Recent advances in organic molecule reactions on metal surfaces.

Chemical reactions of organic molecules on metal surfaces have been intensively investigated in the past decades, where metals play the role of catalysts in many cases. In this review, first, we summarize recent works on spatial molecules, small H2O, O2, CO, CO2 molecules, and the molecules carrying silicon groups as the new trends of molecular candidates for on-surface chemistry applications. Then, we introduce spectroscopy and DFT study advances in on-surface reactions. Especially, in situ spectroscopy technologies, such as electron spectroscopy, force spectroscopy, X-ray photoemission spectroscopy, STM-induced luminescence, tip-enhanced Raman spectroscopy, temperature-programmed desorption spectroscopy, and infrared reflection adsorption spectroscopy, are important to confirm the occurrence of organic reactions and analyze the products. To understand the underlying mechanism, the DFT study provides detailed information about reaction pathways, conformational evolution, and organometallic intermediates. Usually, STM/nc-AFM topological images, in situ spectroscopy data, and DFT studies are combined to describe the mechanism behind on-surface organic reactions.

Characterize and Mediate Assembly of Triptycenes on Au(111) Surface.

Herein, we report the assembly behavior of triptycenes with aldehyde (Trip-1) and amino (Trip-2) groups on pristine and iodine-passivated Au(111) surfaces by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and density functional theory (DFT) calculation. On Au(111) surface, Trip-1 forms long trimer chains and two-dimensional islands via aldehyde-aldehyde hydrogen bonding in one dimension and π-π stacking of adjacent benzene rings in the other dimension. In contrast, Trip-2 lies as individuals or in disorderly stacked islands. Trip-2 and Trip-1 can be mixed in an arbitrary ratio. And Trip-2 molecules disrupt the ordered self-assembly structure of Trip-1 due to the formation of stronger aldehyde-amino hydrogen bonding. DFT, XPS, and Raman spectra confirm the conformational difference of Trip-1 and -2, as well as the aldehyde-amino hydrogen bonding formation in Trip-1 and Trip-2 mixture. On the iodine-passivated Au(111) surface, Trip-1 forms single-molecule chains and a hexagonal closely packed structure due to iodine interlayer mediation. Trip-2 molecules disrupt the hexagonal closely packed structure of Trip-1.

Metal Atoms Participate in the Self-Assembly and On-Surface Reaction Behaviors of 1,4-DBN on Ag(111) Surface.

Herein, using 1,4-dibromonaphthalene (1,4-DBN) as the precursor molecule and Ag(111) surface as the substrate, we have characterized the various coordination and covalent structures formed by 1,4-DBN by low-temperature scanning tunnelling microscopy. We observed that there are three ordered structures (phase I, II, III) and one metal-organic short-chain structure (phase IV) at high coverage, meanwhile a new type of chiral structure (phase V) is observed coexisting with phase II, III, IV at low coverage. Surprisingly, all these structures have surface Ag adatoms incorporated. In addition, the phase III should be formed by a dissymmetric dehalogenation reaction of 1,4-DBN. Furthermore, we showed that the Ullmann coupling and cyclodehydrogenation of 1,4-DBN to form the armchair-shaped graphene nanoribbons will occur after thermal annealing. Combining the experiment data and density functional theory simulations, our results show that the surface Ag adatoms play a critical role in both the self-assembly and the on-surface reaction.

Surface Characterization of the Solution-Processed Organic-Inorganic Hybrid Perovskite Thin Films.

The surface properties of organic-inorganic hybrid perovskites can strongly affect the efficiency and stability of corresponding devices. Even though different surface passivation methods are developed, the microscopic structures of solution-processed perovskite film surfaces are not systematically studied. This study uses low-temperature scanning tunneling microscopy to study the organic-inorganic hybrid perovskite thin films, MA0.4 FA0.6 PbI3 and MAPbI3 , synthesized by the spin-coating method. Flat surface structures, atomic steps, and crystal grain boundaries are resolved at an atomic resolution. The surface imperfections are also characterized, as well as the dominant defects. Simulations on different types of iodine vacancy configurations are performed by density functional theory calculations. In addition, it is observed that the surface iodine lattice structure is unstable during scanning. Tip scanning can also cause the vertical migration of surface iodine ions. The measurements provide the direct visualizations of the surface imperfections of the solution-processed perovskite films. They are essential for understanding the surface-related optoelectronic effects and rationally designing more efficient surface passivation methods.

