Vinylene Derivatives Research Articles

Identification and Quantification of Defect Structures in Poly(2,5-thienylene vinylene) Derivatives Prepared via the Dithiocarbamate Precursor Route by Means of NMR Spectroscopy on 13C-Labeled Polymers

During the past decades several synthetic routes toward the low band gap polymer poly(2,5-thienylene vinylene) (PTV) and derivatives have been studied. This study describes an extensive NMR characterization of 13C-labeled 3-octyl-PTV and its precursor polymer prepared via the dithiocarbamate route which is, since stable monomers are available, a promising route toward PTV derivatives. By introducing 13C-labeled vinylene carbons, we were able to characterize these polymers in a quantitative way, taking the end groups and structural polymerization defects, which disturb the conjugated system, into account. Several NMR techniques and the synthesis of model compounds were used to fully assign the proton and carbon chemical shifts. Moreover, the classically used thermal conversion of the precursor toward the conjugated polymer has been compared to a smoother, acid-induced elimination procedure.

Electron traps in semiconducting polymers: Exponential versus Gaussian trap distribution

The low electron currents in poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives and their steep voltage dependence are generally explained by trap-limited conduction in the presence of an exponential trap distribution. Here we demonstrate that the electron transport of several PPV derivatives can also be well described with a trap distribution that is Gaussianly distributed within the band gap. In contrast to the exponential distribution the trap-limited electron currents can now be modeled using the same Gaussian trap distribution for the various PPV derivatives. © 2011 American Physical Society.

Multistimuli-Responsive Benzothiadiazole-Cored Phenylene Vinylene Derivative with Nanoassembly Properties

A trifluoromethyl-substituted benzothiadiazole-cored phenylene vinylene fluorophore (1) was synthesized and displayed piezo- and vapochromism and thermo-induced fluorescence variation in solid phase. Grinding could disrupt the crystalline compound 1 with orange emission into amorphous compound 1 with green emission, and heating treatment could change the amorphous compound 1 into crystalline compound 1. Ultraviolet-visible (UV-vis) absorption spectra, (13)C nuclear magnetic resonance (NMR), and powder X-ray diffraction (PXRD) characterizations demonstrated that crystalline and amorphous compound 1 possess different molecular packing. A differential scanning calorimetry (DSC) measurement revealed that the emission switching was due to the exchange between the thermodynamic-stable crystalline and metastable amorphous states. The ground sample exhibited vapochromic fluorescence property. Furthermore, compound 1 showed interesting supramolecular assembly characteristics in solution. Slowly cooling the hot N,N-dimethylformamide (DMF) solution of compound 1 resulted in the formation of orange fluorescent fibers, whereas sonication treatment of the cooling solution led to the generation of organic molecular gel. The field emission scanning electronic microscope (FESEM) and fluorescent microscopy images revealed smooth nano- or microfiber and network morphology properties. The PXRD spectra confirmed that these nano- or microstructures had a similar molecular-packing model with the crystalline state of compound 1. Slow evaporation of the toluene solution of compound 1 could produce green emissive microrods, which exhibited interesting thermo-induced fluorescence variation.

Synthesis, characterization, and photophysical properties of novel poly(p‐phenylene vinylene) derivatives with conjugated thiophene as side chains

AbstractTwo novel poly(p‐phenylene vinylene) (PPV) derivatives with conjugated thiophene side chains, P1 and P2, were synthesized by Wittig‐Horner reaction. The resulting polymers were characterized by 1H‐NMR, FTIR, GPC, DSC, TGA, UV–Vis absorption spectroscopy and cyclic voltammetry (CV). The polymers exhibited good thermal stability and film‐forming ability. The absorption spectra of P1 and P2 showed broader absorption band from 300 to 580 nm compared with poly[(p‐phenylene vinylene)‐alt‐(2‐methoxy‐5‐octyloxy‐p‐phenylene vinylene)] (P3) without conjugated thiophene side chains. Cyclic voltammograms displayed that the bandgap was reduced effectively by attaching conjugated thiophene side chains. This kind of polymer appears to be interesting candidates for solar‐cell applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Stabilizing the Dynamic p−i−n Junction in Polymer Light-Emitting Electrochemical Cells

A poly(ethylene oxide) (PEO) oligomer capped with methacrylate end groups can conduct ionic species for the formation of p−i−n junction in polymer light-emitting electrochemical cells (LECs). The polymerization of the methacrylate during the junction formation can mitigate the nonuniformity of the doping frontiers and enhance the uniformity and stability of the junction. Sandwich-structured LECs employing this ionic conductor exhibit both high electroluminescent efficiency and stability. Luminous efficiency greater than 15 lm/W has been obtained using a yellow light-emitting poly(1,4-phenylene vinylene) derivative (Super-Yellow by Covion). In accelerated lifetime testing, the brightness dropped to 75% of the maximum 2200 cd/m2 of luminance (T75%) in 102 h under constant current operation; this corresponds to a lifetime (T75%) of 27 000 h at 100 cd/m2 of peak brightness. Both the efficiency and lifetime are comparable to the best-performing polymer OLEDs optimized for commercial applications.

