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On-Surface Formation of One-Dimensional Polyphenylene through Bergman Cyclization
Abstract
On-surface fabrication of covalently interlinked conjugated nanostructures has attracted significant attentions mainly due to their high stability and efficient electron transportability. Here, from the interplay of scanning tunneling microscopy imaging and density functional theory calculations we have for the first time reported on-surface formation of one-dimensional polyphenylene chains through Bergman cyclization followed by radical polymerization on Cu(110). The formed surface nanostructures are further corroborated by the results of the ex-situ synthesized molecular product after Bergman cyclization. These findings are of particular interest and importance to the construction of molecular electronic nanodevices on surfaces.
... Ullmann coupling [16, 65, 79, 80, 85, 90, 91, 103, 106, 108, 111-113, 119, 122, 132, 136, 141, 143, 157, 168-170, 172, 179] Dehalogenative coupling [60-62, 66, 67, 151] Dehalogenative coupling of alkynyl bromide [63][64][65] Sonogashira coupling [63] Glaser coupling [24,25,71,72,144,145,148,149] Alkyne cyclotrimerization [74,174] Alkyne addition [69] Bergman cyclization [142] Acetylene polymerization [160,161] Dehydrogenative coupling [26,27,76,79] Cyclodehydrogenation [19,21,79,80,90,103,106,119,122,142] [1 + 1+1 + 1] cycloaddition [98] [2 + 2] cycloaddition [85,158] C-X and C-H cross-coupling [18] Superhydrogenation facilitated coupling [86] Decarbonylation reaction [89] H-F elimination-based zipping [95] Desilylative homocoupling [150] Decarboxylative coupling [159] α-diazo ketones chemistry [162] Acetyl polymerization [175] Condensation of boric acid [81,82] Schiff base reaction [83,84] (c) Reaction pathway from OM dimer to covalent product via regioselective aryl-aryl coupling. Adapted from [18] with permission of The Royal Society of Chemistry. ...
... Ullmann coupling [16, 65, 79, 80, 85, 90, 91, 103, 106, 108, 111-113, 119, 122, 132, 136, 141, 143, 157, 168-170, 172, 179] Dehalogenative coupling [60-62, 66, 67, 151] Dehalogenative coupling of alkynyl bromide [63][64][65] Sonogashira coupling [63] Glaser coupling [24,25,71,72,144,145,148,149] Alkyne cyclotrimerization [74,174] Alkyne addition [69] Bergman cyclization [142] Acetylene polymerization [160,161] Dehydrogenative coupling [26,27,76,79] Cyclodehydrogenation [19,21,79,80,90,103,106,119,122,142] [1 + 1+1 + 1] cycloaddition [98] [2 + 2] cycloaddition [85,158] C-X and C-H cross-coupling [18] Superhydrogenation facilitated coupling [86] Decarbonylation reaction [89] H-F elimination-based zipping [95] Desilylative homocoupling [150] Decarboxylative coupling [159] α-diazo ketones chemistry [162] Acetyl polymerization [175] Condensation of boric acid [81,82] Schiff base reaction [83,84] (c) Reaction pathway from OM dimer to covalent product via regioselective aryl-aryl coupling. Adapted from [18] with permission of The Royal Society of Chemistry. ...
... Other routes are also employed on surfaces to attain the polyphenylene by either careful precursor designing or different activating modes. Figure 12(a) shows a reaction pathway from enediyne precursor, which undergoes Bergman cyclization into biradicals, followed by radical addition into the ultimate 1D polyphenylene on Cu(110) [142]. The STM morphologies for intact monomer and polymer chain are shown in figures 12(b) and (c), respectively. ...
On-surface synthesis, as an alternative to traditional in-solution synthesis, has become an emerging research field and attracted extensive attention over the past decade due to its ability to fabricate nanoarchitectures with exotic properties. Compared to wet chemistry, the on-surface synthesis conducted on atomically flat solid surfaces under ultrahigh vacuum exhibits unprecedented characteristics and advantages, opening novel reaction pathways for chemical synthesis. Various low-dimensional nanostructures have been fabricated on solid surfaces (mostly metal surfaces) based on this newly developed approach. This paper reviews the classic and latest works regarding carbon-based low-dimensional nanostructures since the arrival of on-surface synthesis era. These nanostructures are categorized into zero-, one- and two-dimensional classes and each class is composed of numerous sub-nanostructures. For certain specific nanostructures, comprehensive reports are given, including precursor design, substrate choice, synthetic strategies and so forth. We hope that our review will shed light on the fabrication of some significant nanostructures in this young and promising scientific area.
... Cascade cyclizations: accessing TM-polyacene hybrid materials using i-BC. The Bergman cyclization of enediynes is a promising tool for generating carbon-rich materials such as carbon nanotubes and carbon-nano-onions 61,62 . However, its utility in polymer and material sciences is limited due to its inherent drawbacks, such as the self-quenching of diradicals, the high reactivity of diradicals, and the unwanted 5-endo cyclization leading to fulvene radicals causing large polydispersities 61 . ...
... The Bergman cyclization of enediynes is a promising tool for generating carbon-rich materials such as carbon nanotubes and carbon-nano-onions 61,62 . However, its utility in polymer and material sciences is limited due to its inherent drawbacks, such as the self-quenching of diradicals, the high reactivity of diradicals, and the unwanted 5-endo cyclization leading to fulvene radicals causing large polydispersities 61 . In this regard, our investigation of i-BC triggered by the Au(I)PMe 3 /BH 2 couple provides a viable alternative for initiating a defect-free polymerization (Figs. 12, 13). ...
With the rapid growth in artificial intelligence, designing high-speed and low-power semiconducting materials is of utmost importance. This investigation provides a theoretical basis to access covalently bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. Systematic optimization of substrates containing remotely placed boryl groups and the transition metals produced the zwitterions via ionic Bergman cyclization (i-BC) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC, the subsequent steps were barrierless, which involved structureless transition regions. Multivariate analysis revealed the strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman (r-BC), ionic Bergman (i-BC), and ionic Schreiner-Pascal (i-SP) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by the three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP.
... The advantage of on-surface synthesis relates to the highcontrollability of the structures, which can be defined by the employed precursor molecules. [24] So far, various surface reactions have been demonstrated, such as Ullmann-type coupling, [18,22,25] Glaser-type coupling, [26][27][28] Bergman-type reaction, [29,30] Sonogashira-type coupling, [31][32][33] and dehydrogenated coupling. [34,35] Very recently, CÀ Si bond coupling was obtained by reacting bromo-substituted molecules to Si atoms directly on Au(111), leading to the formation of 1,4disilabenzene bridged covalent organic frameworks (COFs) and graphene nanoribbons (GNRs). ...
The incorporation of Si atoms into organic compounds significantly increases a variety of functionality, facilitating further applications. Recently, on‐surface synthesis was introduced into organosilicon chemistry as 1,4‐disilabenzene bridged nanostructures were obtained via coupling between silicon atoms and brominated phenyl groups at the ortho position on Au(111). Here, we demonstrate a high generality of this strategy via syntheses of silole derivatives and nanoribbon structures with eight‐membered sila‐cyclic rings from dibrominated molecules at the bay and peri positions on Au(111), respectively. Their structures and electronic properties were investigated by a combination of scanning tunneling microscopy/spectroscopy and density functional theory calculations. This work demonstrates a great potential to deal with heavy group 14 elements in on‐surface silicon chemistry.
... The advent of bond-resolved non-contact atomic force microscopy (nc-AFM) 31,32 accelerated the precise fabrication of covalent organic nanoarchitectures [33][34][35][36] . Specifically, aryl-alkynes proved useful in a plethora of on-surface reactions 37,38 , such as homo-coupling [39][40][41][42][43][44] , cycloaddition 32,[45][46][47][48] , and cross-coupling with other functional groups [49][50][51][52] . The versatile chemical properties of terminal alkynes limit the control over the reaction pathways, inevitably entailing side products, incidentally mainly appearing in the form of enynes [53][54][55][56][57] . ...
