On-Surface Hydrogenation of Buckybowls: From Curved Aromatic Molecules to Planar Non-Kekulé Aromatic Hydrocarbons.

Functionalization of surfaces with derivatives of Buckminsterfullerene fragment molecules seems to be a promising approach toward bottom-up fabrication of carbon nanotube modified electrode surfaces. The modification of a Cu(100) surface with molecules of the buckybowl pentaindenocorannulene has been studied by means of scanning tunneling microscopy, carbon monoxide-modified noncontact atomic force microscopy, time-of-flight secondary mass spectrometry, and quantum chemical calculations. Two different adsorbate modes are identified, in which the majority is oriented such that the bowl cavity points away from the surface and the convex side is partially immersed into a four-atom vacancy in the Cu(100) surface. A minority is oriented such that the convex side points away from the surface with the five benzo tabs oriented basically parallel to the surface. Thermal annealing leads to hydrogenation and planarization of the molecules in two steps under specific C-C bond cleavage. The benzo tabs of the convex side up species serve as a hydrogen source. The final product has an open-shell electron structure that is quenched on the surface.

Heterochiral recognition among functionalized heptahelicenes on noble metal surfaces.

Chiral recognition among three differently functionalized heptahelicene derivatives on Ag(111) and Au(111) surfaces has been studied with scanning tunnelling microscopy. All three species were found to self-assemble into racemic zigzag structures, with alternation of (M)- and (P)-enantiomers.

Fivefold Symmetry and 2D Crystallization: Self-Assembly of the Buckybowl Pentaindenocorannulene on a Cu(100) Surface.

The modification of metal electrode surfaces with functional organic molecules is an important part of organic electronics. The interaction of the buckminsterfullerene fragment molecule pentaindenocorannulene with a Cu(100) surface is studied by scanning tunneling microscopy, dispersion-enabled density functional theory, and force field calculations. Experimental and theoretical methods suggest that two adjacent indeno groups become oriented parallel to the surface upon adsorption under mild distortion of the molecular frame. The binding mechanism between molecule and surface is dominated by strong electrostatic interaction owing to Pauli repulsion. Two-dimensional aggregation at room temperature leads to a single lattice structure in which all molecules are oriented unidirectionally. Their relative arrangement in the lattice suggests noncovalent intermolecular interaction through C-H⋅⋅⋅π bonding.

The fate of bromine after temperature-induced dehydrogenation of on-surface synthesized bisheptahelicene.

The on-surface synthesis of bisheptahelicene by Ullmann coupling of 9-bromoheptahelicene on Au(111) and its temperature-induced dehydrogenation is studied using temperature-programmed reaction spectroscopy and time-of-flight secondary ion mass spectrometry. Specific dehydrogenation products of bisheptahelicene after loss of 6, 8 and 10 hydrogen atoms are identified, corresponding to molecules having undergone Diels-Alder transformations and intramolecular C-C coupling reactions. By combining with atomic hydrogen produced by dehydrogenation, the Ullmann coupling side-product bromine desorbs as HBr. H2 desorption emerges only after all Br has desorbed. Such characteristic behavior is explained by a kinetic model which explicitly considers the coverage of transient atomic H on the surface. Heating experiments performed with saturated layers of different Br-containing molecules reveal that the onset of HBr desorption depends strictly on the dehydrogenation step and therefore on the structure of the molecules.

Diastereoselective Ullmann Coupling to Bishelicenes by Surface Topochemistry.

The comparison of the self-assembly 9,9'-bisheptahelicene on the Au(111) surface, studied with scanning tunneling microscopy, with the self-assembly of the same species obtained by on-surface synthesis via Ullmann coupling from 9-bromoheptahelicene reveals a diastereomeric excess for the ( M, P)- meso-form of 50%. The stereoselectivity is explained by a topochemical effect, in which the surface-alignment of the starting material and the organometallic intermediate sterically favor the ( M, P)-transition state over the homochiral transition states.

Diastereoselective self-assembly of bisheptahelicene on Cu(111).

Stereochemical effects during two-dimensional crystallization of bisheptahelicene diastereomers on a Cu(111) surface have been studied with scanning tunnelling microscopy. The (M,M)- and (P,P)-enantiomers crystallize into a monolayer racemate lattice, whereas the (M,P)-diastereomers aggregate into their own monolayer phase.

Stereospecific Autocatalytic Surface Explosion Chemistry of Polycyclic Aromatic Hydrocarbons.

