Toward 2-Thiophyne: Ketocarbene versus Hetaryne Intermediates from 2-(Trimethylsilyl)thiophen-3-yl Triflate.

The reaction of 2-(trimethylsilyl)thiophen-3-yl triflate with CsF in the presence of 2,3,4,5-tetraphenylcyclopentadienone affords 4,5,6,7-tetraphenylbenzo[b]thiophene, as it would be expected from the hypothesized generation and trapping of 2-thiophyne. However, a detailed experimental and computational study discards the intermediacy of this elusive 5-membered hetaryne. Instead, a complex mechanism involving the generation of an intermediate ketocarbene, which adds to the cyclopentadienone to give an isolable tricyclic intermediate, followed by thermal rearrangements, is proposed.

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

2,6,10-Triphenylenotriyne: a star-shaped trisaryne.

During the last two decades aryne and bisaryne equivalents have been increasingly used as privileged building blocks for the synthesis of polycyclic aromatic hydrocarbons (PAHs). Here we report the synthesis and reactivity of an efficient precursor of the 2,6,10-triphenylenotriyne synthon, which constitutes the best example to date of a trisaryne equivalent on a benzofused polyaromatic core.

Dodecacene Generated on Surface: Reopening of the Energy Gap.

The acene series represents a model system to investigate the intriguing electronic properties of extended π-electron structures in the one-dimensional limit, which are important for applications in electronics and spintronics and for the fundamental understanding of electronic transport. Here, we present the on-surface generation of the longest acene obtained so far: dodecacene. Scanning tunneling spectroscopy gives access to the energy position and spatial distribution of its electronic states on the Au(111) surface. We observe that, after a progressive closing of the gap and a stabilization to about 1 eV at the length of decacene and undecacene, the energy gap of dodecacene unexpectedly increases to 1.4 eV. Considering the acene series as an exemplary general case, we discuss the evolution with length of the single tunneling resonances in comparison with ionization energy, electronic affinity, and optical gap.

Revisiting Kekulene: Synthesis and Single-Molecule Imaging.

Four decades after the first (and only) reported synthesis of kekulene, this emblematic cycloarene has been obtained again through an improved route involving the construction of a key synthetic intermediate, 5,6,8,9-tetrahydrobenzo[m]tetraphene, by means of a double Diels-Alder reaction between styrene and a versatile benzodiyne synthon. Ultra-high-resolution AFM imaging of single molecules of kekulene and computational calculations provide additional support for a molecular structure with a significant degree of bond localization in accordance with the resonance structure predicted by the Clar model.

Synthesis of Nanographenes, Starphenes, and Sterically Congested Polyarenes by Aryne Cyclotrimerization.

In recent years, synthetic transformations based on aryne chemistry have become particularly popular, mostly due to the spread of methods to generate these highly reactive intermediates in a controlled manner under mild reaction conditions. In fact, aryne cycloadditions such as the Diels-Alder reaction are nowadays widely used for the efficient preparation of polycyclic aromatic compounds. In 1998, our group discovered that arynes can undergo transition metal-catalyzed reactions, a finding that opened new perspectives in aryne chemistry. In particular, Pd-catalyzed [2 + 2 + 2] cycloaddition of arynes allowed the straightforward synthesis of triphenylene derivatives such as starphenes or cloverphenes. We found that this reaction is compatible with different substituents and sterically demanding arynes as starting materials. This transformation is especially useful to increase the molecular complexity in one single step, transforming molecules formed by n cycles in structures with 3n + 1 cycles. In fact, we took advantage of this protocol to prepare a large variety of sterically congested polycyclic aromatic hydrocarbons such as helicenes or twisted polyarenes. Soon after the discovery of the reaction, the co-cyclotrimerization of arynes with other reaction partners, such as electron deficient alkynes, significantly expanded the potential of this transformation. Also the use of catalysts based on alternative metals besides Pd (e.g., Ni, Cu, Au) or the use of other strained intermediates such as cycloalkynes or cycloallenes added value to this reaction. In addition, we realized that the Pd-catalyzed aryne cyclotrimerization reaction is particularly useful for the bottom-up preparation of well-defined nanographenes by chemical methods. Although the extreme insolubility of these planar nanographenes hampered their manipulation and characterization by conventional methods, recent advances in single molecule on-surface characterization by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with functionalized tips under ultrahigh vacuum (UHV) conditions, permitted the impressive visualization of these nanographenes with submolecular resolution, together with the examination of the corresponding molecular orbital densities. Moreover, arynes have been shown to possess a rich on-surface chemistry. In particular, arynes have been generated and studied on-surface, showing that the reactivity is preserved even at cryogenic temperatures. On-surface aryne cyclotrimerization was also demonstrated to obtain large starphene derivatives. Therefore, it is expected that the combination of aryne cycloadditions and on-surface synthesis will provide notable findings in the near future, including the "à la carte" preparation of graphene materials or the synthesis of elusive molecules with unique properties.

