Overcoming the Sticking Point: Electrical Conductivity of Carbon Nanotube Networks Containing 3<i>d</i> Metals

Carbon nanotubes have excellent electrical conductivity along the length of the tubes. Yet, the electrical conductivity across the nanotube–nanotube intersections is weak and severely limits device performance. Here, we show that the incorporation of 3d metal (period 4) atoms into networks of semiconducting (8,0) carbon nanotubes significantly enhances the electrical conductivity within the network. Our calculations using quantum mechanical methods and semiclassical Boltzmann transport theory predict the changes to the electronic structure and provide directional information about the flow of electrons within the network. The ligand field splitting of the transition metals exerts strong effects on the conductivity. Interestingly, networks doped with Sc, V, or Fe can become insulating along certain directions or have higher conductivity across the junction than along the tubes. This finding suggests that doping with transition metals removes a bottleneck of charge transport within carbon nanotube films

Overcoming the Sticking Point: Electrical Conductivity of Carbon Nanotube Networks Containing 3<i>d</i> Metals

Carbon nanotubes have excellent electrical conductivity along the length of the tubes. Yet, the electrical conductivity across the nanotube–nanotube intersections is weak and severely limits device performance. Here, we show that the incorporation of 3d metal (period 4) atoms into networks of semiconducting (8,0) carbon nanotubes significantly enhances the electrical conductivity within the network. Our calculations using quantum mechanical methods and semiclassical Boltzmann transport theory predict the changes to the electronic structure and provide directional information about the flow of electrons within the network. The ligand field splitting of the transition metals exerts strong effects on the conductivity. Interestingly, networks doped with Sc, V, or Fe can become insulating along certain directions or have higher conductivity across the junction than along the tubes. This finding suggests that doping with transition metals removes a bottleneck of charge transport within carbon nanotube films

Thermoelectric Properties of p‑Type Cu₂O, CuO, and NiO from Hybrid Density Functional Theory

The electronic transport coefficients of three Earth-abundant metal oxides Cu₂O, CuO, and NiO were investigated using hybrid density functional theory (DFT). Hybrid DFT methods combined with local Gaussian-type basis sets enabled band structure studies on both non-magnetic and magnetic p-type metal oxides without empirical corrections. The CRYSTAL code was used for obtaining the wavefunction, and the transport properties were calculated with two different methodologies to benchmark their accuracy: a numerical approach as implemented in the BoltzTraP code and an analytical approach recently implemented in CRYSTAL17. Both computational methods produce identical results in good agreement with experimental measurements of the Seebeck coefficient. The predicted electrical conductivities are overestimated, owing likely to the used approximation of a constant electronic relaxation time in the calculations, as explicit electron scattering is neglected and relaxation time is considered only as a free parameter. The obtained results enable us to critically review and complement the available theoretical and experimental literature on the studied p-type thermoelectric metal oxide materials

Bulk Synthesis and Structure of a Microcrystalline Allotrope of Germanium (<i>m-allo</i>-Ge)

An easy to reproduce and scale-up method for the preparation of a microcrystalline allotrope of germanium is presented. Based on the report of the oxidation of a single crystal of Li₇Ge₁₂ the synthesis and structure determination of a powdered sample of Li₇Ge₁₂ is investigated. Besides the known oxidation of Li₇Ge₁₂ with benzophenone a variety of protic solvents such as alcohols and water were used as oxidants. Electron energy loss spectroscopy (EELS) proves that the reaction products do not contain Li. The structure determination of the powder samples based on selected area electron diffraction (SAED), powder X-ray diffraction, quantum chemical calculations (DFT-B3LYP level of theory), and simulated powder X-ray diffraction diagrams obtained using the DIFFaX and FAULTS software packages show that the microcrystalline powders do not match any of the existing structures of germanium including the rough model of so-called <i>allo</i>-Ge. It is shown that the structural motif of layered Ge slabs of the precursor Li₇Ge₁₂ that contain five-membered rings is retained in <i>m</i>icrocrystalline <i>allo</i>-Ge (<i>m-allo</i>-Ge). The covalent connectivity between the slabs and the statistic of the layer sequence is determined. According to B3LYP-DFT calculations of a periodic approximate model a direct band gap is expected for <i>m-allo-</i>Ge