Autocatalytic processes are important in many fields of science, including surface chemistry. A better understanding of its mechanisms may improve the current knowledge on heterogeneous catalysis. The thermally induced decomposition of eight different polycyclic aromatic hydrocarbons (PAHs) on a saturated monolayer of atomic oxygen on a Cu(100) surface is studied using temperature-programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). 9-Bromo-heptahelicene decomposes autocatalytically in a narrow temperature range into CO2 and H2O, while non-halogenated heptahelicene decomposes into the same products but does not show autocatalytic behavior. Fixation of the hydrocarbon to the surface via the organometallic bond after elimination of the bromine is identified as a prerequisite for the autocatalytic reaction mechanism. Of all the hydrocarbons studied, only those being sterically overcrowded decompose autocatalytically. Such an observation can be explained by facile dehydrogenation of the overcrowded PAHs. The reaction of such hydrogen with oxygen creates vacancies in the oxygen layer which act as active sites and catalyze further decomposition.

Pauli Repulsion Versus van der Waals: Interaction of Indenocorannulene with a Cu(111) Surface.

Modification of metal electrode surfaces with functional organic molecules is an important step toward organic electronics. The interaction of the buckybowl indenocorannulene with a Cu(111) surface and the two-dimensional self-assembly on the same surface was studied by means of scanning tunneling microscopy and dispersion-enabled density functional theory. Based on the conjecture of maximizing van der Waals interaction with the surface one would expect the indeno group to be aligned parallel to the surface. Theoretical investigations predict a nonparallel arrangement with the benzo ring of the indeno group located higher above the surface than the bowl rim connected to the indeno group. This adsorbate geometry is due to strong electronic interaction between molecule and surface, including substantial Pauli repulsion. The long-range ordered monolayer shows differences for two molecules of the unit cell in scanning tunneling microscopy contrast, suggesting either different polar alignments, and therefore a different tilt of the indeno group, or occupation of different adsorption sites.

Erecting buckybowls onto their edge: 2D self-assembly of terphenylcorannulene on the Cu(111) surface.

A 2D self-assembly of a C32H12 buckybowl on the Cu(111) surface has been studied by means of scanning tunnelling microscopy. Additional aromatic rings at the rim of the corannulene core cause the bowl-shaped molecule to stand on its edge. This adsorption mode allows distinct π-π and C-Hπ interactions between the convex bowl surfaces as well as between the hydrogen-terminated rim and the convex bowl faces.

Heterochiral to Homochiral Transition in Pentahelicene 2D Crystallization Induced by Second-Layer Nucleation.

Gaining insight into molecular recognition at the molecular level, in particular, during nucleation of crystallites, is challenging and calls for studying well-defined model systems. Investigated by means of submolecular resolution scanning tunneling microscopy and theoretical molecular modeling, we report chiral recognition phenomena in the 2D crystallization of the helical chiral aromatic hydrocarbon pentahelicene on a Cu(111) surface. Homochiral, van der Waals bonded dimers constitute building blocks for self-assembly but form heterochiral as well as homochiral long-range-ordered structures. 2D racemate crystals, built up by homochiral dimers of both enantiomers, are observed at coverages close to a full monolayer. As soon as the coverage leads to second-layer nucleation, the dense racemate phase in the first layer disappears and a homochiral dimer conglomerate phase of lower 2D density appears. Our results show that, at the onset of second-layer nucleation, a local change of enantiomeric composition in the first layer occurs, causing the transition from a 2D racemate to a 2D conglomerate.

Surface-assisted diastereoselective Ullmann coupling of bishelicenes.

Ullmann coupling of chiral 2-bromo[4]helicene has been performed on a Cu(100) surface. Only homochiral 2,2'-bis[4]helicene as the product is observed using STM. Such stereoselectivity is based on the fact that the surface will favour a configuration with the central part of the molecule on the surface, causing the outer ends to spiral away from the surface.

Aggregation of C70-Fragment Buckybowls on Surfaces: π-H and π-π Bonding in Bowl Up-Side-Down Ensembles.

The self-assembly of the C38H14-buckybowl, a fragment bowl of the C70 fullerene, has been studied with scanning tunneling microscopy on the Cu(111) surface. Isolated molecules adsorb bowl opening-up with the center C6 ring parallel to the surface. In extended 2D islands, however, 1/3 of the molecules are oriented such that the bowl opening points down. From a detailed analysis of relative orientation of the molecules, the nature of intermolecular lateral interactions is identified. In densely packed islands, π-π bonding between convex sides of the bowls dominate, while π-H bonding between rim and convex sides plays the important role in small molecular 2D clusters.