Oligo Phenylenevinylene Research Articles

Color-Tunable Amphiphilic Segmented π-Conjugated Polymer Nano-Assemblies and Their Bioimaging in Cancer Cells

We report a unique color tunable amphiphilic segmented π-conjugated polymer design and their π-stack driven diverse self-assembled nanostructures and demonstrate their application as a new classes of aqueous luminescent nanoparticle probes for bioimaging in cervical and breast cancer cells. Oligo-phenylenevinylene (OPV) was employed as rigid luminescent π-core and oligo-ethyleneoxy chains were used as flexible spacers to construct new amphiphilic segmented π-conjugated polymers by Witting–Horner polymerization route. The rigidity of the π-core was varied using tricyclodecanemethyleneoxy, 2-ethylhexyloxy or methoxy pendants and appropriate π-core geometry was optimized to achieve maximum aromatic π-stacking interactions. Solvent-induced chain aggregation of the polymers exhibited a morphological transition from one-dimensional helical nanofibrous to three-dimensional spherical nanoassemblies in good/bad solvent combinations. This morphological transformation was accompanied by the fluorescence color change...

Emergent Molecular Recognition through Self-Assembly: Unexpected Selectivity for Hyaluronic Acid among Glycosaminoglycans.

Oligophenylenevinylene (OPV)-based fluorescent (FL) chemosensors exhibiting linear FL responses toward polyanions were designed. Their application to FL sensing of glycosaminoglycans (heparin: HEP, chondroitin 4-sulfate: ChS, and hyaluronic acid: HA) revealed that the charge density encoded as the unit structure directs the mode of OPV self-assembly: H-type aggregate for HEP with 16-times FL increase and J-type aggregate for HA with 93-times FL increase, thus unexpectedly achieving the preferential selectivity for HA in contrast to the conventional HEP selective systems. We have found that the integral magnitude of three factors consisting of binding mechanism, self-assembly, and FL response can amplify the structural information on the target input into the characteristic FL output. This emergent property has been used for a novel molecular recognition system that realizes unconventional FL sensing of HA, potentially applicable to the clinical diagnosis of cancer-related diseases.

One-pot synthesis and study of spectroscopic properties of oligo(phenylenevinylene)s

Two series of OPVs (oligo(phenylenevinylene)), that is, ((1E,1′E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives and 4-((E)-4-((E)-styryl)styryl)pyridine derivatives with different functional groups of varying electronic properties have been synthesized by one-pot Wittig–Heck methodology. The synthesized derivatives have been studied for their optical properties. Amongst them the ((1E,1′E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives with appropriate changes in the end group showed a significant impact on the UV absorption and emission spectra. Particularly NO2–OPV showed distinct solvatochromism in the wavelength range of 218nm in different solvents. Whereas 4-((E)-4-((E)-styryl)styryl)pyridine derivatives showed clear acidochromism which can be detected visually as well as spectroscopically.

Synthesis of novel phenylenevinylene linkers with electron-donating substituents by the Heck reaction

Three new carboxylic-acid substituted oligo-phenylenevinylenes (OPVs) with electron-donating substituents in the central ring were synthesized in high yield by the Mizoroki–Heck reaction. The linkers were optically characterized by UV–vis absorption spectrophotometry, fluorescence spectroscopy in solution and the solid state, and measurement of the emission quantum yield. A comparison of substituted and unsubstituted OPVs shows that the electron-donating groups significantly affect the luminescent properties of the linkers. As the electron-donating strength of the substituent increases, the absorption bands strengthen, the emission wavelength shifts bathochromically, and the emission quantum yield increases. Moreover, fluorescence analysis of the OPVs in the solid state shows that the nature of the substituent significantly affects the inter-chromophore interactions. These results suggest that the new linkers have potential for electro-optic applications in which high emission efficiency is required, such as chemical sensing.