Extraordinary Large Refractive Index Change of Poly(phenylene vinylene) Derivatives induced by Photo- or Quantum Beam Irradiation

Poly(phenylene vinylene) structure is one of the chromophore with large two photon absorption (TPA) cross section, which is expected as the materials for high density optical data storage with the TPA induced reaction. Photo-irradiation to a poly(phenylene vinylene) derivative changes the refractive index of the polymer by the magnitude as high as 0.7. The mechanism involves photo-induced electron transfer from the TPA dye to matrix for the reaction of a distyrylarene derivative. The refractive index of the polymer also can be changed by the electron beam irradiation.

Insight into the gelation habit of oligo(para-phenylene vinylene) derivatives: effect of end-groups

This paper reports a study on the gelation habits of organogels prepared in benzyl alcohol from derivatives of oligo(p-phenylene vinylene) (OPV) bearing C16H33 aliphatic wings and a series of end groups on the backbone. Significant differences are observed based on whether or not these end groups can establish hydrogen bonds with adjacent OPV molecules or with the solvent. It is particularly shown that the fibril structure, the morphology and the molecular organization not only depend upon the aliphatic wings and/or the π–π interactions of the backbones but equally on the nature of these end groups. Evidence highlighting a homogeneous nucleation process for gel formation are also presented and discussed. Discrepancies with thermoreversible gels of covalent polymers are also stressed.

A potent fluorescent probe for the detection ofcellapoptosis

A new positively charged fluorescent probe poly(p-phenylene vinylene) derivative (PPV-1) has been developed for the detection of an important biological process, apoptosis, in a simple and fluorescent label-free way.

Synthesis, characterization, and OFET characteristics of 3,4-diaryl substituted poly(thienylene vinylene) derivatives

Poly(thienylene vinylene) derivatives bearing aryl substituents at 3,4-positions have been synthesized in good yield by Stille-type polycondensation. Two types of aryl substituents, either a 4-octylphenyl or a 5-octyl-2-thienyl, were investigated in this study. The polymers were characterized by 1H NMR, GPC, TGA, UV–vis absorption, and photoluminescence spectroscopy. The polymers (P1–P4) showed good to excellent solubility in common organic solvents, and UV–vis absorption spectra in solution exhibit maxima in the range of 511–595 nm. GPC analysis using PPP standards showed a number average molecular weight range of 6.59–8.98 × 103 g mol−1 for the various polymers. The field effect transistors based on these polymers were studied, and the results obtained are correlated to their molecular structure.

A deeper Insight into the Dithiocarbamate Precursor Route: Synthesis of Soluble Poly(thienylene vinylene) Derivatives for Photovoltaic Applications

The synthesis of, two new poly(thienylene vinylene) derivatives is described, i.e. poly(3-octyl-2,5-thienylene vinylene) (O-PTV) and poly(bis[octylphenyl-2,5-thienylene vinylene]) (BOP-PTV). Both polymers have been prepared via the dithiocarbamate (DTC) precursor route. The polymerization protocol of the monomer toward the precursor polymer has been optimized by the use of different bases, leading to improved reproducibility of the polymerization step. Processability has been guaranteed by the introduction of alkyl side chains. Finally the precursor polymers were converted toward conjugated polymers and they were fully characterized by UV/vis, IR, GPC, and cyclic voltammetry. Bulk heterojunction solar cells with PCBM as acceptor showed promising power conversion efficiencies of 0.80% for BOP-PTV and 0.92% for O-PTV.

Copoly(p-phenylene)s containing bipolar triphenylamine and 1,2,4-triazole groups: Synthesis, optoelectronic properties, and applications

Two novel copoly(p-phenylene)s (P1–P2) containing bipolar groups (12.8 and 6.8 mol %, respectively), directly linked hole transporting triphenylamine and electron transporting aromatic 1,2,4-triazole, were synthesized to enhance electroluminescence (EL) of poly(p-phenylene vinylene) (PPV) derivatives. The bipolar groups not only enhance thermal stability but also promote electron affinity and hole affinity of the resulting copoly(p-phenylene)s. Blending the bipolar copoly-(p-phenylene)s (P1–P2) with PPV derivatives (d6-PPV) as an emitting layer effectively improve the emission efficiency of its electroluminescent devices [indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/polymer blend/Ca (50 nm)/Al (100 nm)]. The maximum luminance and maximum luminance efficiency were significantly enhanced from 310 cd m−2 and 0.03 cd A−1 (d6-PPV-based device) to 1450 cd m−2 and 0.20 cd A−1 (blend device with d6-PPV/P1 = 96/4 containing ∼0.5 wt % of bipolar groups), respectively. Our results demonstrate the efficacy of the copoly(p-phenylene)s with bipolar groups in enhancing EL of PPV derivatives. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Photophysical properties of poly(triptycene vinylene) derivatives and effect of nitrotoluene exposure

