Ethynylene Derivatives Research Articles

Arylimidazole-terminated phenylene ethynylene molecules as organic luminescent materials

Three phenylene ethynylene derivatives substituted by arylimidazole groups (benzo[d]imidazole, phenanthro[9,10-d]imidazole and pyreno[4,5-d]imidazole) at both ends were successfully synthesized and characterized for organic electroluminescence devices. The photophysical, electrochemical, and thermal stability of the compounds were systematically investigated to reveal their structure-property relationships. All compounds presented a X-shaped structure, and exhibited excellent thermal stability and intense emission in solution and solid states. Furthermore, the solution-processed doped devices using the blend of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with the phenylene ethynylene derivatives as emitting layers emitted deep-blue emission with maximum external quantum efficiency (EQEmax) of 0.63 % for the benzo[d]imidazole-terminated derivative and blue emission with EQEmax of 1.93 % and 1.45 % for the phenanthro[9,10-d]imidazole- and pyreno[4,5-d]imidazole-terminated derivatives, respectively, suggesting that these compounds have a great potential as the organic emitters for deep-blue and blue devices.

Reactions introducing 4-cyano-5-dicyanomethylene-2-oxo-3-pyrrolin-3-yl group to 4-ethynyl-N,N-dialkylaniline derivatives and characterization of the resulting dyes

Electrophilic reactions of a strong acceptor, 3-chloro-1-butyl-4-cyano-5-dicyanomethylene-2-oxo-3-pyrroline, with 4-ethynylaniline derivatives were investigated. The major products were found to be the cis- or trans-monoadducts of the pyrroline-type acceptor, i.e., 4-(dialkylamino)phenylethene derivatives in which the ethene moiety is substituted by chlorine and 1-butyl-4-cyano-5-dicyanomethylene-2-oxo-3-pyrrolin-3-yl group. Their stereostructures depended on the substituent attached to the ethynyl group on the opposite side of aminophenyl group. The adducts with a chloroethylene linkage were able to convert to the ethynylene derivatives by triethylamine. Among the obtained D-π-A molecules, the cis-compounds respect to the donor and acceptor groups showed the absorption maximum at longer wavelengths compared with the corresponding trans-compound. The calculation results suggested more polarized structures in the cis-forms, resulting in their bathochromic shift of the absorption.

Three-state molecular potentiometer based on a non-symmetrically positioned in-backbone linker

We report on the synthesis and single-molecule conductance of a para-oligo(phenylene)ethynylene derivative with a non-symmetric positioned internal anchor group that gives rise to three conduction pathways of well distinctive conductance and length.

Synthesis and Properties of a Cyclohexa‐2,7‐anthrylene Ethynylene Derivative

AbstractThe synthesis of a cyclohexa‐2,7‐(4,5‐diaryl)anthrylene ethynylene (1) was achieved for the first time by using 1,8‐diaryl‐3,6‐diborylanthracene and 1,8‐diaryl‐3,6‐diiodoanthracene as key synthetic intermediates. Macrocycle 1 possesses a planar conformation of approximately D6h symmetry, because of the triple‐bond linker between the anthracene units at the 2,7‐positions. It was confirmed that macrocycle 1, bearing bulky substituents at the outer peripheral positions, behaves as a monomeric form in solution without π‐stacking self‐association. Macrocycle 1 has an inner‐cavity size that allows specific inclusion of [9]cycloparaphenylene ([9]CPP), but not [8]CPP or [10]CPP, through an aromatic edge‐to‐face CH‐π interaction.

Synthesis and Properties of a Cyclohexa-2,7-anthrylene Ethynylene Derivative.

The synthesis of a cyclohexa-2,7-(4,5-diaryl)anthrylene ethynylene (1) was achieved for the first time by using 1,8-diaryl-3,6-diborylanthracene and 1,8-diaryl-3,6-diiodoanthracene as key synthetic intermediates. Macrocycle 1 possesses a planar conformation of approximately D6h symmetry, because of the triple-bond linker between the anthracene units at the 2,7-positions. It was confirmed that macrocycle 1, bearing bulky substituents at the outer peripheral positions, behaves as a monomeric form in solution without π-stacking self-association. Macrocycle 1 has an inner-cavity size that allows specific inclusion of [9]cycloparaphenylene ([9]CPP), but not [8]CPP or [10]CPP, through an aromatic edge-to-face CH-π interaction.

