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.

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.

Caged CO(2) for the direct observation of CO(2)-consuming reactions.

CO(2)-consuming reactions, in particular carboxylations, play important roles in technical processes and in nature. Their kinetic behavior and the reaction mechanisms of carboxylating enzymes are difficult to study because CO(2) is inconvenient to handle as a gas, exists in equilibrium with bicarbonate in aqueous solution, and typically yields products that show no significant spectroscopic differences from the reactants in the UV/Vis range. Here we demonstrate the utility of 3-nitrophenylacetic acid and related compounds (caged CO(2)) in conjunction with infrared spectroscopy as widely applicable tools for the investigation of such reactions, permitting convenient measurement of the kinetics of CO(2) consumption. The use of isotopically labeled caged CO(2) provides a tool for the assignment of infrared absorption bands, thus aiding insight into reaction intermediates and mechanisms.