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1.
Chem Commun (Camb) ; 2020 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-32525154

RESUMO

Strings of gold-organic oligomers of polar units have been formed by on-surface synthesis and investigated with non-contact atomic force microscopy. The mutual alignment of dipoles within the strings is analyzed. While an alternating head-to-tail alignment might be expected from dipolar interactions, a more complicated alignment order is observed. The data suggests that coordination bonding to additional gold adatoms leads to stabilization of parallel pairs of molecules, suppressing a head-to-tail alignment order.

2.
J Am Chem Soc ; 2020 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-32589029

RESUMO

Although methods for a periodic perforation and heteroatom doping of graphene sheets have been developed, patterning closely spaced holes on the nanoscale in graphene nanoribbons is still a challenging task. In this work, nitrogen-doped porous graphene nanoribbons (N-GNRs) were synthesized on Ag(111) using a silver-assisted Ullmann polymerization of brominated tetrabenzophenazine. Insights into the hierarchical reaction pathways from single molecules toward the formation of one-dimensional organometallic complexes and N-GNRs are gained by a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM) with CO-tip, scanning tunneling spectroscopy (STS), and density functional theory (DFT).

3.
Chemistry ; 26(23): 5264-5269, 2020 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-32022327

RESUMO

Heteroatom substitution into the cores of alternant, aromatic hydrocarbons containing only even-membered rings is attracting increasing interest as a method of tuning their electrical conductance. Here, the effect of heteroatom substitution into molecular cores of non-alternant hydrocarbons, containing odd-membered rings, is examined. Benzodichalcogenophene (BDC) compounds are rigid, planar π-conjugated structures, with molecular cores containing five-membered rings fused to a six-membered aryl ring. To probe the sensitivity or resilience of constructive quantum interference (CQI) in these non-bipartite molecular cores, two C2 -symmetric molecules (I and II) and one asymmetric molecule (III) were investigated. I (II) contains S (O) heteroatoms in each of the five-membered rings, while III contains an S in one five-membered ring and an O in the other. Differences in their conductances arise primarily from the longer S-C and shorter O-C bond lengths compared with the C-C bond and the associated changes in their resonance integrals. Although the conductance of III is significantly lower than the conductances of the others, CQI was found to be resilient and persist in all molecules.

4.
Angew Chem Int Ed Engl ; 59(3): 1334-1339, 2020 Jan 13.
Artigo em Inglês | MEDLINE | ID: mdl-31729821

RESUMO

On-surface synthesis offers a versatile approach to prepare novel carbon-based nanostructures that cannot be obtained by conventional solution chemistry. Graphene nanoribbons (GNRs) have potential for a variety of applications. A key issue for their application in molecular electronics is in the fine-tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non-benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) is a highly appealing strategy. Herein we present the selective on-surface synthesis of a Por-GNR hybrid, which consists of two Pors connected by a short GNR segment. The atomically precise structure of the Por-GNR hybrid has been characterized by bond-resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM). The electronic properties have been investigated by scanning tunneling spectroscopy (STS), in combination with DFT calculations, which reveals a low electronic gap of 0.4 eV.

5.
Nano Lett ; 20(1): 652-657, 2020 01 08.
Artigo em Inglês | MEDLINE | ID: mdl-31797665

RESUMO

Bending and twisting around carbon-carbon single bonds are ubiquitous in natural and synthetic polymers. Force-induced changes were so far not measured at the single-monomer level, owing to limited ways to apply local forces. We quantified down to the submolecular level the mechanical response within individual poly-pyrenylene chains upon their detachment from a gold surface with an atomic force microscope at 5 K. Computer simulations based on a dedicated force field reproduce the experimental traces and reveal symmetry-broken bent and rotated conformations of the sliding physisorbed segment besides steric hindrance of the just lifted monomer. Our study also shows that the tip-molecule bond remains intact but remarkably soft and links force variations to complex but well-defined conformational changes.

