Imaging the Solvation of a One-Dimensional Solid on the Molecular Scale

Tuning Surface Organic Structures by Small Gas Molecules through Catassembly and Coassembly

The field of molecular assembly has seen remarkable advancements across various domains, such as materials science, nanotechnology, and biomedicine. Small gas molecules serve as pivotal modulators, capable of altering the architecture of assemblies via tuning a spectrum of intermolecular forces including hydrogen bonding, dipole-dipole interactions, and metal coordination. Surface techniques, notably scanning tunneling microscopy and atomic force microscopy, have proven instrumental in dissecting the structural metamorphosis and characteristic features of these assemblies at an unparalleled single-molecule resolution. Recent research has spotlighted two innovative approaches for modulating surface molecular assemblies with the aid of small gas molecules: "catassembly" and "coassembly". This Perspective delves into these methodologies through the lens of varying molecular interaction types. The strategies discussed here for regulating molecular assembly structures using small gas molecules can aid in understanding various complex assembly processes and structures and provide guidance for the further fabrication of complex surface structures.

The structure-giving role of Rb+ ions for water-ice nanoislands supported on Cu(111)

We characterize the effect of rubidium ions on water-ice nanoislands in terms of area, fractal dimension, and apparent height by low-temperature scanning tunneling microscopy. Water nanoislands on the pristine Cu(111) surface are compared to those at similar coverage on a Rb+ pre-covered Cu(111) surface to reveal the structure-giving effect of Rb+. The presence of Rb+ induces changes in the island shape, and hence, the water network, without affecting the nanoisland volume. The broad area distribution shifts to larger values while the height decreases from three bilayers to one or two bilayers. The nanoislands on the Rb+ pre-covered surface are also more compact, reflected in a shift in the fractal dimension distribution. We relate the changes to a weakening of the hydrogen-bond network by Rb+.

The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.

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.

Hydration at Highly Crowded Interfaces

Understanding the molecular and electronic structure of solvated ions at surfaces requires an analysis of the interactions between the surface, the ions, and the solvent environment on equal footing. Here, we tackle this challenge by exploring the initial stages of Cs^{+} hydration on a Cu(111) surface by combining experiment and theory. Remarkably, we observe "inside-out" solvation of Cs^{+} ions, i.e., their preferential location at the perimeter of the water clusters on the metal surface. In addition, water-Cs complexes containing multiple Cs^{+} ions are observed to form at these surfaces. Established models based on maximum ion-water coordination and conventional solvation models cannot account for this situation, and the complex interplay of microscopic interactions is the key to a fundamental understanding.

Surface Diffusion Aided by a Chirality Change of Self-Assembled Oligomers under 2D Confinement

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures.

Local Chiral Inversion of Thymine Dimers by Manipulating Single Water Molecules

Water, as one of the most important and indispensable small molecules in vivo, plays a crucial role in driving biological self-assembly processes. Real-space detection and identification of water-induced organic structures and further capture of dynamic dehydration processes are important yet challenging, which would help to reveal the cooperation and competition mechanisms among water-involved noncovalent interactions. Herein, introduction of water molecules onto the self-assembled thymine (T) structures under ultrahigh vacuum (UHV) conditions results in the hydration of hydrogen-bonded T dimers forming a well-ordered water-involved T structure. Reversibly, a local dehydration process is achieved by in situ scanning tunneling microscopy (STM) manipulation on single water molecules, where the adjacent T dimers connected with water molecules undergo a local chiral inversion process with the hydrogen-bonding configuration preserved. Such a strategy enables real-space identification and detection of the interactions between water and organic molecules, which may also shed light on the understanding of biologically relevant self-assembly processes driven by water.

Micro- vs Macrosolvation in Reichardt's Dyes

Solvation is a complex phenomenon involving electrostatic and van der Waals forces as well as chemically more specific effects such as hydrogen bonding. To disentangle global solvent effects (macrosolvation) from local solvent effects (microsolvation), we studied the UV-vis and IR spectra of a solvatochromic pyridinium-N-phenolate dye (a derivative of Reichardt's dye) in rare gas matrices, in mixtures of argon and water, and in water ice. The π-π* transition of the betaine dye in the visible region and its C-O stretching vibration in the IR region are highly sensitive to solvent effects. By annealing argon matrices of the betaine dye doped with low concentrations of water, we were able to synthesize 1:1 water-dye complexes. Formation of hydrogen-bonded complexes leads to small shifts of the π-π* transition only, as long as the global polarity of the matrix environment does not change. In contrast, changes of the global polarity result in large spectral band shifts. Hydrogen-bonded complexes of the betaine dye are more sensitive to global polarity changes than the dye itself, explaining why E_T values determined with Reichardt's dyes are very different for protic and nonprotic solvents, even if the relative permittivities of these solvents are similar.

Forming electron traps deactivates self-assembled crystalline organic nanosheets toward photocatalytic overall water splitting

Most biological photoredox reactions occur in sophisticated molecular assemblies consisting of highly organized light-harvesting moieties and catalytic centers. Mimicking these prototypes by creating supramolecular assemblies could be a potentially viable approach toward artificial photosynthesis. Although self-assembled organic materials are known to carry out water splitting reactions, developing self-assembled organic materials for photocatalytic overall water splitting still remains a critical challenge. Herein, we first demonstrate that crystalline organic nanosheets assembled from linear oligo(phenylene butadiynylene) (OPB) are able to catalyze overall water splitting under visible light irradiation. Further investigations reveal that the photocatalytic activity of self-assembled organic structures is closely related to the crystalline structure along with the corresponding electronic structure. Structural disorders in OPB nanosheets and extrinsic factors such as adsorbed water molecules will induce the formation of electron traps which can make the OPB nanosheets thermodynamically unfavorable for photocatalytic overall water splitting. The deactivation mechanism unveiled in this study provides crucial insights into the assembling of artificial organic materials for future solar-to-chemical energy conversion.

Hydration of a 2D Supramolecular Assembly: Bitartrate on Cu(110)

Hydration layers play a key role in many technical and biological systems, but our understanding of these structures remains very limited. Here, we investigate the molecular processes driving hydration of a chiral metal–organic surface, bitartrate on Cu(110), which consists of hydrogen-bonded bitartrate rows separated by exposed Cu. Initially water decorates the metal channels, hydrogen bonding to the exposed O ligands that bind bitartrate to Cu, but does not wet the bitartrate rows. At higher temperature, water inserts into the structure, breaks the existing intermolecular hydrogen bonds, and changes the adsorption site and footprint. Calculations show this process is driven by the creation of stable adsorption sites between the carboxylate ligands, to allow hydration of O–Cu ligands within the interior of the structure. This work suggests that hydration of polar metal–adsorbate ligands will be a dominant factor in many systems during surface hydration or self-assembly from solution.

Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene

The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the "up" position results in the singlet state and switching into the "down" position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.