The photophysical properties of two conjugated polymers containing triptycene group were studied. These systems are highly fluorescent and time-resolved fluorescent measurements show that there are different types of excited species whose relative population is correlated to a heterogeneous collection of chromophore sites. A fluorescence quenching is observed upon DNT exposure because of a dynamic electron transfer process from the polymer to the quencher. The reversibility of the quenching process has been demonstrated by a simple air flow method which allows the re-use of the sensor for cycling measurement of the nitroaromatic analyte.

Synthesis and characterization of low bandgap poly(dithienosilole vinylene) derivatives

Two low bandgap conjugated polymers containing vinylene unit on their main chains, poly(dithienosilole vinylene) derivatives P1 and P2, were synthesized by Pd-catalyzed Stille-coupling method. The two polymers show good solubility in common organic solvents, broad absorption bands from 500 nm to 700 nm and lower energy bandgap of 1.72 eV for P1 and 1.77 eV for P2. The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry. The HOMO and LUMO energy levels of P1 and P2 are at −5.03 eV, −5.18 eV and −3.39 eV, −3.43 eV, respectively. The polymers P1 and P2 containing dithienosilole units show lower HOMO and LUMO energy levels, in comparison with other poly(thienylene vinylene) derivatives.

Correlation Analysis of Fluorescence Intensity and Fluorescence Anisotropy Fluctuations in Single‐Molecule Spectroscopy of Conjugated Polymers

Single-molecule spectroscopy techniques are used to investigate time fluctuations of the fluorescence properties of two different types of conjugated polymer, a polythiophene derivative (PDOPT) and a phenylene vinylene derivative (MEH-PPV), at 100 and 293 K. Linear correlation coefficients between fluorescence intensity and polarization are used to characterize fluctuations. We are able to distinguish between different blinking and bleaching effects on the polarization. Furthermore, the polarization data reveal clear differences in the topology of these two polymers, which is related to the ordered conformation of the MEH-PPV. Plots of correlation coefficients appear to be very different for the two polymers and are also very sensitive to temperature. These observations prove that correlation analysis is a useful tool to understand fluorescence fluctuations in multi-chromophoric systems.

Synthesis and Characterization of Ethylene Glycol Substituted Poly(phenylene Vinylene) Derivatives

We report the synthesis of a series of water-soluble, fluorescent, conjugated polymers via the Gilch reaction with an overall yield greater than 40%. The yield for the Gilch reaction decreases with the increase in the length of the side chain (ethylene glycol repeat units), presumably due to the steric effects inhibiting the linking of monomeric units. The hydrophilic side chain enhances the solubility of the polymer in water and concomitantly leads to a side-chain-dependent conformation and solvent-dependent quantum efficiency. An increase in the ethylene glycol repeat units on the polymer side chain structure results in changes in chain packing; hence, the crystallinity evolves from semicrystalline to liquid crystalline to completely amorphous. An increase in the length of the side chain leads to changes in the polymer-solvent interaction as manifested in the photophysical properties of these polymers. These novel polymers exhibit two glass transition temperatures, which can be readily rationalized by differences in microstructure when casted from hydrophobic and hydrophilic solvents. Cyclic voltammograms of polymer 1d-3d suggest two-electron transfer, as compared to P1 which has one complete redox pair. The potential of having a nanoscaled domain structure and stabilizing two electrons on a polymer chain signifies the potential of these polymers in fabricating electronic and photovoltaic devices.

Solvated poly-(phenylene vinylene) derivatives: conformational structure and aggregation behavior

We study dilute solutions of poly(2,3-diphenyl phenylene vinylene) with hexyl (DP6-PPV) and decyl (DP10-PPV) side chains in two solvents, chloroform and toluene. For this purpose, atomistic and coarse-grained models are parametrized using quantum-chemical calculations and structure-based coarse-graining, respectively. Our simulations indicate that the difference in the aggregation behavior of two derivatives can not be rationalized just in terms of the greater steric hindrance imposed by the longer side chains of DP10-PPV. The coarse-grained model describes qualitatively the DP10-PPV derivative in chloroform, where aggregation does not occur. Although the computed structure factors for this system qualitatively agree with experiments for low concentrations, the calculated persistence length is bigger than the one experimentally reported, hinting at the presence of defects in polymer chains.

