Surface-Assisted Cyclodehydrogenation; Break the Symmetry, Enhance the Selectivity.

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.

Immobilised molecular catalysts and the role of the supporting metal substrate.

This work demonstrates that immobilising molecular catalysts on metal substrates can attenuate their reactivity. In particular, the reactivity towards molecular oxygen of both ruthenium tetraphenyl porphyrin (Ru-TPP) and its Ti analogue (Ti-TPP) on Ag(111) was studied as benchmark for the interaction strength of such metal-organic complexes with possible reactants. Here, Ru-TPP proves to be completely unreactive and Ti-TPP strongly reactive towards molecular oxygen; along with comparison to work in the literature, this suggests that studies into immobilised catalysts might find fruition in considering species traditionally seen as too strongly interacting.

How surface bonding and repulsive interactions cause phase transformations: ordering of a prototype macrocyclic compound on Ag(111).

We investigated the surface bonding and ordering of free-base porphine (2H-P), the parent compound of all porphyrins, on a smooth noble metal support. Our multitechnique investigation reveals a surprisingly rich and complex behavior, including intramolecular proton switching, repulsive intermolecular interactions, and density-driven phase transformations. For small concentrations, molecular-level observations using low-temperature scanning tunneling microscopy clearly show the operation of repulsive interactions between 2H-P molecules in direct contact with the employed Ag(111) surface, preventing the formation of islands. An increase of the molecular coverage results in a continuous decrease of the average intermolecular distance, correlated with multiple phase transformations: the system evolves from an isotropic, gas-like configuration via a fluid-like phase to a crystalline structure, which finally gives way to a disordered layer. Herein, considerable site-specific molecule-substrate interactions, favoring an exclusive adsorption on bridge positions of the Ag(111) lattice, play an important role. Accordingly, the 2D assembly of 2H-P/Ag(111) layers is dictated by the balance between adsorption energy maximization while retaining a single adsorption site counteracted by the repulsive molecule-molecule interactions. The long-range repulsion is associated with a charge redistribution at the 2H-P/Ag(111) interface comprising a partial filling of the lowest unoccupied molecular orbital, resulting in long-range electrostatic interactions between the adsorbates. Indeed, 2H-P molecules in the second layer that are electronically only weakly coupled to the Ag substrate show no repulsive behavior, but form dense-packed islands.

Photoinduced anisotropy and polarization holographic gratings formed in Ag/TiO2 nanocomposite films.

Formation of anisotropy and polarization holographic gratings (s-s, p-p, and s-p) in Ag/TiO2 nanocomposite films were investigated using Nd:YAG lasers (532 nm) as pumping and writing source, respectively. The observations can be well explained by the anisotropic photodissolution of Ag nanoparticles, the photomobility and reduction of Ag+ ions, and the light scattering induced by Ag nanoparticles. Taking these effects into account, a phenomenological model based on simultaneous formation of the absorption grating and the two coupling phase gratings is proposed and found to be in good agreement with the measurements. The main differences in the three polarization grating formation processes are also discussed.

Photo-assisted preparation and patterning of large-area reduced graphene oxide-TiO(2) conductive thin film.

A mild photo-assisted reduction method was developed to fabricate large-area, uniform reduced graphene oxide-TiO(2) films and micropatterns on various substrates in air at room temperature from a composite insulating film of graphene oxide sheets and TiO(2) nanoparticles, with a conductivity (0.5-2 Omega cm) comparable to chemically reduced graphene.