Shape‐Persistent, Sterically Crowded Star Mesogens: From Exceptional Columnar Dimer Stacks to Supermesogens

Hexasubstituted C3 -symmetric benzenes with three oligophenylenevinylene (OPV) arms and three pyridyl or phenyl substituents are shape-persistent star mesogens that are sterically crowded in the center. Such molecular structures possess large void spaces between their arms, which have to be filled in condensed phases. For the neat materials, this is accomplished by an exceptional formation of dimers and short-range helical packing in columnar mesophases. The mesophase is thermodynamically stable for the pyridyl compound. Only this derivative forms filled star-shaped supermesogens in the presence of various carboxylic acids. The latter do not arrange as dimers, but as monomers along the columnar stacks. In this liquid crystal (LC) phase, the guests are completely enclosed by the hosts. Therefore, the host can be regarded as a new LC endoreceptor, which allows the design of columnar functional structures in the future.

A perfect match: fullerene guests in star-shaped oligophenylenevinylene mesogens.

Star-shaped oligophenylenevinylene (OPV) mesogens are shape-persistent and possess formally large void space. A mesogen with three styrene repeating units packs densely in a columnar helical arrangement. Attachment of one fullerene through a short spacer results in an exceptional increase of the mesophase stability. X-ray scattering and modeling evidence a triple-helical arrangement in which the fullerene perfectly fills the void space between the arms of the star mesogen.

Variation of formal hydrogen-bonding networks within electronically delocalized π-conjugated oligopeptide nanostructures.

This photophysical study characterizes the generality of intermolecular electronic interactions present within nanomaterials derived from self-assembling oligopeptides with embedded π-conjugated oligophenylenevinylene (OPV) subunits stilbene and distyrylbenzene that in principle present two distinct β-sheet motifs. Two different synthetic approaches led to oligopeptides that upon self-assembly are expected to self-assemble into multimeric aggregates stabilized by β-sheet-like secondary structures. The target molecules express either two C-termini linked to the central OPV core (symmetric peptides) or the more common N-termini to C-termini polarity typical of natural oligopeptides (nonsymmetric peptides). Both peptide secondary structures were shown to form extended 1-D peptide aggregates with intimate intermolecular π-electron interactions. Differences in length of the π-conjugated OPV segments resulted in differing extents of intermolecular interactions and the resulting photophysics. The peptides containing the shorter stilbene (OPV2) units showed little ground state interactions and resulted in excimeric emission, while the longer distyrylbenzene (OPV3) peptides had different ground state interactions between adjacent π-conjugated subunits resulting in either perturbed electronic properties arising from exciton coupling or excimer-like excited states. Molecular dynamics simulations of nascent aggregate formation predict peptide dimerization to be a spontaneous process, possessing thermodynamic driving potentials in the range 2-6 kcal/mol for the four molecules considered. Antiparallel stacking of the peptides containing an OPV3 subunit is thermodynamically favored over the parallel orientation, whereas both arrangements are equally favored for the peptides containing an OPV2 subunit. This study validates the generality of peptide-π-peptide self-assembly to provide electronically delocalized supramolecular structures and suggests flexibility in peptide sequence design as a way to tune the material properties of π-conjugated supramolecular polymers.

A multi-functional molecular device based on oligo phenylenevinylene and graphene

Using ab initio method, we study the electronic transport for a molecular device consisting of an oligo(p-phenylenevinylene) (OPV) molecule sandwiched between two zigzag-edged graphene nanoribbon (ZGNR) electrodes. Interestingly, a number of electrical functions, including switching, spin-filtering, negative differential resistance and spin-diode, are numerically observed in the device with different OPV conformations respect to ZGNRs and spin configurations in two electrodes. By analyzing the spatial distribution of local density of states, the performance of spin-filtering and -rectifying is explained to the asymmetry distribution of the central molecular orbitals as well as the corresponding coupling to the electrodes.

