Phenylene Oligomers Research Articles

Photoinduced Single Electron Reduction of the 4-O-5 Linkage in Lignin Models for C-P Coupling Catalyzed by Bifunctional N-Heterocyclic Carbenes.

Catalytic activation of Caryl-O bonds is considered as a powerful strategy for the production of aromatics from lignin. However, due to the high reduction potentials of diaryl ether 4-O-5 linkage models, their single electron reduction remains a daunting challenge. This study presents the blue light-induced bifunctional N-heterocyclic carbene (NHC)-catalyzed one-electron reduction of diaryl ether 4-O-5 linkage models for the synthesis of trivalent phosphines. The H-bond between the newly devised bifunctional NHC and diaryl ethers is responsible for the success of the single electron transfer. Furthermore, this approach demonstrates selective one-electron reduction of unsymmetric diaryl ethers, oligomeric phenylene oxide, and lignin model.

Negative differential resistance in oligomeric phenylene ethynylenes molecular device with C2N-h2D nanoribbon electrodes

Using non-equilibrium Green’s function (NEGF) combined with the density functional theory (DFT) first-principles method, we perform a theoretical study for oligomeric phenylene ethynylenes (OPE) molecule sandwiched between different C2N-h2D nanoribbon electrodes. The results point to the conclusion that the electronic transport properties of the devices depend greatly on the edge and width of the C2N-h2D nanoribbon electrodes. A significant NDR effect can be observed for the molecular devices with armchair-edged C2N-h2D and zigzag-edged C2N-h2D nanoribbons electrodes when W = 1.0, which suggests that C2N-h2D nanoribbons have great potential in the field of molecular electronics.

Spectroscopic study of radical cation species formed on sulfated TiO2 upon benzene adsorption

Sulfated TiO2 is well known for its high acidity, which is due to the presence of sulfate ions that create strong Brönsted and Lewis acid sites. The Lewis acid sites can also exhibit oxidizing properties. As previously observed for sulfated ZrO2, the greater the strength of the acid site, the greater the oxidative potential.The oxidative ability can be evaluated by the interaction with a substance with high ionization potential such as benzene. Benzene was adsorbed on sulfated TiO2 at room temperature, and the species formed from the interaction was investigated by Raman and electron spin resonance (ESR) spectroscopy. Resonance Raman spectra of adsorbed benzene showed bands that are distinct from bands observed in the spectra of neutral molecules. Furthermore, from the absorption spectra in the visible and near-infrared it was possible to identify these species as phenylene oligomer radical cations. Also, aromatic radical cations were detected by ESR spectra. Although they were present in small amounts, these cations may be responsible for the bands observed in the Raman spectra because of the resonance Raman effect signal enhancement.

Theoretical Investigation of the Intermolecular Charge‐Transfer Interactions of Poly‐p‐Phenylene Oligomers

A composite molecule (CM) model is used to investigate the nature of intermolecular interactions in the excited states of π‐conjugated oligomers of poly‐(p‐phenylene) (PPP). The dependence of the charge‐transfer (CT) interactions in low‐lying excited states as a function of intermolecular distance, conjugation length, and the translation shifts is examined. We demonstrate how the amplitude of CT mixing in two low‐lying excited states of PPP dimer is sensitive to various aspects of molecular packing. The largest CT mixing in the two excited states is consistently obtained for cofacial packing. However, from a detailed examination of molecular orbital shapes and interactions, it is seen that specific arrangements promote the CT mixing in two low‐lying transitions. With cofacial arrangements, it shows a larger percentage of CT in the first excited states than the corresponding second excited states for phenyl rings on the top of Csp2─Csp2 single bonds. This work illustrates the importance of molecular packing–CT mixing relationship of functional π‐conjugated materials.

