Phenylenevinylene Oligomers Research Articles

Laser gain and excited-state absorption measurements in phenylene-vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene)

The pump–probe technique, was used to measure the effective stimulated emission and excited-state absorption cross-sections of phenylene—vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene) dissolved in toluene for different probe wavelengths. The effective stimulated emission cross-section corresponding to the S 1→S 0 transition was found to be positive. The possible laser tuning ranges have been determined. Time-resolved excited-state absorption permitted simultaneous measurements of the excited-state absorption cross section and the lifetime of the lowest triplet state T 1 and was found to be of the order of 11 ms.

Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer.

Electrical and Optical Properties of Conducting Polymer/Porous Silicon Structures

Electrical and optical properties of diode structures based on porous silicon (PS) and thin films of phenylene vinylene oligomer (PVO) have been studied. Steady-state photoluminescence spectroscopy show that the structure of the luminescence band depends on the PS morphology. We assign the observed effect to the morphology-dependent penetration of PVO material into the pores. Current-voltage characteristics of the PVO/PS diodes are studied and interpreted assuming Schottky emission and hopping transport of carriers aspossible mechanisms of d.c. electrical conduction.

Crown ether substituted phenylenevinylene oligomers: Synthesis and electroluminescent properties

Novel phenylenevinylene oligomers substituted with crown ether rings (CE-OPV) were synthesized using a Wittig or Horner–Wadsworth–Emmons reaction. The resulting oligomers give strong fluorescence emission at 515 nm with a quantum efficiency of ca. 80% in MeCN. While light-emitting diodes (LEDs) with the structure ITO/CE-OPV/Al show similar emission to the photoluminescence at a relatively high turn-on voltage of ca. 9 V/100 nm, the corresponding light-emitting electrochemical cells (LECs) with the structure ITO/CE-OPV+LiCF3SO3/Al emit bluish-green light at a rather low turn-on voltage of ca. 3 V for both the forward and reverse bias. These results indicate that the chemically-grafted crown ether rings assist ionic conductivity, and thus the CE-OPV is of promise for LEC applications.

Supramolecular Materials from Triblock Rodcoil Molecules Containing Phenylene Vinylene

We report here on the synthesis and characterization of several triblock rodcoil molecules containing conformationally rigid and flexible sequences and luminescent chromophores based on phenylene vinylene. The molecules consist of blocks of oligostyrene, oligoisoprene, and a rigid rod block. The synthesis of the rod blocks required the preparation of new phenylene vinylene oligomers. On the basis of electron microscopy and X-ray diffraction experiments, these systems were found to self-organize into supramolecular nanostructures which in turn assemble into monolayers. The nanostructures have diameters between 3.5 and 5 nm, and the regularity of the nanostructures depends on rod-to-coil volume fraction. The layer spacing of one system is 6.0 nm when cast from solution but increases to 7.8 nm upon annealing at 150 °C, possibly as a result of an increase in aggregation number. Interestingly, the formation of supramolecular nanostructures and the observed hierarchical order in the materials formed by rodcoil ...

On the nature of electronic excitations in poly(paraphenylenevinylene): A quantum-chemical investigation

Correlated quantum-chemical calculations are performed on phenylenevinylene oligomers containing up to eleven repeat units, to characterize the nature of the electronic excitations relevant for the photophysical properties of the corresponding polymer. The focus is first on the nonlinear optical response of model conjugated chains and the simulation of their frequency-dependent (third-harmonic generation, electroabsorption, and two-photon absorption) response. From the assignment of the calculated resonance features, the excited states dominating the third-order nonlinear polarizability are identified and their chain-length dependence is investigated. On that basis, we build an essential-state single-chain model (that includes the 1Bu, 2Ag, mAg, and nBu states) and apply it to the interpretation of recent experimental data reported for poly(paraphenylenevinylene) and derivatives. We then examine how the exciton binding energy, here defined as the difference between the energies of the charge-separated nBu and the strongly optically allowed 1Bu excited states, is affected by both intrachain and interchain polarization effects.

Effective stimulated emission and excited state absorption measurements in the phenylene–vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene))

The pump probe technique was used to measure the effective stimulated emission and excited state absorption cross-sections of the phenylene–vinylene oligomer (1,4-bis-(alpha-cyanostyryl)-2,5-dimethoxybenzene) dissolved in toluene for different probe wavelengths. The effective stimulated emission cross-section was calculated from σ SE-eff= σ SE− σ ESA and found to be positive. The possible laser tuning ranges have been determined. The spectral and temporal behavior of the optical transitions were investigated. The dynamics of these optical transitions permitted direct measurements of the lifetime of the lowest triplet state, and was found to be of the order of 11 ms.

