Phenylene Ethynylene Oligomers Research Articles

Quantum Chemical Studies on Possible Molecular Devices Based on Electric Field-Induced Intramolecular Charge Transfer.

We report quantum chemical studies on possible molecular devices working based on electric field-induced intramolecular charge transfer (EFIMCT). In the case of donor-acceptor (DA)-type molecular systems, intramolecular charge transfer (IMCT) can be induced by applying the external electric field to molecular systems along the charge transport direction, providing a possible switching mechanism which does not depend upon the electron-phonon coupling effect and is different from the negative differential resistance mechanism observed in the well-known NO2-substituted phenylene ethynylene oligomers. When the EFIMCT proceeds, the molecular systems have strong static electron correlation effects, where the standard nonequilibrium Green's function-density functional theory (DFT) approach cannot be applied to the molecular junction. As a first step toward practical switching devices, we do quantum chemical studies on the EFIMCT in such molecular systems as an isolated molecule, instead of using the electrode-junction-electrode open quantum system model. A prototype molecule P1 is designed as a tentative candidate molecule where the EFIMCT can proceed. The complete active space self-consistent field (CASSCF) molecular orbital calculations on P1 indicate that the EFIMCT can proceed at the external electric field intensity of 0.003 au, corresponding to about 2.25 V bias voltage. This calculated result strongly suggests that the development of this type of switching devices working at practically low bias voltage is feasible if the molecular system is properly designed. Broken symmetry unrestricted Hartree-Fock and spin-polarized Kohn-Sham DFT calculations also qualitatively reproduce the CASSCF results on P1, to some extent, indicating that these approaches can be employed for rough estimations on the EFIMCT such as the first screening of a large quantity of candidate molecules for this type of molecular devices. The possibility of molecular memory devices based on the EFIMCT is also discussed by analyzing the ground and excited potential energy surface model. Remaining challenges to develop practical molecular devices are discussed.

Photoinduced Intramolecular Electron Transfer in Phenylene Ethynylene Naphthalimide Oligomers.

This paper reports a photophysical investigation of a series of phenylene ethynylene oligomers (OPE) that are end-substituted with a 1,8-naphthalene imide (NI) acceptor. The NI acceptor is attached to the terminus of the OPEs via an ethynylene (-C≡C-) unit that is linked at the 4-position of the NI unit. A series of three oligomers is investigated, OPE1-NI, OPE3-NI, and OPE5-NI, which contain 1, 3, and 5 phenylene ethynylene repeat units, respectively. The properties of the OPEn-NI series are compared to a corresponding set of unsubstituted OPEs, OPE3 and OPE5, which contain 3 and 5 phenylene ethynylene repeats, respectively. The photophysics of all the compounds are interrogated using a variety of techniques including steady-state absorption, steady-state fluorescence, two-photon absorption, time-resolved fluorescence, and transient absorption spectroscopy on femtosecond-to-microsecond time scales. The effect of solvent polarity on the properties of the oligomers is examined. The results show that the NI-substituted oligomers feature a lowest charge transfer (CT) excited state, where the OPE segment acts as the donor and the NI moiety is the acceptor (OPEn•+-NI•-). The absorption spectra in one-photon and two-photon exhibit a clear manifold of absorption features that can be attributed to direct CT absorption. In moderately polar solvents, the emission is dominated by a broad, solvatochromic band that is due to radiative decay from the CT excited state. Ultrafast transient absorption provides evidence for initial population of a locally excited state (LE) which in moderately polar solvents rapidly (∼1 ps) evolves into the CT excited state. The structure, spectroscopy, and dynamics of the CT state are qualitatively similar for OPE3-NI and OPE5-NI, suggesting that delocalization in the OPE segment does not have much effect on the structure or energetics of the CT excited state.