Large rectification ratio induced by nitrogen (boron) doping in graphene nanoribbon electrodes for OPE junctions

Using nonequilibrium Green's function method combined with density functional theory, we present electronic transport properties for a molecular device constructed by an oligomeric phenylene ethynylenes (OPE) molecule between two zigzag-edge graphene nanoribbon (zGNR) electrodes. Nitrogen and boron dopants are introduced into one electrode, which makes the molecular junctions to have obvious rectifying behaviors. The conductance properties are closely related to the position and the kind of impurity atoms in electrodes. Moreover, the I–V curve of the junction, which is boron doping at the edge, displays unidirectional features in the studied bias region, and large rectification ratios up to 103 are obtained. The mechanism of the performance is analyzed in terms of transmission spectra combined with molecular projected self-consistent Hamiltonian and electrode band structures under different bias voltages. Symmetry analyses of the Bloch wave functions of the corresponding subbands and the frontier molecular orbitals are also applied to reveal the rectifying behavior.

Comprehensive Analysis of Fragment Orbital Interactions to Build Highly π‐Conjugated Thienylene‐Substituted Phenylene Oligomers

π-Conjugated thienylene-phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO-LUMO gap, and the role of intramolecular non-covalent cumulative interactions in the construction of π-conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the π orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π(*) orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S⋅⋅⋅F and H⋅⋅⋅F interactions in the fluorinated phenylene-thienylene compounds add to the S⋅⋅⋅O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter-ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the π system, were corroborated by a narrow HOMO-LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD-DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene-(fluorinated and dialkoxylated phenylene) five-ring oligomers agree with the above results and show the formation of quasi-planar conformations with non-covalent S⋅⋅⋅O, H⋅⋅⋅F, and S⋅⋅⋅F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.

Controlling the Structural and Electrical Properties of Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films

The preparation, characterization and electrical properties of Langmuir-Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4'-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good-quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X-ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one-layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently -COO-Au junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na(+) counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H-bonds with neighbouring molecules give rise to a supramolecular structure. STM-based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single-molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in -COO-Au molecular junctions, but also in terms of the presence of lateral H-bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.

Remarkable Effects of Terminal Groups and Solvents on Helical Folding of o-Phenylene Oligomers

Although o-phenylene oligomers (OP(n)R) made of dimethoxyphenylene units are thought to be intrinsically dynamic due to π-electronic repulsion, we show that they fold into a regular helical geometry in CH(3)CN when they carry terminal groups such as CH(3), CH(2)OH, Br, CO(2)Bn, and NO(2). We evaluated their helical inversion kinetics via optical resolution of long-chain oligomers (e.g. 16- and 24-mers) by chiral HPLC. OP(24)Br at 298 K shows a half-life for the optical activity of 5.5 h in CH(3)OH/water (7/3 v/v) and requires 34 h for complete racemization. The perfectly folded helical conformers of OP(n)R, unlike their imperfectly folded ones, are devoid of extended π-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.

Synthesis and Properties of Sulfonated Poly(arylene ether)s with Flexible Oligomeric Phenylene Ether Segments

Novel sulfonated poly(arylene ether)s derived from ω,ω′-dihydroxypoly(1,4-phenylene ether)s (DHPE) and mixed dihalide monomers of 3,3′-disulfo-4,4′-dichlorodiphenylsulfone disodium salt (S-DCDPS) and 2,6-dichlorobenzonitrile were synthesized. As DHPE has molecular weight distribution with phenylene rings lower than ca. 10 units, experimental determination of its average molecular weight made it possible to be used as a monomer to prepare sulfonated poly(arylene ether)s. Membranes derived from resultant polymers showed a similar level of proton conductivity to those of other common sulfonated poly(arylene ether)s; however, its softening temperature was much lower. Therefore, good MEA was able to be obtained by hot-pressing at relatively lower temperature without shrinking around electrode. The membrane with an ion exchange capacity of 0.9 showed lower methanol permeability and dimensional change in methanol solution assuming a condition of DMFC than standard perfluorosulfonic acid polymer membrane (PFSA).