HOMO Level Pinning in Molecular Junctions: Joint Theoretical and Experimental Evidence.

A central issue in molecular electronics in order to build functional devices is to assess whether changes in the electronic structure of isolated compounds by chemical derivatization are retained once the molecules are inserted into molecular junctions. Recent theoretical studies have suggested that this is not always the case due to the occurrence of pinning effects making the alignment of the transporting levels insensitive to the changes in the electronic structure of the isolated systems. We explore here this phenomenon by investigating at both the experimental and theoretical levels the I/ V characteristics of molecular junctions incorporating three different three-ring phenylene ethynylene derivatives designed to exhibit a significant variation of the HOMO level in the isolated state. At the theoretical level, our NEGF/DFT calculations performed on junctions including the three compounds show that, whereas the HOMO of the molecules varies by 0.61 eV in the isolated state, their alignment with respect to the Fermi level of the gold electrodes in the junction is very similar (within 0.1 eV). At the experimental level, the SAMs made of the three compounds have been contacted by a conducting AFM probe to measure their I/ V characteristics. The alignment of the HOMO with respect to the Fermi level of the gold electrodes has been deduced by fitting the I/ V curves, using a model based on a single-level description (Newns-Anderson model). The extracted values are found to be very similar for the three derivatives, in full consistency with the theoretical predictions, thus providing clear evidence for a HOMO level pinning effect.

Insulator-protected mechanically controlled break junctions for measuring single-molecule conductance in aqueous environments

We present a method to fabricate insulated gold mechanically controlled break junctions (MCBJ) by coating the metal with a thin layer of aluminum oxide using plasma enhanced atomic layer deposition. The Al2O3 thickness deposited on the MCBJ devices was varied from 2 to 15 nm to test the suppression of leakage currents in deionized water and phosphate buffered saline. Junctions coated with a 15 nm thick oxide layer yielded atomically sharp electrodes and negligible conductance counts in the range of 1 to 10−4 G0 (1 G0 = 77 μS), where single-molecule conductances are commonly observed. The insulated devices were used to measure the conductance of an amphiphilic oligophenylene ethynylene derivative in deionized water.

Intramolecular Charge-Transfer Interaction of Donor–Acceptor–Donor Arrays Based on Anthracene Bisimide

We designed anthracene bisimide (ABI) derivatives having two triphenylamine (TPA) groups as donor units at the 9,10-positions to form a novel π-conjugated donor-acceptor system. These compounds and their analogues with ethynylene linkers were synthesized by Suzuki-Miyaura and Sonogashira coupling reactions, respectively. In UV-vis spectra, the linker-free derivatives showed broad absorption bands arising from intramolecular charge-transfer interactions. Introducing ethynylene linkers resulted in a considerable red shift of the absorption bands. In fluorescence spectra, the ethynylene derivatives showed intense emission bands at 600-650 nm. Their photophysical and electrochemical properties were compared with those of the corresponding mono TPA derivatives on the basis of theoretical calculations and cyclic voltammetry to evaluate the intramolecular electronic interactions between the donor and acceptor units.

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.

Synthesis and Characterization of Poly(p-phenylene ethynylene)s with nitroxyl radical endgroups

ABSTRACTThe generation of terminal N-Hydroxyl substituents in p-phenylene ethynylene based compounds is presented. P-phenylene ethynylene derivatives were synthesized in a Sonogashira coupling reaction. N-Hydroxyl groups could be introduced by lithiation of iodine moieties and subsequent reaction with the 2-methyl-2-nitrosopropane generated by the cleavage of its dimer. The synthesis by lithiation was found to be more effective compared to the reaction with the Grignard reagent and the chloro-derivative. The resulting compounds were characterized by 1H NMR, UV and PL spectroscopy and were shown to be sensitive towards oxidation. This new approach of introducing additional charge carriers by nitroxyl endgroups might enable conjugated polymers with enhanced conductivity.

