For the first time, scientists have achieved real-time in situ accurate monitoring of the chirality of single molecules

On May 15, 2023, Beijing time, Professor Guo Xuefeng’s team and collaborators of Peking University published a report entitled “Real-time monitoring of reaction stereochemistry through single-molecule observations of chirality-induced spin selectivity” in the journal Nature Chemistry “.

In this study, a carbon-based single-molecule spin valve was constructed for the first time, and the intrinsic physical mechanism of chiral-induced spin selectivity, which has been controversial for a long time, was elucidated. This mechanism was further used to monitor chiral changes in the chemical reaction in real time at single-molecule resolution.

The corresponding authors of the paper are Professor Guo Xuefeng of Peking University, Professor Yonatan Dubi of Ben Gurion University in Israel, and Professor Kendall N. Houk of the University of California, Los Angeles, and the first authors are Yang Chen and Li Yanwei.

Chirality is an inherent property of the three-dimensional configuration of molecules, which is the breaking of its spatial inversion symmetry. In nature, a pair of enantiomers exhibit completely different properties in life activities, chemical reactions and physical processes, so the accurate detection of chirality and the accurate regulation of asymmetric reaction paths are the consistent goals of chemists, and they are also the hot spots of research at home and abroad for decades. Existing chiral detection technology: column chromatography, thermodynamic measurement, etc., need to tailor “chiral aptamers”; Circular dichromatography, on the other hand, requires the target chiral molecule to have a specific chromophore and high purity. Under the background of high-throughput synthesis and precision chemistry, the sensitivity and universality of chiral detection need to be further improved. Single-molecule detection reaches the limit of analytical chemical sensitivity, while chiral-related information in ordinary single-molecule signals is often decomposed and difficult to resolve. Therefore, chiral measurement based on single-molecule detection is worth looking forward to, but there is still a long way to go.

Guo Xuefeng’s research group in the School of Chemistry and Molecular Engineering of Peking University has been engaged in the research of single-molecule science and technology for a long time, and has made breakthroughs in the development of universal methods for the preparation of stable single-molecule devices, constructed the world’s first stable and controllable single-molecule electronic switch, developed a unique technology for single-molecule detection, and opened up a new field of single-molecule cross-scientific research. In particular, graphene-based monomolecular junctions were constructed, and photochemical reactions were successively realized by electrical or photoelectric multimodal characterization methods according to the close relationship between molecular conductance and its structure (Science 2016, 352, 1443; Nat. Commun. 2019, 10, 1450), molecular conformational changes (Nano Lett. 2017, 17, 856; Matter 2021, 5,1224), Non-covalent Interactions (Sci. Adv. 2016, 2, e1601113; Nat. Commun. 2018, 9, 807; Sci. Adv. 2021,7,eabe4365; Chem 2022, 8, 243; Adv. Sci. 2022, 2200022), Basic Organic Reactions (Nano Lett. 2018, 18, 4156; Sci. Adv. 2018, 4, eaar2177; J. Am. Chem. Soc. 2022, 144, 3146; Sci. Adv. 2021, 7, eabf0689; Adv. Mater. 2022, 2204827), Metal-organic catalysis (Nat. Nanotechnol. 2021, 16, 1214; Nat. Commun. 2022, 13, 4552), organic small molecule catalysis (Matter 2021, 4, 2874), and the emergence of increased complexity of reaction scale (J. Am. Chem. Soc. 2023, 145, 6577), solvent interaction (JACS Au 2021, 1, 2271), etc. Based on these systematic studies, he was invited to write a review entitled “Reactions in Single-Molecule Junctions” (Nat. Rev. Mater. 2023, 8, 165)。 These studies have broken through the ensemble average, depicted dynamic and multidimensional single-molecule behavior images, and opened up a new direction for the visualization of single-molecule reaction kinetics, and were praised by relevant experts as the frontier research direction of “Chinese labels”.

