Strong Π-interactions Research Articles

Tunable Charge Transport and Spin Dynamics in Two-Dimensional Conjugated Metal-Organic Frameworks.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have attracted increasing interest in electronics due to their (semi)conducting properties. Charge-neutral 2D c-MOFs also possess persistent organic radicals that can be viewed as spin-concentrated arrays, affording new opportunities for spintronics. However, the strong π-interaction between neighboring layers of layer-stacked 2D c-MOFs annihilates active spin centers and significantly accelerates spin relaxation, severely limiting their potential as spin qubits. Herein, we report the precise tuning of the charge transport and spin dynamics in 2D c-MOFs via the control of interlayer stacking. The introduction of bulky side groups on the conjugated ligands enables a significant dislocation of the 2D c-MOFs layers from serrated stacking to staggered stacking, thereby spatially weakening the interlayer interactions. As a consequence, the electrical conductivity of 2D c-MOFs decreases by 6 orders of magnitude, while the spin density achieves more than a 30-fold increase and the spin-lattice relaxation time (T1) is increased up to ∼60 μs, hence being superior to the reference 2D c-MOFs with compact stackings whose spin relaxation is too fast to be detected. Spin dynamics results also reveal that spinless polaron pairs or bipolarons play critical roles in the charge transport of these 2D c-MOFs. Our strategy provides a bottom-up approach for enlarging spin dynamics in 2D c-MOFs, opening up pathways for developing MOF-based spintronics.

Modulation of iron spin states in highly distorted iron(III) porphyrins: H-bonding interactions and implications in hemoproteins.

A family of five- and six-coordinated Fe-porphyrins which enable us to scrutinize the effects of non-covalent interactions on the out-of-plane displacement of iron and its spin-states and axial ligand orientation in a single distorted macrocyclic environment has been reported. Combined analysis using single-crystal X-ray structure determination and EPR spectral investigation revealed the stabilization of the high-spin state of iron in the five-coordinate complex FeIII(TPPBr8)(OCHMe2), while six-coordinate complexes [FeIII(TPPBr8)(MeOH)2]ClO4, [FeIII(TPPBr8)(H2O)2]ClO4 and [FeIII(TPPBr8)(1-MeIm)2]ClO4 stabilize admixed-high, admixed-intermediate and low-spin states, respectively. The H-bonding interactions between the weak axial H2O/MeOH and perchlorate anion resulted in an elongation of the Fe-O bond which eventually shortened the Fe-N(por) distances leading to the stabilization of the admixed spin state of iron which, otherwise, stabilizes the high-spin (S = 5/2) state only. In addition, the iron atom in [FeIII(TPPBr8)(H2O)2]ClO4 is displaced by 0.02 Å towards one of the water molecules engaged in the H-bonding interactions leading to two different Fe-O (H2O) distances of 2.098(8) and 2.122(9) Å. In contrast, iron in [FeIII(TPPBr8)(MeOH)2]ClO4 sits on the plane of the porphyrin since both the axial methanol units are engaged in similar H-bonding interactions with the ClO4- ion. Moreover, the X-ray structure of low-spin FeII(TPPBr8)(1-MeIm)2 revealed a dihedral angle of 63.0° between two imidazoles which deviates largely from the expected angle of 90° (perpendicular orientations) since the axial imidazole protons are engaged in strong intermolecular C-H⋯π interactions which thereby restrict the axial ligand movement. The complex also displays the shortest Fe-N(1-MeIm) bond along with smallest dihedral angles of 7.8° and 22.4° between the axial imidazole ring and the closest Fe-Np axis due to strong π-interactions between iron and the axial imidazole ligand. Our work highlights the influence of non-covalent interactions on the out-of-plane displacement and spin state of iron and axial ligand orientations which are indeed important steps in the functioning of various hemoproteins.

