Chemical reactions can be induced on surfaces to obtain novel products that cannot be synthesized using conventional methods. Writing in Nature Communications, an international team reports the first on-surface reaction that involves multiple steps, leading to the formation of new, complex products owing to subsequent thermal annealing.
On-surface synthesis, unlike synthesis in solution, can be used to react non-soluble molecules; thus, the production of new compounds is possible. To date, several reactions have been demonstrated, but low yields and an incomplete understanding of the underlying processes are drawbacks that still need to be addressed. Moreover, to obtain complex products, multistep reactions — which are common in solution-based chemistry — have to be developed.
The researchers deposit a precursor molecule on a Cu(111) surface in ultrahigh vacuum and induce reactions by thermal annealing. They use scanning probe techniques — in particular, high-resolution atomic force microscopy, which reveals atomic-scale details of the molecular structures — to directly identify the reaction products after each annealing step. This information, combined with density functional theory calculations, enables them to follow the different stages of the reaction in great detail.
The organic precursor of choice is the planar dehydrobenzo annulene, which is composed of benzene rings and acetylene moieties. Owing to the high reactivity of the Cu(111) surface, following deposition at −123 °C, an initial reaction occurs in which two triple bonds are transformed to double bonds, each incorporating one H atom (even in ultrahigh vacuum, H atoms are always present on the surface). After annealing at 200 °C, two more H atoms are added and a structure composed of organic hexagons and pentagons — which has never been synthesized before — is obtained. The reaction is energetically favorable, such that it occurs even without annealing if the sample is kept at room temperature for several hours. No side products are observed, testifying to the high selectivity of the process.
“On-surface chemical reactions can produce new molecules, which we can expect to exhibit new functionalities”
Final annealing at 400 °C results in the cleavage of two H atoms and the formation of a new C–C bond, leading to another compound that has never been observed before. External energy is required for this transformation to occur; thus, this final step cannot happen without annealing. All the isolated molecules initially present on the surface react to form the final molecule, which can, in principle, be collected by thermal desorption. “By employing a precursor with a confined molecular backbone, we obtained an extremely high selectivity for the final product. This is a very important finding, because the purification of molecules cannot be carried out on surfaces” notes Shigeki Kawai, first author of the study.
Any compound can be used for on-surface chemistry, which should enable the synthesis of very complex products. “On-surface chemical reactions can produce new molecules, which we can expect to exhibit new functionalities,” explains Kawai. “We are focusing on the fundamental science, but the synthesis of nanostructures useful for applications can be foreseen in a near future”.
Kawai, S. et al. Thermal control of sequential on-surface transformation of a hydrocarbon molecule on a copper surface. Nat. Commun. 7, 12711 (2016)
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Pacchioni, G. On-surface chemistry: Some like it hot. Nat Rev Mater 1, 16081 (2016). https://doi.org/10.1038/natrevmats.2016.81