We often face the “difficulty of choice”. For example, when you graduate, you choose to continue your studies or go to social job hunting, such as choosing a stable investment method when investing, or investing in a risky but potentially profitable investment. . In short, the two ends of the choice always have advantages and disadvantages, need to be weighed, and break the stable balance.
Today we will explore the “choice” problem at the atomic and molecular levels.
How can I “see” atoms and molecules?
We know that atoms are the basic elements of the material world, and atoms can form molecules through the action of chemical bonds. The radius of an atom is generally on the order of 10 -11 ~ 10 -10 m, while the resolution of the human eye can only reach 10 -4 m. These fine structures are indistinguishable by the human eye.
To this end, scientists have invented scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Scanning tunneling microscopy, as its name implies, uses quantum tunneling effects to represent the atomic properties of the surface of matter, while atomic force microscopy utilizes the forces between atoms.
With the rapid development of computer performance, scientists also use “first-principles calculations” to study the basic properties of complex systems. The combination of scanning tunneling microscopy, atomic force microscopy and first-principles calculations has become a powerful tool for scientists to study surface physics and chemical processes at the atomic and molecular levels.
Activation of organic molecules “choice”
The controlled selective functionalization of organic molecules is important for achieving accurate construction of low-dimensional materials at the atomic scale. An activating molecule refers to an active state in which a molecule is converted from a ground state to a chemical reaction. A chemical reaction occurs when the activated molecules collide with each other.
The energy required to activate different functional groups in the molecule is different. For similar groups in the molecule, the activation conditions required to activate them are almost identical. These similar groups are distributed at different positions in the molecule, and any one of them is activated and a chemical reaction occurs, often resulting in products of distinct properties. How to activate one of the groups in a targeted manner and then obtain the preset product is a historical problem in traditional chemical synthesis.
For a symmetrical molecule, the group at the equivalent position is identical, so it is called an equivalent group. Selective activation of these equivalent groups appears to be an impossible task. Since the traditional chemical synthesis method can’t be realized, it is better to change the road.
Surface synthesis is a synthetic method that has received much attention in recent years. Using the catalytic and confinement effects of the surface of the metal single crystal, a large number of low-dimensional functional nanomaterials that cannot be synthesized by conventional wet chemical methods are synthesized. So, can the metal surface be selectively activated by equivalent groups in organic molecules?
Use copper to “force” symmetric molecules to make “choices”
Recently, scientists from the Institute of Physics of the Chinese Academy of Sciences, Suzhou University, and Giessen University in Germany have experimented with activation “selection” in a symmetric molecule. This symmetrical molecule is a 4,4″ diamino-p-terphenyl (DATP) molecule which grows like this.
The scientists placed the DATP molecule on the surface of copper Cu (111) and found that the symmetry breaks when DATP molecules are adsorbed on the surface of Cu(111). At this time, the two amino groups equivalent to each other in the DATP molecule exhibit structural and electronic properties, and it is possible to selectively activate one of them.
When the mirror-symmetric DATP molecule adsorbs on the surface of Cu(111), the long axis of the molecule along the direction of [11-2] and its equivalent crystal orientation, the end of the molecule exhibits a fuzzy characteristic, and the mirror symmetry of the molecule is broken, so that the two ends are Amino groups are not equivalent .
DATP molecules are lattice mismatched with Cu(111). It can be understood that Dad is holding a child to walk. The length of the DATP molecule is like the step size of Dad, and the lattice of Cu is the step size of the children. The steps of the two people are big and small, and they cannot always match.
This lattice mismatch allows two equivalent amino functional groups in the DATP molecule to be adsorbed at different positions on the copper surface, one end adsorbed at the top of the copper atom and the other end adsorbed at the hollow position of the surface Cu atom (fcc) .
At the same time, the scientists found that the amino group adsorbed at the top is closer to the copper surface than the other end, and the interaction with the surface is stronger, so the activation is stronger, so that it is under the influence of the tip. Non-steady state features, and preferentially hydrogen bonding with 2-triphenylene oxide (TPCA) molecules .
The molecular asymmetric adsorption caused by lattice mismatch of DATP molecules and Cu(111) surface and the enhanced activity of specific functional groups provide a widely used approach for the selective functionalization of symmetric molecular equivalent groups, on the surface. The asymmetric chemical reaction provides a new idea and allows us to have a better understanding and understanding at the atomic and molecular levels.