ß-Disubstituted Pentacene Derivatives: Thin Film Structural Properties and Four-Probe Field Effect Mobility.

2,9- and 2,10-diphenylpentacene were synthesized by direct C-H borylation of ketal-protected pentacene, followed by halodeboronation, resolution of the dihalo isomers, Suzuki arylation, cleavage of the ketals and decarbonylation in the solid state. They were studied as main active components in organic field effect transistors (OFETs). Diphenyl substitution of pentacene affects the unit cell dimensions only slightly, preserving a face to edge molecular packing in the first layers of thin films evaporated on SiO2 substrates. Both isomers self-assemble into nanoribbons during the thin film growth upon vapor deposition. The similarity between the surface induced phases of the 2,9-isomer and unsubstituted pentacene leads to similar 4-probe hole mobilities, i. e. 0.13 cm2 V-1 s-1 for the former. Whereas 2,9-disubstitution thus does essentially preserve the thin film characteristics of unsubstituted pentacene, 2,10-disubstitution is detrimental to the molecular ordering in the thin films and therefore to the field effect mobility which is only 0.07 cm2 V-1 s-1. The known strong enhancement of field effect mobility observed upon diphenyl substitution of anthracene can thus not be emulated analogously with pentacene.

Extending on-surface synthesis from 2D to 3D by cycloaddition with C60.

As an efficient molecular engineering approach, on-surface synthesis (OSS) defines a special opportunity to investigate intermolecular coupling at the sub-molecular level and has delivered many appealing polymers. So far, all OSS is based on the lateral covalent bonding of molecular precursors within a single molecular layer; extending OSS from two to three dimensions is yet to be realized. Herein, we address this challenge by cycloaddition between C60 and an aromatic compound. The C60 layer is assembled on the well-defined molecular network, allowing appropriate molecular orbital hybridization. Upon thermal activation, covalent coupling perpendicular to the surface via [4 + 2] cycloaddition between C60 and the phenyl ring of the molecule is realized; the resultant adduct shows frozen orientation and distinct sub-molecular feature at room temperature and further enables lateral covalent bonding via [2 + 2] cycloaddition. This work unlocks an unconventional route for bottom-up precise synthesis of three-dimensional covalently-bonded organic architectures/devices on surfaces.

Tuning the Diradical Character of Pentacene Derivatives via Non-Benzenoid Coupling Motifs.

The development of functional organic molecules requires structures of increasing size and complexity, which are typically obtained by the covalent coupling of smaller building blocks. Herein, with the aid of high-resolution scanning tunneling microscopy/spectroscopy and density functional theory, the coupling of a sterically demanded pentacene derivative on Au(111) into fused dimers connected by non-benzenoid rings was studied. The diradical character of the products was tuned according to the coupling section. In particular, the antiaromaticity of cyclobutadiene as the coupling motif and its position within the structure play a decisive role in shifting the natural orbital occupancies toward a stronger diradical electronic character. Understanding these structure-property relations is desirable not only for fundamental reasons but also for designing new complex and functional molecular structures.

Preparative-scale synthesis of nonacene.

During the last years we have witnessed progressive evolution of preparation of acenes with length up to dodecacene by on-surface synthesis in ultra-high vacuum or generation of acenes up to decacene in solid matrices at low temperatures. While these protocols with very specific conditions produce the acenes in amount of few molecules, the strategies leading to the acenes in large quantities dawdle behind. Only recently and after 70 years of synthetic attempts, heptacene has been prepared in bulk phase. However, the preparative scale synthesis of higher homologues still remains a formidable challenge. Here we report the preparation and characterisation of nonacene and show its excellent thermal and in-time stability.

Subsurface-Carbon-Induced Local Charge of Copper for an On-Surface Displacement Reaction.

Transition-metal carbides have sparked unprecedented enthusiasm as high-performance catalysts in recent years. Still, the catalytic properties of copper carbide remain unexplored. By introducing subsurface carbon to Cu(111), a displacement reaction of a proton in a carboxyl acid group with a single Cu atom is demonstrated at the atomic scale and room temperature. Its occurrence is attributed to the C-doping-induced local charge of surface Cu atoms (up to +0.30 e/atom), which accelerates the rate of on-surface deprotonation via reduction of the corresponding energy barrier, thus enabling the instant displacement of a proton with a Cu atom when the molecules adsorb on the surface. This well-defined and robust Cuδ+ surface based on subsurface-carbon doping offers a novel catalytic platform for on-surface synthesis.

On-Surface Decarboxylation Coupling Facilitated by Lock-to-Unlock Variation of Molecules upon the Reaction.