Molecular Packing in Unsubstituted Semiconducting Phenylenevinylene Oligomer and Polymer

p-Bis(p-styrylstyryl)benzene, C38H30, a five-ring phenylene−vinylene model compound for unsubstituted PPV, has been obtained in single-crystal form and the crystal structure has been resolved. The ...

Electrical properties of a phenylene vinylene oligomer derivative

Abstract The electrical properties of 4,4′-bis(4-Dimethyl Amino Styryl) benzene (DMASB) were investigated by studying the current-voltage temperature characteristics of metal-DMASB-metal structures. Rectifying behavior of the devices occurred at an applied field higher than 10 7 Vm −1 which depends on the nature of the electrode metal. At high temperature, a thermally activated process was observed and was related to the Poole-Frenkel effect while at low temperature a hopping conduction between localized states likely occurs.

Electrical properties of a phenylene vinylene oligomer

Abstract The electrical properties of 4,4′-bis(4-styryl-styryl) benzene oligomer based diodes were investigated by d.c and a.c measurements. At high field, the charge carriers are injected through potential barriers at the interface and the transport occurs via a field assisted thermal ionization process in the bulk of the oligomer. The influence of the metal electrode revealed in the current-voltage characteristics as well as in the dielectric data suggests that there might be interactions between the metal and the oligomer.

Optical spectroscopic investigations of phenylene vinylene oligomers

A series of phenylene vinylene oligomers containing from 1 to 6 phenylene rings was studied by absorption, Raman and photoluminescence spectroscopies. The results are discussed by examining the influence of chain length on the spectra and compared to those of PPV films obtained from the Wessling precursor route.

Singlet Oxygen as a Reactive Intermediate in the Photodegradation of Phenylenevinylene Oligomers

Singlet molecular oxygen (a1Δg) is shown to be the principal reactive intermediate in the photoinduced oxygen-dependent decomposition of a series of phenylenevinylene oligomers. The reaction rate between singlet oxygen and the oligomer decreases (1) with a decrease in the extent of phenylenevinylene conjugation (i.e., oligomer chain length) and (2) upon the incorporation of electron-withdrawing substituents on the oligomer. The reaction rate, however, does not appear to depend solely on the electron density of the oligomer π system, as calculated using ab initio methods. For oligomers with vinyl-substituted cyano group(s), the reaction with singlet oxygen is very slow, thus making such compounds good candidates for the production of stable electroluminescent materials.

Electroluminescent Behavior of a Homologous Series of Phenylenevinylene Oligomers

Finally, the film emission spectra of both the DEHF/PER and the DEHF/CNSt copolymers are unaffected bythermal annealing at 200˚C for 48 h, suggesting that noemissive excimer is formed in the annealed copolymers.The dramatic effect of annealing on the DEHF homopoly-mer is shown in Figure 3b. Although excimer formation isless pervasive in the branched DEHF derivative relative tothe di-n-hexylfluorene (DHF) homopolymer, a broad andunstructured longer-wavelength excimer emission (500–550 nm) that increases with annealing time and tempera-ture is observed.In summary, the synthesis of thermally stable, easily pro-cessable random copolymers of 9,9-di-2¢-ethylhexylfluor-ene and either perylene or a-cyanostilbene can be accom-plished by nickel-mediated coupling reaction. Here, thebranched DEHF units provide adequate solubility whenunsubstituted perylene and a-cyanostilbene units are incor-porated into the conjugated main chain. We have demon-strated that formation of excimers in thin films of poly-(fluorene) derivatives upon thermal annealing can be sup-pressed by incorporation of lower-bandgap chromophoricsegments, which serve as efficient excitonic energy trapsupon excitation of the poly(fluorene) chromophoric seg-ments. This study provides indirect evidence of the possiblerole of the anthracene subunits in DHF/ANT copolymersin suppressing excimer formation.

Molecular-wire behaviour in p -phenylenevinylene oligomers

Electron transfer from electron-donor to electron-acceptor molecules via a molecular ‘bridge’ is a feature of many biological andchemical systems. The electronic structure of the bridge component in donor–bridge–acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer1,2. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor–acceptor distance — effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires3,4,5,6. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p -phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor–acceptor separations as large as 40 A, with rate constants of the order of 1011 s−1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.

Quantum mechanical calculations of electronic-conformational defects in phenylene vinylene oligomers

We investigate the quasiparticle conformational defects (polarons, bipolarons and excitons) in the phenylene vinylene oligomers (PVOs). The most probable conformations are determined by means of the minimization of the total Hartree-Fock (HF) energy calculated at the 3–21G level. The Hartree-Fock calculations of the excited states are followed by the Cl-Singles calculation. On the basis of these results the infra-red (IR) vibrational transitions are calculated. The theoretical transitions are compared with the IR vibrational spectrum of the poly(phenylene vinylene) (PPV).