Zirconocene-mediated selective synthesis of 1,4-bis(alkynyl)benzenes

A series of novel 1,4-bis(alkynyl)benzene derivatives were synthesized from trimethylsilyl-substituted alkynes by the mediation of zirconocene with excellent regioselectivity in high yields. The 3,6-bis(trimethylsilyl)-4,5-dialkylphthalic acid dimethyl esters were prepared by cycloaddition of 2,5-bis(trimethylsilyl)zirconacyclopentadienes to dimethyl acetylenedicarboxylate. After iodination with iodine monochloride, 3,6-diiodo-4,5-dialkylphthalic acid dimethyl esters reacted with terminal alkynes to prepare the corresponding 1,4-bis(alkynyl)benzene derivatives by Sonogashira coupling reactions. After removal of trimethylsilyl, 4,5-dibutyl-3,6-bis(ethynyl)phthalic acid dimethyl ester (compound 3) reacted with 4-iodobenzoic acid ethyl ester and 2-iodothiophene, respectively, to obtain the corresponding products 4a and 4c. Compound 3 can be extended to higher oligomers, which reacted with 1-bromo-4-iodobenzene and phenylacetylene in a stepwise manner under Sonogashira conditions to give the phenylene-ethynylene oligomer 5 in an isolated yield of 85%. The structures of the products were confirmed by 1H NMR spectroscopy, 13C NMR spectroscopy, and MS. The optical properties of the 1,4-bis(alkynyl)benzene derivatives were studied by UV-Vis spectroscopy and fluorescence spectra. The results indicated that some can be developed into potential photovoltaic materials.

Sensitizing TADF Absorption Using Variable Length Oligo(phenylene ethynylene) Antennae.

Beyond their applications in organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF) materials can also make good photonic markers. Time-gated measurement of their delayed emission enables “background-free” imaging in, for example, biological systems, because no naturally-occurring compounds exhibit such long-lived emission. Attaching a strongly-absorbing antenna, such as a phenylene ethynylene oligomer, to the TADF core would be of interest to increase their brightness as photonic markers. With this motivation, we study a sequence of TADF-oligomer conjugates with oligomers of varying length and show that, even when the absorption of the oligomer is almost resonant with the charge-transfer absorption of the TADF core, the antenna transfers energy to the TADF core. We study this series of compounds with time resolved emission and transient absorption spectroscopy and find that the delayed fluorescence is essentially turned-off for the longer antennae. Interestingly, we find that the turn-off of the delayed fluorescence is not caused by quenching of the TADF charge-transfer triplet state due to triplet energy transfer of the lower-lying triplet state to the antenna, but must be associated with a decrease in the reverse intersystem crossing rate. These results are of relevance for the further development of TADF “dyes” and also, in the broader context, for understanding the dynamics of TADF molecules in the vicinity of energy donors/acceptors (i.e., in fluorescent OLEDs wherein TADF molecules are used as an assistant dopant).

2D mesoporous atomic monolayers of phenyleneethynylene oligomers on pyrolytic graphite surfaces analyzed by scanning tunneling microscopy

We report on the scanning tunneling microscopy and small-wide angle X-ray scattering and diffraction of four linear and rigid π-conjugated p-phenyleneethynylene (PPE) oligomers. The four compounds possess two dodecanoxy side chains on the central PPE moiety, differ in conjugated length (3 and 5 phenyleneethynylene moieties) and are benzyl, Bz3PEBz, Bz5PEBz, or carboxylic acid, Ac3PEAc, Ac5PEAc, end-capped. We found that in the benzylbenzoate terminated oligomers, a staggered arrangement of molecules is obtained endorsed by the π-π interaction between the benzyls. In contrast, for the carboxylic acid terminated oligomers, a rather linear type arrangement is promoted by the hydrogen bond created between the carboxylic acid groups. In general, all of the molecules form a lamellar like pattern adopting face-on arrangements, where the conjugated backbones are flat-lying on the HOPG surface. The lamella length corresponds to the oligomer conjugation length, while within the lamellae, the calculated distances correspond to the spacing between conjugated backbones with an average value of 2.06 nm, consistent with that found by XRD, SAXS and DFT/def-SVP for the oligomers that self-assemble with the lateral alkyl chains interdigitated. The lack of more dodecanoxy side chains on the rest of the pheneyleneethynylene moieties gives rise to mesoporous type self-assembled monolayers on HOPG.

