Jurjen Wildeman

Novel high efficiency copolymer laser dye in the blue wavelength region

The lasing performance and wavelength tunability of a novel high efficiency copolymer poly[(2, 5, 2‘, 5‘‐tetraoctyl)‐p‐terphenyl‐4,4‘‐ylene vinylene‐p‐phenylene vinylene], TOP‐PPV, pumped with third harmonic radiation of a Nd:YAG laser, was studied in various organic solvents. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP‐PPV copolymer in hexane exceeds that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm (Δλ=38 nm).

Conjugated block copolymers for light-emitting diodes

In this communication we report results obtained in our laboratory on the design, the synthesis, and the characterization of new light-emitting copolymers. We are interested in block copolymers containing well defined conjugated sequences, which give rise to luminescence. Those chromophores, which are (pi) -conjugated oligomers like oligo(phenylenevinylene)s, are linked to each other by non (pi) -conjugated oligomers, for example oligo(silanylene)s, or directly in a non-coplanar way. In such systems color tuning of the luminescence is achieved through precise control of the length of the conjugated blocks and by changing the chemical structure of those blocks. Variations of the chemical structures of both the luminescent and interrupting blocks also give control over properties like flexibility and solubility of the polymers, which are relevant for device fabrication and optimization.

Sexithiophene-C60 blends as model systems for photovoltaic devices

Large photovoltaic responses have been recently observed in devices based on donor-acceptor blends. Their enhanced performance, which relies on the formation of a bicontinuous network of internal heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion range. Photovoltaic cells based on the blends are demonstrated.

Hysteresis-free electron currents in poly(p-phenylene vinylene) derivatives

The interpretation of electron currents in conjugated polymers is strongly hindered by the occurrence of hysteresis. We investigate the transport of electrons in electron-only devices based on derivatives of poly(p-phenylene vinylene) (PPV) for various hole-blocking bottom electrodes as well as purification of the polymer. The use of a variety of hole blocking bottom contacts, as metallic electrodes and n-type doped polymers, did not give any improvement in the observed hysteresis. By purification of the PPV, hysteresis free electron-only currents can be obtained. The deep traps responsible for hysteresis, with a concentration in the 10 16 cm -3 range, are not responsible for the trap-limited electron transport as observed in purified PPV.

Retention Time and Depolarization in Organic Nonvolatile Memories Based on Ferroelectric Semiconductor Phase-Separated Blends

Resistive switches have been fabricated using a phase-separated blend film of ferroelectric random copolymer poly(vinylidene fluoride-co-trifluoroethylene) with the organic semiconductor regio-irregular poly(3-hexylthiophene) (rir-P3HT). Spin-coated blend films have been contacted with symmetrical Ag top and Ag bottom electrodes, yielding switching diodes. The ferroelectric polarization modulates the injection barrier, yielding an injection-limited off-state and a space-charge-limited on -state. To study the effect of depolarization, an additional polyphenylenevinylene-type semiconductor layer with the highest occupied molecular orbital energy that is comparable to that of rir-P3HT has been inserted in the diode stack. When the ad-layer is the injecting contact, the current modulation ratio goes to unity. The origin is a decrease in the effective band bending at the contact with increasing ad-layer thickness. When the counter electrode at the blend interface is the injecting contact, the diode can be switched, but the on-state is only stable when an electric field that is larger than the coercive field is applied. Upon field removal, the ferroelectric depolarizes, and the current drops to that of an unpoled pristine diode. The depolarization is confirmed by capacitance-voltage and retention time measurements. To realize bistable diodes with excellent retention times, the thickness of the semiconducting wetting layer may not be at most 10 nm.

LEDs based on conjugated PPV block copolymers

A way to control the bandgap in semi-conducting polymers is by preparing polymers with a partially conjugated backbone. In our laboratory, three conjugated copolymers containing PPV trimers as light emitting chromophores have been synthesized, which emit in the blue, green and orange wavelength region. The copolymers have a well defined conjugated backbone consisting of regularly alternating terphenylene and p-phenylene-vinylene blocks. The desired control of conjugation length is achieved through steric interaction induced by the side-chains in the terphenyl blocks of the copolymer. In this paper we evaluate the electrical and optical properties of LED devices based on these conjugated PPV block copolymers.

Electric-Field-Induced Fluorescence Quenching in Polyfluorene, Ladder-Type Polymers, and MEH-PPV Evidence for Field Effects on Internal Conversion Rates in the Low Concentration Limit

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

Substituent effects on the excited states of phenyl-capped phenylene vinylene tetramers

The singlet and triplet excited states of phenyl-capped tetramers of phenylene vinylene with different alkyl, alkoxy or cyano substituents, were investigated in benzene solution. The lowest singlet states were studied by laser flash-photolysis with time-resolved microwave conductivity and fluorescence. The triplets were generated by pulse radiolysis, via energy transfer from the solvent excited singlet followed by inter-system crossing or by energy transfer from the naphthalene triplet. Their absorption spectra and rates of quenching by oxygen were determined.

Improved Synthesis of 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, a Key Intermediate for Drimane-related Sesquiterpenes.

Abstract The titel compound is conveniently prepared in 65% overall yield by a two step synthesis starting from the commercially available β-ionone.

The Effects of Side-Chain-Induced Disorder on the Emission Spectra and Quantum Yields of Oligothiophene Nanoaggregates: A Combined Experimental and MD-TDDFT Study

Oligomeric thiophenes are commonly used components in organic electronics and solar cells. These molecules stack and/or aggregate readily under the processing conditions used to form thin films for these applications, significantly altering their optical and charge-transport properties. To determine how these effects depend on the substitution pattern of the thiophene main chains, nanoaggregates of three sexithiophene oligomers having different alkyl substitution patterns were formed using solvent-poisoning techniques and studied using steady-state and time-resolved emission spectroscopy. The results indicate the substantial role played by the side-chain substituents in determining the emissive properties of these species. Both the measured spectral changes and their dependence on substitution are well-modeled by combined quantum chemistry and molecular dynamics simulations. The simulations connect the side-chain-induced disorder, which determines the favorable chain-packing configurations within the aggregates, with their measured electronic spectra.