PPV Chain Research Articles

Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D ...

Hybrid transdermal drug delivery patch made from poly(p‐phenylene vinylene)/natural rubber latex and controlled by an electric field

AbstractA flexible, natural rubber (NR) patch was developed for electrically controllable transdermal drug delivery. NR latex was crosslinked at various crosslinking ratios under the UV curing method. Ibuprofen (Ibu) was the model drug and was used as the dopant for poly(p‐phenylene vinylene) (PPV) acting as the drug encapsulating host. For the pristine Ibu‐loaded NR patch, the amount of Ibu permeation increased with decreasing crosslink density and increasing electrical potential. For the Ibu‐doped PPV/NR patch, the amount of Ibu release−permeation also increased with increasing electrical potential and was higher than that of the pristine NR matrices. Without an applied electric field, the drug remained attached to the PPV during an initial period of 6 h. Under an applied electric field, the oxidation state of the conductive polymer was altered, the iontophoretic effect, pore formation in the NR matrix, expansion of the pore size in hair follicles and PPV chain expansion combined to increase the Ibu release−permeation amount. Thus, the flexible PPV/NR transdermal drug delivery patch was demonstrated to be effective in drug release−permeation based on the strength of the electrical potential, the crosslinking density and the presence of PPV as the encapsulation host. © 2018 Society of Chemical Industry

(Invited) DAN Assisted Formation of Charge Transfer Complex from Highly Charged Fullerenes and Conjugated Polymer

Water-soluble, same charged, cationic conjugated poly(phenylenevinylene) (PPV) and cationic fullerenes were complexed with negatively charged single stranded DNA and double stranded DNA via electrostatic and pi-pi interactions. The photoinduced charge transfer complex (PPV/DNA/C60) has a charge transfer and separation efficiencies at least comparable to those observed from oppositely charged, cationic PPV and anionic fullerene. DNA serves as a template that straightens the PPV chain conformation which leads to an enhanced quantum yield. Meanwhile, DNA allows incorporation of C60 by overcoming the electrostatic repulsion between PPV and C60. While oppositely charge donor and acceptor molecules feature charge transfer in the form of static quenching, for DNA-assisted charge transfer complexes, the charge transfer proceeds as a combination of static and dynamic quenching for ssDNA and purely dynamic for dsDNA.

Preparation of fluorescent conjugated polymer fibrous membranes for rapid recognition of aromatic solvents.

Fluorescent poly(phenylenevinylene) (PPV)/poly(vinyl alcohol) (PVA) fibrous membrane was prepared via electrospinning of PPV precursor and PVA aqueous solution followed by thermal elimination. Further cross-linking produced the cross-linked membrane PPV/CPVA. Both PPV/PVA and PPV/CPVA membranes were found to have similar morphology and photophysics. These membranes showed a great fluorescence quenching response to aromatic solvents and a much smaller response to other organic solvents. Water also effectively quenched the fluorescence of PPV/PVA but not that of PPV/CPVA. This was attributed to un-cross-linked PVA being able to dissolve in water and the cross-linking improving the resistance of the membrane toward water. The sensing behavior was found to have good reversibility. The contact angle study showed that addition of only about 1% of PPV into the matrix reduced the hydrophilicity of the membrane significantly, suggesting that the PPV chains would be located at the surface of the fibers. X-ray photoelectron spectroscopy (XPS) investigation further confirmed such surface enrichment of PPV in the binary polymer blends. The PPV chain on the surface facilitated the π-π interaction between the polymer backbones and the aromatic molecules, thus leading to good selectivity and fast response of the two fibrous membranes toward aromatic solvents.

Torsional effects on the optical properties of PPV and PPP

The optical properties of infinite and isolated chains of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PPP) are studied using Density Functional Theory (DFT) in the Local Density Approximation (LDA) with norm-conserving pseudopotentials to replace the core electrons. We investigate theoretically the influence of several structural modifications of conjugated polymers on the band gap and the optical properties. Our results show that the band gap for isolated chains of PPV depends on the chain length. We also find that for both PPV and PPP one-dimensional chain polymers, the optical properties are strongly affected by the torsion angles.