Effect of Amyloid Fibril Oxidation on its Seeding Potency

Extracellular plaques composed of β-sheet rich amyloid-beta (Aβ) fibrils are believed to cause neurodegeneration and cognitive decline in Alzheimer's disease. Fibrils can form through a primary nucleation-dependent aggregation pathway or secondary nucleation mechanism in which preformed fibrils cause Aβ monomers to misfold and aggregate into amyloid fibrils. As Aβ aggregation is believed to occur during the prolonged asymptomatic stage before neurodegeneration, it is important to gain insights into the aggregation process. Photodynamic therapy utilizes photosensitizing molecules to produce reactive oxygen species, which in turn can oxidize an array of biomolecules including proteins. We have recently shown that a novel Aβ fibril sensor, the anionic phenylene ethynylene oligomer OPE1-, exhibits controllable photosensitizing activity. When bound to Aβ fibrils, OPE1- becomes highly fluorescent, sensitizes the production of singlet oxygen, and induces fibril oxidation upon irradiation. As the singlet oxygen species is short-lived, the controlled photosensitization thus lowers off-target oxidation. In this study we evaluated the effects of OPE1- induced Aβ40 fibril oxidation on its seeding potency. Aβ40 aggregation seeded by non-oxidized and oxidized fibrils were characterized by Thioflavin-T fluorescence assay and compared. Morphologies of the fibrils produced by both seeding conditions were imaged by transmission electron microscopy and fibril secondary structures were analyzed by circular dichroism spectroscopy. Our results thus far show that oxidation of the fibril seeds does not alter Aβ40 aggregation rates or its seeding potency after 3 days of incubation. Furthermore, fibrils produced in the presence of oxidized and non-oxidized fibril seeds showed similar morphologies on TEM images. We plan to further investigate the effect of oxidation on fibril cell toxicity. Results from this study contribute to a better understanding of amyloid protein aggregation via secondary nucleation pathway, which is important in understanding Alzheimer's disease process.

Detergent-induced self-assembly and controllable photosensitizer activity of diester phenylene ethynylenes

Photodynamic therapy, in which malignant tissue is killed by targeted light exposure following administration of a photosensitizer, can be a valuable treatment modality but currently relies on passive transport and local irradiation to avoid off-target oxidation. We present a system of excited-state control for truly local delivery of singlet oxygen. An anionic phenylene ethynylene oligomer is initially quenched by water, producing minimal fluorescence and no measurable singlet oxygen generation. When presented with a binding partner, in this case an oppositely charged surfactant, changes in solvent microenvironment result in fluorescence unquenching, restoration of intersystem crossing to the triplet state, and singlet oxygen generation, as assayed by transient absorption spectroscopy and chemical trapping. This solvation-controlled photosensitizer model has possible applications as a theranostic agent for, for example, amyloid diseases.

BIASING FIELD EFFECTS ON ELECTRONIC PROPERTIES IN HALOGENATED PHENYLENE ETHYNYLENE OLIGOMERS

In this paper results from ab initio simulations of the electronic structure properties of a class of halogenated phenylene ethynylene oligomers (OPE) are presented. These molecular species are investigated because of their suitable properties for application as single-molecule switches in the future emerging molecular electronic devices. Combined Hartree-Fock and Density Functional Theory approach is applied to investigate the biasing field effects on the relevant electronic properties, such as potential energy of the ground states, potential barrier height, localization of frontier molecular orbitals and the HOMO-LUMO gap. Special attention is also paid on the effects of substitution of the hydrogen atoms in the central phenylene ring of basic OPE molecule with halogen atoms. The analyses of the obtained results undoubtedly show that the biasing field has a strong impact on the potential barrier height, transition probabilities and band gap. Halogenation of the central phenylene ring does not have such a strong influence on the aforementioned properties, but it could be a useful way for fine tuning of some of the properties, especially the potential barrier height, enabling control of the torsional stochastic switching, inherent for the studied species.

Stimuli-responsive side chains for new function from conjugated materials

This article describes several projects in our laboratory relating to stimuli-responsive conjugated materials using functional side chain moieties. Although side chain engineering has become increasingly important in controlling the packing of conjugated materials, these groups have typically been reserved for imparting solubility to otherwise insoluble materials. By incorporating solubilizing side chains through photocleavable nitrobenzyl linkers, new conjugated polymers behave as negative photoresists upon exposure to ultraviolet light with minimal photobleaching. Our approach enables photopatterning and solution-based fabrication of multilayer conjugated polymer films without the use of orthogonally soluble materials. Work to understand the electronic effects of the aromatic side chain substituents in these materials led to the subsequent discovery of aromatic interactions between side chains and main chains that control the conformations of solid-state phenylene-ethynylene oligomers, including highly twisted conjugated backbones. Such materials, when appropriately substituted, display reversible piezochromic and mechanofluorochromic behavior.

Tip-enhanced Raman spectroscopic imaging shows segregation within binary self-assembled thiol monolayers at ambient conditions.