Synthesis and properties of phenylene ethynylene derivatives

Synthesis and properties of phenylene ethynylene derivatives

Metal Cluster Terminated “Molecular Wires”

The gold complexes Au(C≡CC6H4C≡CC6H4Me)(PPh3) (3) and {Au(PPh3)}2(μ-C≡CC6H4C≡CC6H4C≡CC6H4C≡C) (6), prepared from the reaction of AuCl(PPh3) with the corresponding terminal or trimethylsilyl protected alkynes, react readily with Ru3(CO)10(μ-dppm) to afford phenylene ethynylene derivatives featuring the Ru3(μ-AuPPh3)(μ-C2R)(CO)7 cluster “end-caps”. The hydrido cluster Ru3(μ-H)(μ-C2C6H4C≡CC6H4Me)(CO)7 (4a) has also been obtained. There are significant differences in the absorption spectra of the organic precursors, the gold complexes and the clusters indicate a mixing of electronic states between the cluster and phenylene ethynylene moieties, while the presence of the Ru3 and in particular Ru3(μ-AuPPh3) cluster end-caps leads to a quenching of the phenylene ethynylene centred emission. The crystallographically determined structures of 3, 4a and Ru3(μ-AuPPh3) (μ-C2C6H4C≡CC6H4Me)(CO)7 (4b) are reported.

Immobilization of π-conjugated molecules on Au using dendrimer-based templates

Abstract In this paper, we demonstrate immobilization of two kinds of π-conjugated molecules on Au using dendrimer-based templates; one is a p -phenylene ethynylene derivative and the other is a 4,4′-bis(phenylethynyl)-2,2′-bipyridine derivative. Both molecules were bound to benzyl-ether dendrons through an alkali-labile, ester linkage to give two different generation dendrimers. The removal of the dendrons from the self-assembled monolayers of the dendrimers left the isolated π-conjugated-molecules as well as small collections of the molecules on the surfaces, which was confirmed by surface Fourier transform infrared (FT-IR) spectroscopy and scanning tunneling microscopy (STM). Insertion of the dendrimer into n -dodecanethiol monolayer instead of the direct adsorption on a bare gold gave only the isolated, single molecules on the surface after the dendrons were removed.

Facially amphiphilic phenylene ethynylenes

AbstractFacially amphiphilic poly(phenylene ethynylenes) were synthesized with various non‐polar alkyl and polar amine side chains by Sonogashira‐Hagihara coupling conditions. para‐Phenylene ethynylene derivatives with pentoxy and tertiary amine side chains were dissolved in organic solvent, such as toluene or 1,2,4‐trichlorobenzene, and introduced into an acidic, aqueous environment. The polymer molecules were found to self‐assemble and stabilize these oil‐water interfaces as visualized by confocal scanning laser microscopy. meta Derivatives containing dodecyl and primary amine side chains were also prepared and found to be active at the air‐water interface. When analyzed by Langmuir trough experiments, this polymer was found to have an area of approximately 45 Å2 per repeat unit, which correlates to an edge‐on orientation at the air‐water interface where the polar groups are immersed in the water sub‐phase and both the hydrocarbon backbone and dodecyl chains extend away from the interface. Due to the facially amphiphilic architecture and large hydrocarbon side chains, these polymers were found to self‐assemble from aqueous solutions into large aggregates with some order. Copyright © 2005 John Wiley & Sons, Ltd.

Morphological Characterization of the Molecular Superstructure of Polyphenylene Ethynylene Derivatives

Two different polyphenylene ethynylene derivatives, one partly hydrophobic and one hydrophilic, were investigated with a combination of X-ray and light scattering techniques and hydrodynamic techniques, as well as scanning force microscopy and transmission electron microscopy to elucidate their molecular structure and aggregation behavior in tetrahydrofuran and water, respectively. It turns out that both polymers possess a rod-like molecular architecture which, however, is the basis of a cascade of aggregation steps. Both, electron microscopy and X-ray analysis support the concept of a primary back-to-back aggregation of polymer chains into cylindrically shaped aggregates with high anisometry. The thickness of these aggregates was between 4.0 and 4.5 nm. The aggregates of the hydrophobic polymer further associate into fibrils and these fibrils form clusters of globular shape, though with high internal anisometry.

Acyclic Diyne Metathesis (ADIMET), an Efficient Route to Poly(phenylene)ethynylenes (PPEs) and Nonconjugated Polyalkynylenes of High Molecular Weight

Conjugated and nonconjugated diynes can be readily converted to poly(phenylene)‐ethynylene derivatives and polyalkynylenes by acyclic diyne metathesis. The catalyst for these condensation reactions is [(tBuO)3 WCtBu], and they proceed with cleavage of volatile alkynes such as 2‐butyne and 3‐hexyne. The chain lengths of the polymers are controlled by reaction temperature and pressure.