra of gas-phase stilbene.

Experimental and theoretical studies have shown that the π→π* transition from the ground state to the first excited state is achieved by excitation of the stilbene molecule by UV light, thereby changing the hybridization characteristics of the alkene bond carbon atoms. The excitation process causes the hydrogen atoms inside the molecule to be turned on in an off-plane motion mode, an ultrafast process that causes the carbon atoms of the alkene bond to make a conical motion, thereby de-exciting the entire system to a flat area of the potential energy surface (Figure 5p*). In this flat area, the system begins to rotate on the axis of the alkene bond. The combination of the above two motions promotes the entire chemical reaction to pass through the cone-shaped cross-region of the potential energy surface, and transmits the layout of the excited state from the first excited state coherent to the ground state, completing the isomerization process.

Figure 3: Time-resolved photoelectron spectra of liquid phase distyrene.

Changes in the probability of ionization due to nuclear motion, as well as changes in vertical ionization energy during the evolution of excited molecules, have been observed experimentally. More importantly, through the analysis of the Fourier transform of the experimental results (Figure 4), the characteristic frequencies associated with the torsional motion of the alkene bond were experimentally confirmed, and the characteristic frequencies of the liquid phase were redshifted. This suggests that the torsional motion becomes slower in the liquid phase environment, and given the large movement of the benzene ring, the authors propose that the “friction effect” of the liquid phase environment is the hypothesis that causes the chemical reaction to slow down. Theoretical calculations have better reduced the experimental results of the liquid phase by considering a time-dependent coefficient of friction in the molecular dynamics simulation, thus supporting the deceleration of the solvation effect on the large movement of molecules as a possible cause of slow chemical reactions (Figure 3). Despite the “friction effect”, the study shows that the isomerization process maintains a high degree of coherence in the liquid phase environment, which partly explains why the apparent purpurulite is naturally selected as an important component involved in the visual process. At the same time, under the same experimental conditions, the “one-to-one” study of the excitation state dynamics of gas and liquid molecules using time-resolved photoelectron spectroscopy provides a new idea for people to understand the effect of solvation effect on photochemical reactions.

Figure 4: Fourier transform analysis results of time-resolved photoelectron spectra.

Figure 5: Schematic diagram of the excited state dynamics process.

(Source: Science Network)

Related paper information:https://doi.org/10.1038/s41557-022-01012-0



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