Exploring a Route to Cyclic Acenes by On-Surface Synthesis.

A route to generate cyclacenes by on-surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels-Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning-tunneling-microscopy(STM)-based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO-functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.

Synthesis and reactivity of a trigonal porous nanographene on a gold surface.

The synthesis of porous nanographenes is a challenging task for solution chemistry, and thus, on-surface synthesis provides an alternative approach. Here, we report the synthesis of a triporous nanographene with 102 sp2 carbon atoms by combining solution and surface chemistry. The carbon skeleton was obtained by Pd-catalyzed cyclotrimerization of arynes in solution, while planarization of the molecule was achieved through two hierarchically organized on-surface cyclodehydrogenation reactions, intra- and inter-blade. Remarkably, the three non-planar [14]annulene pores of this nanographene further evolved at higher temperatures showing interesting intra-porous on-surface reactivity.

Controlled Fragmentation of Single Molecules with Atomic Force Microscopy by Employing Doubly Charged States.

By atom manipulation we performed on-surface chemical reactions of a single molecule on a multilayer insulating film using noncontact atomic force microscopy. The single-electron sensitivity of atomic force microscopy allows us to follow the addition of single electrons to the molecule and the investigation of the reaction products. By performing a novel strategy based on long-lived doubly charged states a single molecule is fragmented. The fragmentation can be reverted by again changing the charge state of the system, characterizing a reversible reaction. The experimental results in addition to density-functional theory provide insight into the charge states of the different products and reaction pathways. Similar molecular systems could be used as charge-transfer units and to induce reversible chemical reactions.

[19]Dendriphene: A 19-Ring Dendritic Nanographene.

The synthesis of a threefold symmetric nanographene with 19 cata-fused benzene rings distributed within six branches is reported. This flat dendritic starphene, which is the largest unsubstituted cata-condensed PAH that has been obtained to date, was prepared in solution by means of a palladium-catalyzed aryne cyclotrimerization reaction and it was characterized on surface by scanning probe microscopy with atomic resolution.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses.

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining an in-solution Pd-catalyzed cycloaddition reaction and on-surface Au-promoted cyclodehydrogenation. The structure and electronic properties of the resulting three-fold symmetric C66H24 molecule have been characterized by scanning probe microscopy with atomic resolution and corroborated by theoretical modelling.

Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface.

On-surface synthesis provides a powerful method for the generation of long acene molecules, making possible the detailed investigation of the electronic properties of single higher acenes on a surface. By means of scanning tunneling microscopy and spectroscopy combined with theoretical considerations, we discuss the polyradical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. We present energy and spatial mapping of the tunneling resonances of hexacene, heptacene, and decacene, and discuss the role of molecular orbitals in the observed tunneling conductance maps. We show that the energy gap between the first electronic tunneling resonances below and above the Fermi energy stabilizes to a finite value, determined by a first diradical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the main contributor to the ground state of acenes remains the lowest-energy closed-shell electronic configuration.

Atomic Force Microscopy Identifying Fuel Pyrolysis Products and Directing the Synthesis of Analytical Standards.

Here we present a new method that integrates atomic force microscopy (AFM) with analytical tools such as high-performance liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) absorbance, and mass spectrometry (MS) along with synthetic chemistry. This allows the detection, identification, and quantification of novel polycyclic aromatic hydrocarbons (PAH) in complex molecular mixtures. This multidisciplinary methodology is employed to characterize the supercritical pyrolysis products of n-decane, a model fuel. The pyrolysis experiments result in a complex mixture of both unsubstituted as well as highly methylated PAH. We demonstrate the AFM-driven discovery of a novel compound, benz[ l]indeno[1,2,3- cd]pyrene, with the chemical structure assignment serving as input for the chemical synthesis of such molecule. The synthesis is verified by AFM, and the synthesized compound is used as a reference standard in analytical measurements, establishing the first-ever unequivocal identification and quantification of this PAH as a fuel product. Moreover, the high-resolution AFM analysis detected several five- to eight-ring PAH, which represents novel fuel pyrolysis and/or combustion products. This work provides a route to develop new analytical standards by symbiotically using AFM, chemical synthesis, and modern analytical tools.

Hexacene generated on passivated silicon.

On-surface synthesis represents a successful strategy to obtain designed molecular structures on an ultra-clean metal substrate. While metal surfaces are known to favor adsorption, diffusion, and chemical bonding between molecular groups, on-surface synthesis on non-metallic substrates would allow the electrical decoupling of the resulting molecule from the surface, favoring application to electronics and spintronics. Here, we demonstrate the on-surface generation of hexacene by surface-assisted reduction on a H-passivated Si(001) surface. The reaction, observed by scanning tunneling microscopy and spectroscopy, is probably driven by the formation of Si-O complexes at dangling bond defects. Supported by density functional theory calculations, we investigate the interaction of hexacene with the passivated silicon surface, and with single silicon dangling bonds.