Wavelike electronic energy transfer in donor-acceptor molecular systems through quantum coherence.

Quantum-coherent intermolecular energy transfer is believed to play a key role in light harvesting in photosynthesis and photovoltaics. So far, a direct, real-space demonstration of quantum coherence in donor-acceptor systems has been lacking because of the fragile quantum coherence in lossy molecular systems. Here, we precisely control the separations in well-defined donor-acceptor model systems and unveil a transition from incoherent to coherent electronic energy transfer. We monitor the fluorescence from the heterodimers with subnanometre resolution through scanning tunnelling microscopy induced luminescence. With decreasing intermolecular distance, the dipole coupling strength increases and two new emission peaks emerge: a low-intensity peak blueshifted from the donor emission, and an intense peak redshifted from the acceptor emission. Spatially resolved spectroscopic images of the redshifted emission exhibit a σ antibonding-like pattern and thus indicate a delocalized nature of the excitonic state over the whole heterodimer due to the in-phase superposition of molecular excited states. These observations suggest that the exciton can travel coherently through the whole heterodimer as a quantum-mechanical wavepacket. In our model system, the wavelike quantum-coherent transfer channel is three times more efficient than the incoherent channel.

Desilylative Coupling Involving C(sp2)-Si Bond Cleavage on Metal Surfaces.

Desilylative coupling involving C-Si bond cleavage has emerged as one of the most important synthetic strategies for carbon-carbon/heteroatom bond formation in solution chemistry. However, in on-surface chemistry, C-Si bond cleavage remains a synthetic challenge. Here, we report the implementation of C(sp2)-Si bond cleavage and subsequent C-C bond formation on metal surfaces. The combination of scanning tunneling microscopy and density functional theory calculation successfully reveals that the incorporation of the C-Br group on the arylsilanes is critical to the success of this desilylative coupling reaction on metal surfaces. Our study represents a promising approach for the removal of protecting silyl groups in on-surface chemistry.

Upregulation of wild-type p53 by small molecule-induced elevation of NQO1 in non-small cell lung cancer cells.

The tumor suppressor p53 is usually inactivated by somatic mutations in malignant neoplasms, and its reactivation represents an attractive therapeutic strategy for cancers. Here, we reported that a new quinolone compound RYL-687 significantly inhibited non-small cell lung cancer (NSCLC) cells which express wild type (wt) p53, in contract to its much weaker cytotoxicity on cells with mutant p53. RYL-687 upregulated p53 in cells with wt but not mutant p53, and ectopic expression of wt p53 significantly enhanced the anti-NSCLC activity of this compound. RYL-687 induced production of reactive oxygen species (ROS) and upregulation of Nrf2, leading to an elevation of the NAD(P)H:quinoneoxidoreductase-1 (NQO1) that can protect p53 by inhibiting its degradation by 20S proteasome. RYL-687 bound NQO1, facilitating the physical interaction between NQO1 and p53. NQO1 was required for RYL-687-induced p53 accumulation, because silencing of NQO1 by specific siRNA or an NQO1 inhibitor uridine, drastically suppressed RYL-687-induced p53 upregulation. Moreover, a RYL-687-related prodrug significantly inhibited tumor growth in NOD-SCID mice inoculated with NSCLC cells and in a wt p53-NSCLC patient-derived xenograft mouse model. These data indicate that targeting NQO1 is a rational strategy to reactivate p53, and RYL-687 as a p53 stabilizer bears therapeutic potentials in NSCLCs with wt p53.

Diamantanethiols on Metal Surfaces: Spatial Configurations, Bond Dissociations, and Polymerization.

We report the on-surface chemistry of diamantanethiols on metal surfaces by combining low-temperature STM studies with quantum mechanical density functional theory computations. First, we examined the spatial configurations of diamantanethiols on metal surfaces, in which the thiol-substrate confinement plays a key role. We then thermally desorbed the diamantanethiols from the substrate surfaces to determine whether the C-S or S-metal bonds preferentially break. Finally, we explored diamantane-4,9-dithiol and its polymerization on metal surfaces, forming linear nanodiamond disulfur chains. This work broadens the fundamental knowledge of functionalized diamondoid behavior on surfaces and provides a novel approach to link diamantane as necklace-chain nanodiamond hybrid materials.

