Enrique Guitián

Bond-order discrimination by atomic force microscopy.

Visualizing Bond Order Bond lengths in conjugated molecules closely reflect individual bond order and are usually determined by diffraction methods. It is valuable to know bond order for rationalizing aromaticity, and reactivity and for chemical structure determination. Gross et al. (p. 1326; see the Perspective by Perez and the cover) differentiated the bond orders in individual molecules in the fullerene C60 and in polyaromatic hydrocarbons by imaging with noncontact atomic force microscopy (AFM). The molecules were adsorbed onto a copper surface, and the AFM tip was decorated with a CO molecule, which was used to measure tip frequency shifts above the bonds and their apparent lengths. Multiple bonds appeared brighter in the images because of stronger Pauli repulsion, and their shorter length was amplified by bending of the CO at the tip apex. Images detected with an atomic force microscope tip decorated with a carbon monoxide molecule could distinguish Pauling bond order. We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)–functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.

EFFICIENT PALLADIUM-CATALYZED CYCLOTRIMERIZATION OF ARYNES : SYNTHESIS OF TRIPHENYLENES

Found at the core of many discotic liquid crystals, triphenylenes can now be synthesized efficiently by the palladium-catalyzed trimerization of arynes, including those containing donor and acceptor substituents [Eq. (a); R=OMe, F].

Selected strategies for the synthesis of triphenylenes

This tutorial review surveys the most useful strategies for the synthesis of triphenylenes. It is aimed at organic chemists in general and, in particular, synthetic chemists interested in the development of advanced materials for organic electronic devices. The synthetic strategies considered are classified according to the structures of their key intermediates. Selected examples illustrate the variety of target structures and the method(s) of choice for their synthesis.

On-surface generation and imaging of arynes by atomic force microscopy

Reactive intermediates are involved in many chemical transformations. However, their characterization is a great challenge because of their short lifetimes and high reactivities. Arynes, formally derived from arenes by the removal of two hydrogen atoms from adjacent carbon atoms, are prominent reactive intermediates that have been hypothesized for more than a century. Their rich chemistry enables a widespread use in synthetic chemistry, as they are advantageous building blocks for the construction of polycyclic compounds that contain aromatic rings. Here, we demonstrate the generation and characterization of individual polycyclic aryne molecules on an ultrathin insulating film by means of low-temperature scanning tunnelling microscopy and atomic force microscopy. Bond-order analysis suggests that a cumulene resonance structure is the dominant one, and the aryne reactivity is preserved at cryogenic temperatures. Our results provide important insights into the chemistry of these elusive intermediates and their potential application in the field of on-surface synthesis.

From Perylene to a 22‐Ring Aromatic Hydrocarbon in One‐Pot

The successful synthesis of a threefold symmetric C78H36 molecule with 22 fused benzene rings is reported. This clover-shaped nanographene was characterized on an ultrathin insulating film with atomic resolution by scanning probe microscopy.

Substrate-Independent Growth of Atomically Precise Chiral Graphene Nanoribbons

Contributing to the need for new graphene nanoribbon (GNR) structures that can be synthesized with atomic precision, we have designed a reactant that renders chiral (3,1)-GNRs after a multistep reaction including Ullmann coupling and cyclodehydrogenation. The nanoribbon synthesis has been successfully proven on different coinage metals, and the formation process, together with the fingerprints associated with each reaction step, has been studied by combining scanning tunneling microscopy, core-level spectroscopy, and density functional calculations. In addition to the GNR’s chiral edge structure, the substantial GNR lengths achieved and the low processing temperature required to complete the reaction grant this reactant extremely interesting properties for potential applications.

[16]Cloverphene: a Clover-Shaped cata-Condensed Nanographene with Sixteen Fused Benzene Rings.

Abstract Cloverphenes are graphene-type polyarenes, the structures of which resemble the threefold symmetry of clover leafs. In their Communication (10.1002/anie.201104935), D. Pena and co-workers present the synthesis and properties of a nanosized [16]cloverphene derivative, which consists of 16 fused benzene rings (22 benzene rings in total) and 102 sp(2) -hybridized atoms. The synthesis involves sequential [4+2] and [2+2+2] aryne cycloadditions.

Die erste effiziente Palladium‐katalysierte Cyclotrimerisierung von Arinen: Synthese von Triphenylenen

Von reaktiven Zwischenstufen zu nutzlichen Produkten gelangt man durch die Palladium-katalysierte Trimerisierung intermediar gebildeter 1,2-Didehydrobenzole, die auch Donor- oder Acceptor-substituiert sein konnen [Gl. (a); R=OMe bzw. F]. Die dabei unter milden Bedingungen erhaltenen Triphenylene sind die zentrale Struktureinheit vieler Verbindungen, die discotische Flussigkristalle bilden.

Highly Selective Insertion of Arynes into a C(sp)−O(sp3) σ Bond

Arynes react with ethoxyacetylene to afford 2-ethoxyethynylaryl derivatives through a highly chemo- and regioselective formal insertion of the aryne into the C(sp)-O(sp(3)) bond of the alkyne. Computational studies suggest that the reaction does not proceed through a mechanism initiated by the nucleophilic addition of the oxygen atom to the aryne as previously proposed but by the addition of the triple bond of the alkyne to the aryne.

Aryne Insertion into I–I σ-Bonds

A new protocol for the efficient synthesis of o-diiodoarenes has been developed. This method allows the synthesis of substituted and polycyclic o-diiodoarenes, which are difficult to obtain by classical methods. This diiodination process involves the formal insertion of arynes into the I-I σ-bond.