Palladium-catalyzed cocyclotrimerization of arynes with a pyramidalized alkene.

The metal-catalyzed [2+2+2] cocycloaddition of arynes with pyramidalized alkenes is presented. The generation of a highly reactive pyramidalized alkene in the presence of a large excess of in situ-produced arynes led to the corresponding cocyclotrimerization (1 : 2)-adducts in good yields, establishing the first example of a palladium-based reaction of a pyramidalized alkene.

Site-selective reversible Diels-Alder reaction between a biphenylene-based polyarene and a semiconductor surface.

Understanding the mechanisms involved in the covalent attachment of organic molecules to surfaces is a major challenge for nanotechnology and surface science. On the basis of classical organic chemistry mechanistic considerations, key issues such as selectivity and reactivity of the organic adsorbates could be rationalized and exploited for the design of molecular-scale circuits and devices. Here we use tris(benzocyclobutadieno)triphenylene, a singular Y-shaped hydrocarbon containing antiaromatic cyclobutadienoid rings, as a molecular probe to study the reaction of polycyclic conjugated molecules with atomic scale moieties, dangling-bond (DB) dimers on a hydrogen-passivated Ge(001):H surface. By combining molecular design, synthesis, scanning tunneling microscopy and spectroscopy (STM/STS) and computational modeling, we show that the attachment involves a concerted [4+2] cycloaddition reaction that is completely site-selective and fully reversible. This selectivity, governed by the bond alternation induced by the presence of the cyclobutadienoid rings, allows for the control of the orientation of the molecules with respect to the surface DB-patterning. We also demonstrate that by judicious modification of the electronic levels of the polycyclic benzenoid through substituents, the reaction barrier height can be modified. Finally, we show that after deliberate tip-induced covalent bond cleavage, adsorbed molecules can be used to fine tune the electronic states of the DB dimer. This power to engineer deliberately the bonding configuration and electronic properties opens new perspectives for creating prototypical nanoscale circuitry.

Studying an antiaromatic polycyclic hydrocarbon adsorbed on different surfaces.

Antiaromatic and open-shell molecules are attractive because of their distinct electronic and magnetic behaviour. However, their increased reactivity creates a challenge for probing their properties. Here, we describe the on-surface and in-solution generation and characterisation of a highly reactive antiaromatic molecule: indeno[1,2-b]fluorene (IF). In solution, we generated IF by KI-induced dehalogenation of a dibromo-substituted precursor molecule and found that IF survives for minutes at ambient conditions. Using atom manipulation at low temperatures we generated IF on Cu(111) and on bilayer NaCl. On these surfaces, we characterised IF by bond-order analysis using non-contact atomic force microscopy with CO-functionalised tips and by orbital imaging using scanning tunnelling microscopy. We found that the closed-shell configuration and antiaromatic character predicted for gas-phase IF are preserved on the NaCl film. On Cu(111), we observed significant bond-order reorganisation within the s-indacene moiety because of chemisorption, highlighting the importance of molecule surface interactions on the π-electron distribution.

On-Surface Route for Producing Planar Nanographenes with Azulene Moieties.

Large aromatic carbon nanostructures are cornerstone materials due to their increasingly active role in functional devices, but their synthesis in solution encounters size and shape limitations. New on-surface strategies facilitate the synthesis of large and insoluble planar systems with atomic-scale precision. While dehydrogenation is usually the chemical zipping reaction building up large aromatic carbon structures, mostly benzenoid structures are being produced. Here, we report on a new cyclodehydrogenation reaction transforming a sterically stressed precursor with conjoined cove regions into a planar carbon platform by incorporating azulene moieties in their interior. Submolecular resolution STM is used to characterize this exotic large polycyclic aromatic compound on Au(111) yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction. The resulting polycyclic aromatic carbon structure shows a [18]annulene core hosting peculiar pore states confined at the carbon cavity.

Generation and Characterization of a meta-Aryne on Cu and NaCl Surfaces.

We describe the generation of a meta-aryne at low temperature (T = 5 K) using atomic manipulation on Cu(111) and on bilayer NaCl on Cu(111). We observe different voltage thresholds for dehalogenation of the precursor and different reaction products depending on the substrate surface. The chemical structure is resolved by atomic force microscopy with CO-terminated tips, revealing the radical positions and confirming a diradical rather than an anti-Bredt olefin structure for this meta-aryne on NaCl.

Decacene: On-Surface Generation.

Acenes are intriguing molecules with unique electronic properties. The difficulties in their preparation owing to low stability under ambient conditions are apparent because successful syntheses of long unsubstituted acenes are still scarce, in spite of the great attention they have attracted. Only unsubstituted acenes up to heptacene have been isolated in bulk, with nonacene being the largest acene detected to date. Herein we use on-surface assisted reduction of tetraepoxy decacene precursors on Au(111) as the key step to generate unprecedented decacene which is visualized and its electronic resonances studied by scanning tunneling microscopy (STM) and spectroscopy (STS).