Belonging to the enyne family, enetriynes comprise a distinct electron-rich all-carbon bonding scheme. However, the lack of convenient synthesis protocols limits the associated application potential within, e.g., biochemistry and materials science. Herein we introduce a pathway for highly selective enetriyne formation via tetramerization of terminal alkynes on a Ag(100) surface. Taking advantage of a directing hydroxyl group, we steer molecular assembly and reaction processes on square lattices. Induced by O2 exposure the terminal alkyne moieties deprotonate and organometallic bis-acetylide dimer arrays evolve. Upon subsequent thermal annealing tetrameric enetriyne-bridged compounds are generated in high yield, readily self-assembling into regular networks. We combine high-resolution scanning probe microscopy, X-ray photoelectron spectroscopy and density functional theory calculations to examine the structural features, bonding characteristics and the underlying reaction mechanism. Our study introduces an integrated strategy for the precise fabrication of functional enetriyne species, thus providing access to a distinct class of highly conjugated π-system compounds.
... 59,60 However, due to undesirable self-quenching of diradicals, side reaction due to high reactivity of diradicals, and the unwanted 5-endo cyclization leading to fulvene radicals causing large polydispersities, its utility in polymer and material sciences is limited. 59 In this regards, our discovery of i-BC-triggered by Au(I)PMe3/boron couple ( Figure 12 and Table 1), provides a lucrative alternate for initiating a defect-free polymerization. For the model study, we chose polyenynes that could produce three consecutive aromatic rings. ...
With a rapid growth in artificial intelligence, designing high-speed and low power semiconducting material is of utmost importance. This investigation provides a theoretical basis to access covalently-bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. A systematic optimization of substrates containing remotely-placed boronyl groups and the transition metals produced the zwitterions via ionic Bergman cyclization ( i-BC ) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC , the subsequent steps showed plateau-TS involving structureless transition regions. A multivariate analysis revealed a strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman ( r-BC ), ionic Bergman ( i-BC ), and ionic Schreiner-Pascal ( i-SP ) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP .
... [2][3][4] In this regard, on-surface synthesis has proven to be a powerful tool, offering two primary advantages: (1) The employment of ultrahigh vacuum (UHV) conditions and catalytic metal surfaces provide a fertile playground for synthesizing unstable products [5][6][7] which are hard to obtain with traditional solution-based chemistry; (2) reaction intermediates and products can be accessed in unprecedented details via surface sensitive techniques, thus providing key mechanistic information on the reaction pathway. [8][9][10][11] Several important chemical reactions have been developed for preparing new compounds and covalently interlinked nanostructures, such as Ullman coupling, [12][13][14] imine formation, 15,16 dehydration of boronic acids, 17 Bergman cyclization, 18,19 Glaser coupling, 20,21 click reaction, 22 to mention a few. Most notably, the formation of intermolecular C-C bonds after activations of the C-X (X = Br, I, or H) bonds has been most extensively studied to fabricate a spectrum of carbon nanostructures. ...
On-surface synthesis has been a subject of intensive research during the last decade. Various chemical reactions have been developed on surfaces to prepare compounds and carbon nanostructures, most of which are centered on the carbon-carbon bond formation. Despite the vast progress so far, the diversity of functional groups in organic chemistry has been far less explored in on-surface synthesis. Herein, we study the surface-assisted synthesis of ethers through the homocoupling of hydroxymethyl substituents on Ag(111). By using two hydroxymethyl substituent functionalized molecular precursors with different symmetries, we have achieved the formation of ether chains and rings. High-resolution scanning tunneling microscopy complemented with density functional theory calculations are used to support our findings and offer mechanistic insights into the reaction. This work expands the toolbox of on-surface reactions for the bottom-up fabrication of more sophisticated functional nanostructures.
... 163 Terminal alkyne groups may also undergo cyclization reactions such as the Bergman cyclization. 168,171,172 Cycloaddition reactions, such as the [2+2+2] 164, 173-175 and [1+1+1+1] 176 types are another possible route towards generating conjugated cyclic linkages. Other varieties such as cascade-type cyclization reactions 34,177 have also been studied, and allow the formation of polymers after the cyclization step. ...
The mature research field of conventional solution-based chemistry has allowed for a continuous generation of increasingly refined materials with amazing physical and chemical properties. This has contributed to great advances in the field of organic nanomaterials. The development of the recently revealed surface-supported synthesis under ultrahigh vacuum conditions has opened new doors for the formation of defect-free low-dimensional carbon nanostructures that are not achievable by conventional means. This new field, typically termed on-surface synthesis (OSS), is lately showing a booming growth that resonates with almost all the requirements for promising quantum size materials with tunable physicochemical properties.
In this thesis, this novel approach is applied for the growth of a variety of materials, followed by a comprehensive characterization of the structural, chemical and electronic properties of the intermediates and end-products. This is achieved mainly by scanning tunneling microscopy operating at low temperature (4.3 K) in various measurement modes, assisted by theoretical calculations (DFT and probe particle methods). In particular, the following systems have been studied:
(i) Metal-organic chemistry, in which the stereospecific coupling motif of gold thiolates is found to be controllable by varying the on-surface reaction parameters (i.e. coverage and substrate temperature).
(ii) Pyrene-based chemistry. With alkyne-functionalized pyrene molecules, the impact of the presence of metal-organic complexes on alkyne coupling reactions is investigated. The complexes do not only modify the reaction outcome, but also reduce its activation temperature threshold, becoming an excellent example of second-order on-surface synthesis (i.e. OSS of catalyst to steer following OSS reactions). Secondly, pyrenes are used that are functionalized with alkynes and also Br atoms, allowing for alkyne homo- and cross-coupling reactions. The resulting products and their relative abundance are explored thoroughly after different annealing steps, revealing a dominant Glaser-coupling product upon heating. The absence of the latter when using non-halogenated precursor evidenced the role of Br in this coupling reaction. Importantly, a systematic comparison of the electronic properties of the different product structures has allowed drawing important structure-property relations with regard to the various inter-pyrene coupling motifs.
(iii) Acene-based chemistry. The synthesis of higher acenes is studied, in particular heptacene on Ag(001), starting from tetrahydroheptacene molecular precursors with and without additional functionalization by Br atoms. A variety of stable reaction intermediates is found, often resulting from hydrogen migration events. However, heptacene is the end-product from the two precursors and displays a fully charged LUMO orbital. Focusing on the intermediates, two sp3-hybridized carbon atoms present at the second ring of dihydroheptacene are shown as sufficient self-decoupling factors. Magnetic Kondo fingerprints associated to specific charging scenarios are investigated. Finally, the impact of the surface structure on coupling reactions with acene-based precursors is then explored. More specifically, Au(111) and Au(110) surfaces, as well as anthracene precursors functionalized with halogen and methyls are used with this aim. On Au(111), the atomically flat surface, five covalent coupling motifs are found, with the 3-fold symmetric starphene showing the highest occurrence upon annealing. In contrast, on Au(110), being a 2×1 reconstructed surface, the uniaxial symmetry of the surface promotes the formation of mainly linear non-benzenoid polymers along the grooves of this substrate.
This thesis contributes to the surface science field by showing the possibility to synthesize a variety of novel carbon-based nanomaterials using on-surface synthesis methods. The second main contribution is the detailed characterization of the produced materials using surface-sensitive techniques assisted by theoretical calculations. A deep understanding of the physicochemical properties of these materials is a crucial step forward towards their ultimately progressing applications.
... [5][6][7] Among these are coupling protocols yielding products similar to Ullmann, 8,9 Glaser, 10,11 and Sonogashira reactions, 12,13 as well as intramolecular processes like Bergman cyclization. 14,15 Moreover, on-surface synthesis enables the construction of molecules that are challenging or not accessible via conventional solution chemistry. [16][17][18][19] Therefore, the exploration of new on-surface chemistry is highly desirable to complement the limited tool kits and realize increasingly complex architectures. ...
Intramolecular methyl–methyl coupling on Au (111) is explored as a new on-surface protocol for edge extension in graphene nanoribbons (GNRs). Characterized by high-resolution scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy, the methyl–methyl coupling is proven to indeed proceed at the armchair edges of the GNRs, forming six-membered rings with sp3- or sp2-hybridized carbons.