Helical Self-Assemblies of Segmented Poly(phenylenevinylene)s and Their Hierarchical Donor–Acceptor Complexes

We report aromatic π-stack-driven helical self-assemblies of segmented poly(phenylenevinylene)s and their hierarchical helical donor–acceptor assemblies with electron deficient molecules by a solvent-induced self-organization process. New segmented PPVs were designed and synthesized having tricyclodecane-substituted oligophenylenevinylene (OPV) π-core with flexible methylene chains of variable carbon atoms 4, 8, and 12. The polymers were obtained in high molecular weights with very good solubility in common organic solvents. The polymers were found to be amorphous, and their glass transition temperature varied from 100 to 170 °C with respect to the increase in the rigidity of the polymer backbone. The flexible segmented polymer underwent aromatic π-stack interaction to produce π-conjugated polymer aggregates in methanol and tetrahydrofuran (THF) solvent combinations. Electron microscopic studies confirmed that these π-aggregates appeared as bundles of helical assemblies. An electron deficient perylenebisimide derivative was complexed with segmented polymers to produce stable and helical donor–acceptor hierarchical assemblies. The formation of D–A assemblies was confirmed by detail photophysical studies such as absorbance, emission, time-resolved fluorescence decay, and FRET mechanism. A controlled experiment with structurally identical rigid polymer revealed that appropriate polymer design in the segmented skeleton is essential for making stable helical D–A assemblies. Thus, the present study provides insight into the formation of stable helical donor–acceptor assemblies in π-conjugated polymers that are useful for application in optoelectronics.

Synthesis of symmetrically functionalized oligo(p-phenylenevinylene) by Pd-catalyzed Heck coupling reaction

A facile method for synthesis of end-substituted oligo(p-phenylenevinylene) (OPV) with hydrophilic poly(ethylene glycol) (PEG) segment has been established using Pd(0)-catalyzed Heck coupling reaction. The OPVs are symmetrically end-substituted with a hydrophilic poly(ethylene glycol) segment. The prepared oligo(phenylenevinylene)s exhibit the expected photoluminescence properties.

Raman Scattering in Molecular Junctions: A Pseudoparticle Formulation

We present a formulation of Raman spectroscopy in molecular junctions based on a many-body state representation of the molecule. The approach goes beyond the previous effective single orbital formalism and provides a convenient way to incorporate computational methods and tools proven for equilibrium molecular spectroscopy into the realm of current carrying junctions. The presented framework is illustrated by first principle simulations of Raman response in a three-ring oligophenylene vinylene terminating in amine functional groups (OPV3) junction. The calculated shift in Stokes lines and estimate of vibrational heating by electric current agree with available experimental data. In particular, our results suggest that participation of the OPV3 cation in Raman scattering under bias may be responsible for the observed shift, and that the direction of the shift depends on renormalization of normal modes. This work is a step toward atomistic quantum ab initio modeling of the optical response of nonequilibrium electronic dynamics in molecular junctions.

Theoretical Investigation of the β Value of the π-Conjugated Molecular Wires by Evaluating Exchange Interaction between Organic Radicals

The decay constant β of π-conjugated molecular wire corresponds to the efficiency of electron tunneling through the molecule. We have calculated β values of intramolecular magnetic exchange interactions by density functional theory for organic bis(nitronyl nitroxide)s that have several molecular wire units, namely, oligo-p-phenylene (OPP), oligophenylene ethynylene (OPE), oligophenylene vinylene (OPV), oligophenylene imine (OPI), bicyclo[1.1.1]pentane oligomer (BCP), β,meso,β-triply fused porphyrin oligomer (TFP), and meso,meso-linked porphyrin oligomer (MLP). For OPP, OPE, OPV, OPI, BCP, TFP, and MLP, β was calculated to be 0.41, 0.25, 0.22, 0.32, 0.77, 0.03, and 0.65 Å–1, respectively. The obtained β values are in good agreement with the experimental value of the molecular tunneling conductance, suggesting similarity of the mechanism between exchange interaction and molecular conductance. This method can be used to estimate the efficiency of electron tunneling through the molecular wire.

Synthesis and photophysical properties of new oligophenylene vinylenes showing amplified spontaneous emission

Three new phenylene vinylene oligomers containing either a bis(oligoether)phenyl ring or a benzothiadiazole central unit and either carbazole or triphenylamine end capping groups were synthesized and characterized. The photophysical characterization in solution reveals a significant variation in properties depending on the combined moieties. Amplified spontaneous emission in neat drop cast films of two of these compounds was studied. One of them was also characterized in terms of its two-photon photoluminescence.

