Meta-phenylene Ethynylene Research Articles

A Facile Synthetic Route to Amphiphilic Poly(Meta-Phenylene Ethynylene) and Poly(Meta-Phenylene Ethynylene)-Block-Polyisocyanide Using a Single Catalyst.

An optically active, amphiphilic meta-phenylene ethynylene (m-PE) bearing a chiral amide pendant was designed and synthesized. Living polymerization of m-PE using alkyne-Pd(II) as the initiator afforded well-defined poly(meta-phenylene ethynylene) (m-PPE). These m-PPEs were found to have a stable helical conformation in THF, 1,4-dioxane, and CH3CN and showed split Cotton effects over the range of 245–400 nm. The positive first Cotton effect was observed at a wavelength of approximately 308 nm, and the negative second Cotton effect was observed at a wavelength of approximately 289 nm. The m-PPEs exhibited helical conformational changes in different mixed solvents and showed effective solvent-dependent helix inversion in CHCl3/THF solutions. The sign of the Cotton effect of m-PPE was inverted at 25 °C by varying the mixing ratio of THF and CHCl3. Finally, amphiphilic poly(meta-phenylene ethynylene)-block-polyisocyanide containing hydrophilic PPE and hydrophobic PPI segments were facilely prepared using Pd(II)-terminated m-PPE as the macroinitiator. This block copolymer can self-assemble into well-defined spherical nanostructures in a selective THF/CH3OH solution. This efficient polymerization will open up enormous opportunities for the preparation of functional amphiphilic block copolymers in a wide variety of fields.

Supramolecular catalysis in the methylation of meta-phenylene ethynylene foldamer containing N,N-dimethylaminopyridine

DFT investigations show that the methylation reaction of N,N-dimethylaminopyridine (DMAP)-modified meta-phenylene ethynylene foldamer can be catalyzed by the noncovalent interactions between the foldamer and the methyl sulfonate esters.

Synthesis and Self-Assembly of Cyclic 2,7-Anthrylene Ethynylene 1,3-Phenylene Ethynylene Trimer with a Planar Conformation.

Cyclic arylene ethynylene hexamer 1, composed of alternating 2,7-anthrylene ethynylene units and meta-phenylene ethynylene units, was synthesized. It shows C3 symmetry and possesses a flat and rigid conformation with a large equilateral triangle-like cavity. Macrocycle 1 self-associates through π-π stacking interactions between the anthracene-containing macrocyclic aromatic cores with indefinite-association constant KE =6980 m-1 in CDCl3 at 303 K. Macrocycle 1 also self-assembles into π-stacked nanofibers in the drop-cast film.

Hierarchically self-assembled helical aromatic conjugated polymers

We review recent progress in hierarchically self-assembled helical aromatic π-conjugated polymers (ACPs) by focusing on (i) the intrachain helicity of ACPs in which the main chain forms a helicene-like helical conformation and (ii) the interchain helicity of ACPs in which the main chains are helically π-stacked with one another. In Part 1, intrachain helical poly(meta-phenylene ethynylene) derivatives are discussed in terms of thermodynamic stability, usage of a chiral template, and feasibility of chiral recognition. Intrachain helical poly(meta-phenylene) derivatives are discussed from viewpoints of hierarchically self-assembled whiskers, magnetically forced macroscopic alignment, and chemical modification producing poly(thiaheterohelicene)s with stiff intrachain helicity. In Part 2, both polythiophene derivatives and poly(para-phenylene vinylene) derivatives are discussed in terms of interchain helicity and the effect of chiral organisation on chiroptical properties. Subsequently, thienylene-based diblock copolymers are discussed in terms of formation of hierarchical superhelices. Finally, thienylene- and phenylene-based homopolymers and copolymers are discussed in terms of an interchain helically π-stacked structure; reversible photoswitching of circularly polarised luminescence; and the evolution of RGB- and white-coloured, circularly polarised luminescence.

Optically Active, Amphiphilic Poly(meta-phenylene ethynylene)s: Synthesis, Hydrogen-Bonding Enforced Helix Stability, and Direct AFM Observation of Their Helical Structures

Optically active, amphiphilic poly(meta-phenylene ethynylene)s (PPEa) bearing L- or D-alanine-derived oligo(ethylene glycol) side chains connected to the backbone via amide linkages were prepared by microwave-assisted polycondensation. PPEa's exhibited an intense Cotton effect in the π-conjugated main-chain chromophore regions in various polar and nonpolar organic solvents due to a predominantly one-handed helical conformation stabilized by an intramolecular hydrogen-bonding network between the amide groups of the pendants. The stable helical structure was retained in the bulk and led to supramolecular column formation from stacked helices in oriented polymer films as evidenced by X-ray diffraction. Atomic force microscopy was used to directly visualize the helical structures of the polymers in two-dimensional crystalline layers with molecular resolution, and, for the first time, their absolute helical senses could unambiguously be determined.

Single-Turn Helix–Coil Strands Stabilized by Metal···Metal and π–π Interactions of the Alkynylplatinum(II) Terpyridyl Moieties in meta-Phenylene Ethynylene Foldamers

Dinuclear alkynylplatinum(II) terpyridyl complexes with oligomeric bridge consisting of five repeating meta-phenylene ethynylene (mPE) units have been found to exhibit a strong tendency to fold back onto themselves to form short helical strands through the stabilization of Pt···Pt and π-π interactions. The steric bulk of the terpyridine ligands and the length of the oligomeric bridge have been found to affect the extent of the intramolecular Pt···Pt interaction that governs the stabilization of the short helical strand in solution. Their folding properties via Pt···Pt and π-π stacking interactions have been studied by (1)H NMR, 2D ROESY NMR, electronic absorption, and emission spectroscopies.