Harvesting Fluorescence from Efficient T_<i>k</i> → S_<i>j</i> (<i>j</i>, <i>k</i> > 1) Reverse Intersystem Crossing for ππ* Emissive Transition-Metal Complexes

Using a bimetallic Au­(I) complex bearing alkynyl-(phenylene)₃-diphosphine ligand (<b>A</b>-<b>3</b>), we demonstrate that the fluorescence can be exquisitely harvested upon T₁ → T_<i>k</i> (<i>k</i> > 1) excitation followed by T_<i>k</i> → S_<i>j</i> (<i>j</i>, <i>k</i> > 1) intersystem crossing (ISC) back to the S₁ state. Upon S₀ → S₁ 355 nm excitation, the S₁ → T₁ intersystem crossing rate has been determined to be 8.9 × 10⁸ s^–1. Subsequently, in a two-step laser pump–probe experiment, following a 355 nm laser excitation, the 532 nm T₁ → T_<i>k</i> probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump–probe signal correlates well with the lifetime of the T₁ state. Careful examination reveals the efficiency of T_<i>k</i> → S<i>_j</i> (<i>j</i>, <i>k</i> > 1) reverse intersystem crossing to be 5.2%. The result is rationalized by a mechanism incorporating substantial involvement of metal-to-ligand charge transfer (MLCT) in the T_<i>k</i> (S_<i>j</i>) states, enhancing the rate of T_<i>k</i> → S_<i>j</i> ISC, which is competitive with the rate of T_k → T₁ internal conversion. This mechanism is also proven to be operative in the <b>A</b>-<b>3</b> solid film and should be universally applicable to the transition-metal complexes possessing a dominant ππ* configuration in the lowest-lying states. From an energy point of view, the UV fluorescence (375 nm) generated by green (532 nm) excitation can be recognized as a signal up-conversion process

Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for Coordination Self-Assembly

A family of cyanide-bridged heterometallic aggregates has been constructed of the chromophoric cycloplatinated metalloligands and coordinatively unsaturated d¹⁰ fragments {M­(PPh₃)_<i>n</i>}. The tetranuclear complexes of general composition [Pt­(C^N)­(CN)₂M­(PPh₃)₂]₂ [C^N = ppy, M = Cu (<b>1</b>), Ag (<b>2</b>); C^N = tolpy (Htolpy = 2-(4-tolyl)-pyridine), M = Cu (<b>4</b>), Ag (<b>5</b>); C^N = F₂ppy (HF₂ppy = 2-(4, 6-difluorophenyl)-pyridine), M = Cu (<b>7</b>), Ag (<b>8</b>)] demonstrate a squarelike arrangement of the molecular frameworks, which is achieved due to favorable coordination geometries of the bridging ligands and the metal ions. Variation of the amount of the ancillary phosphine (for M = Ag) afforded compounds [Pt­(C^N)­(CN)₂Ag­(PPh₃)]₂ (C^N = ppy, <b>3</b>; C^N = tolpy, <b>6</b>); for the latter one an alternative cluster topology, stabilized by the Pt–Ag metallophilic and η¹-C_ipso(C^N)–Ag bonding, was observed. The solid-state structures of all of the title species <b>1</b>–<b>8</b> were determined crystallographically. The complexes exhibit moderately strong room-temperature phosphorescence as crystalline powders (Φ_em = 16–34%, λ_em = 470–511 nm). The luminescence studies and time-dependent density functional theory computational analysis indicate that the photophysical behavior is dominated by the ³π–π* electronic transitions localized on the cyclometalated fragment and mixed with M_PtLCT contribution, while the d¹⁰-phosphine motifs have a negligible contribution into the frontier orbitals and therefore show a little influence on the emission performance of the described compounds

Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes Defines Structural Diversity