... In the reaction, thermal catalysis with metal surface atoms and/or adatoms leads to transformation and conjugation of molecules. [19][20][21] Since the first systematic demonstration by Grill et al., [19] many efforts have been devoted to develop various on-surface reactions, such as Ullmann-type reaction, [19,22,23] Glaser-type reaction, [24][25][26] Bergman-type reaction [27,28] and so on. In contrast to wet chemistry, the onsurface reaction is free from the solubility issue and thus becomes of central importance to synthesize large polycyclic aromatic hydrocarbons (PAHs) such as graphene nanoribbons. ...
Cyclo‐dehydrogenation is of importance to induce the planarization of molecules on noble surfaces upon annealing. In contrast to a number of successful syntheses of polycyclic aromatic hydrocarbons by forming carbon–carbon bonds, it is still rare to conduct conjugation and cleavage of carbon–nitrogen bonds in molecules. Here, we present a systematic transformation of the C−N bonds in11,11,12,12‐tetraphenyl‐1,4,5,8‐tetraazaanthraquinodimethane as well as three other derivatives on Au(111). With bond‐resolved scanning tunneling microscopy, we discovered novel the “heterocyclic segregation” reaction of one pyrazine ring with two nitrogen atoms to form two quinoline rings with one nitrogen each. Density functional theory calculations showed that the intramolecular ring‐forming and ‐opening of N‐heterocycles are strongly affected by the initial hydrogen–substrate interaction.
... For example, the state-of-the-art resolution can be routinely obtained with tip apex functionalized by a small CO molecule. 10 The seminal work of covalent coupling of brominated tetraphenyl-porphyrins demonstrated in 2007 by Grill et al. ignited an explosive growth in the field of on-surface reactions 7 inspired by their in-solution counterparts, such as Ullmann, 7,11 Bergman, 12,13 Glaser, 14,15 Sonogashira, 16 Schiff-base, 17 and dehydrogenation 18,19 reactions. A variety of carbon-based nanostructures have been successfully synthesized, including nanographenes (NGs), 20 graphene nanoribbons (GNRs), 21 and covalent organic frameworks (COFs). ...
On-surface synthesis has been developed into a promising research field for fabricating low-dimensional materials with great potential in tailoring structures, functionality, and, thus, desired chemical and physical properties. Thus far, most surface-assisted reactions are conducted on single-crystal metal surfaces, which serve as catalysts. However, the metal surface severely quenches the intrinsic electronic or optical properties of the adsorbed functional material. In view of potential applications, in particular device fabrication, direct integration of functional molecular systems on technologically relevant insulating or semiconducting surfaces is highly desirable. Recently, significant efforts have been made toward realizing chemical reactions on nonmetallic substrates; however, details of their catalytic mechanisms are still unclear and require further investigation. On the other hand, various approaches have been demonstrated to replace the catalytic functionality of metals with, for example, photochemistry or direct tip manipulation. In this Perspective, we review early advances in this field of nonmetallic surface confined reactions and highlight upcoming opportunities and challenges. We start by describing recent advances in reaction types, followed by presenting various external catalytic methods and end with pointing out promising future directions.
... Other sources of diradical formation can be utilized in addition to the most commonly employed carbon-halogen cleavages, such as the Bergman cyclization of diethinyl arene species. [172][173][174] In an on-surface process, this ring closure is followed by polymerization. Remarkably, to be practically useful in solution, this ring closure is performed in the presence of a cyclohexadiene as a hydrogen source to avoid coupling between radicals. ...
Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects of nanoscience. This field of research includes their synthesis after deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.
... Bottom-up fabrication of stable organic architectures linked by covalent bonds as demanded has drawn tremendous interest over the past decades [1][2][3], driven by their appealing application potential in both fundamental science and industrial fields, such as nanoelectronics and sensing devices [4,5]. Under this context, a collection of covalent coupling reactions has been extensively explored inspired from the traditional bulk chemistry, including homocoupling of alkynes and various dehalogenative coupling [6][7][8][9][10]. Especially, the dehalogenated aryl-aryl coupling of selected precursors in the presence of coinage metal atoms under UHV conditions, also known as surface-assisted Ullmann coupling, has been witnessed as an effective and controllable approach to link precursors into oligomers, polymer chains towards building high-stability covalent nanostructures at the single atom level [11][12][13][14][15][16][17][18][19]. ...
Ullmann coupling on coinage metals has attracted significant attention in surface chemistry due to its atomic precision of constructing extended covalent nanostructures with predictability, while the elementary reaction mechanism behind has not been comprehensively understood yet. In this study, we demonstrate the active contribution of surface atoms in promoting Ullmann coupling of 2,7-dibromopyrene (Br2Py) on Au(111) accompanied with the structure transformation from self-assembly to covalently bound one-dimension polymer chains via a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory calculations (DFT). Even the organometallic intermediate on Au substrate has been occasionally reported before, it is interesting herein to observe organometallic intermediates with the gold-carbon bonding, and demonstrate that surface adatoms specially play an important role in the dehalogenation process of Br2Py on Au(111) as elucidated by DFT simulations. Step-wise annealing facilitates the decoupling of the coordinated Au atoms from organometallic oligomers and the ultimate formation of extended polymers, which has been consistently interpreted by XPS and DFT calculations. Our study might shed new insights on the atomistic understanding of Ullmann coupling chemistry on Au substrates and proposes an effective approach to comprehensively elucidate the effect of organometallic intermediates towards on-surface Ullmann coupling reaction.
... The transmission is complex, time-consuming and expensive. Therefore, exploring other possible chemical reactions (for instance, Bergman cyclization will be accomplished by thermal activation or light [121] and, dehydrogenative coupling can be enabled to happen directly [122] ) to fabricate the GNRs has become a strong desire on studies of nonmetallic substrate. ...
Graphene, monolayer of graphite, is predicted to be one of the most promising materials to replace silicon for future electronic instruments. Despite its extraordinary electronic and thermal properties, the lack of an electronic band gap, severely hampers its potential for applications in digital electronics. In contrast, narrow stripes of graphene, so called graphene nanoribbons (GNRs) are semiconductors, due to the quantum confinement with the tunable band gap by variation with the width and edge structure that is armchair, zigzag or the combination of both edges. In this review, it was covered the recent progress in the several processes of bottom‐up approach based on the surface‐catalyzed assembly of molecular precursor, and in tuning the electronic properties of GNRs via fabricating atomically precise GNRs of different widths, various edge structures as well as doped structures. In addition, this review also indicates the challenge in ahead and possible promising ways to finely tune the electronic structures of GNRs via mediating the molecular configuration of precursor slightly.
... On-surface reaction of alkyne has attracted increasing attention recently due to its great potential for the synthesis of graphdiyne via Glaser coupling 24,25 and the creation of biradical phenyl via Bergman cyclization of cis-enediyne units [26][27][28] . The introduction of chirality to the alkynes for on-surface reactions is interesting for advancing understanding of alkyne reactivity at surfaces and as a model system for addressing the challenging problem of steering chirality in coupling reactions at surfaces. ...
Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces remains a great challenge. Here, competing reactions of a prochiral alkyne on Ag(111): twodimensional (2D) homochiral Glaser coupling and heterochiral cross-coupling with a Bergman cyclization step have been examined. We demonstrate control strategies in steering the reactions between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters, as confirmed by high-resolution scanning probe microscopy (SPM). Control experiments and density functional theory (DFT) calculations reveal that the template effect of organometallic chains obtained under specific kinetic conditions enhances Glaser coupling between homochiral molecules. In contrast, for the reaction of free monomers, the kinetically favorable reaction pathway is the cross-coupling between two heterochiral molecules (one of them involving cyclization). This work demonstrates the application of kinetic control to steer chiral organic coupling pathways at surfaces.
... On-surface reaction of alkyne has attracted increasing attention recently due to its great potential for the synthesis of graphdiyne via Glaser coupling 24,25 and the creation of biradical phenyl via Bergman cyclization of cis-enediyne units [26][27][28] . The introduction of chirality to the alkynes for on-surface reactions is interesting for advancing understanding of alkyne reactivity at surfaces and as a model system for addressing the challenging problem of steering chirality in coupling reactions at surfaces. ...
Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces remains a great challenge. Here, competing reactions of a prochiral alkyne on Ag(111): two-dimensional (2D) homochiral Glaser coupling and heterochiral cross-coupling with a Bergman cyclization step have been examined. We demonstrate control strategies in steering the reactions between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters, as confirmed by high-resolution scanning probe microscopy (SPM). Control experiments and density functional theory (DFT) calculations reveal that the template effect of organometallic chains obtained under specific kinetic conditions enhances Glaser coupling between homochiral molecules. In contrast, for the reaction of free monomers, the kinetically favorable reaction pathway is the cross-coupling between two heterochiral molecules (one of them involving cyclization). This work demonstrates the application of kinetic control to steer chiral organic coupling pathways at surfaces.
π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (−C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (−C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal −CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.
Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.
The incorporation of Si atoms into organic compounds significantly increases a variety of functionality, facilitating further applications. Recently, on‐surface synthesis was introduced into organosilicon chemistry as 1,4‐disilabenzene bridged nanostructures were obtained via coupling between silicon atoms and brominated phenyl groups at the ortho position on Au(111). Here, we demonstrate a high generality of this strategy via syntheses of silole derivatives and nanoribbon structures with eight‐membered sila‐cyclic rings from dibrominated molecules at the bay and peri positions on Au(111), respectively. Their structures and electronic properties were investigated by a combination of scanning tunneling microscopy/spectroscopy and density functional theory calculations. This work demonstrates a great potential to deal with heavy group 14 elements in on‐surface silicon chemistry.
Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.
On-surface synthesis is a powerful methodology for the fabrication of low-dimensional functional materials. The precursor molecules usually anchor on different metal surfaces via similar configurations. The activation energies are therefore solely determined by the chemical activity of the respective metal surfaces. Here, we studied the influence of the detailed adsorption configuration on the activation energy on different metal surfaces. We systematically studied the desulfonylation homocoupling for a molecular precursor on Au(111) and Ag(111) and found that the activation energy is lower on inert Au(111) than on Ag(111). Combining scanning tunneling microscopy observations, synchrotron radiation photoemission spectroscopy measurements, and density functional theory calculations, we elucidate that the phenomenon arises from different molecule-substrate interactions. The molecular precursors anchor on Au(111) via Au-S interactions, which lead to weakening of the phenyl-S bonds. On the other hand, the molecular precursors anchor on Ag(111) via Ag-O interactions, resulting in the lifting of the S atoms. As a consequence, the activation barrier of the desulfonylation reactions is higher on Ag(111), although silver is generally more chemically active than gold. Our study not only reports a new type of on-surface chemical reaction but also clarifies the influence of detailed adsorption configurations on specific on-surface chemical reactions.
From scanning tunnealing microscopy and density functional theory calculations, we demonstrate that different annealing mechanisms could modulate distinct reaction pathways, where in stepwise annealing procedure the detached Br atoms may reduce the activation barrier of CH activation resulting in hierarchical cross dehydrogenative coupling, while for one-step annealing only Ullmann coupling products are observed.
Conjugated ladder polymers (CLPs) are an attractive type of macromolecule consisting of periodically repeated fused aromatic rings. Their rigid and fully conjugated backbones endow CLPs with intriguing optical, electronic, and magnetic properties. Current challenges facing CLP chemistry are difficulty in regulating their synthesis due to unfavorable side reactions (e.g. cross-linking and branching) and their low solubility. Template-confined synthesis is a viable strategy for controlling CLP structures at the atomic scale. In this review, we describe recent developments and future perspectives in the confined synthesis of CLPs utilizing templates, including metal surfaces and nanoporous materials. Access to CLPs with greater diversity will contribute to the ability to control their intrinsic properties, paving the way to their future applications in many fields.
Immobilization of organic molecules on metal surfaces and their coupling via thermally induced C–C bond formation is an important technique in organic and polymer synthesis. Using this approach, insoluble and reactive carbon nanostructures can be synthesized and the reactions monitored in situ using scanning probe microscopy methods. The diversity of conceivable products, however, is limited by the number and variety of known on-surface reactions. Here, we introduce the on-surface synthesis of polyarylenes by intermolecular oxidative coupling of isopropyl substituents of arenes. This [3+3] dimerization reaction forms a new phenylene ring and can be regarded as a formal cycloaromatization. The synthetic value of this reaction is proved by the synthesis of polyarylenes and co-polyarylenes, which we demonstrate by synthesizing poly(2,7-pyrenylene-1,4-phenylene). Scanning tunnelling microscopy and non-contact atomic force microscopy studies, complemented by density functional theory calculations, offer mechanistic insight into the on-surface cycloaromatization reaction. On-surface methods can be used to synthesize organic molecules, polymers and nanomaterials, however, the diversity of conceivable products is limited by the number of known on-surface reactions. Now, a phenylene ring-forming reaction on a gold surface by intermolecular oxidative coupling of isopropyl substituents on arenes is reported. The reaction is probed using bond-resolved imaging and computational modelling.
Various methodologies have been well established to construct self-assembled structures on surfaces that are aesthetic and eye-catching. How to practically apply these self-assemblies still remains a great challenge. In particular, how to develop the surface molecular self-assembly into a new and applicable strategy to control on-surface reactions plays a crucial role in the construction of covalently bonded structures on surfaces. In this chapter, recent progress in the development of such a self-assembly strategy is overviewed. After a brief description of the fundamentals in surface reaction kinetics, a series of typical proof-of-principle case studies are summarised, focusing on the application of the self-assembly strategy to control on-surface reactions. Such a strategy may be exploited as an efficient bottom-up approach to constructing covalently bonded and thermally stable structures that are urgently needed in nanoscience and nanotechnology. In specific, the emphasis is to utilise the molecular self-assembly strategy to steer on-surface reactions via mediations of the reaction pathway, selectivity, and site. Some remaining challenges and perspectives relating to the self-assembly strategy are also provided for future explorations.
The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces.
Die Silylen‐verknüpfte Sonogashira‐Kreuzkupplung ermöglicht die Synthese von partiell fluorierten Phenylenethinylen‐Oligomeren auf Ag(111), wie Akihiro Orita, Shigeki Kawai et al in ihrer Zuschrift berichten (DOI: 10.1002/ange.202102882). Die desilylierende Sonogashira‐Reaktion erreicht eine hohe Chemoselektivität von bis zu 75 %, während die konkurrierende Ullmann‐Homokupplung und die desilylierende Glaser‐Kupplung unterdrückt werden.
Purpose
Technology presents e-commerce as an alternative buying and selling place that is accepted by the public. The high growth of e-commerce has an impact on the sustainability of both the economic dimension, the social dimension and the environmental dimension. Indonesia is the country with the fastest-growing e-commerce but also has the second-largest plastic waste in the world. The synergy of sustainability for e-commerce is an interesting and awaited innovation. This is because sustainability has become the responsibility of all countries in the world.
Design/methodology/approach
A theoretical understanding of the context of sustainability in e-commerce separately focuses on a company perspective and the use of green products from a consumer perspective. It requires the involvement of e-commerce stakeholders as a whole to get comprehensive research results. The use of qualitative research methods with exploratory approaches is used in this study to reveal the concept of sustainability in e-commerce in Indonesia.
Findings
This study found similarities in the topic of acceptance of sustainability in e-commerce with a unified theory of acceptance and use of technology (UTAUT) including performance expectancy, effort expectancy, facilitating conditions, social influence and habits. Changes to the variables were revealed due to changes in the e-commerce phase. The variable trust is in the introduction phase and builds trust in e-commerce. Currently, in Indonesia, the e-commerce phase is in a phase of growth and value formation. Habit creation and dependence is a requirement for value formation. Several new topics were proposed in this study, namely, awareness, security, logistics and user interface and user experience (UX). The establishment of an e-commerce identity through UX clearly shows its target market. The e-commerce phase and the topics involved in it can become a reference for e-commerce regulation-making in Indonesia.
Research limitations/implications
This study is limited to e-commerce in Indonesia with data processing limited to February 2020.