Anionic PPV polymerization from the sulfinyl precursor route: Block copolymer formation from sequential addition of monomers

The sulfinyl precursor route for the synthesis of poly(p-phenylene vinylene) (PPV) materials via an anionic polymerization procedure employing dedicated initiators is evaluated in depth. Reaction kinetics are investigated to gain more control over the polymerization, since polymerization proceeds to full conversion already on the timescale of mixing of the reaction components. Even at −78 °C almost full conversion of the monomer is observed after few seconds. BEH-PPVs are obtained in the range of 3000 to 16,000 g mol−1, whereby dispersity decreases with decreasing molecular weight, allowing for materials with a PDI of 1.1 for the smallest PPV chain. Block copolymerizations were performed via sequential addition of monomers to make use of the living PPV chain ends. Bimodal product mixtures are obtained, consisting of block copolymer as well as PPV homopolymer. The block copolymer PPV-b-poly(tert-butyl acrylate) could nevertheless be separated by selective precipitation as well as preparative chromatography techniques.

Macro-counterions in a precursor to poly(phenylene vinylene): Toward defect-free luminescent films

Thermal elimination of a water soluble poly(xylylidene) precursor to poly(phenylene vinylene) (PPV) was accomplished in air at 80 °C using the macro-counterion poly(ethylene glycol)-4-nonylphenyl-3-sulfopropyl ether (PEGNOPS). The high relative photoluminescence efficiency was ascribed to the low carbonyl content, with a possible contribution from PPV chain separation, which minimizes radiationless interchain exciton formation. In addition, the PEGNOPS precursor showed no carbonyl formation on long term storage, in contrast to the precursors with dodecylbenzene sulfonate and chloride counterions.

Photophysics of conjugated polymers: interplay between Förster energy migration and defect concentration in shaping a photochemical funnel in PPV

Recent single molecule experiments have suggested the existence of a photochemical funnel in the photophysics of conjugated polymers, like poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV). The funnel is believed to be a consequence of the presence of conformational or chemical defects along the polymer chain and efficient non-radiative energy transfer among different chromophore segments. Here we address the effect of the excitation energy dynamics on the photophysics of PPV. The PPV chain is modeled as a polymer with the length distribution of chromophores given either by a Gaussian or by a Poisson distribution. We observe that the Poisson distribution of the segment lengths explains the photophysics of PPV better than the Gaussian distribution. A recently proposed version of an extended 'particle-in-a-box' model is used to calculate the exciton energies and the transition dipole moments of the chromophores, and a master equation to describe the excitation energy transfer among different chromophores. The rate of energy transfer is assumed to be given here, as a first approximation, by the well-known Förster expression. The observed excitation population dynamics confirms the photochemical funneling of excitation energy from shorter to longer chromophores of the polymer chain. The time scale of spectral shift and energy transfer for our model polymer, with realistic values of optical parameters, is in the range of 200-300 ps. We find that the excitation energy may not always migrate towards the longest chromophore segments in the polymer chain as the efficiency of energy transfer between chromophores depends on the separation distance between the two and their relative orientation.

Dynamical study on charge injection and transport in a metal/poly(p-phenylene vinylene)/metal structure

On the basis of the tight-binding Su-Schrieffer-Heeger model， the dynamical process of charge injection from metal electrode to a polymer in a metal/poly(p-phenylene vinylene) (PPV)/metal structure has been investigated by using a nonadiabatic dynamic approach. It is found that the injected charges form wave packets due to the strong electron-lattice interaction in PPV. The formation of the wave packet sensitively depends on the applied voltage， the electric field， and the contact between PPV and the metal electrode. In the absence of external fields， the charge can be injected into PPV chain and form wave packet under the critical bias voltage. With the increase of the electric field， the charge carriers can be ejected from PPV to the right metal electrode. But when the electric field becomes high enough， the wave packet can not be formed because it moves too fast in PPV. It is also shown that the number of ejected charges is not an integral multiple of the elementary charge e.

Wavelength-Resolved Studies of Förster Energy Transfer in Azobenzene-Modified Conjugated Polymers: The Competing Roles of Exciton Migration and Spectral Resonance

We report results from wavelength-resolved fluorescence lifetime measurements of azobenzene-modified poly-(p-phenylenevinylene) (PPV). The introduction of an azobenzene side chain enables reversible phototriggered modulation of the PPV emission. Intensity modulation is possible because Forster-type energy transfer from the PPV backbone to the azobenzene side-chain acceptors is more efficient for the cis-azobenzene isomer than for the trans. Here we explore how side-chain quenching competes with intrinsic PPV exciton dynamics. By probing the red and blue edges of the PPV emission, we evaluate the photophysical effects of the side chain on different exciton populations. Prior to UV exposure, when the azobenzene side chains are trans, three exciton subpopulations are detected: (i) rapidly diffusing and unquenched, (ii) nondiffusing and quenched, and (iii) low-energy, nondiffusing and unquenched. Exciton diffusion on the far blue edge is extremely efficient and mitigates the effects of side-chain quenching, giving rise to the first population. The second population is due to spectral resonance between side chains and nondiffusing excitons. The third population consists of excitons residing on the longest PPV chain segments. After UV-induced trans - cis photoisomerization, we observe a greater degree of quenching on the red edge as spectral overlap improves and the lowest-energy chromophores become efficiently quenched. Hence, cis-azobenzene quenching of (iii) contributes greatly to the efficient photomodulation of these PPV derivatives. In addition, rapid exciton migration on the blue edge may help to improve modulation efficiencies by effectively competing with energy transfer to trans-azobenzene. These findings provide improved understanding of the underlying mechanisms of energy transfer to side chains in this important class of polymers.