Parent o-Phenylene Oligomers: Synthesis, Conformational Behavior, and Characterization

The o-phenylenes are an unusual class of conjugated polymer, defined largely by substantial steric twisting along their backbones. Consequently, they exhibit limited conjugation but also interesting conformational behavior: they have been shown to adopt well-defined helical secondary structures, both in the solid state and in solution. While several examples of functionalized o-phenylene oligomers have been reported, most of the basic properties of the parent compounds are unknown. Here we report the synthesis and characterization of the series of unsubstituted o-phenylene oligomers up to the octamer. Through a combination of NMR spectroscopy, including dynamic NMR (EXSY), and computational chemistry, we have found that these compounds adopt compact helical conformations in solution with three repeat units per turn. Although formally conjugated, the oligomers have a very short effective conjugation length of necl ≈ 4 (based on UV–vis spectra), significantly shorter than most other conjugated systems. Also, unlike other (substituted) o-phenylenes, no hypochromicity is observed in their UV–vis spectra. The fluorescence spectra of the series exhibit a systematic blue shift with increasing length. We believe this unusual property results from increased steric congestion in the longer oligomers, which are therefore less able to accommodate structural relaxation in the excited state.

ChemInform Abstract: Photoinduced Electron and Energy Transfer in Phenylene Oligomers

AbstractReview: 48 refs.

Dark Antimicrobial Mechanisms of Cationic Phenylene Ethynylene Polymers and Oligomers against Escherichia coli

The interactions of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and oligo-phenylene ethynylenes (OPEs) with E. coli cells are investigated to gain insights into the differences in the dark killing mechanisms between CPEs and OPEs. A laboratory strain of E. coli with antibiotic resistance is included in this work to study the influence of antibiotic resistance on the antimicrobial activity of the CPEs and OPEs. In agreement with our previous findings, these compounds can efficiently perturb the bacterial cell wall and cytoplasmic membrane, resulting in bacterial cell death. Electron microscopy imaging and cytoplasmic membrane permeability assays reveal that the oligomeric OPEs penetrate the bacterial outer membrane and interact efficiently with the bacterial cytoplasmic membrane. In contrast, the polymeric CPEs cause serious damage to the cell surface. In addition, the minimum inhibitory concentration (MIC) and hemolytic concentration (HC) of the CPEs and OPEs are also measured to compare their antimicrobial activities against two different strains of E. coli with the compounds’ toxicity levels against human red blood cells (RBC). MIC and HC measurements are in good agreement with our previous model membrane perturbation study, which reveals that the different membrane perturbation abilities of the CPEs and OPEs are in part responsible for their selectivity towards bacteria compared to mammalian cells. Our study gives insight to several structural features of the PPE-based CPEs and OPEs that modulate their antimicrobial properties and that these features can serve as a basis for further tuning their structures to optimize antimicrobial properties.

Cationic Phenylene Ethynylene Polymers and Oligomers Exhibit Efficient Antiviral Activity

The antiviral activities of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using two model viruses, the T4 and MS2 bacteriophages. Under UV/visible light irradiation, significant antiviral activity was observed for all of the CPEs and OPEs; without irradiation, most of these compounds exhibited high inactivation activity against the MS2 phage and moderate inactivation ability against the T4 phage. Transmission electron microscopy (TEM) and SDS polyacrylamide gel electrophoresis (SDS-PAGE) reveal that the CPEs and OPEs exert their antiviral activity by partial disassembly of the phage particle structure in the dark and photochemical damage of the phage capsid protein under UV/visible light irradiation.

Ph2N-Susbtituted Ethylene-Bridged p-Phenylene Oligomers: Synthesis and Photophysical and Redox Properties

For a series of p-phenylene-based oligomers terminated with two triphenylamines, their absorption, photoluminescence, and band gaps show a pattern of extensive π-conjugation with increasing array size. Oligomers with large central arrays have greater quantum yields than their small analogues. Cyclic voltammetric (CV) measurements indicated two-step oxidations of the two diphenylamino groups for compounds 1-5 and one-step oxidations for the two amines of large oligomers 6 and 7.

Photoinduced electron and energy transfer in phenylene oligomers

Phenylene oligomers represent a borderline case between very strongly π-conjugated molecular wires such as oligo-p-phenylene vinylenes and saturated molecular bridges. Even subtle chemical modifications of phenylene oligomers can therefore have a strong impact on charge transfer rates and mechanisms. On the basis of recently published selected case studies, this tutorial review discusses the key factors that affect charge transfer kinetics in phenylene oligomers with particular focus on the role of donor-bridge energy matching. Selected examples of triplet-triplet energy transfer reactions across phenylene oligomers are also discussed.