Practical implications
The results of this study provide an overview of increasing the intention to use e-commerce through human acceptance and engineering dimensions. This research also reveals the stages of e-commerce in Indonesia that can be used as a reference for determining the right regulations for e-commerce and the trade-offs for sustainability.
Originality/value
This study produces additional references to the intention to use technology by completing the UTAUT model. This study reveals changes in variables in perceived value that are interesting for further research along with technological developments and changes in people’s habits. Exploration carried out can add references to the application of sustainability in e-commerce, especially in developing countries.
A coupling module composed of (trimethylsilyl)ethynyl and chlorophenyl groups selectively accelerated sluggish desilylative on-surface Sonogashira coupling with homocoupling efficiently suppressed. Bond-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) with a CO-functionalized tip enabled investigation of this coupling reaction and evaluation of its chemoselectivity (see picture).
Abstract
On-surface synthesis is a powerful method for the fabrication of π-conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl- and chloro-substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (-Me2Si-) through CH3−Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.
On‐surface synthesis is a powerful method for the fabrication of π‐conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl‐ and chloro‐substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond‐resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (‐Me2Si‐) through CH3−Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.
For years, concepts and models relevant to the fields of molecular electronics and organic electronics have been invented in parallel, slowing down progress in the field. This book illustrates how synthetic chemists, materials scientists, physicists, and device engineers can work together to reach their desired, shared goals, and provides the knowledge and intellectual basis for this venture.
Supramolecular Materials for Opto-Electronics covers the basic principles of building supramolecular organic systems that fulfil the requirements of the targeted opto-electronic function; specific material properties based on the fundamental synthesis and assembly processes; and provides an overview of the current uses of supramolecular materials in opto-electronic devices. To conclude, a “what’s next” section provides an outlook on the future of the field, outlining the ways overarching work between research disciplines can be utilised.
Postgraduate researchers and academics will appreciate the fundamental insight into concepts and practices of supramolecular systems for opto-electronic device integration.
The combination of alkyne and halogen functional groups in the same molecule allows for the possibility of many different reactions when utilized in on-surface synthesis. Here, we use a pyrene-based precursor with both functionalities to examine the preferential reaction pathway when it is heated on an Au(111) surface. Using high-resolution bond-resolving scanning tunneling microscopy, we identify multiple stable intermediates along the prevailing reaction pathway that initiate with a clearly dominant Glaser coupling, together with a multitude of other side products. Importantly, control experiments with reactants lacking the halogen functionalization reveal the Glaser coupling to be absent and instead show the prevalence of non-dehydrogenative head-to-head alkyne coupling. We perform scanning tunneling spectroscopy on a rich variety of the product structures obtained in these experiments, providing key insights into the strong dependence of their HOMO-LUMO gaps on the nature of the intramolecular coupling. A clear trend is found of a decreasing gap that is correlated with the conversion of triple bonds to double bonds via hydrogenation and to higher levels of cyclization, particularly with nonbenzenoid product structures. We rationalize each of the studied cases.
On-surface synthesis via covalent coupling of adsorbed precursor molecules on metal surfaces has emerged as a promising strategy for the design and fabrication of novel organic nanoarchitectures with unique properties and potential applications in nanoelectronics, optoelectronics, spintronics, catalysis, etc. Surface-chemistry-driven molecular engineering (i.e., bond cleavage, linkage, and rearrangement) by means of thermal activation, light irradiation, and tip manipulation plays critical roles in various on-surface synthetic processes, as exemplified by the work from the Ernst group in a prior issue of ACS Nano. In this Perspective, we highlight recent advances in and discuss the outlook for on-surface syntheses and molecular engineering of carbon-based nanoarchitectures.
A novel N‐heterocyclic segregation reaction with tetraazaanthraquinodimethane was performed on Au(111) by bond‐resolved scanning tunneling microscope at 4.3 K. A combination of high‐resolution imaging and density functional theory calculations prove that the intramolecular ring‐forming and ‐opening are strongly affected by the initial hydrogen–substrate interaction, while the product is identical.
Abstract
Cyclo‐dehydrogenation is of importance to induce the planarization of molecules on noble surfaces upon annealing. In contrast to a number of successful syntheses of polycyclic aromatic hydrocarbons by forming carbon–carbon bonds, it is still rare to conduct conjugation and cleavage of carbon–nitrogen bonds in molecules. Here, we present a systematic transformation of the C−N bonds in11,11,12,12‐tetraphenyl‐1,4,5,8‐tetraazaanthraquinodimethane as well as three other derivatives on Au(111). With bond‐resolved scanning tunneling microscopy, we discovered novel the “heterocyclic segregation” reaction of one pyrazine ring with two nitrogen atoms to form two quinoline rings with one nitrogen each. Density functional theory calculations showed that the intramolecular ring‐forming and ‐opening of N‐heterocycles are strongly affected by the initial hydrogen–substrate interaction.
Wet-chemical bottom-up synthesis methods for two-dimensional (2D) layered materials are less explored than the top-down exfoliation of bulk materials. Here, we set out to synthesize a graphyne-type material by a wet-chemical synthesis method using Sonogashira–Hagihara cross-coupling polycondensation of a multifunctional monomer, 2, bearing alkyne and vinyl bromide functionalities. Spectroscopic and chemical analysis revealed that upon C–C bond formation, an unanticipated Bergman cyclization occurred to give an aromatic 2D porous carbon material (2D-PCM). 2D-PCM is a black material with graphene-like layers and a bulk structure that is similar to irregular graphite. It is porous with a hierarchical pore structure and an apparent Brunauer–Emmett–Teller surface area of 575 m² g⁻¹. The material has excellent electrochemical performance as an electrode in supercapacitors with a specific capacitance of 378 F g⁻¹ at the current density of 0.1 A g⁻¹, which surpasses state-of-the-art carbon materials, suggesting that wet-chemical methods might give functional benefits over top-down processing routes.
TEM has long been used as the ultimate characterization technique for nanomaterials at the atomic level, as discussed briefly in this chapter. Here, the importance of the use of graphene in TEM is also presented to introduce the properties that makes it an indispensable for the characterization of other nanomaterials and study their properties and van der Waals interactions. This review provides a broad overview of the importance of using TEM for the study of nanomaterials and the rise of graphene as a superior substrate for the study of different kinds of low‐dimensional materials. A review of the study of morphology, properties and behaviour of a range of nanomaterials is presented, with a specific focus on how graphene facilitates these studies due to its unique influence and interaction with the specific materials under TEM. The main objective of the review is to present and discuss the various studies and characterization that have been carried out on a range of nanomaterials using TEM with the use of graphene, and the influence of its interaction with nanomaterials under TEM.
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The carbon-carbon coupling of 4,4′-dibromo-p-terphenyl (DBTP) on Pd(1 1 1) surface has been investigated at a single molecular level by scanning tunneling microscopy. DBTP molecules undergo debromination partially and are aligned along the [1–10] direction of the surface after deposition at RT. Annealing at 338 K promotes the formation of V-shaped and zigzag-shaped oligomers. The high resolution STM images revel that such carbon-carbon coupling benefits form Pd-catalyzed meta-CH activation.
On-surface Ullmann-type reactions are widely used to fabricate various carbon nanostructures in a bottom-up approach, by conjugating small hydrocarbons via dehalogenation. In the reaction, the dissociated halogen atoms remain on the substrate and are usually removed by high-temperature annealing. Here, we demonstrate an alternative method, in which most of bromine atoms can be desorbed from Au(111) just by depositing silicon atoms. A combination of scanning tunneling microscopy and density functional theory calculations revealed that the highly volatile silicon tetrabromine is synthesized and consequently desorbs from the surface even at room temperature. This low temperature removal of the halogen atoms may increase flexibility in on-surface chemical reactions towards synthesis and characterization of further functionalized carbon nanomaterials.