Solid State Assemblies and Photophysical Characteristics of Linear and Bent-Core π-Conjugated Oligophenylenevinylenes

New classes of luminescent linear, bent-core, and star-shaped oligophenylenevinylenes (OPVs) having 1,4-para and 1,3-meta rigid aromatic cores were designed and developed. 3-Pentadecylphenol, a renewable resource molecule, was chosen as the flexible unit at the longitudinal or middle position of the OPV aromatic core for solid state ordering. Depending upon the nature of the π-core, the OPVs exhibited either mosaic-type liquid crystalline textures or spherulitic crystalline solids. The enthalpies of melting transitions revealed that the bent-core OPV structure showed enhanced solid state packing compared to linear or star-shaped OPVs. Small and wide-angle X-ray diffraction analysis confirmed layered-like assemblies in OPV molecules. Photophysical experiments such as excitation, emission, and time-resolved fluorescence decay dynamics were carried out to trace the molecular self-organization of OPV chromophores. Time correlated single photon counting technique (TCSPC) luminescent decay profiles and decay lifetimes (τ1 and τ2 values) revealed that the OPV chromophores showed faster exciton decay in the tightly packed bent-core structure. The weakly packed star-shaped OPV showed enhanced excited state luminescence stability up to 10 ns. A direct correlation between the OPV chemical structure, solid state ordering, and photophysical characteristics was established.

Role of Geometric Distortion and Polarization in Localizing Electronic Excitations in Conjugated Polymers

Five different Density Functional Theory (DFT) models (ranging from pure GGA to long-range-corrected hybrid functionals) were used to study computationally the nature of the self-trapped electronic states in oligophenylene vinylenes. The electronic excitations in question include the lowest singlet (S1) and triplet (T1(†)) excitons (calculated using Time Dependent DFT (TD-DFT) method), positive (P(+)) and negative (P(-)) polarons, and the lowest triplet (T1) states (computed with the Self-Consistent Field (SCF) scheme). The polaron formation (spatial localization of excitations) is observed only with the use of range-corrected hybrid DFT models including long-range electronic exchange interactions. The extent of localization for all studied excitations is found to be invariant with respect to the size of the oligomer chain in their corresponding optimal geometries. We have analyzed the interdependence between the extent of the geometrical distortion and the localization of the orbital and spin density, and have observed that the localization of the P(+) and P(-) charged species is quite sensitive to solvent polarization effects and the character of the DFT functional used, rather than the structural deformations. In contrast, the localization of neutral states, S1 and T1(†), is found to follow the structural distortions. Notably, T1 excitation obtained with the mean field SCF approach is always strongly localized in range-corrected hybrid DFT models. The molecular orbital energetics of these excitations was further investigated to identify the relationship between state localization and the corresponding orbital structure. A characteristic stabilization (destabilization) of occupied (virtual) orbitals is observed in hybrid DFT models, compared to tight-binding model-like orbital filling in semilocal GGA functionals. The molecular and natural orbital representation allows visualization of the spatial extent of the underlying electronic states. In terms of stabilization energies, neutral excitons have higher binding energies compared to charged excitations. In contrast, the polaronic species exhibit the highest solvation energies among all electronic states studied.

Molecular sensing: modulating molecular conduction through intermolecular interactions

We observe changes in the molecular conductivity of individual oligophenylene-vinylene (OPV) molecules due to interactions with small aromatic molecules. Fluorescence experiments were correlated with scanning tunneling microscopy measurements in order to determine the origin of the observed effect. Both nitrobenzene and 1,4-dinitrobenzene decreased fluorescence intensity and molecular conductivity, while toluene had no effect. The observed changes in the fluorescence and conduction of OPV correlate well with the electron withdrawing ability of the interacting aromatic molecules. These results demonstrate the potential usefulness of OPV as a sensor for aromatic compounds containing electron withdrawing groups.