Modelling the effect of nonplanarity on charge transport along conjugated polymer chains

Conjugated polymers show interesting properties that make them appropriated for nanoelectronics. Several studies of poly(p-phenylene vinylene) (PPV) have suggested that each polymer chain consists of several planar segments, with conjugation length of nanoscale dimension, linked by twists or kinks. A pronounced twist between two planar segments in a PPV chain not only causes loss of main-chain conjugation but it may also alter electron and hole mobility along the chain, which has further implications for the percolation of charge through the polymer film.We used self-consistent quantum molecular dynamics calculations to provide information on the electric field needed to move the injected charges (either electrons or holes) along the planar segments of PPV and to cross the twist between two planar segments perpendicular to each other. Field-dependent charge mobility was also estimated for conjugated segments of various lengths. Our results suggest that electrons can cross the twist between adjacent planar segments for lower applied electric fields than holes if there is no more than one electronic charge (electron or hole) on the PPV chain, otherwise similar fields are needed.

The influence of PPV chain aggregated structure on optical properties

A comparison has been made on the fluorescence of the poly ( p-phenylenevinylene) (PPV) that without filtering the low molecular weight molecules (sample A) with the PPV of the low molecular weight molecules filtered (sample B). Although there is no obvious difference found in the FT-IR and Raman spectra between two samples, in the photoluminescence spectrum of the sample A, the peak at 510 nm is not appeared and the fluorescence intensity for the peak at 550 nm is increased. Under the high pressure condition the fluorescence peak at 520 nm, which corresponds to the peak at 510 nm of the usual PPV, is observed. Applying high pressure to the sample A or by filtering the low molecular weight molecules, the chain aggregated structure is modified, resulting in a change in the PPV chain distortion degree.

The influence of surface treatments on cathode formation and stability in polymer light emitting diodes

We studied the stability of metal/polymer interfaces by measuring the diffusion of calcium into a polymer (OC1C10 PPV) layer during and after deposition of the metal using low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS).During deposition the calcium diffusion depth in the PPV was found to be comparable for untreated samples and samples prepared in oxygen ambient (10−7mbar). In both cases diffusion depths up to ∼7nm were observed. For PPV layers treated with atomic oxygen, the diffusion depth during deposition was slightly smaller.After deposition, it was observed that calcium diffusion in OC1C10 PPV continues for several hours. When oxygen was present during calcium deposition or during the spin coating of the PPV, the diffusion coefficient for calcium in PPV was decreased considerably. In these cases accumulation of oxygen (adsorbed in the PPV during deposition or spin coating) at the calcium/PPV interface continued for several hours after deposition. Treatment of the PPV with atomic oxygen before calcium deposition resulted in a strong decrease of the calcium diffusion coefficient after deposition.From XPS measurements it was observed that calcium interacts with the chemically bonded oxygen in the PPV and also with the oxygen absorbed in the PPV layer. It can be concluded that oxygen, either chemically bonded to the PPV chain or adsorbed in the film, reduces the calcium diffusion coefficient.The initial performance of PLEDs with atomic oxygen treated PPV layers and PLEDs with calcium deposited in oxygen ambient was worse than the performance of untreated devices, but the stability in the life-test was better.

Charge and energy dynamics in photo-excited poly(para-phenylenevinylene) systems

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrodinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain.

Ground-state potential energy curves of phenylenevinylene oligomers

In this paper we calculate the energies of the different configurations of a poly( p-phenylene vinylene) trimer (OPV3) using ab inito Hartree–Fock (HF) methods. In order to determine sufficiently the energy configurations of the trimer, we used two dihedral angles, θ and φ, as variables of the potential energy surface, as opposed to a single dihedral angle as commonly found in the literature. We also determine the effects of the number of monomer units to the rotational barrier. We show that stilbene is not long enough to fully account for conjugation effects in the rotation of a phenyl ring within a polymer. In addition to this, we also show that computationally intensive calculations of oligomers longer than trimers are unnecessary in determining the energetics of a segment in a PPV chain.