Nanographenes (NGs) have gained increasing attention due to their immense potential as tailor-made organic materials for nanoelectronics and spintronics. They exhibit a rich spectrum of physicochemical properties that can be tuned by controlling the size, the edge structure or by introducing structural defects in the honeycomb lattice. Here, we report the design and on-surface synthesis of NGs containing several odd-membered polycycles induced by a thermal procedure on Au(111). Our scanning tunneling microscopy, non-contact atomic force microscopy and scanning tunneling spectroscopy measurements, complemented by computational investigations, describe the formation of two non-benzenoid NGs (2A and 2B) containing four embedded azulene units in the polycyclic framework, via on-surface oxidative ring-closure reactions. Interestingly, we observe surface-catalyzed skeletal ring rearrangement reactions in the NGs, which lead to the formation of additional hep-tagonal rings as well as pentalene and as-indacene units in 2A and 2B, respectively. Both 2A and 2B on Au(111) exhibit narrow experimental frontier electronic gaps of 0.96 and 0.85 eV, respectively, and Fermi level pinning of their HOMO to-gether with considerable electron transfer to the substrate. Ab initio calculations estimate moderate open-shell biradical characters for the NGs in gas phase.
On-surface synthesis has emerged as a powerful approach for atomically precise fabrication of molecular architectures with potential applications in nanotechnology. However, since conjugated structures prefer to adsorb parallel on the surface for maximizing the molecular-substrate interaction, it is challenging to synthesize molecular structures that protrude from the surface such as polymer chains with upright conformations. Here, we show up-standing phenanthrene polymer chains with all-cis configuration obtained by on-surface synthesis upon highly selective C-H activation. Using bond-resolved nc-AFM imaging, the reaction route of polymers from in-plane to all-cis upright conformation is fully characterized and the reaction mechanism is further revealed by density functional theory calculations. Our results on this selective dehydrogenation induced upright oriented polymer chains will enrich the toolbox for on-surface synthesis of novel molecular structures and provide insights on precursors design to fabricate three-dimensional molecular frameworks on surfaces.
Metal-organic frameworks prepared on surfaces (SMOFs) have been considered to have potential applications in various research fields. Traditionally, the SMOFs are prepared by co-adsorbing organic ligands and metal atoms on surfaces. In this article, we successfully construct the SMOFs via the dehydrogenation reactions of aromatic amines on the Cu(111) surfaces. The dehydrogenated nitrogen radicals interact with the copper adatoms, forming the N-Cu-N bonds. Combining with the scanning tunneling microscopy (STM) and the density functional theory (DFT) calculations, we obtain the structural models of the SMOFs.
Metal oxides hold promise for the development of future organic electronics and low-cost, high-efficient heterogeneous catalysts due to their wide range of crystallographic, electronic and magnetic structures. As the size of today’s electronic devices reaches the nanometer scale, and as interest has grown in establishing the catalytic mechanisms of metal oxide catalysts, the interface interactions between metal oxides and adsorbates are at the focus of current research. Likewise, functionalization of surfaces by organic molecules has been an important topic during the last two decades. Especially the potential arising from new macromolecular structures on surfaces created by self-assembly has triggered a lot of research. While this area of research is relatively mature when it comes to adsorption and self-assembly on metal surfaces, studies on oxide surfaces are still rare. This thesis aims at performing fundamental, comparative studies of the adsorption of large organic molecules on cobalt oxides and copper by using scanning tunneling microscopy (STM), in order to gain a fundamental understanding of the adsorption behavior, such as the adsorption geometry of the molecules, the interactions between the molecules and the substrate surfaces, and possible on-surface reactions. By this, the similarities of and differences between the adsorption of molecules on metal and metal oxide surfaces are revealed. The results will open a door towards designing and fabricating functional molecular nanostructures on metal oxide surfaces. Therefore, an important aspect of this thesis is to study the adsorption properties of porphyrin molecules on the Cu(111) surface to set the appropriate basis for a comparison. The adsorption geometry of free-base and cobalt metalated 5,15-diphenylporphyrin (2H-DPP and Co-DPP) are studied by scanning tunneling microscopy and density functional theory. The self-assembly structures as a function of molecular coverage are subsequently explored. Both the 2H- and Co-DPP exhibit chirality on the surface that derives from the saddle shape distortion of the molecules. It is shown that and how such a molecular chirality transfers to self-assembly structures. The self-assembly structures of 2H-DPP are independent of the molecular coverage. Conversely, the Co-DPP is more mobile on the copper surface, which allows structural phase transitions as function of molecular coverage. The molecule-molecule and the molecule-substrate interactions play important roles in the transition of Co-DPP self-assembly structures from chirally separated, enantiopure to racemic networks as the molecular coverage increases. On-surface reactions of DPP molecules on a metal substrate is another important aspect that is discussed in this thesis. By using unfunctionalized 2H-DPP molecules, the possible mechanism of their intramolecular dehydrogenative coupling is revealed. The reaction products further act as precursors for the subsequent intermolecular organometallic and dehydrogenative coupling. The competition between these two on-surface reactions is studied as a function of sample temperature, annealing time, molecular coverage and molecular geometries. The last aspect in this thesis is to study the adsorption of 2H-DPP and Co-DPP molecules on rock salt and spinel type cobalt oxide films, which are epitaxially grown on Ir(100). On rock salt type cobalt oxide films of ~2.6nm (~11BL) thickness, both 2H-DPP and Co-DPP are weakly adsorbed due to the π-anion interactions. The metalation of 2H-DPP with Co ions from the cobalt oxide film is observed and studied by STM and x-ray photoelectron spectroscopy. The adsorption of Co-DPP is affected by the cobalt oxide film thickness. When the film is thicker than 0.26nm (1BL), the Co-DPP is found to rotate at 80K on the rock salt type cobalt oxide film. The time average of this rotation imaged by STM reveals the potential landscape of the molecule-molecule and molecule-substrate interactions. At approximately 470K, the molecules start to react with the substrate and eventually decompose. On spinel type cobalt oxides, the interaction between the DPP molecules and the film surface is much stronger. As a consequence, the metalation by cobalt ions from the oxide and the (partial) decomposition appear at almost the same temperature.
Organometallic chains can be successfully achieved via surface-assisted synthesis on Ag(111) surface by choosing prochiral 4,4'-dibromo-2,2'-bis(2-phenylethynyl)-1,1'-biphenyl (DBPB) molecule as designed precursor. High-resolution scanning tunneling microscopy investigation reveals that prochiral molecule shows chirality on surface, and can evolve to organometallic chains on Ag(111) surface based on Ullmann coupling. Due to special structural features of DBPB molecule, chiral selectivity will lose in organometallic chains. This result may provide an important basis for selecting suitable precursors to fabricate of chiral covalent nanostructures on surface.
With the rapid development of scanning probe microscopy, it has become possible to study polymerization processes on suitable surfaces at the atomic level and in real space. In the two-dimensional confinement of a surface, polymerization reactions can give rise to the formation of unprecedented polymers with unique structures and properties, not accessible in solution. After a little over one decade since the discovery of covalent on-surface polymerization, we give an overview of the field, analyse the crucial aspects and critically reflect on the status quo. Specifically, we provide some general considerations about fundamental mechanisms as well as kinetics and thermodynamics of on-surface polymerization processes. The important role of the surface is detailed in view of its ability to control polymer formation with regard to structure, dimensionality and composition. Furthermore, examples that allow for locally induced polymerization are highlighted. Finally, we provide an analysis of scientific challenges in the field and outline future prospects.
Intramolecular dehalogenation reaction as an essential step is generally involved in the deprotection of carbon-carbon double bonds in solution chemistry. However, harsh reaction conditions often limit the functional-group compatibility and cause the selectivity problems. In comparison with traditional solution reactions, on-surface chemical reactions always occur on metal substrates, which could serve as catalysts to effectively reduce the reaction barriers in many cases. On the other hand, on-surface intermolecular dehalogenative homocoupling reactions have been extensively demonstrated, while, to our knowledge, intramolecular dehalogenation reactions are less discussed on surfaces. Herein, by the combination of high-resolution STM imaging and DFT calculations, we introduce the vicinal dibromo group on chemically different surfaces, such as, Cu(110), Ag(110) and Au(111), respectively, to investigate the generality of intramolecular dehalogenation reactions. As a result, we successfully achieved the direct formation of C-C double bonded products in a facile manner on all three surfaces.