Nevertheless, enantiomers differ little, and conductance is often degenerate and difficult to distinguish. Guo Xuefeng’s research group has anchored chiral cyclodextrin, realized chiral detection of a variety of amino acids through chiral-chiral-specific interaction, and established a “fingerprint” (Sci. Adv. 2021,7, eabe4365)。 However, the universality of detection still needs to be expanded, and the real-time tracking of chiral changes in the reaction process needs a breakthrough from “0” to “1”. Recently, they and their collaborators have used the intrinsic property of electrons in electrical detection, spin, combined with the universal chiral-induced spin selectivity (CISS) to achieve real-time in situ monitoring of asymmetric breaking during the reaction (Figure 1).

Figure 1: Schematic diagram of monitoring chiral changes in single molecules using the CISS effect.

CISS is the selective filtering effect of enantiomers on passing spin polarization currents, which makes chiral molecules show extraordinary potential in constructing spintronic devices. However, the intrinsic mechanism of this phenomenon has long been controversial. The mainstream view is that chiral molecules produce a huge “solenoid magnetic field”, but it is still difficult to explain the fact that the spin polarizability in the experiment is orders of magnitude higher than predicted by the theory. First, in order to elucidate the intrinsic mechanism of the CISS effect, Guo Xuefeng’s group replaced one side of the gold electrode of the traditional device with a magnetized ferromagnetic electrode to achieve spin injection, and designed the chiral reaction center (Maleimide’s Michael addition) outside the conductive channel (Figure 1b). The enantiomer of the product also showed a fairly high spin selectivity, indicating the successful construction of a carbon-based single-molecule spin valve (Figure 2), which is the first in the world. This experimental phenomenon also shows that the “solenoid magnetic field” of chiral small molecules only provides an initial but essential symmetry break, and the CISS effect can be attributed to the alignment of the surface magnetization direction (induced by spin orbit) in metal (Au) with the induction solenoid field caused by chiral molecules, that is, the spin filtering of the electrode, so that a fairly high spin polarizability can be obtained. In order to verify this mechanism, after obtaining the basic parameters of single-molecule devices based on Landauer’s equation fitting the I−V curve without magnetization, the current filtering effect under spin injection can be accurately simulated, which is basically consistent with the experiment (Figure 2), thereby clarifying the long-controversial CISS mechanism and laying a foundation for the further development of spintronics.

Figure 2: Experimental results and theoretical simulation of graphene-based single-molecule spin valve filtering different spin polarization currents by S-configuration products as chiral centers.

Based on this, they used graphene-based single-molecule spin valves to further monitor the Michael addition reaction in real time. Compared with conventional monitoring signals, the conductivity values of the product are demerged, and the enantiomers become distinguishable. By comparing it with the absolute conductance of the fixed configuration product, the absolute (S or R) configuration of the two conductance states can be accurately identified (Figure 3). Based on the visualization of the reaction trajectory, real-time in situ monitoring of chiral changes in the reaction can be realized, and the trend of symmetry breaking during the reaction can be clearly understood (Figure 3).

Figure 3: Real-time in situ monitoring of chiral changes in chemical reactions.

Different from the enantiomeric excess value of the product in macroscopic experiments, which needs to wait for a period of time before monitoring the enantiomer excess value of the product, this single-molecule technology can achieve strict synchronization of the signal and the molecular state, so as to directly observe the intermediate related to the chirality of the reaction coordinates and judge the stereoscopic configuration of a single product in real time. Therefore, a large amount of microscopic individual information that breaks through the ensemble average can be discovered in the complex behavior of (chiral) single molecules, including key latent chiral intermediates, symmetry-breaking reaction trajectories, stereoselective interactions between molecules, chiral transport and amplification mechanisms, and the evolution of asymmetric reaction characteristics over time at the molecular scale. This single-molecule chirality monitoring technology is based on the coupling relationship between molecular chirality and electron spin, and has universal applicability. The electrical platform for real-time monitoring of catalytic reactions provides new ideas and methods for monitoring the mechanism of asymmetric reactions, and provides unlimited possibilities for revealing new reaction processes and guiding macroscopic asymmetric synthesis.

The research was jointly funded by the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Beijing National Research Center for Molecular Sciences. (Source: Science Network)

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