Foldamer Structuring by Covalently Bound Macromolecules

We used fluorescence and electronic absorption spectroscopy to study the molecular weight dependence of macromolecule-induced folding in a chain-centered meta-phenylene ethynylene (mPE) oligomer. Analogous to the ability of intrinsically unstructured proteins (IUPs) to induce folding of globular proteins in cellular environments, we show that macromolecules attached to both ends of an mPE dodecamer induce the foldamer to collapse into a presumed helical conformation. The collapse is especially prominent once the macromolecule segments become larger than ca. 50 kDa. For sufficiently large macromolecules, the conformational structuring occurs even in solvents that normally denature the foldamer. Based on these findings, chain-centered foldamers might find use as models to investigate the fundamental macromolecular physics of IUPs.

Apolar ortho-phenylene ethynylene oligomers: conformational ordering without intermolecular aggregation

This paper describes the characterization of solvent induced folding behavior for non-polar (NP) alkoxy substituted ortho-phenylene ethynylene (o-PE) oligomers. Oligomers of lengths up to nine units have been shown to adopt helical conformations in heptane by NMR and CD spectroscopy, while chloroform promotes extended conformations. Surprisingly, the molar ellipticity values found in heptane for these oligomers are very small compared to other literature values of meta-phenylene ethynylene (m-PE) folded systems; however, comparable molar ellipticity values were found for a closed macrocyclic o-PE suggesting the weak ellipticity is a molecular-feature rather than a quality of folding indicator.

Interaction of Antimicrobial Oligomers with Lipids Studied by Solid-State NMR

A family of synthetic mimic of antimicrobial peptides (SMAMP), amphiphilic meta-phenylene ethynylene (mPE) molecules show a wide range of antimicrobial activity and specificity. The interaction of a specifically active mPE molecule (AMO-2) with mixed DOPE/DOPG lipid was studied by solid-state NMR. The AMO-2 molecules do not preferentially interact more strongly with either lipid component, but rather are well dispersed in the lipid matrix. AMO-2 intimately interacts with all parts of lipid molecules, including head groups. Magic-angle spinning sideband analysis indicated that in samples with co-existing lamellar and inversed hexagonal phases (HII), neither lipid component aggregate in either phase. The presence of AMO-2 molecules causes dynamic disorder in lipid head groups, as evidenced by the broadening of both static and MAS 31P spectra. AMO-2 molecules do not massively transform the lamellar lipid into HII phase.

Activity of an antimicrobial peptide mimetic against planktonic and biofilm cultures of oral pathogens.

Antimicrobial peptides (AMPs) are naturally occurring, broad-spectrum antimicrobial agents that have recently been examined for their utility as therapeutic antibiotics. Unfortunately, they are expensive to produce and are often sensitive to protease digestion. To address this problem, we have examined the activity of a peptide mimetic whose design was based on the structure of magainin, exhibiting its amphiphilic structure. We demonstrate that this compound, meta-phenylene ethynylene (mPE), exhibits antimicrobial activity at nanomolar concentrations against a variety of bacterial and Candida species found in oral infections. Since Streptococcus mutans, an etiological agent of dental caries, colonizes the tooth surface and forms a biofilm, we quantified the activity of this compound against S. mutans growing under conditions that favor biofilm formation. Our results indicate that mPE can prevent the formation of a biofilm at nanomolar concentrations. Incubation with 5 nM mPE prevents further growth of the biofilm, and 100 nM mPE reduces viable bacteria in the biofilm by 3 logs. Structure-function analyses suggest that mPE inhibits the bioactivity of lipopolysaccharide and binds DNA at equimolar ratios, suggesting that it may act both as a membrane-active molecule, similar to magainin, and as an intracellular antibiotic, similar to other AMPs. We conclude that mPE and similar molecules display great potential for development as therapeutic antimicrobials.

Towards Photocontrol over the Helix–Coil Transition in Foldamers: Synthesis and Photoresponsive Behavior of Azobenzene‐Core Amphiphilic Oligo(meta‐phenylene ethynylene)s

Introduction of photochromic azobenzene units into amphiphilic oligo(meta-phenylene ethynylene)s allowed photocontrol over the helix-coil transition in this important class of foldamers. Two design principles were followed in efforts to accommodate cis- and trans-azobenzene moieties within the helical structure to selectively turn the helical state on and off, respectively. Several oligomer series with varying connectivities to the central azobenzene chromophore were synthesized and these photochromic oligomers were investigated with regard to their folding behavior in both dark and irradiated states. Both the foldamers' chain lengths and the electronic structures of the azobenzene moieties had to be optimized to ensure folding differences and selective excitation of the photochrome. The design of such stimuli-responsive macromolecules, displaying large structural changes upon irradiation, should guide the design of future materials in, for example, "smart" delivery applications.

Conformational Changes of Facially Amphiphilic meta-Poly(phenylene ethynylene)s in Aqueous Solution

Design and control of polymer secondary conformation is a challenging problem. Facially amphiphilic polymers based on rigid but conformationally flexible meta-phenylene ethynylene backbones were pr...

Theoretical investigation of the role of π–π interactions for the stability of phenylene ethynylene aggregates

A theoretical investigation of phenylene ethynylene macrocycles and oligomers aggregation was carried out. Two types of stacked supramolecular organization were investigated at the MP2 level: the intermolecular aggregation of macrocycles and the intramolecular conformational ordering of long oligomers. For the neutral monomer and ortho- and meta-macrocycles bound states of π–π stacked pairs were found, having binding energies of ∼−0.061, −0.378 and −0.81 eV, respectively. The relative stability between ortho- and meta-phenylene ethynylene chains and helices as function of the oligomer length n, in gas phase was also studied. The results show that π–π interactions stabilize the helical structures for long oligomers.