Comparative study of interzeolite transformed metallosilicates for the chemisorption of benzene, toluene, and xylenes

Zeolitic metallosilicates with Lewis acidity are effective in the chemisorption of benzene, toluene, and xylenes (BTX) because of the strong π-interactions between the Lewis acid sites and BTX adsorbates. In this study, we prepared boro-, nickel-, and zincosilicate delMWW (two-dimensional delaminated MWW) and beta zeolite (*BEA) adsorbents by using the interzeolite transformation method and compared them with their mother MWWs for BTX chemisorption. Different structural properties, framework-atom species, and hydroxyl and Lewis acid site concentrations were systematically characterized by using various analytical methods. Among the studied adsorbents, nickel silicate beta zeolite (Ni-BEA) exhibited the highest concentration of Lewis acid sites and a hydrophobic character because of the stable framework-bound Ni species. In addition, its advantageous textural properties, large pore size, high surface area, and nano-sized crystals, induced the highest chemisorption content and a strong chemisorption ability of BTX at high-temperature (120 °C) adsorption under humid conditions.

Molecular salts of the isoniazid derivatives. Expanding the scope of sulfonate-pyridinium synthon to design materials

Understanding the intermolecular interactions is central to crystal engineering and is pivotal in designing of the new materials. Of recent we have reported sulfonate-pyridinium interaction as robust synthon and herein we utilize the interaction to develop molecular complexes: [(ISN–I3C–H)+(5-SSA-H)ˉ] (1), [(ISN-pV-H)+(5-SSA-H)ˉ.MeOH·H2O] (2), and [(ISN-oV-H)+(5-SSA-H)ˉ.H2O] (3). 1–3 have been obtained by reaction of isoniazid derivatives: ((1H-indol-3-yl)methylene)isonicotinohydrazide (ISN–I3C), isonicotinic acid (4-hydroxy-3-methoxy-benzylidene)-hydrazide (ISN-pV) and isonicotinic acid (2-hydroxy-3-methoxy-benzylidene)-hydrazide (ISN-oV) with 5-sulfosalicylic acid (5-SSA-2H). Molecular salts 1 and 2 manifest direct charge assisted sulfonate-pyridinium interaction, while, in 3, the synthon is masked by lattice water. The Schiff base cations in 1 and 2 stack together through strong π-interactions forming 2-dimensional layers and 1-dimensional tapes, respectively, which, in turn, are bridged by the organo-sulfonate anions into a 3-dimensional ionic solids. While the crystal formers in 3, the hydrogen bonds are utilized to form three molecule thick sheets, which aggregate through C–H⋯O and π-π interactions to form a 3-dimensional supramolecule. Hirshfeld studies corroborate with structural studies and validate that reciprocal O–H/H–O interactions are the chief intermolecular interactions in these oxygen rich compounds while weak dispersive H–H contacts are the second dominant interactions responsible for the molecular packing. Framework energy, as well as individual contribution of columbic, potential and dispersive forces towards molecular packing, are visualized through framework energy analyses of 1–3, which also substantiate that very robust sulfonate-pyridinium contact is not the strongest intermolecular contact. The resultant multi-component crystals exhibit enhanced optical properties, which are attributed to proton transfer and/or π-stacking interactions. The band gap values of 1–3 observed in the Tauc plots corroborate with charge transfer interactions. The products also exhibit enhanced solubility in methanol, higher than the isoniazid derivatives, and the thermal stability up to 240 ​°C. The molecular salts have also been screened for preliminary biological studies.

Crystal Engineering of Room Temperature Phosphorescence in Organic Solids.

We report a series of highly emissive azatriangulenetrione (TANGO) solids in which the luminescent properties are controlled by engineering the molecular packing by adjusting the steric size of substituents. The co-alignment of "phosphorogenic" carbonyl groups within the π-stacks results in an almost pure triplet emission in HTANGO, TCTANGO, TBTANGO and TITANGO, while their rotation by ≈60° in the sterically hindered tBuTANGO leads to an almost pure singlet emission. Despite strong π-interactions, aggregation-induced quenching and triplet-triplet annihilation are avoided in HTANGO and TCTANGO which display efficient phosphorescence in the solid state. To our knowledge, HTANGO with the solid-state phosphorescence quantum yield of 42 % at room temperature is the most efficient phosphor composed of the 1st /2nd raw elements only.

Synthesis, crystal structures, photoluminescence, electrochemistry and DFT study of aluminium(III) and gallium(III) complexes containing a novel tetradentate Schiff base ligand.

