Tautomerization of single asymmetric oxahemiporphycene molecules on Cu(111).

We have studied 22-oxahemiporphycene molecules by a combination of scanning tunneling microscopy at low temperatures and density functional theory calculations. In contrast to other molecular switches with typically two switching states, these molecules can in principle exist in three different tautomers, due to their asymmetry and three inequivalent binding positions of a hydrogen atom in their macrocycle. Different tautomers are identified from the typical appearance on the surface and tunneling electrons can be used to tautomerize single molecules in a controllable way with the highest rates if the STM tip is placed close to the hydrogen binding positions in the cavity. Characteristic switching processes are explained by the different energy pathways upon adsorption on the surface. Upon applying higher bias voltages, deprotonation occurs instead of tautomerization, which becomes evident in the molecular appearance.

Adsorption and thermal evolution of [C1C1Im][Tf2N] on Pt(111).

In the context of ionic liquid (IL)-assisted catalysis, we have investigated the adsorption and thermal evolution of the IL 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C1Im][Tf2N]) on Pt(111) between 100 and 800 K by angle-resolved X-ray photoelectron spectroscopy and scanning tunneling microscopy. Defined amounts of IL in the coverage range of a complete first wetting layer were deposited at low temperature (100-200 K), and subsequently heated to 300 K, or directly at 300 K. At 100 K, the IL adsorbs as an intact disordered layer. Upon heating to 200 K, the IL stays intact, but forms an ordered and well-oriented structure. Upon heating to 250 K, the surface order increases, but at the same time STM and XPS indicate the onset of decomposition. Upon heating to 300 K, decomposition progresses, such that 50-60% of the IL is decomposed. The anion-related reaction products desorb instantaneously, and the cation-related products remain on the surface. Thereby, the surface is partly passivated, enabling the remaining IL to still be adsorbed intact at 300 K. For IL deposition directly at 300 K, a fraction of the IL instantaneously decomposes, with the anion-related products desorbing, opening free space for further deposition of IL. Hence, cation-related species accumulate at the expense of anions, until one fully closed wetting layer is formed. As a consequence, a higher dose is required to reach this coverage at 300 K, compared to 100-200 K.

In situ hydrolysis of imine derivatives on Au(111) for the formation of aromatic mixed self-assembled monolayers: multitechnique analysis of this tunable surface modification.

This paper presents a novel method for preparing aromatic, mixed self-assembled monolayers (SAMs) with a dilute surface fraction coverage of protonated amine via in situ hydrolysis of C═N double bond on gold surface. Two imine compounds, (4'-(4-(trifluoromethyl)benzylideneamino)biphenyl-4-yl)methanethiol (CF(3)-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, TFBABPMT) and (4'-(4-cyanobenzylideneamino)biphenyl-4-yl)methanethiol (CN-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, CBABPMT), self-assembled on Au(111) to form highly ordered monolayers, which was demonstrated by infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). A nearly upright molecular orientation for CF(3)- and CN-terminated SAM was detected by near edge X-ray absorption fine structure (NEXAFS) measurements. Afterward, the acidic catalyzed hydrolysis was carried out in chloroform or an aqueous solution of acetic acid (pH = 3). Systematic studies of this hydrolysis process for CN-terminated SAM in acetic acid at 25 °C were performed by NEXAFS measurements. It was found that about 30% of the imine double bonds gradually cleaved in the first 40 min. Subsequently, a larger hydrolysis rate was observed due to the freer penetration of acetic acid in the SAM and resultant more open molecular packing. Furthermore, the molecular orientation in mixed SAMs did not change during the whole hydrolysis process. This partially hydrolyzed surface contains a controlled amount of free amines/ammonium ions which can be used for further chemical modifications.

Time- and Temperature-Dependent Growth Behavior of Ionic Liquids on Au(111) Studied by Atomic Force Microscopy in Ultrahigh Vacuum.

We deposited defined amounts of [C1C1Im][Tf2N] on Au(111) at different temperatures and investigated the morphology and wetting behavior of the deposited films by atomic force microscopy. For multilayer coverages, we observe a drastically different growth behavior when comparing deposition at room temperature (RT) and deposition below 170 K followed by slow annealing to RT. Upon deposition at RT, we find the formation of 2-30 nm high and 50-500 nm wide metastable 3D droplets on top of a checkerboard-type wetting layer. These droplets spread out into stable 2D bilayers, on the time scale of hours and days. The same 2D bilayer structure is obtained after deposition below 170 K and slow annealing to RT. We present a statistical analysis on the time-dependent changes of the shape and volume of the 3D droplets and the 2D bilayers. We attribute the stabilization of the 2D bilayers on the wetting layer and on already formed bilayers to the high degree of order in these layers. Notably, the transformation process from the 3D droplets to 2D bilayer islands is accelerated by tip effects and also X-ray radiation.

Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface.

Prototypical molecular switches such as azobenzenes exhibit two states, i.e., trans and cis, with different characteristic physical properties. In recent years various derivatives were investigated on metallic surfaces. However, bulk insulators as supporting substrate reveal important advantages since they allow electronic decoupling from the environment, which is key to control the switching properties. Here, we report on the light-induced isomerization of an azobenzene derivative on a bulk insulator surface, in this case calcite (101̅4), studied by atomic force microscopy with submolecular resolution. Surprisingly, cis isomers appear on the surface already directly after preparation, indicating kinetic trapping. The photoisomerization process is reversible, as the use of different light sources results in specific molecular assemblies of each isomer. The process turns out to be very efficient and even comparable to molecules in solution, which we assign to the rather weak molecular interaction with the insulator surface, in contrast to metals.