6.
Nat Nanotechnol ; 14(10): 957-961, 2019 10.
Artigo em Inglês | MEDLINE | ID: mdl-31527843

RESUMO

One of the main challenges to upscale the fabrication of molecular devices is to achieve a mechanically stable device with reproducible and controllable electronic features that operates at room temperature1,2. This is crucial because structural and electronic fluctuations can lead to significant changes in the transport characteristics at the electrode-molecule interface3,4. In this study, we report on the realization of a mechanically and electronically robust graphene-based molecular junction. Robustness was achieved by separating the requirements for mechanical and electronic stability at the molecular level. Mechanical stability was obtained by anchoring molecules directly to the substrate, rather than to graphene electrodes, using a silanization reaction. Electronic stability was achieved by adjusting the π-π orbitals overlap of the conjugated head groups between neighbouring molecules. The molecular devices exhibited stable current-voltage (I-V) characteristics up to bias voltages of 2.0 V with reproducible transport features in the temperature range from 20 to 300 K.

7.
Nanoscale ; 11(27): 13117-13125, 2019 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-31268079

RESUMO

Using graphene as electrodes provides an opportunity for fabricating stable single-molecule field-effect transistors (FETs) operating at room temperature. However, the role of the unique graphene band structure in charge transport of single-molecule devices is still not clear. Here we report the Dirac-cone induced electrostatic gating effects in single-molecule FETs with graphene electrodes and a solid-state local bottom gate. With the highest occupied molecular orbital (HOMO) as the dominating conduction channel and the graphene leads remaining intrinsic at zero gate voltage, electrostatic gating on the HOMO and the density of states of graphene at the negative gate polarity reinforces each other, resulting in an enhanced conductance modulation. In contrast, gating effects on the HOMO and the graphene states at the positive gate polarity are opposite. Depending on the gating efficiencies, the conductance can decrease, increase or remain almost unchanged when a more positive gate voltage is applied. Our observations can be well understood by a modified single-level model taking into account the linear dispersion of graphene near the Dirac point. Single-molecule FETs with Dirac-cone enhanced gating have shown high performances, with the modulation of a wide range of current over one order of magnitude. Our studies highlight the advantages of using graphene as an electrode material for molecular devices and pave the way for single-molecule FETs toward circuitry applications.

8.
Chemphyschem ; 20(18): 2360-2366, 2019 09 17.
Artigo em Inglês | MEDLINE | ID: mdl-31087751

RESUMO

On-surface synthesis is a unique tool for growing low-dimensional carbon nanomaterials with precise structural control down to the atomic level. This novel approach relies on carefully designed precursor molecules, which are deposited on suitable substrates and activated to ultimately form the desired nanostructures. One of the most applied reactions to covalently interlink molecular precursors is dehalogenative aryl-aryl coupling. Despite the versatility of this approach, many unsuccessful attempts are also known, most of them associated to the poor capability of the activated precursors to couple to each other. Such failure is often related to the steric hindrance between reactants, which may arise due to their coplanarity upon adsorption on a surface. Here, we propose a copolymerization approach to overcome the limitations that prevent intermolecular homocoupling. We apply the strategy of using suitable linkers as additional reactants to the formation of fully conjugated polycyclic nanowires incorporating non-benzenoid rings.

9.
Nano Lett ; 19(5): 2750-2757, 2019 05 08.
Artigo em Inglês | MEDLINE | ID: mdl-30933563

RESUMO

The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on a Ag(111) metal substrate show different charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric fields, whereas their charge state is fixed otherwise. This enables the activation of selected molecules inside islands by vacancy creation from scanning-probe-based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on one hand, and the implementation of specific functionality into otherwise homogeneous monolayers, on the other. Activated molecules in the direct neighborhood influence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which offers the prospect of implementing complex functionality in such structures in the future.

10.
Nanoscale ; 11(3): 1437-1441, 2019 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-30608494

RESUMO

A combination of a preexfoliated nanographene (NG) dispersion and fused electron donor-acceptor tetrathiafulvalene-perylenediimide (TTF-PDI) results in a noncovalent functionalization of NG. Such novel types of nanohybrids were characterized by complementary spectroscopic and microscopic techniques. The design strategy of the chromophoric and electroactive molecular conjugate renders a large and planar π-extended system with a distinct localization of electron-rich and electron-poor parts at either end of the molecular conjugate. Within the in situ formed nanohybrid, the conjugate was found to couple electronically with NG preferentially through the electron accepting PDI rather than the electron donating TTF and to form the one-electron reduced form of PDI, which corresponds to p-doping of graphene.