Single crystals of the aluminium and gallium complexes of 6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenol), namely diaqua(6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ4O1,N,N',O1')aluminium(III) nitrate ethanol monosolvate, [Al(C22H18N2O4)(H2O)2]NO3·C2H5OH, 1, and diaqua(6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ4O1,N,N',O1')gallium(III) nitrate ethanol monosolvate, [Ga(C22H18N2O4)(H2O)2]NO3·C2H5OH, 2, were obtained after successful synthesis in ethanol. Both complexes crystallized in the triclinic space group P-1, with two molecules in the asymmetric unit. In both structures, in one of the independent molecules the tetradentate ligand is almost planar while in the other independent molecule the ligand shows significant distortions from planarity, as illustrated by the largest distance from the plane constructed through the central metal atom and the O,N,N',O'-coordinating atoms of the ligand in 1 of 1.155 (3) Å and a distance of 1.1707 (3) Å in 2. The possible reason for this is that there are various strong π-interactions in the structures. This was confirmed by density functional theory (DFT) calculations, as were the other crystallographic data. DFT was also used to predict the outcome of cyclic voltammetry experiments. Ligand oxidation is more stabilized in the gallium complex. Solid-state photoluminescence gave an 80 nm red-shifted spectrum for the gallium complex, whereas the aluminium complex maintains the ligand curve with a smaller red shift of 40 nm.

1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon.

Reacting (NHC)(cyclopropenyl)gold(I) complexes with cationic gold complexes [(IPr)AuX] afforded extremely reactive allylium-1,1-diido-bridged digold intermediates. We prove the existence and constitution of this structure with FT-ICR-MS/MS, NMR, and UV-vis-NIR experiments and isolated the nucleophilic addition product [(Me)(Ph)(CCHC){Au(IPr)}2(SOMe2)]NTf2 with DMSO. Our computational investigation unveiled that the bonding situation of this μ-allylium-1,1-diido digold domain was best described as a three-center-four-electron bond with a π-backbond. The valence orbitals showed extreme delocalization and strong π-interactions between the three centers Au1-C1-Au2. The bridging carbon atom C1 was best described as trigonal planar sp-hybridized carbon in this structure. Excitation succeeded in UV-vis-NIR measurements with energies as low as near-IR radiation.

A nanocomposite consisting of an amorphous seed and a molecularly imprinted covalent organic framework shell for extraction and HPLC determination of nonsteroidal anti-inflammatory drugs.

A novel nanocomposite consisting of an amorphous seed and a molecularly imprinted covalent organic framework shell was prepared via a heterogeneous nucleation and growth method. By using ibuprofen as the dummy template, a molecularly imprinted covalent organic framework (MICOF) with a large surface area was prepared from 1,3,5-triformylbenzene and 4,4'-diaminobiphenyl. It was placed on the surface of monodisperse amorphous seeds. Owing to strong π-interaction, the MICOF@SiO2 nanocomposite displays fast binding kinetics, large adsorption capacities and selectivity for nonsteroidal anti-inflammatory drugs (NSAIDs). Following desorption from the MICOF@SiO2 with methanol containing 1% ammonium hydroxide, the NSAIDs ketoprofen, ibuprofen, diclofenac, indomethacin, flurbiprofen and naproxen were quantified by HPLC with UV detection. Under optimized conditions, the method exhibits good linearity within the range of 0.002-1.0μgmL-1, low limits of detection (0.38-2.92μgL-1), and acceptable repeatability. The recoveries of NSAIDs at three spiking levels range from 77 to 112%, and the RSDs are <9.4%. The method was successfully applied to the analysis of NSAIDs in spiked environmental water samples. Graphical abstract A molecular imprinted covalent organic framework nanocomposite (MICOF@SiO2) was prepared by heterogeneous nucleation and growth method. It was explored as a sorbent for the solid phase extraction of nonsteroidal anti-inflammatory drugs before determination by HPLC with UV detection.