On-surface enantioselective covalent coupling of non-functionalized porphyrins is demonstrated without utilizing chirality transfer from a self-assembled enantiopure precursor structure. We achieved to synthesize chiral porphyrin tapes on the Ag (110) surface by thermally induced dehydrogenative coupling using 5,15-diphenylporphyrin (2H-DPP) as a precursor. We employ scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to study the properties of the precursor molecules and resulting covalently bonded structures. Our analysis shows that the enantioselectivity is due to the specific molecule-substrate interaction confining the orientation of the diffusing molecules and stearic hindrance making non-enantiopure bonds energetically unfavorable.
Hydroxyl is a key reaction intermediate in many surface catalyzed redox reactions, yet establishing the phase diagram for water/hydroxyl adsorption on metal surfaces remains a considerable challenge for interfacial chemistry. While the structures formed on close packed metal surfaces have been discussed widely, the phase diagram on more reactive, open metal surfaces, is complex and the H-bonding structures are largely unknown. Based on scanning tunnelling microscopy and density functional theory calculations, we report the phase diagram for water/hydroxyl on Cu(110), providing a complete molecular description of the complex hydrogen bonding structures formed. Three distinct phases are observed as the temperature is decreased and the water/hydroxyl ratio increased: pure OH dimers, extended 1H2O:1OH chains, aligned along the close-packed Cu rows, and finally a distorted 2D hexagonal c(2 × 2) 2H2O:1OH network. None of these phases obey the conventional ‘ice rules’, instead their structures can be understood based on weak H donation by hydroxyl, which favours H-bonding structures dominated by water donation to hydroxyl, and competition between hydroxyl adsorption sites. Hydroxyl binds in the Cu bridge site in the 1D chain structures, but is displaced to the atop site in the 2D network in order to accommodate water in its preferred atop binding geometry. The adsorption site and stability of hydroxyl can therefore be tuned simply by changing the surface temperature and water content, giving a new insight as to how the open metal template influences the water/hydroxyl structures formed and the activity of hydroxyl.
We present the design and performance of a high-pressure scanning tunneling microscope (HP-STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10-10-1000 mbar) on a routine basis. The HP-STM is integrated in a gold-plated high-pressure cell with a volume of only ~0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.
The native copper adatoms get trapped in a self-assembled molecular nanostructure which is mainly formed by the intermolecular van der Waals interactions, and two dominating specific binding modes between the adatoms and the molecules are revealed at the atomic scale by high-resolution STM imaging.
The covalent linking of acetylenes presents an important route for the fabrication of novel carbon-based scaffolds and two-dimensional materials distinct from graphene. To date few attempts have been reported to implement this strategy at well-defined interfaces or monolayer templates. Here we demonstrate through real space direct visualization and manipulation in combination with X-ray photoelectron spectroscopy and density functional theory calculations the Ag surface-mediated terminal alkyne Csp−H bond activation and concomitant homo-coupling in a process formally reminiscent of the classical Glaser–Hay type reaction. The alkyne homo-coupling takes place on the Ag(111) noble metal surface in ultrahigh vacuum under soft conditions in the absence of conventionally used transition metal catalysts and with volatile H2 as the only by-product. With the employed multitopic ethynyl species, we demonstrate a hierarchic reaction pathway that affords discrete compounds or polymeric networks featuring a conjugated backbone. This presents a new approach towards on-surface covalent chemistry and the realization of two-dimensional carbon-rich or all-carbon polymers.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
In contrast to the many methods of selectively coupling olefins, few protocols catenate saturated hydrocarbons in a predictable manner. We report here the highly selective carbon-hydrogen (C-H) activation and subsequent dehydrogenative C-C coupling reaction of long-chain (>C(20)) linear alkanes on an anisotropic gold(110) surface, which undergoes an appropriate reconstruction by adsorption of the molecules and subsequent mild annealing, resulting in nanometer-sized channels (1.22 nanometers in width). Owing to the orientational constraint of the reactant molecules in these one-dimensional channels, the reaction takes place exclusively at specific sites (terminal CH(3) or penultimate CH(2) groups) in the chains at intermediate temperatures (420 to 470 kelvin) and selects for aliphatic over aromatic C-H activation.
Bergman cyclization of enediynes, regarded as a promising strategy for anticancer drugs, now finds its own niche in the area of polymer chemistry and material science. The highly reactive aromatic diradicals generated from Bergman cyclization can undergo polymerization acting as either monomers or initiators of other vinyl monomers. The former, namely homopolymerization, leads to polyphenylenes and polynaphthalenes with excellent thermal stability, good solubility, and processability. The many remarkable properties of these aromatic polymers have further endowed them to be manufactured into carbon-rich materials, e.g., glassy carbons and carbon nanotubes. Whereas used as initiators, enediynes provide a novel resource for high molecular weight polymers with narrow polydispersities. The aromatic diradicals are also useful for introducing oligomers or polymers onto pristine carbonous nanomaterials, such as carbon nano-onions and carbon nanotubes, to improve their dispersibility in organic solvents and polymer solutions.
The construction of electronic devices from single molecular building blocks, which possess certain functions such as switching or rectifying and are connected by atomic-scale wires on a supporting surface, is an essential goal of molecular electronics. A key challenge is the controlled assembly of molecules into desired architectures by strong, that is, covalent, intermolecular connections, enabling efficient electron transport between the molecules and providing high stability. However, no molecular networks on surfaces 'locked' by covalent interactions have been reported so far. Here, we show that such covalently bound molecular nanostructures can be formed on a gold surface upon thermal activation of porphyrin building blocks and their subsequent chemical reaction at predefined connection points. We demonstrate that the topology of these nanostructures can be precisely engineered by controlling the chemical structure of the building blocks. Our results represent a versatile route for future bottom-up construction of sophisticated electronic circuits and devices, based on individual functionalized molecules.
Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), by chemical reactions readily on surfaces is the first step to a true advance in the field of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. Recently, a collection of different reactions leading to large tailor-made organic molecules on single-crystal metal surfaces has been reported. The fundamental mechanisms controlling these reactions can be investigated from a surface science perspective. This discipline skillfully combines the use of characterization techniques at the nanoscale, with single-crystal metallic surfaces able to catalyse these reactions. We present a tutorial review that highlights the relevance of the new bottom up strategies and classifies most of the different molecular on-surface reactions involving aromatic organic molecules that have been published up to date.
Graphene nanoribbons-narrow and straight-edged stripes of graphene, or single-layer graphite-are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.
Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C(60) from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C(60) and the triazafullerene C(57)N(3) from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.
Using scanning tunneling microscopy, we demonstrate that the 1,3-dipolar cycloaddition between a terminal alkyne and an azide can be performed under solvent-free ultrahigh vacuum conditions with reactants adsorbed on a Cu(111) surface. XPS shows significant degradation of the azide upon adsorption, which is found to be the limiting factor for the reaction.
Molecular electronics describes the field in which molecules are utilized as the active (switching, sensing, etc.) or passive (current rectifiers, surface passivants) elements in electronic devices. This review focuses on experimental aspects of molecular electronics that researchers have elucidated over the past decade or so and that relate to the fabrication of molecular electronic devices in which the molecular components are readily distinguished within the electronic properties of the device. Materials, fabrication methods, and methods for characterizing electrode materials, molecular monolayers, and molecule/electrode interfaces are discussed. A particular focus is on devices in which the molecules or molecular monolayer are sandwiched between two immobile electrodes. Four specific examples of such devices, in which the electron transport characteristics reflect distinctly molecular properties, are discussed.
A new polymer, poly(3,4-bis(phenylethynyl)styrene), has been synthesized that undergoes a Bergman reaction upon heating to yield a highly aromatic material. The occurrence of the cycloaromatization reaction is characterized through thermal analysis and absorption spectroscopy. The plasma etch resistance of the polymer has been measured by reactive ion etching (RIE) in sulfur hexafluoride and oxygen plasmas and is shown to be superior to other conventional organic plasma etch barriers.