11.
Nanoscale ; 10(38): 18131-18134, 2018 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-30256379

RESUMO

A novel strategy to regulate the tunneling mechanism for charge transport through an organoborane wire via Lewis acid-base interactions has been developed. A change from LUMO- to HOMO-dominated charge transport upon the addition of the fluoride is verified both experimentally and theoretically.

12.
Inorg Chem ; 57(10): 6076-6083, 2018 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-29741382

RESUMO

A pentanuclear CuII5-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand ( trans-1,4-cyclohexanediyl-4,4'-bi(1,2,4-triazole) ( tr2 cy)), MoVI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}4- anions, which represent heteroleptic derivatives of γ-type [Mo8O26]4- further interlinked by [Cu3(OH)2]4+ cations through [ N- N] bridges. The framework structure of the resulting compound [Cu5(OH)2( tr2 cy)2Mo6O22]·6H2O (1) is thus built up from neutral heterometallic {Cu5(OH)2Mo6O22} n layers pillared with tetradentate tr2 cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8O26]4- cluster into which CuII inserts corresponds with the site that has the lowest defect ("MoO2 vacancy") formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = -500(40) K (-348(28) cm-1), J2 = -350(10) K (-243(7) cm-1) and g = 2.32(2), χ2 = 6.5 × 10-4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH-, O2-, and tr2 cy bridging ligands.

13.
Nanoscale ; 10(3): 1337-1344, 2018 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-29296988

RESUMO

Advances in organic chemistry allow the synthesis of large, complex and highly functionalized organic molecules having potential applications in optoelectronics, molecular electronics and organic solar cells. Their integration into devices as individual components or highly ordered thin-films is of paramount importance to address these future prospects. However, conventional sublimation techniques in vacuum are usually not applicable since large organic compounds are often non-volatile and decompose upon heating. Here, we prove by atomic force microscopy and scanning tunneling microscopy, the structural integrity of complex organic molecules deposited onto an Au(111) surface using electrospray ionisation deposition. High resolution AFM measurements with CO-terminated tips unambiguously reveal their successful transfer from solution to the gold surface in ultra-high vacuum without degradation of their chemical structures. Furthermore, the formation of molecular structures from small islands to large and highly-ordered self-assemblies of those fragile molecules is demonstrated, confirming the use of electrospray ionisation to promote also on-surface polymerization reactions of highly functionalized organic compounds, biological molecules or molecular magnets.

14.
Inorg Chem ; 56(21): 12952-12966, 2017 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-29019398

RESUMO

We investigated the coordination ability of the bis(1,2,4-triazolyl) module, tr2pr = 1,3-bis(1,2,4-triazol-4-yl)propane, toward the engineering of solid-state structures of copper polyoxomolybdates utilizing a composition space diagram approach. Different binding modes of the ligand including [N-N]-bridging and N-terminal coordination and the existence of favorable conformation forms (anti/anti, gauche/anti, and gauche/gauche) resulted in varieties of mixed metal CuI/MoVI and CuII/MoVI coordination polymers prepared under hydrothermal conditions. The composition space analysis employed was aimed at both the development of new coordination solids and their crystallization fields through systematic changes of the reagent ratios [copper(II) and molybdenum(VI) oxide precursors and the tr2pr ligand]. Nine coordination compounds were synthesized and structurally characterized. The diverse coordination architectures of the compounds are composed of cationic fragments such as [CuII3(µ2-OH)2(µ2-tr)2]4+, [CuII3(µ2-tr)6]6+, [CuII2(µ2-tr)3]4+, etc., connected to polymeric arrays by anionic species (molybdate MoO42-, isomeric α-, δ-, and ß-octamolybdates {Mo8O26}4- or {Mo8O28H2}6-). The inorganic copper(I,II)/molybdenum(VI) oxide matrix itself forms discrete or low-dimensional subtopological motifs (0D, 1D, or 2D), while the organic spacers interconnect them into higher-dimensional networks. The 3D coordination hybrids show moderate thermal stability up to 230-250 °C, while for the 2D compounds, the stability of the framework is distinctly lower (∼190 °C). The magnetic properties of the most representative samples were investigated. The magnetic interactions were rationalized in terms of analyzing the planes of the magnetic orbitals.


