Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.

The fabrication and control of coordination compounds or architectures at well-defined interfaces is a thriving research domain with promise for various research areas, including single-site catalysis, molecular magnetism, light-harvesting, and molecular rotors and machines. To date, such systems have been realized either by grafting or depositing prefabricated metal-organic complexes or by protocols combining molecular linkers and single metal atoms at the interface. Here we report a different pathway employing metal-organic chemical vapor deposition, as exemplified by the reaction of meso-tetraphenylporphyrin derivatives on atomistically clean Ag(111) with a metal carbonyl precursor (Ru3(CO)12) under vacuum conditions. Scanning tunneling microscopy and X-ray spectroscopy reveal the formation of a meso-tetraphenylporphyrin cyclodehydrogenation product that readily undergoes metalation after exposure to the Ru-carbonyl precursor vapor and thermal treatment. The self-terminating porphyrin metalation protocol proceeds without additional surface-bound byproducts, yielding a single and thermally robust layer of Ru metalloporphyrins. The introduced fabrication scheme presents a new approach toward the realization of complex metal-organic interfaces incorporating metal centers in unique coordination environments.

The electric field of a laser pulse can be described as Here we report the first method permitting absolute CEP detection with a solid-state detector applicable in ambient conditions. Recently, we have shown that the strong electric field of an intense, linearly-polarized, visible/near-infrared (VIS/NIR), few-cycle laser pulse can rapidly increase the (ac) conductivity of a solid insulator, allowing electric currents to be induced and switched with the field of visible light [22]. In these experiments, we exposed amorphous silicon dioxide (bandgap g 9 eV E ≈ ) to a strong, controlled electric field ( ) F t of a few-cycle pulse with a carrier photon energy of ∆ is a consequence of dispersive pulse broadening inside the glass wedges. However, in our experiments P ( ) Q l ∆ was still detectable above the noise level for values of 400 µm l ∆ > , corresponding to a pulse duration of more than 9 fs (FWHM of the time-dependent cycle-averaged intensity). Subsequently, PQ was calibrated with respect to the absolute CEP of the laser pulse via stereo-ATI measurements performed with identical pulses [4]. After the measurement of P ( ) Q l ∆ with the solid-state device, a mirror was inserted into the beam path, deflecting the 5 pulses into a stereo-ATI apparatus located -together with the solid-state detector -in the same vacuum chamber (Fig. 1). Here, the CEP of the incident laser pulse was detected by analyzing the kinetic energy distribution of electrons that are photoemitted from Xe atoms, see Methods Summary. An uncertainty due to a Gouy phase shift in both foci can be neglected since in both experiments, the sample was placed exactly in the region of the highest laser intensity.We set 17 different propagation lengths l ∆ , ranging from 21.5 µm − to 27.5 µm +. For each of them, 500 single-shot stereo-ATI measurements were performed. Because consecutive laser pulses had a CEP-shift of π , which is only required for the accurate detection of P ( ) Q l ∆ , only spectra from odd-numbered pulses were considered for the stereo-ATI measurements. As shown in [4], CEϕ can then be reconstructed by calculating two asymmetry parameters ( , ) X Y by integrating the averaged time-of-flight spectra L,R TOF ( ) n t of the electrons photoemitted from Xe atoms by the intense few-cycle VIS/NIR pulses in two different regions. The parametric plot of ( , ) X Y in Fig. 2(a) was obtained by calculating, for each. The photoelectron spectra L,R TOF ( ) n twere measured with the left (L) and right (R) micro-channel plates (MCPs) of the set-up in Fig. 1 We have compared the results of the solid-state-based phase retrieval with the predictions of two quantum mechanical models. The first model, which was earlier employed in Ref.[24] to describe the ultrafast increase in conductivity of SiO 2 nanojunctions, is based on the nearestneighbor tight-binding approximation. The second model, presented in detail in Ref.[25], describes quantum dynamics in a one-dimensional pseudopotential (see the Methods Summary for details). In both models, the electric fi...

We investigated the synthesis of one-dimensional nanostructures via Schiff base (imine) formation on three close-packed coinage metal (Au, Ag, and Cu) surfaces under ultrahigh vacuum conditions. We demonstrate the feasibility of forming pyrene-fused pyrazaacene-based oligomers on the Ag(111) surface by thermal annealing of tetraketone and tetraamine molecules, which were designed to afford cyclocondensation products. Direct visualization by scanning tunneling microscopy of reactants, intermediates, and products with submolecular resolution and the analysis of their statistical distribution in dependence of stoichiometry and annealing temperature together with the inspection of complementary X-ray photoelectron spectroscopy signatures provide unique insight in the reaction mechanism, its limitations, and the role of the supporting substrate. In contrast to the reaction on Ag(111), the reactants desorb from the Au(111) surface before reacting, whereas they decompose on the Cu(111) surface during the relevant thermal treatment.