π-Conjugated polymer–Eu3+complexes: versatile luminescent molecular probes for temperature sensing

We report π-conjugated polymer–Eu3+ ion complexes as new potential luminescent thermo-sensitive molecular probes. Carboxylic acid functionalized segmented π-conjugated polymers having oligophenylenevinylene (OPV) chromophores in the poly(ethyleneoxide) or polymethylene backbones were custom designed, synthesized and utilized as efficient photosensitizers for Eu3+ ions. These π-conjugated polymer–Eu3+ ion complexes were found to be thermo-sensitive and behaved as reversible ‘turn-on’ or ‘turn-off’ luminescent switches in solution and in solid state. Luminescent decay studies revealed that the red-emission from the Eu3+ ion excited state was highly sensitive to temperature which drove the functioning of optical switches. The decay rate constants followed a typical Arrhenius trend over a wide temperature range having similar activation energies. Both the nature as well as length of the segmented polymer chain that tied the OPV optical chromophores in the backbone determine the temperature range of the luminescent on–off process. The emission characteristics of the oligomer–Eu3+ ion complex were found to be non-thermosensitive which emphasized the need for the segmented π-conjugated polymer ligand structure for the probes based on Eu3+ ion complexes. The present strategy opens up new concept and molecular design principles for π-conjugated polymer–lanthanide ion complexes as potential candidates for temperature sensitive luminescent molecular probes.

Herringbone and Helical Self-Assembly of π-Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self-organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self-assembly in π-conjugated molecules based on custom-designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single-crystal X-ray structures were resolved for these OPV synthons and the existence of long-range multiple-arm CH/π interactions was revealed in the crystal lattices. Alignment of these π-conjugated OPVs in the solid state was found to be crucial in producing either right-handed herringbone packing in the crystal or left-handed helices in the liquid-crystalline mesophase. Pitch- and roll-angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self-assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π-conjugated materials.

Copper(II) ion-sensing mechanism of oligo-phenylene vinylene derivatives: syntheses and theoretical calculations

Oligo-phenylene vinylene (oligo-PV) with two picolinamide side-groups and six methoxy end-groups was synthesized in order to be a fluorescent sensing molecule. Various metal ion solutions (1.5×10−4 M) were added to the 1.5×10−6 M acetonitrile solution of the fluorescent molecule. The fluorescent emission spectra showed that, at about the same concentration (1.5×10−4 M), only Cu(II) ion can quench the fluorescent emission of the picolinamide-PV solution. Possible metal ion-sensing mechanisms could be either the binding at picolinamide side-groups or methoxy end-groups, or interchain-stacking driven by the metal ions. Hence, oligo-PVs with six methoxy end-groups but without substituted side-groups, and another oligo-PV with six methoxy end-groups and two ethoxy side-groups were synthesized for comparison. These molecules turned out to be inactive to any metal ion solutions. Moreover, quantum calculation was used to confirm the result. Binding energy and conformation were calculated and simulated. It could be concluded that nitrogen and oxygen atoms of the picolinamide group and one oxygen atom from the methoxy group are involved in the metal ion-binding process.

D-π-A-A-π-D Prototype 2,2′-Bipyridine Dyads Exhibiting Large Structure and Environment-Sensitive Fluorescence: Synthesis, Photophysics, and Computation

A series of 4,4'-π-conjugated-2,2'-bipyridine chromophores (MS 1-8) were synthesized, and their photophysical and thermal properties were investigated. The title "push-pull' chromophores", except MS 1, were integrated with both alkoxy and alkylamino donor functionalities that differ in their donation capabilities. The oligophenylenevinylene (OPV) chromophores MS 4-8 are associated with a π-extended backbone in which the position and the number of alkoxy donors were systematically varied. All of the studied systems possess a D-π-A-A-π-D dyad archetype in which the A-A is the central 2,2'-bipyridine acceptor core that is electronically attached with the donor termini through π-linkers. The fluorescence quantum yields of the synthesized chromophores are found to be sensitive to the molecular archetype and the solvent medium. Out of the eight fluorescent compounds reported in this article, the compound MS 5 exhibits fluorescence in the solid state also. The modulating effect of the nature, position, and number of donor functionalities on the optical properties of these classes of compounds has further been comprehended on the basis of DFT and TD-DFT computation in a solvent reaction field.