Using scanning tunneling microscopy, we demonstrate that the 1,3-dipolar cycloaddition between a terminal alkyne and an azide can be performed under solvent-free ultrahigh vacuum conditions with reactants adsorbed on a Cu(111) surface. XPS shows significant degradation of the azide upon adsorption, which is found to be the limiting factor for the reaction.

The adsorption of Ca on poly(3-hexylthiophene) (P3HT) has been studied by adsorption microcalorimetry, atomic beam/surface scattering, X-ray photoelectron spectroscopy (XPS), low-energy He(+) ion scattering spectroscopy (LEIS), and first-principles calculations. The sticking probability of Ca on P3HT is initially 0.35 and increases to almost unity by 5 ML. A very high initial heat of adsorption in the first 0.02 ML (625-500 kJ/mol) is attributed to the reaction of Ca with defect sites or residual contamination. Between 0.1 and 0.5 ML, there is a high and nearly constant heat of adsorption of 405 kJ/mol, which we ascribe to Ca reacting with subsurface sulfur atoms from the thiophene rings of the polymer. This is supported by the absence of LEIS signal for Ca and the shift of the S 2p XPS binding energy by -2.8 eV for reacted S atoms. The heat of adsorption decreases above 0.6 ML coverage, reaching the sublimation enthalpy of Ca, 178 kJ/mol, by 4 ML. This is attributed to the formation of Ca nanoparticles and eventually a continuous solid Ca film, on top of the polymer. LEIS and XPS measurements, which show only a slow increase of the signals related to solid Ca, support this model. Incoming Ca atoms are subject to a kinetic competition between diffusing into the polymer to react with subsurface thiophene units versus forming or adding to three-dimensional Ca clusters on the surface. At approximately 1.6 ML Ca coverage, Ca atoms have similar probabilities for either process, with the former dominating at lower coverage. Ultimately about 1.6 ML of Ca (1.2 x 10(15) atoms/cm(2)) reacts with S atoms, corresponding to a reacted depth of approximately 3 nm, or nearly five monomer-unit layers. Density-functional theory calculations confirm that the heat of reaction and the shift of the S 2p signal are consistent with Ca abstracting S from the thiophene rings to form small CaS clusters.

The activation barrier for cis-to-trans isomerization is a key parameter for governing the properties of photoswitchable molecules. This quantity can be computed by using theoretical methods, but experimental determination is not straightforward. Photoswitchable molecules typically do not change their conformation in the pure crystalline state. When the molecules are in solution, the switching is affected by the viscosity and polarity of the solvent and when embedded in polymers, the conformational change is affected by the polymer matrix. Here, we describe a novel approach where the photoswitchable group is integrated in a highly crystalline, porous molecular framework. Sufficiently large pore sizes in such metal-organic frameworks, MOFs, allow unhindered switching and the strictly periodic structure of the lattice eliminates virtually all contributions from inhomogeneities. Using IR spectroscopy to probe the conformational state of azobenzene, the energy barrier separating the cis and the trans state could be determined by an Arrhenius analysis of the data accumulated in a temperature regime between 314 K and 385 K. The result, 1.09 ± 0.09 eV, is in very good agreement with the activation energy reported for the thermal cis-to-trans isomerization of free azobenzene as computed by DFT calculations.

Adsorption of tetrahydroxybenzene (THB) on Cu(111) and Au(111) surfaces is studied using a combination of STM, XPS, and DFT. THB is deposited intact, but on Cu(111) it undergoes gradual dehydrogenation of the hydroxyl groups as a function of substrate temperature, yielding a pure dihydroxy-benzoquinone phase at 370 K. Subtle changes to the adsorption structure upon dehydrogenation are explained from differences in molecule-surface bonding.

Surface coordination networks formed by coadsorption of metal atoms and organic ligands have interesting properties, for example regarding catalysis and data storage. Surface coordination networks studied to date have typically been based on single metal atom centers. The formation of a novel surface coordination network is now demonstrated that is based on network nodes in the form of clusters consisting of three Cu adatoms. The network forms by deposition of tetrahydroxybenzene (THB) on Cu(111) under UHV conditions. As shown from a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations, all four hydroxy groups of THB dehydrogenate upon thermal activation at 440 K. This highly reactive ligand binds to Cu adatom trimers, which are resolved by high-resolution STM. The network creates an ordered array of mono-dispersed metal clusters constituting a two-dimensional analogue of metal-organic frameworks.Metal-organic frameworks (MOFs) have emerged as an important new class of designable nanoporous materials with many potential application areas, not least within gas storage and catalysis. [1] MOFs consist of metal cations interconnected by organic linkers in a crystalline three-dimensional matrix. Motivated by sensing and other interface-related applications, thin-film MOFs have been synthesized by growing or adsorbing MOFs on surfaces. [2] The extreme case of truly two-dimensional, molecular monolayer-thick analogues of MOFs is approached from the complementary direction of surface coordination networks. [3] These structures result from advances in surface supramolecular chemistry under ultrahigh vacuum (UHV) conditions and are synthesized by coadsorbing organic ligands with metal atoms on metal singlecrystal surfaces. Early work in this direction demonstrated formation of metal-organic clusters [4] and networks [5] from coordination between carboxylates and Cu or Fe adatoms. Subsequently, many well-ordered surface coordination networks have been synthesized, [6] notably demonstrating systematic control of network pore size and symmetry from appropriate choice of ligand (size and chemical functionality), metal center, and metal substrate. [7] Bulk MOFs are based on network nodes that in many cases go beyond simple cations, for example, small metal [8] or metal-oxide clusters, such as Zn 4 O clusters in the classic MOF-5. [9] In contrast, surface coordination networks have typically been based on single metal adatom nodes or coordination nodes with two (separated) adatoms. [6c, 10] Achieving more complex network nodes [11] in surface coordination networks is an interesting prospect, as the properties of the network nodes can be key to the functionality of the MOF/surface coordination network, for example, in terms of magnetic properties [10,12] or catalytic activity. [13] Herein we use a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations to demonstrate the synt...

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The adsorption and reaction of formaldehyde (CH 2 O) on the oxidized rutile TiO 2 (110) surface were studied by temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), infrared reflection−absorption spectroscopy (IRRAS), and density functional theory (DFT) calculations. The experimental and theoretical data reveal the presence of various species depending on the temperature and coverage. Exposure to formaldehyde at 65 K leads to the formation of CH 2 O multilayers, which desorb completely upon heating to 120 K. After smaller exposures at low temperatures (45−65 K), STM allowed us to identify individual, isolated CH 2 O monomers. The theoretical results indicate that these monomers are bound to the surface Ti 5c sites via σ-donation and adopt a tilted geometry. Upon heating, the CH 2 O monomers polymerize to form paraformaldehyde (polyoxymethylene, POM) chains, oriented primarily along the Ti 5c rows ([001] direction). Upon further heating, POM is found to decompose around 250 K, releasing CH 2 O into the gas phase. In addition, dioxymethylene (DOM) was detected as minority species formed via reaction of Ti 5c -bound CH 2 O with surface O atoms. For all substrate species, the characteristic IR vibrations were measured. Because these are the first IRRAS data for TiO 2 macroscopic single crystals exposed to formaldehyde, we have performed DFT calculations to aid the assignment of the various bands.

The chemical activity of oxygen vacancies on well-defined, single-crystal CeO2(111)-surfaces is investigated using CO as a probe molecule. Since no previous measurements are available, the assignment of the CO ν1 stretch frequency as determined by IR-spectroscopy for the stoichiometric and defective surfaces are aided by ab initio electronic structure calculations using density functional theory (DFT).