Phase segregation of coadsorbed thiol molecules on a gold surface was investigated with nanoscale chemical imaging using tip-enhanced Raman spectroscopy (TERS). Samples were prepared using mixed solutions containing thiophenol (PhS) and an oligomeric phenylene-ethynylene (OPE) thiol, with 10:1, 2:1, and 1:1 molar ratios. Phase segregation into domains with sizes from ≈30 to 240nm is observed with these molar ratios. A comparison of TERS images with different pixel sizes indicates that a pixel size bigger than 15nm is not reliable in defining nanodomains, because of undersampling. In this study, the formation of nanodomains was clearly evident based on the molecular fingerprints provided by TERS, while ambient scanning tunneling microscopy (STM) was not capable of discerning individual domains via their apparent height difference. TERS therefore allows to image nanodomains in binary self-assembled monolayers, which are invisible to methods solely relying on topographic or electron density characteristics of self-assembled monolayers. Moreover, TERS mapping provides statistical data to describe the distribution of molecules on the sample surface in a well-defined manner. Peak ratio histograms of selected TERS signals from samples prepared with different mixing ratios give a better understanding of the adsorption preference of the thiols studied, and the relationship of their mixing ratio in solution and adsorbed on the surface.

Enzyme-specific sensors via aggregation of charged p-phenylene ethynylenes.

Chemical and biological sensors are sought for their ability to detect enzymes as biomarkers for symptoms of various disorders, or the presence of chemical pollutants or poisons. p-Phenylene ethynylene oligomers with pendant charged groups have been recently shown to have ideal photophysical properties for sensing. In this study, one anionic and one cationic oligomer are combined with substrates that are susceptible to enzymatic degradation by phospholipases or acetylcholinesterases. The photophysical properties of the J-aggregated oligomers with the substrate are ideal for sensing, with fluorescence quantum yields of the sensors enhanced between 30 and 66 times compared to the oligomers without substrate. The phospholipase sensor was used to monitor the activity of phospholipase A1 and A2 and obtain kinetic information, though phospholipase C did not degrade the sensor. The acetylcholinesterase sensor was used to monitor enzyme activity and was also used to detect the inhibition of acetylcholinesterase by three different inhibitors. Phospholipase A2 is a biomarker for heart and circulatory disease, and acetylcholinesterase is a biomarker for Alzheimer's, and indicative of exposure to certain pesticides and nerve agents. This work shows that phenylene ethynylene oligomers can be tailored to enzyme-specific sensors by careful selection of substrates that induce formation of a molecular aggregate, and that the sensing of enzymes can be extended to enzyme kinetics and detection of inhibition. Furthermore, the aggregates were studied through all-atom molecular dynamics, providing a molecular-level view of the formation of the molecular aggregates and their structure.

Intended and Unintended Consequences and Applications of Unnatural Interfaces: Oligop-Phenylene Ethynylene Electrolytes, Biological Cells and Biomacromolecules

This short review focuses on an extended study of synthetic oligomericphenyleneethynylene (OPE) electrolytes that have been investigated for both their antimicrobial activity and their fluorescence sensing properties. In both cases, interfaces between these synthetic electrolytes and naturally occurring materials such as proteins, lipids, nucleic acids and cells are critical to their function or activity. The review contains a general overview with a focus at the end on three recent examples including biological and chemical sensing, induction of spore germination and fluorescent detection of amyloid protein aggregates.

Linear Bimetallic Alkynylplatinum(II) Terpyridyl Complexes Bearing p-Phenylene Ethynylene Oligomers: Synthesis, Characterization, Aggregation, and Photophysical Properties

The series of luminescent bimetallic alkynylplatinum(II) terpyridyl complexes [(tBu3-tpy)PtC≡C(ArC≡C)nPt(tBu3-tpy)](PF6)2 (I–IV; tBu3-tpy = 4,4′,4″-tritertbutyl-2,2′:6′,2″-terpyridine, Ar = C6H2(n-hexyloxy)2-2,5), in which two platinum(II) centers are linked by p-phenylene ethynylene oligomers, were synthesized and characterized. Their electronic absorption and luminescent behaviors were studied by steady-state spectroscopy and transient spectroscopy. Concentration-dependent emission data show that all of the complexes form self-assembled aggregates in certain concentration ranges. Complex IV, bearing a longer p-phenylene ethynylene oligomer, exhibits higher propensity in comparison to I–III to form aggregates at a concentration of >1 × 10–5 M, probably resulting from stronger metal–metal interactions and π–π stacking. All four of the complexes displayed biexponential decays with lifetimes in the range of nanoseconds in dilute solutions.

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.