Sandwich-like graphene-Bi2S3 hybrid derived from (BiO)2CO3 nanosheets as advanced anode materials for lithium/sodium ion batteries

Bismuth sulfides (Bi2S3) are potential electrode materials that have a high theoretical capacity for both lithium and sodium ion batteries. To better utilize the material in practical applications, it is urgent to solve the problem of severe capacity decay caused by low electronic conductivity and large volume change during cycling. Introducing graphene into the system would effectively alleviate these issues on account of the good electrical conductivity, high mechanical flexibility, and large surface area possessed by graphene. However, it is difficult to wrap Bi2S3 with a graphene layer due to the intrinsic hydrophobicity of graphene layers and the strong π-interaction of graphene nanosheets. In this work, sandwich-like graphene-Bi2S3 hybrids (rGO@Bi2S3) are synthesized from two-dimensional (BiO)2CO3 nanosheets (BOC NSs) by electrostatic self-assembly and a subsequent sulfidation process with the assistance of a cationic surfactant. The electrostatic force provides an efficient approach for graphene layers to wrap well around BOC NSs to form a sandwich-like structure that can be retained after the sulfidation process. Benefitting from the merits of high electronic conductivity, robust structure stability, and sufficient space for volume expansion, the as-obtained rGO@Bi2S3 hybrids show both high capacity and cycle stability as the anode of lithium/sodium ion batteries.

Metal-like Boronic-Organic Frameworks: A Design and Computation.

Because of the lack of strong π-interaction in their bonds connecting building units, most of the metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) achieved so far are insulators or wide-bandgap semiconductors. The design of metal-like frameworks based on known chemical components is a challenge. This work reports that aryl borons can be linked together through isocyanides to form stable and easily accessible low-dimensional boronic-organic frameworks (BOFs). Particularly, the boron atoms in the BOFs behave like transition metals, forming the combined σ-donation and π-backdonation bonds instead of the usual electron-sharing bonds with the isocyanide linkers. This peculiar bonding endows BOFs with semimetal and narrow-bandgap semiconductor features, which are different from MOFs and COFs and may be found to be useful in future nanoelectronics. The results open a door to integrating the knowledge of the donor-acceptor chemistry in the main group into materials science.

Highly Efficient Nitrogen and Sulfur Co-Doped Three-Dimensional Graphene-Based Nanocatalysts for the ORR

a, Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China. *Email: wangdl81125@hust.edu.cnb, Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, USA Graphene, a single-atom-thick hexagonally arrayed sp2 carbon atoms bonded sheet, has attracted tremendous interest in renewable energy conversion and storage devices due to its extraordinary properties, especially ultrahigh specific surface area and predominant electron conductivity1. However, graphene based two-dimensional catalysts suffer from an irreversible re-stacking of the nanosheets due to the strong π-interaction during thermal annealing process and electrochemical measurements which would result in a lower surface area, limiting the mass transfer rate and even decreasing the catalytic activity as well as electrochemical stability. Figure 1 (a) TEM image of NSGCB; (b) ORR activity of NSGCB and the corresponding comparative catalysts; (c) TEM image of NSCNT-3; (d) ORR activity of NSCNT-3 and the corresponding comparative catalysts. Herein, we report two strategies for the synthesis of nitrogen and sulfur co-doped three-dimensional graphene intercalated structure nanomaterials as electro-catalysts for ORR in alkaline medium. One is the insertion of Vulcan XC-72 carbon spheres into the graphene layer, forming a graphene-Vulcan XC-72- grapheme “sandwich-like” structure2. The other way is partially unzipping of multi-walled carbon nanotubes to form a graphene nanoribbon-carbon nanotube-graphene nanoribbon structure. Both of the prepared three-dimensional structured nanocomposites were experienced to nitrogen and sulfur co-doping process, and moreover, the resulting doped electro-catalysts exhibited outstanding ORR performance. AcknowledgementsThis work was supported by the National Natural Science Foundation (21306060, 21573083), the Program for New Century Excellent Talents in Universities of China (NCET-13-0237), the Doctoral Fund of Ministry of Education of China (20130142120039). References (1) Li, Y., Zhao, Y., Cheng, H., Hu, Y., Shi, G., Dai, L., & Qu, L. Nitrogen-doped graphene quantum dots with oxygen-rich functional groups. Journal of the American Chemical Society, 2011, 134, 15-18. (2) Wu, M., Wang, J., Wu, Z., Xin, H. L., & Wang, D. Synergistic enhancement of nitrogen and sulfur co-doped graphene with carbon nanosphere insertion for the electrocatalytic oxygen reduction reaction. Journal of Materials Chemistry A, 2015, 3, 7727-7731. Figure 1

Strong π-interactions between tantalum and apical ligands in square pyramidal complexes supported by a rigid tetradentate spectator ligand