Assuntos
Complexos de Coordenação/química , Cobre/química , Molibdênio/química , Polímeros/química , Triazóis/química , Complexos de Coordenação/síntese química , Ligantes , Fenômenos Magnéticos , Modelos Químicos , Conformação Molecular , Estrutura Molecular , Polímeros/síntese química
15.
ACS Nano ; 11(8): 8413-8420, 2017 08 22.
Artigo em Inglês | MEDLINE | ID: mdl-28731327

RESUMO

Electron donor-acceptor molecules are of outstanding interest in molecular electronics and organic solar cells for their intramolecular charge transfer controlled via electrical or optical excitation. The preservation of their electronic character in the ground state upon adsorption on a surface is cardinal for their implementation in such single-molecule devices. Here, we investigate by atomic force microscopy and scanning tunneling microscopy a prototypical system consisting of a π-conjugated tetrathiafulvalene-fused dipyridophenazine molecule adsorbed on thin NaCl films on Cu(111). Depending on the adsorption site, the molecule is found either in a nearly undisturbed free state or in a bound state. In the latter case, the molecule adopts a specific adsorption site, leading to the formation of a chelate complex with a single Na+ alkali cation pulled out from the insulating film. Although expected to be electronically decoupled, the charge distribution of the complex is drastically modified, leading to the loss of the intrinsic donor-acceptor character. The chelate complex formation is reversible with respect to lateral manipulations, enabling tunable donor-acceptor molecular switches activated by on-surface coordination.

16.
Angew Chem Int Ed Engl ; 56(36): 10786-10790, 2017 08 28.
Artigo em Inglês | MEDLINE | ID: mdl-28700106

RESUMO

Structures of the aromatic N-heterocyclic hexaazatriphenylene (HAT) molecular synthon obtained by surface-assisted self-assembly were analyzed with sub-Å resolution by means of noncontact atomic force microscopy (nc-AFM), both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. These results reveal how the crystallization governs the length scale of the network order for non-flexible molecular species without affecting the local bonding schemes. The capability of nc-AFM to accurately resolve structural relaxations will be highly relevant for the characterization of vitreous two-dimensional supramolecular materials.

17.
Nat Commun ; 8: 15388, 2017 05 22.
Artigo em Inglês | MEDLINE | ID: mdl-28530247

RESUMO

Realization of long-range magnetic order in surface-supported two-dimensional systems has been challenging, mainly due to the competition between fundamental magnetic interactions as the short-range Kondo effect and spin-stabilizing magnetic exchange interactions. Spin-bearing molecules on conducting substrates represent a rich platform to investigate the interplay of these fundamental magnetic interactions. Here we demonstrate the direct observation of long-range ferrimagnetic order emerging in a two-dimensional supramolecular Kondo lattice. The lattice consists of paramagnetic hexadeca-fluorinated iron phthalocyanine (FeFPc) and manganese phthalocyanine (MnPc) molecules co-assembled into a checkerboard pattern on single-crystalline Au(111) substrates. Remarkably, the remanent magnetic moments are oriented in the out-of-plane direction with significant contribution from orbital moments. First-principles calculations reveal that the FeFPc-MnPc antiferromagnetic nearest-neighbour coupling is mediated by the Ruderman-Kittel-Kasuya-Yosida exchange interaction via the Au substrate electronic states. Our findings suggest the use of molecular frameworks to engineer novel low-dimensional magnetically ordered materials and their application in molecular quantum devices.

18.
Chemistry ; 23(29): 7104-7112, 2017 May 23.
Artigo em Inglês | MEDLINE | ID: mdl-28317198

RESUMO

The synthesis is reported of a series of homo- and hetero-dinuclear octahedral complexes of the ligand 1, 1,2-bis(1-methyl-benzimidazol-2-yl) ethanol, where the two metal centres are linked by hydrogen bonds between coordinated alcohols and coordinated alkoxides. Homonuclear divalent MII MII , mixed-valent MII MIII and heteronuclear MII M'III species are prepared. The complexes have been characterised by X-ray crystallography and show unusually short O⋅⋅⋅O distances for the hydrogen bonds. Magnetic measurements show the hydrogen-bond bridges can lead to ferromagnetic or antiferromagnetic coupling. The electrochemistry of the dinuclear species is significantly different from the mononuclear systems: the latter show irreversible waves in cyclic voltammograms as a result of the need to couple proton and electron transfer. The dinuclear species, in contrast, show reversible waves, which are attributed to rapid intramolecular proton transfer facilitated by the hydrogen-bonded structure.