The face-capping triphosphine, 1,1,1-tris­(diphenyl­phos­phino)­methane (tppm), together with bridging alkynyl ligands and the counterions, facilitates the formation of a family of silver complexes, which adopt cluster frameworks of variable nuclearity. The hexanuclear compounds [Ag₆­(C₂­C₆H₄-4-X)₃­(tppm)₂­(An^–)₃] (X = H (<b>1</b>), CF₃ (<b>2</b>), OMe (<b>3</b>), An^– = CF₃SO₃^–; X = OMe (<b>4</b>), An^– = CF₃COO^–) are produced for the electron-accepting to moderately electron-donating alkynes and the appropriate stoichiometry of the reagents. <b>1</b> and <b>3</b> undergo an expansion of the metal core when treated with 1 equiv of Ag⁺ to give the species [Ag₇­(C₂­C₆H₄-4-X)₃­(tppm)₂­(CF₃­SO₃)₃]­(CF₃­SO₃) (X = H (<b>5</b>), OMe (<b>6</b>)). The electron-donating substituent (X = NMe₂) particularly favors this Ag₇ arrangement (<b>7</b>) that undergoes geometry changes upon alkynylation, resulting in the capped prismatic cluster [Ag₇­(C₂­C₆H₄-4-NMe₂)₄­(tppm)₂­(CF₃­SO₃)]­(CF₃­SO₃)₂ (<b>8</b>). Alternatively, for the aliphatic <i>^t</i>Bu-alkyne, only the octanuclear complex [Ag₈­(C₂Bu^<i>t</i>)₄­{(PPh₂)₃­CH}₂­(CF₃­SO₃)₂]­(CF₃­SO₃)₂ (<b>9</b>) is observed. The structures of <b>1</b>–<b>4</b> and <b>6</b>–<b>9</b> were determined by X-ray diffraction analysis. In solution, all the studied compounds were found to be stereochemically nonrigid that prevented their investigation in the fluid medium. In the solid state, clusters <b>2</b>, <b>3</b>, <b>5</b>–<b>8</b> exhibit room temperature luminescence of triplet origin (maximum Φ_em = 27%, λ_em = 485–725 nm). The observed emission is assigned mainly to [<i>d</i>(Ag) → π*­(alkyne)] electronic transitions on the basis of TD-DFT computational analysis

Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes Defines Structural Diversity

The face-capping triphosphine, 1,1,1-tris­(diphenyl­phos­phino)­methane (tppm), together with bridging alkynyl ligands and the counterions, facilitates the formation of a family of silver complexes, which adopt cluster frameworks of variable nuclearity. The hexanuclear compounds [Ag₆­(C₂­C₆H₄-4-X)₃­(tppm)₂­(An^–)₃] (X = H (<b>1</b>), CF₃ (<b>2</b>), OMe (<b>3</b>), An^– = CF₃SO₃^–; X = OMe (<b>4</b>), An^– = CF₃COO^–) are produced for the electron-accepting to moderately electron-donating alkynes and the appropriate stoichiometry of the reagents. <b>1</b> and <b>3</b> undergo an expansion of the metal core when treated with 1 equiv of Ag⁺ to give the species [Ag₇­(C₂­C₆H₄-4-X)₃­(tppm)₂­(CF₃­SO₃)₃]­(CF₃­SO₃) (X = H (<b>5</b>), OMe (<b>6</b>)). The electron-donating substituent (X = NMe₂) particularly favors this Ag₇ arrangement (<b>7</b>) that undergoes geometry changes upon alkynylation, resulting in the capped prismatic cluster [Ag₇­(C₂­C₆H₄-4-NMe₂)₄­(tppm)₂­(CF₃­SO₃)]­(CF₃­SO₃)₂ (<b>8</b>). Alternatively, for the aliphatic <i>^t</i>Bu-alkyne, only the octanuclear complex [Ag₈­(C₂Bu^<i>t</i>)₄­{(PPh₂)₃­CH}₂­(CF₃­SO₃)₂]­(CF₃­SO₃)₂ (<b>9</b>) is observed. The structures of <b>1</b>–<b>4</b> and <b>6</b>–<b>9</b> were determined by X-ray diffraction analysis. In solution, all the studied compounds were found to be stereochemically nonrigid that prevented their investigation in the fluid medium. In the solid state, clusters <b>2</b>, <b>3</b>, <b>5</b>–<b>8</b> exhibit room temperature luminescence of triplet origin (maximum Φ_em = 27%, λ_em = 485–725 nm). The observed emission is assigned mainly to [<i>d</i>(Ag) → π*­(alkyne)] electronic transitions on the basis of TD-DFT computational analysis

Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes Defines Structural Diversity

The face-capping triphosphine, 1,1,1-tris­(diphenyl­phos­phino)­methane (tppm), together with bridging alkynyl ligands and the counterions, facilitates the formation of a family of silver complexes, which adopt cluster frameworks of variable nuclearity. The hexanuclear compounds [Ag₆­(C₂­C₆H₄-4-X)₃­(tppm)₂­(An^–)₃] (X = H (<b>1</b>), CF₃ (<b>2</b>), OMe (<b>3</b>), An^– = CF₃SO₃^–; X = OMe (<b>4</b>), An^– = CF₃COO^–) are produced for the electron-accepting to moderately electron-donating alkynes and the appropriate stoichiometry of the reagents. <b>1</b> and <b>3</b> undergo an expansion of the metal core when treated with 1 equiv of Ag⁺ to give the species [Ag₇­(C₂­C₆H₄-4-X)₃­(tppm)₂­(CF₃­SO₃)₃]­(CF₃­SO₃) (X = H (<b>5</b>), OMe (<b>6</b>)). The electron-donating substituent (X = NMe₂) particularly favors this Ag₇ arrangement (<b>7</b>) that undergoes geometry changes upon alkynylation, resulting in the capped prismatic cluster [Ag₇­(C₂­C₆H₄-4-NMe₂)₄­(tppm)₂­(CF₃­SO₃)]­(CF₃­SO₃)₂ (<b>8</b>). Alternatively, for the aliphatic <i>^t</i>Bu-alkyne, only the octanuclear complex [Ag₈­(C₂Bu^<i>t</i>)₄­{(PPh₂)₃­CH}₂­(CF₃­SO₃)₂]­(CF₃­SO₃)₂ (<b>9</b>) is observed. The structures of <b>1</b>–<b>4</b> and <b>6</b>–<b>9</b> were determined by X-ray diffraction analysis. In solution, all the studied compounds were found to be stereochemically nonrigid that prevented their investigation in the fluid medium. In the solid state, clusters <b>2</b>, <b>3</b>, <b>5</b>–<b>8</b> exhibit room temperature luminescence of triplet origin (maximum Φ_em = 27%, λ_em = 485–725 nm). The observed emission is assigned mainly to [<i>d</i>(Ag) → π*­(alkyne)] electronic transitions on the basis of TD-DFT computational analysis

Hybrid Inorganic–Organic Complexes of Zn, Cd, and Pb with a Cationic Phenanthro-diimine Ligand

The phosphonium-decorated phenanthro-imidazolyl pyridine ligand, LP+Br, readily reacts with zinc(II) and cadmium(II) bromides to give inorganic–organic zero-dimensional compounds [LP+ZnBr2]2[ZnBr4] (1) and [(LP+)2Cd2Br4][CdBr4] (2), respectively, upon crystallization. These salts are moderately fluorescent in the solid state under ambient conditions (λem = 458 nm, Φem = 0.11 for 1; λem = 460 nm, Φem = 0.13 for 2). Their emission results from spin-allowed electronic transitions localized on the organic component with the negligible effect of [MBr4]2– and MBr2 units. Contrary to ionic species 1 and 2, lead(II) bromide affords a neutral and water-stable complex [(LP+)2Pb3Br8] (3), showing weak room-temperature phosphorescence arising from spin–orbit coupling due to the heavy atom effect. The emission, which is substantially enhanced for the amorphous sample of 3 (λem = 575 nm, Φem = 0.06), is assigned to the intraligand triplet excited state, which is a rare phenomenon among Pb(II) molecular materials