The synthesis of new tantalum(V) square pyramidal complexes supported by the tert-butyl-substituted (ethenediylidene)tetraphenoxide spectator ligand (TPO) and their reactivity are herein described. The tantalum ethoxide (TPO)Ta(OCH2CH3) (1) and dimethylamide (TPO)Ta(NMe2) (2) complexes were prepared and structurally characterised, giving valuable insight into π-interactions between the metal centre and the apical ligand. The solid-state structures confirmed the geometry of both complexes, with TPO forming the base of the square pyramidal structure and the ethoxide and dimethylamide ligands occupying the apical position. Bond lengths and angles from the structures indicate significant double bond character between the metal and the apical ligand with very short metal–oxygen and metal–nitrogen bonds in 1 and 2, respectively, which results from donation of π-electron density from the heteroatom into empty metal dπ-orbitals. Reaction of (TPO)Ta(OCH2CH3) (1) and (TPO)Ta(NMe2) (2) with trimethylaluminum gave the methyl complex (TPO)Ta(CH3) (3). Complexes 1 and 2 were cleanly regenerated by addition of ethanol and dimethylamine to complex 3. Addition of strong Brønsted acids to these complexes gave mixtures of compounds that could not be characterised, possibly due to competing protonation of the spectator TPO ligand or formation of oligomeric species.

Dynamic tracking of pathogenic receptor expression of live cells using pyrenyl glycoanthraquinone-decorated graphene electrodes.

Expression of specific transmembrane receptors by cells frequently represents an important signature of diseases, but this dynamic event can hardly be monitored directly with live cells due to the limitation of current biochemical techniques. Here we develop a pyrenyl glycoanthraquinone construct that can be firmly immobilized on a graphene-spotted screen printed electrode via strong π-interactions. The inherent current signal produced by the surface-confined glycoquinone can be used to detect selective sugar-protein recognitions with simple electrochemical techniques and portable facilities. Importantly, we demonstrate that the level of pathogenic receptors expressed by different types of live cells can be tracked with the electrode system in a label-free manner, providing a useful tool for the on-demand disease diagnosis as well as basic biochemical studies.

Boron difluoride complexes of 2′-hydroxychalcones and curcuminoids as fluorescent dyes for photonic applications**Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2-6 November, 2014, Ha Long, Vietnam.

The field of fluorescent boron complexes has witnessed tremendous developments in recent years. In that context, we have investigated two series of boron difluoride complexes based on 2′-hydroxychalcone and curcuminoid ligands that represent naturally occurring pigment structures. The dyes display significantly large Stokes shift values, indicating that an ICT state is involved as lower-energy state in the singlet manifold. Remarkably they are also fluorescent in the solid-state, with emission wavelengths usually in the visible and mainly in the near infrared (NIR). It is especially intriguing that those dyes experience strong π-interactions in the crystal phase. We have observed that the formation of those highly stacked structures was not detrimental to solid-state emission and could even be exploited for the generation of efficient NIR emitters. For example, the boron complexes of curcuminoid ligands can be used to generate NIR fluorescent organic nanoparticles with large cross sections for two-photon absorption. The design of organic dyes displaying NIR emission in solution or in the solid-state remains challenging for applications in bioimaging and organic photonics.

Iron L-edge X-ray absorption spectroscopy of oxy-picket fence porphyrin: experimental insight into Fe-O2 bonding.

The electronic structure of the Fe-O(2) center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multideterminant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O(2) heme complex [Fe(pfp)(1-MeIm)O(2)] (pfp ("picket fence porphyrin") = meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O(2)] is further compared to those of [Fe(II)(pfp)(1-MeIm)(2)], [Fe(II)(pfp)], and [Fe(III)(tpp)(ImH)(2)]Cl (tpp = meso-tetraphenylporphyrin) which have Fe(II)S = 0, Fe(II)S = 1, and Fe(III)S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O(2) pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O(2) with the iron, with the latter having implications with respect to the spin polarization of the ground state.

