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1.
Chemistry ; 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32031714

RESUMO

Ring/chain competition in oligomerization reactions represents a long-standing topic of synthetic chemistry and was treated extensively for solution reactions but is not well-understood for the two-dimensional confinement of surface reactions. Here, the kinetic and thermodynamic principles of ring/chain competition in on-surface synthesis are addressed by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and Monte Carlo simulations applied to azulene-based organometallic oligomers on Cu(111). Analysis of experiments and simulations reveals how the ring/chain ratio can be controlled through variation of coverage and temperature. At room temperature, non-equilibrium conditions prevail and kinetic control leads to preferential formation of the entropically favored chains. In contrast, high-temperature equilibrium conditions are associated with thermodynamic control, resulting in increased yields of the energetically favored rings. The optimum conditions for ring formation include the lowest possible temperature within the regime of thermodynamic control and a low coverage. The general implications are discussed and compared to the solution case.

2.
J Am Chem Soc ; 141(44): 17713-17720, 2019 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-31617709

RESUMO

Various two-dimensional (2D) carbon allotropes with nonalternant topologies, such as pentaheptites and phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface synthesis, nanoribbons of the nonalternant graphene allotropes, phagraphene and tetra-penta-hepta(TPH)-graphene, have been obtained by dehydrogenative C-C coupling of 2,6-polyazulene chains. These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tips and density functional theory calculations have been used to elucidate their structural properties. The proposed synthesis of nonalternant carbon nanoribbons from the fusion of synthetic line-defects may pave the way for large-area preparation of novel 2D carbon allotropes.

3.
Chemistry ; 25(68): 15656-15661, 2019 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-31544988

RESUMO

The binary lead fluoride Pb3 F8 was synthesized by the reaction of anhydrous HF with Pb3 O4 or by the reaction of BrF3 with PbF2 . The compound was characterized by single-crystal and powder X-ray diffraction, IR, Raman, and solid-state MAS 19 F NMR spectroscopy, as well as thermogravimetric analysis, XP and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Solid-state quantum-chemical calculations are provided for the vibrational analyses and band assignments. The electronic band structure offers an inside view of the mixed valence compound.

4.
ACS Nano ; 12(10): 10537-10544, 2018 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-30295463

RESUMO

Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly( para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains.

5.
J Phys Condens Matter ; 30(38): 385501, 2018 Sep 26.
Artigo em Inglês | MEDLINE | ID: mdl-30102241

RESUMO

We investigated the compound 1T-Cu x Ta1-x S2 with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu x Ta1-x S2 resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS2 in which tantalum is partially substituted by copper. 1T-Cu x Ta1-x S2 readily decomposes at elevated temperatures and can only be prepared and stabilized by a sufficiently high amount of sulfur excess. XPS and NEXAFS measurements reveal that copper has the oxidation state +I in 1T-Cu x Ta1-x S2, which is supported by quantum chemical calculations. The disorder introduced by copper doping causes an Anderson-type localization of the conduction electrons as manifested by a strong increase of the electrical resistivity and a Curie-type paramagnetism at low temperatures as in other doped systems 1T-M x Ta1-x S2 with higher valent metals. Quantum chemical calculations support this interpretation.

6.
Chem Commun (Camb) ; 54(70): 9741-9744, 2018 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-30084444

RESUMO

Control over the competition between an organometallic hexamer macrocycle and oligomer chains formed from the non-alternant aromatic 1,3-dibromoazulene (DBAz) precursor has been achieved in surface-assisted synthesis on a copper(111) surface. In contrast to kinetic reaction control via the high-dilution principle, the ring formation is achieved here by thermodynamic control, which is based on two-dimensional (2D) confinement and reversible bonds.

7.
Phys Chem Chem Phys ; 18(44): 30643-30651, 2016 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-27790657

RESUMO

The interface formation upon vapor deposition of a metal onto a molecular organic semiconductor was studied using a well-defined complexation reaction between a metal and a porphyrin. Specifically, metallic cobalt (Co) was vapor deposited onto a thin film of 2H-tetraphenylporphyrin (2HTPP) at room temperature. The resulting interface was probed with Hard X-ray Photoelectron Spectroscopy (HAXPES) using photon energies between 2 and 6 keV to obtain a detailed depth profile of the chemical composition. Characteristic changes in the N 1s core level signals reveal the formation of a cobalt tetraphenylporphyrin (CoTPP) layer between the Co and 2HTPP layers. Assuming an abrupt interface between CoTPP and 2HTPP (layer-by-layer model), analysis of the XPS data results in a thickness of the CoTPP reaction layer of 1.6 nm. However, a more advanced numerical analysis allowed us to reconstruct details of the actual depth distribution of the CoTPP interphase layer: up to a depth of 1.5 nm, all 2HTPP molecules were converted into CoTPP. Beyond this depth, the CoTPP concentration decreases sharply within 0.15 nm to zero.

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