Conformational properties of quasi-particles in poly(p-phenylene vinylene)

We investigate the quasi-particle conformational defects (excitons, polarons and bipolarons) in phenylene vinylene oligomers (PVOs) consisting of up to 12 repeat units. The conformations are determined by minimisation of the total Hartree–Fock energy calculated at 3-21G level. The Hartree–Fock calculations of the excited states are followed by the CI–Singles calculation. The bond length alternation along the oligomer chain is discussed. On the basis of these results the vibrational transitions are calculated. To correct the basis set truncation error, the vibrational frequencies are multiplied by a uniform scaling factor. The types of vibrations are assigned. The theoretical spectra are in good agreement with the experimental infrared vibrational spectrum of poly(p-phenylene vinylene) (PPV). © 1998 John Wiley & Sons, Ltd.

Electronic structure of the lowest singlet and triplet excited states in cyano-substituted oligo(phenylene vinglene)s

The lowest singlet and triplet excited states in cyano-substituted phenylene vinylene oligomers are characterized by means of configuration interaction calculations. First, the vertical singlet-singlet, S 0 → S 1, singlet-triplet, S 0 → T 1, and triplet-triplet T 1 → T n , excitation energies are evaluated in oligomers ranging in size from two to five phenylene rings; the spatial extent of the S 1, T 1, and T n excited states is estimated on the basis of a simple analysis of their wavefunctions. We then pay attention to the lattice distortions taking place in the lowest singlet and triplet excited states of these model oligomers. In each case, the results are compared to those obtained for the corresponding unsubstituted oligo(phenylene vinylene)s. Besides the bathochronic shift associated with the electroactive character of the substituents, an overall localization of the excited state wavefunction is found when going from unsubstituted to cyano-substituted oligomers.

CONFORMATIONAL PROPERTIES OF POLY(P-PHENYLENE VINYLENE)

We investigate the quasiparticle conformational defects (excitons, polarons and bioplarons) in the phenylene vinylene oligomers. The conformations are determined by means of the minimization of the total Hartree-Fock energy calculated at 3–21G level. The infrared vibrational transitions are calculated and the types of vibrations are assigned. The theoretical spectra are in good agreement with the experimental vibrational spectrum of poly(p-phenylene vinylene).

Characterization of 6PV phenylene vinylene oligomer thin films

Investigations of phenylene vinylene oligomer thin films with 7 phenyl, 6 vinyl units (6PV) have been performed by optical and electrical measurements. The optical characterizations including infrared, Raman and UV-VIS spectroscopies were discussed by comparing the results with those obtained in poly( p-phenylene vinylene) (PPV). The electrical properties of oligomer based diodes were studied by measuring the current-voltage-temperature characteristics and the thermally stimulated currents (TSC). At high fields, tunneling of carriers through the interfacial barrier would occur while at lower fields, two distinct temperature ranges were observed. In the high temperature range (> 250 K), a field assisted mechanism occurs involving localized states in the oligomer bulk whereas in the low temperature range, hopping conduction is probable. The TSC spectrum shows two relaxation peaks at 219 and 261 K which correspond to trap depths of 0.4 and 0.55 eV. The deep traps appeared as a characteristic of phenylenevinylene oligomers.

Modified spontaneous emission in oligo ( p-phenylene vinylene) planar microcavities

This paper reports on the photoluminescence properties of phenylene vinylene oligomers films (OPV) the emission of which was modified by microcavity effect. Films with 4 and 6 vinyl groups (respectively denoted OPV-4 and OPV-6) were investigated. A strong blue luminescence at 470 nm is observed for OPV-4. OPV-6 shows an emission spectrum quite similar to that of poly phenylene vinylene (PPV). Photoluminescence excitation measurements (PLE) give similar behaviors for the two samples and do not show any change in the shape of the spectra. Ageing under UV irradiation (380 nm) of the unencapsulated OPV-4 layers was evidenced by an exponential decrease of the photoluminescence intensity of about 40% after 3 h a continuous illumination. OPV-6 samples, aged on shelf, for weeks does only show a small decrease (15%) of the PL level over more than 7 h under continuous UV irradiation. Microcavity related effects were observed by inserting the films between two mirrors. These are generally a TiO 2/SiO 2 distributed Bragg reflectors (DBR) as bottom mirror and a Ag film as top mirror. However best results were obtained with two dielectric DBR. As expected, a strong enhancement of the emission at the cavity resonance and pronounced angular effects (emission peaking in the direction normal to the surface of the device) were observed. The importance of the position of the emitting material within the microcavity is evidenced with a thin tris(8-hydroxy) quinoline aluminum (Alq) embedded in SiO 2 with various cavity configurations.