Transparent, Conductive, and Flexible Multiwalled Carbon Nanotube/Graphene Hybrid Electrodes with Two Three-Dimensional Microstructures

Transparent, conductive, and flexible multiwalled carbon nanotube (MWCNT)/graphene hybrids with two three-dimensional microstructures—an interconnected network and a double-layer structure—were prepared. The conductivity and performance of MWCNT/graphene films can be controlled by different microstructures. A photoswitch using a layered heterostructure of a CdTe quantum dot on an interconnected MWCNT/graphene (IN-MWCNT/graphene) electrode shows an enhanced reversible photocurrent with a higher on/off ratio than that of double-layer structures(DL-MWCNT/graphene). Electrochemical capacitors using a IN-MWCNT/graphene network also exhibit an outstanding rate capability and good cycling stability due to a large surface area and high porosity. Results indicate that the IN-MWCNT/graphene hybrid with porous structures and strong π-interaction is an excellent conductive network for multifunctional flexible devices. The performance of MWCNT/graphene hybrid films can be further optimized by the improved interface an...

A Dft Study of Styrene Polymerization using Neutral (2Z, 4E)-4-(Methylimino)Pent-2-En-2-Ol Nickel(Ii)

The recent discovery of the ability of salicylaldiminato Ni(II) complexes to promote styrene polymerization creates the potential for a new class of olefin polymerization catalysts. Styrene polymerization using (2Z,4E)-4-(methylimino)- pent-2-en-2-ol Ni (MPNi) catalyst has been studied using density functional theory at the B3LYP/6-31g (d,p) level. In particular, the reaction mechanisms have been investigated in detail. The results indicate that the reaction involves a four-membered cyclic transition state with a energy barrier of 21.63 kcal mol−1. In addition, the analysis indicates that strong π-interactions between styrene and Ni2+ are very important for the activation of styrene.

Cyclopentadienyl Ligands as Perfect Anion Receptors: Teamwork between π-Anion Interaction and C−H···Anion Hydrogen Bonds

Electron-deficient arenes are now recognized as potent anion receptors, mainly through the establishment of π-anion interactions. Cyclopentadienyl ligands (Cp’s) also belong to the family of arenes that exhibit very low electron density at their centroid due to the presence of the metal on one side. As such, they also work as anion receptors. A closer look at the reported X-ray structures in which an anion is in short contact with the Cp framework reveals that in most cases the five-membered ring is substituted, generally pentamethylated (Cp*). This is quite surprising since Cp* is electron-richer than the unsubstituted Cp ligand. This computational study shows that Cp* functions as a ring bearing a gemstone: the anion establishes a strong π-interaction with the centroid, but this is not sufficient to hold it straight. For that matter, each methyl group develops a C−H···anion hydrogen bond, thus working as a claw to prevent the gem from falling. Although individually small, the interaction energy provided by the five methyl groups makes the M-Cp* a perfect anion receptacle.

Polymers of Pyrenes

Pyrene is one of the most robust and studied chromophores. It has a strong blue emission and a strong tendency to form excimers. Blue emissive polymers are of interest for emissive display materials. As a result placing pyrene groups into π-extended frameworks capable of efficiently transporting charge is of interest for use in light-emitting devices. X-ray structures of the monomer show that pendant phenyl groups are effective at preventing strong π-interactions between neighboring pyrenes to prevent excimer formation.

Electron Transport in Molecular Wires

Numerical simulations have been performed to study the influence of π-orbital interactions and molecule–metal coupling strength (σ1=σ2=σi) on the electronic transport properties of molecular assembly system. The model involves 1,4-dithiolbenzene (DTB) molecules stacked in one dimension ordered structure. The results show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) gap (HLG) is reduced when decreasing the intermolecular distance d and increasing the number of DTB molecular units N. The influences of the geometric and coupling-strength parameters d, N, π-orbital, and σi on the current–voltage (I–V) or the conductance–voltage (G–V) characteristics are investigated. It is found that the conductance gap depends on the HLG. The system in the regime of strong π-interactions shows shorter conductance gap. The conductance gap for a wire made of two DTB molecular units decreases by 2.4 V when d changes from 6.9 to 3.3 Å, while it sharply decreases by 3.3 V when the molecular structure goes from a single molecule to a wire made of six molecular units. The results indicate also that σi is an important factor in determining the current flow. The saturation current for a wire containing four molecular units increases by 150 µA when σi changes from 0.2 to 0.4 eV. In fact, the main factors that determine the I–V or G–V characteristics are clearly related to the magnitude of the HLG, σi and the position of the energy levels of the molecule relative to the Fermi energy contacts.