Polymerization of silanes through dehydrogenative Si-Si bond formation on metal surfaces.

Element-element double bonds of group 14 elements can be formed in solution, but generally only by applying harsh reductive conditions using sterically highly shielded tetryl halides as precursors. The two-dimensional confinement in surface-assisted polymerization represents a valuable alternative to access such reactive compounds, as it allows shielding of the labile entities without requiring bulky residues and catalytic activation of the reactive groups. Here, we demonstrate Si-Si bond formation in on-surface chemistry. Polymerization upon multiple Si-H bond dissociation and subsequent Si-Si bond formation was achieved on Au(111) and Cu(111) surfaces by using two different monomers, each containing two silicon functional groups (CH3SiH2 or SiH3) attached to an aromatic backbone, leading to polymeric disilenes that interact with the surface. A combination of experimental and theoretical studies corroborates the formation of covalent Si-Si bonds between the long, highly ordered polymer chains with high diastereoselectivity. The reactive Si=Si bonds formally generated via double dehydrogenative coupling are stabilized via covalent Si-surface interaction.

Conformational evolution following the sequential molecular dehydrogenation of PMDI on a Cu(111) surface.

Molecular spatial conformational evolution following the corresponding chemical reaction pathway at surfaces is important to understand and optimize chemical processes. Combining experimental and theoretical methods, the sequential N-H and C-H dehydrogenation of pyromellitic diimide (PMDI) on a Cu(111) surface are reported. STM experiments and atomistic modeling allow structural analysis at each well-defined reaction step. First, exclusively the aromatic N-H dehydrogenation of the imide group is observed. Subsequently, the C-H group at the benzene core of PMDI gets activated leading to a dehydrogenation reaction forming metalorganic species where Cu adatoms pronouncedly protruding from the surface are coordinated by one or two PMDI ligands at the surface. All reactions of PMDI induce conformational changes at the surface as confirmed by STM imaging and DFT simulations. Such conformational evolution in sequential N-H and C-H activation provides a detailed insight to understand molecular dehydrogenation processes at surfaces.

Azo bond formation on metal surfaces.

The formation of azo compounds via redox cross-coupling of nitroarenes and arylamines, challenging in solution phase chemistry, is achieved by on-surface chemistry. Reaction products are analyzed with a cryogenic scanning tunneling microscope (STM) and X-ray photoelectron spectroscopy (XPS). By using well-designed precursors containing both an amino and a nitro functionality, azo polymers are prepared on surface via highly efficient nitro-amino cross-coupling. Experiments conducted on other substrates and surface orientations reveal that the metal surface has a significant effect on the reaction efficiency. The reaction was further found to proceed from partially oxidized/reduced precursors in dimerization reactions, shedding light on the mechanism that was studied by DFT calculations.

Aryl Triflates in On-Surface Chemistry.

The reactivity of aryl triflates in on-surface C-C coupling is reported. It is shown that the triflate group in aryl triflates enables regioselective homo coupling with preceding or concomitant hydrodetriflation on Cu(111). Three different symmetrical π-systems with two and three triflate functionalities were used as monomers leading to oligomeric conjugated π-systems. The cascade, comprising different intermediates at different reaction temperatures as observed for one of the molecules, proceeds via initial removal of the trifluoromethyl sulfonyl group to give an aryloxy radical which in turn is deoxygenated to the corresponding aryl radical. Thermodynamically driven regioselective 1,2-hydrogen atom transfer leads to a translocated aryl radical which in turn undergoes coupling. For a sterically more hindered bistriflate, where one ortho position was blocked, dehydrogenative coupling occurred at remote position with good regioselectivity. Starting materials, intermediates as well as products were analyzed by scanning tunneling microscopy. Structures and suggested mechanism were further supported by DFT calculations.

A Unidirectional Surface-Anchored N-Heterocyclic Carbene Rotor.