Influence of a Mesogenic Dendrimer on the Morphology of Polymer–Fullerene Composites for Photovoltaics

The efficiency of “bulk hetero junction”-type plastic solar cells depends crucially on the morphology of an entangled network of two immiscible compounds that serve as electron donor and electron acceptor, respectively. The present work describes preliminary results about the question how the luminescence and the morphology of a composite containing a fullerene C60 derivative and a p-phenylene vinylene (PPV) derivative are altered, when a dendrimer with a fullerene core and mesogenic side-chains is added. Fluorescence quenching and surface topography of the composites indicate that adding the mesogenic dendrimer can result in smaller domain sizes, which may be useful for photovoltaic applications.

Polymeric coatings for photostability enhancement of poly(p‐phenylene vinylene) derivative films

AbstractPoly(p‐phenylene vinylene) (PPV) derivatives are an important class of conjugated polymers, known for their applications as electroluminescent materials for light‐emitting devices and sensors. These derivatives are highly susceptible to photodegradation by the combined action of oxygen and light. Here, the use of various commercial polymers as protective coatings against the photodegradation of PPV derivatives was explored. Cast films of two similar PPV derivatives, poly[(2‐methoxy‐5‐n‐hexyloxy)‐p‐phenylene vinylene] and poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐p‐phenylene vinylene], were submitted to photodegradation by exposure to white light under atmospheric conditions in order to verify if the type of side chain (linear or branched) had an effect on the photodegradation. No significant differences in the photodegradation behaviour between the two polymers were noticed. The following commercial polymers were tested as protective coatings for the PPV derivative cast films: 99 and 80% hydrolysed poly(vinyl alcohol) (PVA) and starch. The best results were achieved using coatings of 99% hydrolysed PVA, which increased about 700 times the time necessary for complete degradation of the PPV derivative films. The results show the effectiveness of this coating in minimizing and, possibly, controlling the effects of the photodegradation of PPV derivative films, which can be useful in many applications, e.g. oxygen sensors. Copyright © 2009 Society of Chemical Industry

Synthesis and spectroscopic characterization studies of low molecular weight light emitting PPV segmented copolymers

New tuneable segmented alternating copoly( p-phenylene vinylene) (PPV) derivatives were chemically synthesised via the Wittig polycondensation reaction. The chemical and optical properties in solution and in solid state were evaluated. The absorption and photoemission spectra show that incorporating an ethane dioxy spacer limits the conjugation length and produces a blue emitting copolymer, poly[1,2-ethane dioxy 2-methoxy-1,4-phenylene-vinyl 1,4-phenylene-vinyl 3-methoxy 1,4-phenylene] (EDO-PPV- 1) 1. A red shift in the copolymer emission was achieved by adding an electron-donating group, a hexoloxy-substituted PPV segment, poly[1,2-ethane dioxy 2-methoxy-1,4-phenylene-vinyl 2,5-dihexoloxy 1,4-phenylene-vinyl 3-methoxy 1,4-phenylene] (EDO-PPV- 2) 2. The photophysical properties of these block copolymers was compared with the fully conjugated copolymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) 3. The absorption band of the π deloc– π deloc ∗ transition at 400–500 nm is limited in 1 and 2 indicating that the conjugation is effectively interrupted. The effect of torsional motions on the Stokes shift in these segmented copolymers is discussed. The photoluminescence (PL) efficiency and lifetime of 1 and 2 is different, indicating that the contribution from additional non-radiative decay channels due to interchain interactions is significant. The steady state and time-resolved photophysical characteristics of the thin film form of the copolymers indicates a high order of stacking in copolymer 1 and strong interchain interaction leading to strong excimer emission which may be ascribed to the low folding degree and planarity of the copolymers chemical structure. The spectroscopic time-resolved analysis confirms the interchain interaction.

Heterojunction effect on the charge injection and transport

The charge injection and transport across an organic heteroinjection of is investigated. The heteroinjection is formed by a poly-p-phenylene vinylene derivative (MEH-PPV) hole injection layer with a poly(9,9-dioctylfluorene) (PFO) hole accepting layer on top. The electric field in the accepting layer is obtained after correcting the applied voltage for the voltage drop across the injecting layer due to the buildup of space charge. The distribution of electric field in the accepting layer has effect on the transport of charge. At high electric fields, the current across the heterojunction exhibits only a weak dependence on the field. The strong dependence at low fields can be explained by taking the increase of the Fermi level into account, which effectively modifies the barrier for charge carriers waiting for a jump across the heterojunction.