On-surface synthesis (OSS) involving relatively high energy barriers remains challenging due to a typical dilemma: firm molecular anchor is required to prevent molecular desorption upon the reaction, whereas sufficient lateral mobility is crucial for subsequent coupling and assembly. By locking the molecular precursors on the substrate then unlocking them during the reaction, we present a strategy to address this challenge. High-yield synthesis based on well-defined decarboxylation, intermediate transition, and hexamerization is demonstrated, resulting in an extended and ordered network exclusively composed of the newly synthesized macrocyclic compound. Thanks to the steric hindrance of its maleimide group, we attain a preferential selection of the coupling. This work unlocks a promising path to enrich the reaction types and improve the coupling selectivity hence the structual homogeneity of the final product for OSS.

Synthesis and Absorption Properties of Long Acenoacenes.

Acenes, polyaromatic hydrocarbons composed of linearly fused benzene rings have received immense attention due to their performance as semiconductors in organic optoelectronic applications. Their appealing physicochemical properties, such as extended delocalization, high charge carrier mobilities, narrow HOMO-LOMO gaps and partially radical character in the ground state make them very attractive targets for many potential applications. However, the intrinsic synthetic challenges of unsubstituted members such as high reactivity and poor solubility are still limiting factors for their wider exploitation. Herein, we report a simple general synthesis of a new family of angularly fused acenoacenes with improved stability compared to their isoelectronic linear counterparts. The synthesis and comprehensive characterization of pentacenopentacene, pentacenohexacene and hexacenohexacene, with lengths between decacene and dodecacene, are disclosed.

Preparation of Tetrabenzo[4.4.2]undecastarphene by On-Surface Synthesis.

A large dissymmetric starphene molecule, the tetrabenzo[a,c,u,w]naphtho[2,3-l]nonaphene, was obtained by first preparing a soluble precursor which was then sublimated on a Au(111) surface in an ultra-high vacuum. In a second step, controlled annealings from 200 °C to 275 °C initiated two successive cyclodehydrogenation steps with the formation of 3 new carbon-carbon bonds. A second conformer was also stable enough during the annealing step to give another compound in similar yield, the benzodibenzo[7,8,9,10]naphthaceno[2,1-h]phenanthro[9,10-p]hexaphene. The formation of this more-hindered species stresses the importance of strong molecule-surface interactions during the cyclodehydrogenations steps of these large polyaromatic hydrocarbons.

A Large Starphene Comprising Pentacene Branches.

Starphenes are attractive compounds due to their characteristic physicochemical properties that are inherited from acenes, making them interesting compounds for organic electronics and optics. However, the instability and low solubility of larger starphene homologs make their synthesis extremely challenging. Herein, we present a new strategy leading to pristine [16]starphene in preparative scale. Our approach is based on a synthesis of a carbonyl-protected starphene precursor that is thermally converted in a solid-state form to the neat [16]starphene, which is then characterised with a variety of analytical methods, such as 13 C CP-MAS NMR, TGA, MS MALDI, UV/Vis and FTIR spectroscopy. Furthermore, high-resolution STM experiments unambiguously confirm its expected structure and reveal a moderate electronic delocalisation between the pentacene arms. Nucleus-independent chemical shifts NICS(1) are also calculated to survey its aromatic character.

On-Surface Synthesis of Chlorinated Narrow Graphene Nanoribbon Organometallic Hybrids.

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives-organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired.

Long-range ordered and atomic-scale control of graphene hybridization by photocycloaddition.

Chemical reactions that convert sp2 to sp3 hybridization have been demonstrated to be a fascinating yet challenging route to functionalize graphene. So far it has not been possible to precisely control the reaction sites nor their lateral order at the atomic/molecular scale. The application prospects have been limited for reactions that require long soaking, heating, electric pulses or probe-tip press. Here we demonstrate a spatially selective photocycloaddition reaction of a two-dimensional molecular network with defect-free basal plane of single-layer graphene. Directly visualized at the submolecular level, the cycloaddition is triggered by ultraviolet irradiation in ultrahigh vacuum, requiring no aid of the graphene Moiré pattern. The reaction involves both [2+2] and [2+4] cycloadditions, with the reaction sites aligned into a two-dimensional extended and well-ordered array, inducing a bandgap for the reacted graphene layer. This work provides a solid base for designing and engineering graphene-based optoelectronic and microelectronic devices.

On-Surface Synthesis with Atomic Hydrogen.