A template approach was demonstrated to prepare ultrathin ordered porous carbon through Bergman cyclization of enediyne self-assembled monolayers on silica nanoparticles, followed by carbonization and template removal. The sizes of macropores can be finely tuned by adjusting the sizes of silica nanoparticles. The structures of the carbonous products were characterized with scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption isotherm, and Raman spectroscopy. The results showed that these materials consisted of interconnected ordered macropores with ultrathin walls. Large BET surface areas (up to1786 m2/g), large pore volumes (1.24 cm3/g), and partially graphitized frameworks were revealed for these materials. Depending on these excellent properties, potential applications would be expected.
While graphene has attracted significant attention from the research community due to its high charge carrier mobility, important issues remain unresolved that prevent its widespread use in technologically significant applications such as digital electronics. For example, the chemical inertness of graphene hinders integration with other materials, and the lack of a bandgap implies poor switching characteristics in transistors. The formation of ordered organic monolayers on graphene has the potential to address each of these challenges. In particular, functional groups incorporated into the constituent molecules enable tailored chemical reactivity, while molecular-scale ordering within the monolayer provides sub-2 nm templates with the potential to tune the electronic band structure of graphene via quantum confinement effects. Toward these ends, we report here the formation of well-defined one-dimensional organic nanostructures on epitaxial graphene via the self-assembly of 10,12-pentacosadiynoic acid (PCDA) in ultrahigh vacuum (UHV). Molecular resolution UHV scanning tunneling microscopy (STM) images confirm the one-dimensional ordering of the as-deposited PCDA monolayer and show domain boundaries with symmetry consistent with the underlying graphene lattice. In an effort to further stabilize the monolayer, in situ ultraviolet photopolymerization induces covalent bonding between neighboring PCDA molecules in a manner that maintains one-dimensional ordering as verified by UHV STM and ambient atomic force microscopy (AFM). Further quantitative insights into these experimental observations are provided by semiempirical quantum chemistry calculations that compare the molecular structure before and after photopolymerization.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
Scanning tunnelling microscopy (STM) has proved to be a fascinating and powerful technique in the field of surface science. The fact that sets the STM apart from most other surface sensitive techniques is its ability to resolve the structure of surfaces on an atomic scale, that is atom-by-atom, and furthermore its ability to study the dynamics of surface processes. This article presents a survey of recent STM studies of well characterized single crystal metal surfaces under ultra-high vacuum conditions. It particularly addresses STM investigations of clean metal surfaces, adsorbates on metal surfaces, adsorbate-induced restructuring of metal surfaces, chemical reactions on metal surfaces, metal-on-metal growth and finally studies of electron confinement and quantum size effects on metal surfaces.
Beyond supramolecular self-assembly: On-surface covalent coupling of organic precursors is a powerful method to obtain 1D or 2D robust molecular arrays on a substrate (see picture) that have numerous potential applications in nanotechnology. Significant advances have recently been made in this field including imine formation, dehydration of boronic acids, esterification, as well as radical and carbene coupling reactions.
We report on covalent two-dimensional phenylene-boroxine hybrid-networks that were synthesized under ultra-high vacuum conditions from doubly functionalized monomers through thermally activated condensation prior to deposition and successive heterogeneously catalyzed radical addition on Ag(111). Structural properties were characterized in situ by high resolution Scanning-Tunneling-Microscopy (STM).
We report on the assembly of tribromo-substituted dimethylmethylene-bridged triphenylamine (heterotriangulene) on Ag(111). Depending on activation temperature, two-dimensional porous metal-coordination or covalent networks are obtained.
We have studied the organometallic intermediate of a surface-supported Ullmann coupling reaction from 4, 4″-dibromo-p-terphenyl to poly(para-phenylene) by scanning tunneling microscopy/spectroscopy and density functional theory calculations. Our study reveals at a single-molecular level that the intermediate consists of biradical terphenyl (ph)(3) units that are connected by single Cu atoms through C-Cu-C bridges. Upon further increasing the temperature, the neighboring biradical (ph)(3) units are coupled by C-C bonds forming poly(para-phenylene) oligomers while the Cu atoms are released.
The fabrication of large molecular devices, directly on surfaces in UHV conditions, by covalent coupling of smaller precursors has become in the past years an attractive solution for Molecular Electronics. This review presents the state-of-the-art and an analysis of the potential of this new field, from Ullmann type C-C coupling, cyclodehydrogenation, and reactions involving heteroelements to 2D polymerisation on insulating thin films. Mechanistic insights are also mentioned, giving preliminary explanations on the influence of the substrate and the 2D confinement. Potential perspectives for further developments are then evoked.
Interest in thermal and chemical stability of surface-supported organic networks has stimulated recent attempts to covalently interlink adsorbed molecular species into extended nanostructures. We show, using low-temperature scanning tunneling microscopy, that imidization of anhydrides and amines adsorbed on Au(111) can be thermally initiated under controlled ultrahigh vacuum conditions. Using two types of amine-functionalized polyphenyl molecules together with the organic semiconductor PTCDA, monolayer thick linear polymeric strands and a porous polymeric network with nanoscale dimensions are obtained.
We present a theory for tunneling between a real surface and a model probe tip, applicable to the recently developed ‘‘scanning tunneling microscope.’’ The tunneling current is found to be proportional to the local density of states of the surface, at the position of the tip. The effective lateral resolution is related to the tip radius R and the vacuum gap distance d approximately as [(2 Å)(R+d)]1/2. The theory is applied to the 2×1 and 3×1 reconstructions of Au(110); results for the respective corrugation amplitudes and for the gap distance are all in excellent agreement with experimental results of Binnig et al. if a 9-Å tip radius is assumed. In addition, a convenient approximate calculational method based on atom superposition is tested; it gives reasonable agreement with the self-consistent calculation and with experiment for Au(110). This method is used to test the structure sensitivity of the microscope. We conclude that for the Au(110) measurements the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer. Finally, tunneling to semiconductor surfaces is considered. Calculations for GaAs(110) illustrate interesting qualitative differences from tunneling to metal surfaces.
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
A new density functional (DF) of the generalized gradient approximation (GGA) type for general chemistry applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C(6) x R(-6). A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common density functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on standard thermochemical benchmark sets, for 40 noncovalently bound complexes, including large stacked aromatic molecules and group II element clusters, and for the computation of molecular geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for standard functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean absolute deviation of only 3.8 kcal mol(-1). The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the average CCSD(T) accuracy. The basic strategy in the development to restrict the density functional description to shorter electron correlation lengths scales and to describe situations with medium to large interatomic distances by damped C(6) x R(-6) terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chemical method for large systems where dispersion forces are of general importance.
Controlling the molecular growth of organic semiconductors is an important issue to optimize the performance of organic devices. Conjugated molecules, used as building blocks, have an anisotropic shape and also anisotropic physical properties like charge transport or luminescence. The main challenge is to grow highly crystalline layers with molecules of defined orientation. The higher the crystallinity, the closer these properties reach their full intrinsic potential, while the orientation determines the physical properties of the film. Herein we show that the molecular orientation and growth can be steered by the surface chemistry, which tunes the molecule-substrate interaction. In addition, the oxygen reconstruction of the surface, demonstrates the flexibility of the organic molecules to adopt a given surface corrugation and their unique possibility to release stress by tilting.
Oxidative DNA cleavage by the diradical species generated from enediyne or enyne-cumulene progenitor is believed to be at the heart of the biological activity of the naturally occurring enediyne antibiotics. These serve as a perfect example of a prodrug designed by Nature. The promotion of certain members of this class of natural products to the actual clinical stage against certain types of tumors has generated an unprecedented flurry of research activities in the field of chemistry, biology, and medicine in search of new therapeutic agents. Various factors, such as ring strain, the proximity of carbon atoms undergoing covalent connectivity, etc., play important roles in controlling the process of diradical generation via cycloaromatization, and the lead from Mother Nature continues to be followed, as evidenced from the report of numerous synthetic mimics of the natural products. A major focus of research in this field involves the synthesis of designed enediynes to correlate their chemical and biological activity coupled with a novel triggering mechanism. Triggering of enediynes is necessary to activate them to generate the reactive form under appropriate conditions. Variations of pH and photo-irradiation are two important methods for triggering enediynes that are stable under ambient conditions. In this review, the synthesis and reactivity of endiynes and analogous molecules equipped with such triggering devices is discussed along with the analysis of the current level of biological activity achieved thus far.
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