Versatile Bottom‐up Approach to Stapled π‐Conjugated Helical Scaffolds: Synthesis and Chiroptical Properties of Cyclic o‐Phenylene Ethynylene Oligomers

Getackert: Die kleinsten Mitglieder einer Familie von Kohlenstoff-Nanowendeln (CNCs) mit fixierter helikaler Struktur wurden durch Einführung einer oder zweier „Klammern“ in o-Phenylenethinylen-Oligomere synthetisiert. Die chiroptischen Antworten der Systeme mit enantiomerenreinen, von L-Tartrat abgeleiteten Klammern bestätigen die induzierte Helizität. Theoretische Studien besagen, dass diese CNCs pseudoelastisch sind.

Versatile Bottom‐up Approach to Stapled π‐Conjugated Helical Scaffolds: Synthesis and Chiroptical Properties of Cyclic o‐Phenylene Ethynylene Oligomers

Spring loaded: the smallest members of a family of carbon nanocoils (CNCs), adopting a fixed helical structure, have been synthesized by introduction of one or two staples in o-phenylene ethynylene oligomers. The chiroptical responses of the systems having enantiopure L-tartrate-derived staples confirmed the induced helicity. Theoretical studies suggest that these CNCs are pseudoelastic.

Dual conductance, negative differential resistance, and rectifying behavior in a molecular device modulated by side groups

We investigate the electronic transport properties for a molecular device model constructed by a phenylene ethynylene oligomer molecular with different side groups embedding in a carbon chain between two graphene electrodes. Using the first-principles method, the unusual dual conductance, negative differential resistance (NDR) behavior with large peak to valley ratio, and obvious rectifying performance are numerically observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals (MOs) as well as transmission coefficients under various external voltage biases gives an inside view of the observed results, which suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier MOs as well as the corresponding coupling between the molecule and electrodes. But the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule and the rectification as well as the NDR behavior (peak to valley ratio) can be improved by adding different side groups in the device model.

Acetylene Used as a New Linker for Molecular Junctions in Phenylene–Ethynylene Oligomer Langmuir–Blodgett Films

Langmuir and Langmuir–Blodgett films have been fabricated from an acetylene-terminated phenylene–ethynylene oligomer, namely 4-((4-((4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzoic acid (HOPEA). Characterization of the Langmuir film by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveals the formation of a high quality monolayer at the air–water interface. One layer Langmuir–Blodgett (LB) films were readily fabricated by the transfer of HOPEA Langmuir films onto solid substrates by the withdrawal of the substrate. The deposition mode was Z-type. Quartz crystal microbalance (QCM) experiments confirm the formation of directionally oriented, monolayer LB films, in which the HOPEA molecules are linked to the gold substrate by attachment through the acid group. The morphology of these films was analyzed by atomic force microscopy (AFM), which revealed an optimum transference surface pressure of 18 mN m–1 for the formation of homogeneous films. Cyclic voltammetry also showed a si...

Water-soluble meta -poly(phenylene ethynylene) oligomers with stable helical secondary structure

Two novel water-soluble meta-poly(phenylene ethynylene) (mPPE) copolymers were synthesized and characterized, each contained ester and amine functional groups attached to exohelix positions on the phenylene rings and one contained methoxy endohelix functional groups. Secondary structure formation was investigated for these materials in aqueous solutions using ultraviolet and fluorescence spectroscopy. Additionally, the folding behaviors are reported for the mPPEs and their protected amine precursors in other protic and aprotic solvents. Results indicate that both mPPEs are able to form stable helical structures in water, while only the nonmethoxylated polymer exhibited a helical structure in acetonitrile and several alcohols. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Membrane activity of antimicrobial phenylene ethynylene based polymers and oligomers

Poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and cationic phenylene ethynylene oligomers (OPEs) exhibit broad-spectrum antimicrobial activity and one of their main targets is believed to be the bacterial cell wall and membrane. This review focuses on recent progress made in elucidating the membrane perturbation mechanism of these antimicrobial molecules. We first review the membrane action models proposed for naturally occurring antimicrobial peptides (AMPs) because the membrane activities of CPEs and OPEs and AMPs are believed to be mediated by the same physical and chemical driving forces. An overview of the current research progress on the interactions between CPE and OPE compounds and model membranes is then presented, including main parameters controlling the membrane activity of these synthetic antimicrobial compounds. Combined with direct visualization of CPE and OPE induced changes in bacterial cell morphology, evidence to date points to a carpet or detergent-like pathway by which these compounds perturb the lipid membrane. Lastly, general trends between the membrane activity and dark antimicrobial activity of the CPEs and OPEs are reviewed to shed light on the structure–function relationship of this novel class of biocidal compounds.