Controlling Reactivity-Real-Space Imaging of a Surface Metal Carbene

Increased Selectivity in Photolytic Activation of Nanoassemblies Compared to Thermal Activation in On-Surface Ullmann Coupling

On-surface synthesis is a powerful method that has emerged recently to fabricate a large variety of atomically precise nanomaterials on surfaces based on polymerization. It is very successful for thermally activated reactions within the framework of heterogeneous catalysis. As a result, it often lacks selectivity. We propose to use selective activation of specific bonds as a crucial ingredient to synthesize desired molecules with high selectivity. In this approach, thermally nonaccessible products are expected to arise in photolytically activated on-surface reactions with high selectivity. We demonstrate for assembled 2,2'-dibromo biphenyl clusters on Cu(111) that the thermal and photolytic activations yield distinctly different products, combining submolecular resolution of individual product molecules in real-space imaging by scanning tunneling microscopy with chemical identification in X-ray photoelectron spectroscopy and supported by ab initio calculations. The photolytically activated Ullmann coupling of 2,2'-dibromo biphenyl is highly selective, with only one identified product. It starkly contrasts the thermal reaction, which yields various products because alternate pathways are activated at the reaction temperature. Our study extends on-surface synthesis to a directed formation of thermally inaccessible products by direct bond activation. It promises tailored reactions of nanomaterials within the framework of on-surface synthesis based on the photolytic activation of specific bonds.

Light-Induced Increase of the Local Molecular Coverage on a Surface

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.

On-Surface Synthesis and Real-Space Visualization of Aromatic P3 N3

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P3 N3 ), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P3 N3 Cl6 precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D3h -symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P3 N21 via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.

Chirality control of a single carbene molecule by tip-induced van der Waals interactions

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification.

C-C Coupling of Carbene Molecules on a Metal Surface in the Presence of Water

A novel surface-confined C-C coupling reaction involving two carbene molecules and a water molecule was studied by scanning tunneling microscopy in real space. Carbene fluorenylidene was generated from diazofluorene in the presence of water on a silver surface. While in the absence of water, fluorenylidene covalently binds to the surface to form a surface metal carbene, and water can effectively compete with the silver surface in reacting with the carbene. Water molecules in direct contact with fluorenylidene protonate the carbene to form the fluorenyl cation before the carbene can bind to the surface. In contrast, the surface metal carbene does not react with water. The fluorenyl cation is highly electrophilic and draws electrons from the metal surface to generate the fluorenyl radical which is mobile on the surface at cryogenic temperatures. The final step in this reaction sequence is the reaction of the radical with a remaining fluorenylidene molecule or with diazofluorene to produce the C-C coupling product. Both a water molecule and the metal surface are essential for the consecutive proton and electron transfer followed by C-C coupling. This C-C coupling reaction is unprecedented in solution chemistry.