Synthesis and characterization of triangulene

Triangulene, the smallest triplet-ground-state polybenzenoid (also known as Clar's hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized1. Despite containing an even number of carbons (22, in six fused benzene rings), it is not possible to draw Kekulé-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons2. Synthesis and characterization of unsubstituted triangulene has not been achieved because of its extreme reactivity1, although the addition of substituents has allowed the stabilization and synthesis of the triangulene core3, 4 and verification of the triplet ground state via electron paramagnetic resonance measurements5. Here we show the on-surface generation of unsubstituted triangulene that consists of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free-molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric molecular structure. The unique topology of such non-Kekulé hydrocarbons results in open-shell π-conjugated graphene fragments6 that give rise to high-spin ground states, potentially useful in organic spintronic devices7, 8. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level

Polyyne formation via skeletal rearrangement induced by atomic manipulation

Rearrangements that change the connectivity of a carbon skeleton are often useful in synthesis, but it can be difficult to follow their mechanisms. Scanning probe microscopy can be used to manipulate a skeletal rearrangement at the single-molecule level, while monitoring the geometry of reactants, intermediates and final products with atomic resolution. We studied the reductive rearrangement of 1,1-dibromo alkenes to polyynes on a NaCl surface at 5 K, a reaction that resembles the Fritsch-Buttenberg-Wiechell rearrangement. Voltage pulses were used to cleave one C-Br bond, forming a radical, then to cleave the remaining C•-Br bond, triggering the rearrangement. These experiments provide structural insight into the bromo-vinyl radical intermediates, showing that the C=C•-Br unit is nonlinear. Long polyynes, up to the octayne Ph-(C≡C)8-Ph, have been prepared in this way. The control of skeletal rearrangements opens a new window on carbon-rich materials and extends the toolbox for molecular synthesis by atom manipulation

Polyyne formation via skeletal rearrangement induced by atomic manipulation

Rearrangements that change the connectivity of a carbon skeleton are often useful in synthesis, but it can be difficult to follow their mechanisms. Scanning probe microscopy can be used to manipulate a skeletal rearrangement at the single-molecule level, while monitoring the geometry of reactants, intermediates and final products with atomic resolution. We studied the reductive rearrangement of 1,1-dibromo alkenes to polyynes on a NaCl surface at 5 K, a reaction that resembles the Fritsch-Buttenberg-Wiechell rearrangement. Voltage pulses were used to cleave one C-Br bond, forming a radical, then to cleave the remaining C•-Br bond, triggering the rearrangement. These experiments provide structural insight into the bromo-vinyl radical intermediates, showing that the C=C•-Br unit is nonlinear. Long polyynes, up to the octayne Ph-(C≡C)8-Ph, have been prepared in this way. The control of skeletal rearrangements opens a new window on carbon-rich materials and extends the toolbox for molecular synthesis by atom manipulation

Believe in the force

Nearly a decade after discovering molecular chirality in 1848, Louis Pasteur changed research direction and began investigating fermentations. Conflicting explanations have been given for this switch to microbiology, but the evidence strongly suggests that Pasteur’s appointment in 1854 to the University of Lille—an agricultural- industrial region where fermentation-based manufacturing was of great importance— and an appeal for help in 1856 by a local manufacturer experiencing problems in his beetroot-fermentation-based alcohol production played a significant role. Thus began, in late 1856, Pasteur’s pioneering studies of lactic and alcoholic fermentations. In 1857, reportedly as a result of a laboratory mishap, he found that in incubations of ammonium (6)-tartrate with unidentified microorganisms (1)-tartaric acid was consumed with con- siderable preference over (2)-tartaric acid. In 1860, he demonstrated a similar enantio- selectivity in the metabolism of tartaric acid by Penicillium glaucum, a common mold. Chance likely played a significant role both in Pasteur’s shift to microbiology and his discovery of enantioselective tartrate fermentations, but he rejected pure serendipity as a significant factor in experimental science and in his own career. Pasteur’s mile- stone discovery of biological enantioselectivity began the process that in the long run established the fundamental importance of molecular chirality in biology. Chiralit