Polymer-C 60 excitation mixing

The one-electron spectrum of C 60 cluster which is coupled to PPV chain is calculated. It shows that non-rigid band scheme has to be considered for this composite system. The splitting of states in the closest valence and conduction bands is discussed.

Torsional structure in poly(para-phenylene vinylene)

Abstract A direct approach to calculate the phonon dispersion curves in poly( para -phenylene vinylene, PPV) by using a parameterized tight-binding force constant method is presented. The calculated phonon dispersion curves for the first time show a pronounced soft mode in responding to a specific twisted structure in PPV, in very good agreement with experiments. A torsion angle of 22 ° is predicted in PPV chain, consistent with experimental data in oligomers. I

Formation of layer structures inH2SO4intercalatedpoly(p-phenylene vinylene)

X-ray-diffraction studies of highly oriented $\mathrm{poly}(p$-phenylene vinylene) (PPV) films before and after vapor-phase or liquid-phase intercalation by ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ have been performed. In both cases the data support a structural evolution dominated by a direct transformation of the original PPV polymer to a single, heavily doped phase. The best-fit model obtained from structure-factor refinements to the ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}$-PPV equatorial data is fully consistent with the previously proposed ``stage-1'' layered structure in which the PPV chains and the sulfate ions form an alternating sequence of cation- and anion-radical layers with a monoclinic unit cell dominated by the PPV placement and lattice parameters of $a=9.98$ \AA{}, $b=6.80$ \AA{}, $c=6.6$ \AA{}, and $\ensuremath{\gamma}=94.5\ifmmode^\circ\else\textdegree\fi{}$. Nonequatorial scattering data suggest that at room temperature the ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ layer is not well ordered. Additional data sets probing the anisotropy within the equatorial plane (perpendicular to the PPV chain axis) clearly indicate that the insertion of ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ layers occurs by creating galleries that are parallel to $\mathbf{a}$-$\mathbf{c}$ plane of the PPV host polymer. This direction is effectively perpendicular to the commonly accepted intercalation scheme, which has these layers parallel to the pristine PPV [100] planes.

Excitonic fs-luminescence rise in poly(phenylenevinylene), PPV

In the so-called molecular approach a realistic PPV chain is considered to be a collection of segmental subunits separated by a distribution of breaks, which arises by statistical disorder and predicts (i) a variation of conjugation lengths and (ii) a density of states (DOS) of localized S 1-levels. In this picture, the asymptotic optical dynamics is triggered by energy relaxation toward the low-energy tail states of the DOS and gives rise (i) to transient red shifts and (ii) to fs-luminescence patterns dependent on the spectral position of the luminescence window. In this work the delicate fs-interplay of both evolving and decaying luminescence photons caused by interference, due to inhomogeneous broadening (IHB), and the vibrational spacings in the Franck-Condon (FC) progression, has been studied. High-precision fs-luminescence rise-profiles have been probed at ε = 2.25 and 2.10 eV, respectively, that provide, for the first time, direct experimental evidence for the sequential dynamics of molecular hopping excitations in the segmental environment of PPV.

Excitons in the optical absorption spectra of the electroluminescent polymer poly( para-phenylenevinylene)

The component of photoexcited states with large spatial extent in the optical absorption spectra of the electroluminescent polymer poly( para-phenylenevinylene) (PPV) is investigated by using the intermediate-exciton theory. We find that there is a peak due to long-range excitons on the higher-energy side of the lowest main feature of the optical spectra. The energy position is nearly the same as that of the Hartree - Fock (HF) energy gap. The oscillator strengths of long-range excitons are larger when the electric field of the light is perpendicular to the chain axis. The fact that the onset of the long-range excitons is located near the HF gap might be related to the mechanisms of the large photocurrents measured in this energy region. Next, we calculate the ratio of the oscillator strengths due to long-range excitons to the sum of all of the oscillator strengths of the absorption, as a function of the PPV monomer number. The ratio depends strongly on the system size when the monomer number is low, but the magnitude of the ratio becomes almost constant when the monomer number is more than 10 and near 20, even though slight boundary condition effects persist. Finally, we discuss two different theoretical assignments of the characters of the main features in the absorption of PPV. We show that a large value of the hopping integral is realistic for optical excitations in the neutral PPV chain. The experimental feature at around 5.8 eV is mainly due to localized excitons.