19.
Inorg Chem ; 56(5): 2662-2676, 2017 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-28260389

RESUMO

New tetranuclear and octanuclear mixed-valent cobalt(II/III) pivalate clusters, namely, [NaCo4(O2CCMe3)6(HO2CCMe3)2(teaH)2(N3)]·2H2O (in two polymorphic modifications, 1 and 1a) and [Co8(O2CCMe3)10(teaH)4(N3)](Me3CCO2)·MeCN·H2O (2) have been synthesized by ultrasonic treatment of a dinuclear cobalt(II) pivalate precursor with sodium azide and triethanolamine (teaH3) ligand in acetonitrile. The use of Dy(NO3)3·6H2O in a similar reaction led to the precipitation of a tetranuclear [NaCo4(O2CCMe3)4(teaH)2(N3)(NO3)2(H2O)2]·H2O (3) cluster and a heterometallic hexanuclear [Co3Dy3(OH)4(O2CCMe3)6(teaH)3(H2O)3](NO3)2·H2O (4) cluster. Single-crystal X-ray analysis showed that 1 (1a) and 3 consist of a tetranuclear pivalate/teaH3 mixed-ligand cluster [CoII2CoIII2(O2CCMe3)4(teaH)2(N3)]+ decorated with sodium pivalates [Na(O2CCMe3)2(HO2CCMe3)2]- (1 or 1a) or sodium nitrates [Na(NO3)2]- (3) to form a square-pyramidal assembly. In 2, the cationic [Co8(O2CCMe3)10(teaH)4(N3)]+ cluster comprises a mixed-valent {CoII4CoIII4} core encapsulated by an azide, 4 teaH2- alcoholamine ligands, and 10 bridging pivalates. Remarkably, in this core, the µ4-N3- ligand joins all four CoII atoms. The heterometallic hexanuclear compound 4 consists of a cationic [CoIII3DyIII3(OH)4(O2CCMe3)6(teaH)3(H2O)3]2+ cluster, two NO3- anions, and a crystallization water molecule. The arrangement of metal atoms in 4 can be approximated as the assembly of a smaller equilateral triangle defined by three Dy sites with a Dy···Dy distance of 3.9 Å and a larger triangle formed by Co sites [Co···Co, 6.1-6.2 Å]. The interpretation of the magnetic properties of clusters 2-4 was performed in the framework of theoretical models, taking into account the structural peculiarities of clusters and their energy spectra. The behavior of clusters 2 and 3 containing CoII ions with orbitally nondegenerate ground states is determined by the zero-field splitting of these states and Heisenberg exchange interaction between the ions. To get a good understanding of the observed magnetic behavior of cluster 4, we take into consideration the crystal fields acting on the DyIII ions, the ferromagnetic coupling of neighboring DyIII ions, and the intercluster antiferromagnetic exchange. For all examined clusters, the developed models describe well the observed temperature dependence of the magnetic susceptibility and the field dependence of magnetization. The computational results apparently show that in cluster 4 two DyIII ions with similar nearest surroundings demonstrate single-molecule-magnet (SMM) behavior, while the strong rhombicity of the ligand surrounding hinders the SMM behavior of the third DyIII ion.

20.
ACS Nano ; 10(6): 5782-8, 2016 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-27219352

RESUMO

Organic-organic heterojunctions are nowadays highly regarded materials for light-emitting diodes, field-effect transistors, and photovoltaic cells with the prospect of designing low-cost, flexible, and efficient electronic devices.1-3 However, the key parameter of optimized heterojunctions relies on the choice of the molecular compounds as well as on the morphology of the organic-organic interface,4 which thus requires fundamental studies. In this work, we investigated the deposition of C60 molecules at room temperature on an organic layer compound, the salt bis(benzylammonium)bis(oxalato)cupurate(II), by means of noncontact atomic force microscopy. Three-dimensional molecular islands of C60 having either triangular or hexagonal shapes are formed on the substrate following a "Volmer-Weber" type of growth. We demonstrate the dynamical reshaping of those C60 nanostructures under the local action of the AFM tip at room temperature. The dissipated energy is about 75 meV and can be interpreted as the activation energy required for this migration process.

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