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Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

The coordination assembly of 1,3,5-trispyridylbenzene with Cu on a Au(111) surface has been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. An open two-dimensional (2D) metal-organic network of honeycomb structure is formed as the 2D network covers partial surface. Upon the 2D network coverage of the entire surface, further increment of molecular density on the surface results in a multistep nonreversible structural transformation in the self-assembly. The new phases consist of metal-organic networks of pentagonal, rhombic, zigzag, and eventually triangular structures. In addition to the structural change, the coordination configuration also undergoes a change from the two-fold Cu-pyridyl binding in the honeycomb, pentagonal, rhombic and zigzag structures to the three-fold Cu-pyridyl coordination in the triangular structure. As the increment of molecular packing density on the surface builds up intrinsic in-plane compression pressure in the 2D space, the transformation of the structure, as well as the coordination binding mode, is attributed to the in-plane compression pressure. The quantitative structural analysis of the various phases upon molecular density increment allows us to construct a phase diagram of network structures as a function of the in-plane compression.

A new pair of multifunctional Zn(II)-Yb(III) complex enantiomers based on a chiral amine-phenol ligand, [YbZn2(SS/RR-L)2(H2O)4](ClO4)3•5H2O (S-1 and R-1) [H2L = ((SS/RR)-cyclohexane-1,2-diylbis(azanediyl))-bis(methylene))-bis(2-methoxyphenol))], were synthesized and structurally characterized. In S-1 and R-1,...

Tungsten trioxide (WO3) nanowires were synthesized by thermal evaporation of tungsten powder in two steps: tungsten suboxide (WO3−x) nanowires were synthesized, and then oxidized in O2 ambient and transformed into WO3 nanowires. Raman spectroscopy was applied to study the thermochromic phase transition of one-dimensional WO3 nanowires. From the temperature dependence of the characteristic mode at 33cm−1 in WO3, the phase transition temperature was determined. It was found that the phase transition of WO3 nanowires was reversible and the phase transition temperatures were even lower than that of WO3 nanopowder.

Crystalline molecular materials exhibiting both proton conduction and single-molecule magnet (SMM) behaviors would offer a great opportunity for applications in fuel cells, molecular spintronics and high-density data storage technologies. However,...

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Luminescent metal−organic frameworks (MOFs) coupled with proton conduction offer wide applications in clean energy and luminescence sensors. High stability and remarkable proton conductivity and sensing are requisites for practical applications. Specifically, high thermal and chemical stabilities under harsh conditions are important but challenging to achieve. Herein, an ultra-stable Cu(I)-tetrazolate MOF, NaCu 3 (mtz) 4 (1, mtz = 5-methyltetrazolate), was prepared. It possesses a 3D framework with 1D channels and exhibits outstanding thermal and chemical stabilities under various conditions, including water, organic solvents, and acidic and basic solutions of wide pH range. The compound exhibits strong green emissions, which can be ascribed to metal-toligand charge transfer, and selective fluorescence sensing for pollutant nitrobenzene. Moreover, compound 1 presents a high proton conductivity of over 10 −2 S cm −1 at 70 °C and 100% relative humidity. In 1, abundant uncoordinated nitrogen atoms on the pore walls act as H-bonding acceptors to achieve its high proton conductivity under waterassisted conditions. The compound is an unprecedented water-dependent proton conductor of the tetrazole-based MOF, providing a new avenue to design robust and high-performance proton-conductive MOFs.