Breaking free of chiral symmetry

Helical macromolecules are ubiquitous in nature, and almost always adopt a one-handed conformation. Synthetic systems, in contrast, are typically obtained in racemic right- and left-handed mixtures. A helical phenylene oligomer has now been prepared that forms a non-racemic mixture on crystallization, and on oxidation locks one conformation in.

Metal–Molecule–Metal Junctions in Langmuir–Blodgett Films Using a New Linker: Trimethylsilane

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.

Covalent Grafting of m-Phenylene-Ethynylene Oligomers to Oxide Surfaces

Oligomeric phenylene ethynylene is potentially a useful molecule for organic photovoltaics, light emitting diodes, sensor materials, or as intrinsically porous polymers for gas storage and separation. In the former two applications, the organic molecules are supported on an inorganic material for structural support or for charge collection at an electrode. Here, a 12 repeat unit oligomer of m-phenylene ethylene is covalently attached to high surface area and planar oxide supports as an alternate route to materials development, which displays novel properties and applications due to the covalent connection to the surface. Oligomers and model dimers are attached to alkyl-azide decorated surfaces via Cu-catalyzed 1,3-dipolar Huisgen cycloaddition or covalently linked with silane-bearing surfaces via Rh-catalyzed hydrosilylation. Surface structures and densities are verified by FTIR, TGA, transmission UV−visible spectroscopy, N2 physisorption, and transmission electron microscopy. On high-surface-area materia...

Hydration effect on the electronic transport properties of oligomeric phenylene ethynylene molecular junctions

A first-principles computational method based on the hybrid density functional theory is developed to simulate the electronic transport properties of oligomeric phenylene ethynylene molecular junctions with H2O molecules accumulated in the vicinity as recently reported by Na et al. [Nanotechnology 18 424001 (2007)]. The numerical results show that the hydrogen bonds between the oxygen atoms of the oligomeric phenylene ethynylene molecule and H2O molecules result in the localisation of the molecular orbitals and lead to the lower transition peaks. The H2O molecular chains accumulated in the vicinity of the molecular junction can not only change the electronic structure of the molecular junctions, but also open additional electronic transport pathways. The obvious influence of H2O molecules on the electronic structure of the molecular junction and its electronic transport properties is thus demonstrated.

Molecule Length Directed Self-Assembly Behavior of Tetratopic Oligomeric Phenylene−Ethynylenes End-Capped with Carboxylic Groups by Scanning Tunneling Microscopy

A series of tetratopic C2-symmetric quasi-planar oligomeric phenylene−ethynylenes bearing carboxylic groups (OPE-COOHs), including the shortest 1,4-di((3,5-dicarboxyphenyl)ethynyl)benzene (TCB), midlength 4,4′-di((3,5-dicarboxyphenyl)ethynyl)biphenyl (TCBP), and the longest 4,4′′-di((3,5-dicarboxyphenyl)ethynyl)-p-terphenyl (TCTP), were synthesized via Pd(PPh3)4−CuI-catalyzed Sonogashira coupling reaction. Their molecular structures were characterized by nuclear magnetic resonance spectroscopy (NMR), matrix-assisted laser desorption−ionization time-of-flight mass spectroscopy (MALDI-TOF-MS), and element analyses. The molecule length effect of OPE-COOHs on two-dimensional (2D) assemblies at the octanoic acid−highly ordered pyrolytic graphite (HOPG) interface was investigated by scanning tunneling microscopy (STM) technique at ambient temperature and simulated by molecular modeling. The patterns and domain sizes of OPE-COOHs strongly depend on their molecule length with Kagomé network only for TCB and parallel network for TCBP and TCTP. TCTP with the longest rigid-rod OPE backbone exhibits clearly larger ordered domain size than that of TCB and TCBP. Besides, TCBP exhibits tight boundaries transition among different orientation domains via the acute or obtuse V-shaped chevron arrangements. Those different two-dimensional (2D) assembly behaviors will be favorable to get a further understanding of the condensed architectures in conjugated organic semiconductors.