Surface-assisted synthesis has become a powerful approach for generation of molecular nanostructures, which could not be obtained via traditional solution chemistry. Nowadays there is an intensive search for reactions that could proceed on flat surfaces in order to boost the versatility and applicability of synthesized nano-objects. Here we propose application of atomic hydrogen combined with on-surface synthesis in order to tune the reaction pathways. We demonstrate that atomic hydrogen could be widely applied: (1) as a cleaning tool, which allows removal of halogen residues from the surface after Ullmann couplings/polymerization, (2) by reaction with surface organometallics to provide stable hydrogenated species, and (3) as a reagent for debromination or desulfurization of adsorbed species.

A Practical General Method for the Preparation of Long Acenes.

The field of long acenes, the narrowest of the zig-zag graphene nanoribbons, has been an area of significant interest in the past decade because of its potential applications in organic electronics, spintronics and plasmonics. However the low solubility and high reactivity of these compounds has so far hindered their preparation on large scales. We report here a concise strategy for the synthesis of higher acenes through Diels-Alder condensation of arynes with a protected tetraene ketone. After deprotection by cleavage of the ketal, the obtained monoketone precursors cleanly yield the corresponding acenes through quantitative cheletropic thermal decarbonylation in the solid state, at moderate temperatures of 155 to 205 °C. This approach allows the preparation of heptacene, benzo[a]hexacene, cis- and trans-dibenzopentacene and offers a valuable new method for the synthesis of even larger acenes.

Three-dimensional hydrogen bonding between Landers and planar molecules facilitated by electrostatic interactions with Ni adatoms.

Using a combination of UHV-STM and molecular mechanics calculations, we investigate the surface self-assembly of a complex multi-component metal-molecule system with synergistic non-covalent interactions. Hydrogen bonding between three-dimensional Lander-DAT molecules and planar PTCDI molecules, adsorbed closer to the surface, is found to be facilitated by electrostatic interactions between co-adsorbed Ni adatoms and the flexible molecular DAT groups.

Unimolecular Logic Gate with Classical Input by Single Gold Atoms.

By a combination of solution and on-surface chemistry, we synthesized an asymmetric starphene molecule with two long anthracenyl input branches and a short naphthyl output branch on the Au(111) surface. Starting from this molecule, we could demonstrate the working principle of a single molecule NAND logic gate by selectively contacting single gold atoms by atomic manipulation to the longer branches of the molecule. The logical input "1" ("0") is defined by the interaction (noninteraction) of a gold atom with one of the input branches. The output is measured by scanning tunneling spectroscopy following the shift in energy of the electronic tunneling resonances at the end of the short branch of the molecule.

Bicomponent Supramolecular Architectures at the Vacuum-Solid Interface.

This review aims at giving the readers the basic concepts needed to understand two-dimensional bimolecular organizations at the vacuum-solid interface. The first part describes and analyzes molecules-molecules and molecules-substrates interactions. The current limitations and needs in the understanding of these forces are also detailed. Then, a critical analysis of the past and recent advances in the field is presented by discussing most of the key papers describing bicomponents self-assembly on solid surface in an ultrahigh vacuum environment. These sections are organized by considering decreasing molecule-molecule interaction strengths (i.e. starting from strong directional multiple H bonds up to weaker nondirectional bonds taking into account the increasing fundamental role played by the surface). Finally, we conclude with some research directions (predicting self-assembly, multi-components systems, and nonmetallic surfaces) and potential applications (porous networks and organic surfaces).

Simple and economic elaboration of high purity CaCO3 particles for bone graft applications using a spray pyrolysis technique.

Materials for bone grafting are a major health stake today. The existing synthetic materials only partially answer to the requirements for such uses and do not provide a real alternative solution to the classical way consisting of the reuse of natural osseous materials, known to create other problems. In the present work, we have prepared calcium carbonate powders using spray pyrolysis in the perspective to realize ideal and multi-purposed bricks to form bone graft materials, by bottom-up approaches, using classical techniques. The chemical reactions involved in the synthesis process consist in the thermal decomposition of calcium acetate. This precursor, a classical food additive, is known to be non toxic, easy to use and very cheap. Morphological and chemical characterization was conducted on the obtained materials. Depending on the pyrolysis temperature, we observed partial decomposition of calcium acetate or lime formation. An annealing step is consequently required to finalize the transformation. Finally, it appears that annealed products are pure calcite particles, having a spherical morphology and possessing a low dispersity in the micron size range. These particles exhibit an original internal structure, reinforcing and ensuring the preservation of their morphological characteristics during the sintering process. Furthermore, a porosity value compatible with drug carrier capabilities was measured on the obtained products. All these qualities allow us to consider a multi-purpose use of these particles in almost all types of bone graft materials.

Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene.

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.