A molecular rotor based on N-heterocyclic carbenes (NHCs) has been rationally designed following theoretical predictions, experimentally realized, and characterized. Utilizing the structural tunability of NHCs, a computational screening protocol was first applied to identify NHCs with asymmetric rotational potentials on a surface as a prerequisite for unidirectional molecular rotors. Suitable candidates were then synthesized and studied using scanning tunneling microscopy/spectroscopy (STM/STS), analytical theoretical models, and molecular dynamics simulations. For our best NHC rotor featuring a mesityl N substituent on one side and a chiral naphthylethyl substituent on the other, unidirectional rotation is driven by inelastic tunneling of electrons from the NHC to the STM tip. While electrons preferentially tunnel through the mesityl N substituent, the chiral naphthylethyl substituent controls the directionality. Such NHC-based surface rotors open up new possibilities for the design and construction of functionalized molecular systems with high catalytic applicability and superior stability compared with other classes of molecular rotors.

Intermolecular coupling and intramolecular cyclization of aryl nitriles on Au(111).

The on-surface dimerization reaction of an organic nitrile on Au(111) is reported. The formation of the product, which contains five newly formed σ-bonds and a diazapyrene core structure, was investigated and characterized by scanning tunneling microscopy. Experimental and computational studies of reference compounds support our findings.

On-Surface Reactive Planarization of Pt(II) Complexes.

A series of Pt(II) complexes with tetradentate luminophores has been designed, synthesized, and deposited on coinage metal surfaces with the aim to produce highly planar self-assembled monolayers. Low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations reveal a significant initial nonplanarity for all complexes. A subsequent metal-catalyzed separation of the nonplanar moiety at the bridging unit via the scission of a C-N bond is observed, leaving behind a largely planar core complex. The activation barrier of this bond scission process is found to depend strongly on the chemical nature of both bridging group and coordination plane, and to increase from Cu(111) through Ag(111) to Au(111).

Diamantane Suspended Single Copper Atoms.

Single chains of metal atoms are expected to be perfect one-dimensional nanowires in nanotechnology, due to their quantum nature including tunable electronic or spin coupling strengths. However, it is still rather difficult to fabricate such nanowires with metallic atoms under directional and separation control. Here, we succeeded in building higher-order single diamondoid-chains from the lower-order chains using a chemically well-controlled approach that employs diamondoids on metal surfaces. This approach results in higher-order diamondoid double chains by linking two neighboring single chains, and ultimately forms a central chain consisting of single Cu atoms suspended by the diamantane framework. The suspended Cu atoms are placed above the metal surface with a periodic distance of 0.67 ± 0.01 nm. Our bottom-up approach will allow detailed experimental investigations of the properties of these exciting suspended metal atoms (for example, quantized conductance, spin coupling, as well as transfer, etc.). Furthermore, we also identified different spatial configurations on the metal surfaces in on-surface reaction processes using high-resolution AFM imaging and density functional theory computations. Our findings broaden the on-surface synthesis concept from 2D planar aromatic molecules to 3D bulky aliphatic molecules.

Elucidating the Binding Modes of N-Heterocyclic Carbenes on a Gold Surface.

Tuning the binding mode of N-heterocyclic carbenes on metal surfaces is crucial for the development of new functional materials. To understand the impact of alkyl side groups on the formation of NHC species at the Au(111) surface, we combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We reveal two significantly different binding modes depending on the alkyl chain length. In the case of a short alkyl substituent, an up-standing configuration with one Au adatom is preferred, whereas the longer alkyl groups result exclusively in NHC-Au-NHC complexes lying flat on the surface. Our study highlights how well-defined structural modifications of NHCs allow for controlling the local binding motif on surfaces, which is important to design designated catalytic sites at interfaces.

Reaction Selectivity in On-Surface Chemistry by Surface Coverage Control-Alkyne Dimerization versus Alkyne Trimerization.

This work reports the influence of molecular coverage in on-surface C-C-bond formation on reaction outcome. 6-Ethynyl-2-naphthoic acid (ENA) was chosen as organic component and Ag(111) as substrate. The alkyne moiety in ENA can either react by dimerization to ENA dimers (Glaser coupling or hydroalkynylation) or cyclotrimerization to generate a benzene core as connecting moiety. Dimer formation is preferred at high surface coverage whereas trimerization is the major reaction pathway at low coverage. Mechanistic studies are provided.

α-Diazo Ketones in On-Surface Chemistry.

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,ß-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.

Intermolecular On-Surface σ-Bond Metathesis.

Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynylbenzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process.