Recently, Professor Yu Miao's research group at Harbin Institute of Technology has made important progress in dealing with on-surface synthesis (OSS) of high-reactivity barriers. By designing the "lock-unlock" transformation of molecules on the substrate during surface synthesis, a new strategy is provided for reaction types containing relatively high-energy barriers applied to OSS. The research results were published in the internationally renowned academic journal anguaulte chemie international edition (German Applied Chemistry).
Surface synthesis has shown considerable prospects in the development and synthesis of nanoscale organic semiconductors as well as a new generation of electronic devices. Unlike the many reaction types that can be applied to liquid-phase synthesis, OSS is often limited to a few reaction types with lower energy barriers. When it comes to higher reaction energy barriers, oss are caught in a dilemma: in order to overcome the high barriers, the molecules are required to be locked on the surface to ensure that they still do not desorb at high thermal excitation temperatures, and have a long enough reaction time at the local site; but in order to facilitate the coupling and orderly assembly of molecules after bonding, it is necessary for the molecules to be able to effectively migrate on the surface. In addition, due to competition between chain and ring coupling forms, in most cases, it is difficult to control the structural homogeneity and long-range orderliness of the product.
In response to the above challenges, Professor Yu Miao's team proposed a new strategy, that is, the molecular precursor is firmly locked on the cu(111) surface to ensure a high yield reaction; after the bonding reaction occurs, the molecules are unlocked from the surface to promote coupling and assembly. Bcpm molecules with maleimide and bicarboxylic acid functional groups were employed (Figure 1). The transformation of the molecule "lock-unlock" on the surface of cu (111) is achieved by different adsorption configurations of maleimide functional groups before and after decarboxylation: before decarboxylation, maleimide groups rotate 35 degrees relative to the horizontal direction of the surface, and the oxygen and basal CU atoms on one side have significant charge transport, forming ion bonds, anchoring the molecules to the substrate; after decarboxylation, the maleimide groups rotate, and the oxygen atoms on both sides are far away from the surface, no longer generating charge transmission with the substrate, and unlocking is achieved. The change in this adsorption configuration is related to the rearrangement of intramolecular charges after the decarboxylation reaction: before the reaction, due to the strong absorption of electrons by the bicarboxylic acid functional group, the maleimide group tends to attract foreign aid electrons from the basement CU; after the reaction, the intramolecular charge is rearranged, and the maleimide group no longer needs foreign aid electrons. At the same time, the spatial steric hindrance of the maleimide functional group and the negative charge carried by it are used to achieve the selectivity of the coupling form. Through the above strategy, the high yield surface decarboxylation coupling reaction was realized, and the product was highly selectively hexa-membered cyclized, forming a two-dimensional long-range ordered assembly (Figure 2).
Figure 1. The bcpm molecule and its schematics of decarboxylation, formation of intermediates and hexacycles on the cu(111) surface
Figure 2. STM images and theoretical calculations of bcpm decarboxylation of hexagroup rings on cu(111) surfaces
Based on scanning tunneling microscopy (stm) results combined with density functional calculations and reflective infrared absorption spectra, the system has obvious advantages over the reported surface decarboxylation coupling: the molecular coverage remains unchanged throughout the reaction process; the decarboxylation temperature is more than 30 degrees lower than the reported reaction temperature; the reaction yield is higher than 85%; molecular assembly and reaction are independent of molecular coverage; the final product is long-range ordered; the hexamer ring structure is the only product, and the chain coupling form is completely suppressed. This work opens up an important way to improve the utility of OSS when it comes to high-response barriers, as well as to achieve structural homogeneity and long-range order of the final product.
The research work was supported by the National Natural Science Foundation of China (21473045, 51772066). Professor Yu Miao's research group has long been engaged in the design and performance research of functional carbon-based materials, and has made breakthrough achievements in single atomic layer organic semiconductors, precision functionalization of photographic graphene, seawater desalination and energy battery applications. The results were published in Nature Chemistry, Nature Communications, j. am. chem. soc.、angew. chem. int. ed.、energy environ. Journals such as sci., acs nano, nano lett., nano energy, etc.
on-surface decarboxylation coupling facilitated by lock-to-unlock variation of molecules upon the reaction
shaoshan wang, zhuo li, pengcheng ding, cristina mattioli, wujun huang, yang wang, andré gourdon, ye sun, mingshu chen, lev kantorovich, xueming yang, federico rosei, miao yu
angew. chem. int. ed., 2021, doi: 10.1002/anie.202106477
Instructor introduction
Yu Miao
https://www.x-mol.com/university/faculty/49969
Research group links
http://homepage.hit.edu.cn/miaoyu