D. Ades

The role of carbazole in organic light-emitting devices

Abstract New organic oligomers and polymers based on the carbazole molecule are explored for possible applications in light-emitting devices. In one case, (butyl- or octyl-) carbazole dimers and poly (N-butyl-3,6-carbazolylene) polymer were used as the hole-transporting and light-emitting layer in multilayer light-emitting diodes (LEDs). These devices yielded bright blue light (as much as about 6000 cd m−2) with high external quantum (about 10%) and luminance efficiencies (about 2 1m W−1). The other case involved ([3-octylthiophene]-[bis-(N-ethyl or octyl carbazolylene)]) multiblock copolymers as the active emitting layer in single-layer LEDs. Color tuning was achieved in these devices by changing the number of monomer units contained in the thiophene chain. We also observed an increase of the external quantum efficiency in diodes based on the copolymers with short thiophene segments that we attributed to a more balanced charge injection.

Chain length effect on the electroactivity of poly(N-alkyl-3,6-carbazolediyl) thin films

Abstract Several poly(N-alkyl-3,6-carbazolediyl)s (PCZS) have been obtained by electroreduction of the corresponding dibromomonomers in the presence of a catalytic Ni(0)-based system. Depending on the nature of the alkyl group linked to the nitrogen atom, PCZS are soluble in organic solvents such as nitrobenzene, chloroform and tetrahydrofuran and insoluble in acetonitrile and alcohols. Taking these properties into account thin films have been prepared (in the neutral state) either by casting from solutions or directly by electrodeposition onto conducting substrates in the CH3CN/NBu4BF4 medium. Electrochemical behavior of these materials in the range 0–1.4 V versus saturated calomel electrode (SCE) is strongly dependent on the macromolecular chain length, but is slightly affected by the nature of the alkyl chain. Depending on the potential applied, the films are either colorless, green or blue. The redox processes involved in electrochemical oxidation of the polymer as well as those of the oligomers are discussed.

Synthesis and characterization of poly (N-butyl-3,6-carbazolediyl)

SummaryPoly (N-butyl-3,6-carbazolediyl) was synthesized by electrochemcial reduction of the NiBr2, 2,2′bipyridine/3,6 dibromo N-butylcarbazole system in N,N dimethylacetamide. The favorable interactions between solvent and butyl groups lead to the solubilization of the electroactive polymer in organic medium and allowed the determination of macromolecular and structural features by means of classical methods. From 13C NMR investigations it was concluded to a Π conjugation centered on the nitrogen atom of the carbazolic structure.

Blue organic light emitting diodes based on bicarbazyle derivates: Device stability and multilayer configuration

The device stability of blue light emitting diodes based on N,N′-diethyl-3,3′-bicarbazyle, (EtCz)2, are substantially improved by coevaporation with N,N′-diethyl-3,3′-bicarbazyle-6,6′ dicarbaboxylic acid, (OcCzCOOH)2. We attribute this effect to the formation of a network of hydrogen bonds in the alloy related to the presence of carboxyl groups. A three layer device with transport layers for both electrons and holes improves the quantum efficiency: With an Al cathode we obtain a luminance of 100 cd/m2 at current density levels of 50 mA/cm2. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. The incorporation of an electron transport layer splits and shifts the electroluminescent band to lower energies and we observe a significant additional downshift after a period of three weeks which we attributed to molecular diffusion at the interface between the emitting and elec...

Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder forms

We present absorption and photoluminescence features of four samples of carbazole molecules substituted with various electron–acceptor groups. These molecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron–donor carbazole nucleus and cyanovinylene bearing either another nitrile function, an ethylester, a phenyl, or a para-nitrophenyl groups. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the samples and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowing and shift of the peak emission. We obtain further narrowing of the emission band and further redshifting of the emission when we illuminate, transversely, a glass capillary containing the crystalline sample by an ultraviolet light-emitting diode.

Investigation on the electrocatalyzed step polymerization of soluble electroactive poly(N-alkyl-3,6-carbazolylenes)

Abstract Simultaneous GPC and cyclic voltammetry (CV) measurements were carried out in the course of the electrocatalyzed polymerization of various 3,6-dibromo(N-alkylcarbazoles). The evolution of the molar mass population distribution as a function of the electrolysis time reveals a step polymerization processm while showing a termination of the chain growth due to the occurrence of monobrominated species. The consumption of the catalytic species corresponds in a first step to an increase in the chain length, while after given periods there is no more chain growth related to the continuous consumption of the catalytic precursor. The study of the effect of the initial [monomer]/[catalyst] ratio on the molar mass population evolution indicates that the coupling of Ni(0)-activated species is responsible for the chain growth.

Electrosynthesis and study of phenylene-carbazolylene copolymers

Abstract Phenylene-carbazolylene copolymers with variable composition and properties were prepared by electrocatalyzed dehalogenative polycondensation of 4,4′-dihalobiphenyl and N -alkyl-3,6-dibromocarbazole mixtures in the presence of a zero valent nickel catalyst. The polymers are partly soluble in polar solvents and this solubility depends on the proportion of carbazolylene units in the materials. For a given composition, solubility increases with length of the aliphatic substituent linked to the nitrogen. The conductivity upon doping varies between those of the corresponding homopolymers and is a function of the length of the alkyl substituents. Thin films of these materials can be prepared either by solvent casting or by direct electrodeposition onto various supports. The electrochemical behavior is strongly dependent on the copolymer composition and reveals the existence of two distinct electronic states (two quantum wells): the first one is related to the presence of the carbazolylene units, while the second shows the presence of phenylene moieties.

Comparative two-dimensional NMR analysis of poly(N-butyl-3,6-carbazolylene), poly(N-vinylcarbazole) and model compounds

Abstract The two-dimensional NMR spectrum of poly( N -butyl-3,6-carbazolylene), prepared by electrochemical synthesis, is presented and assigned on the basis of correlations between directly bonded carbons and protons and of those obtained for N -butyl-3,6-dibromocarbazolylene as a model compound. The isolated aromatic protons (numbers 4, 5) were found to be less shielded than their bonded carbons, because of the magnetic influence of the next-neighbour substitutents. The 2D-NMR spectrum of commercial poly( N -vinylcarbazole) was reinvestigated and assigned from both the edited spectrum, the 2D-NMR of N -ethylcarbazole as a model compound and assignments made for 3,6 substituted carbazolylenes. From all the results, complete assignments of the aromatic region were made. Complete splitting of both the carbons and protons was found, owing to the non-equivalence of the two benzene rings fused to the pyrrole ring.

Investigation of the electrocatalysed step polymerization of soluble poly(N-alkyl-3,6-carbazolylene)s

Abstract Simultaneous g.p.c. and cyclic voltammetry measurements were carried out in the course of the electrocatalysed polymerization of various 3,6-dibromo( N -alkylcarbazole)s. The evolution of the molar mass population distribution as a function of the electrolysis time reveals a step polymerization process, while showing a termination of the chain growth due to the occurrence of monobrominated species. The consumption of the catalytic species corresponds in a first step to an increase in the chain length, while after given periods there is no more chain growth related to the continuous consumption of the catalytic precursor. The study of the effect of the initial [monomer]/[catalyst] ratio on the molar mass population evolution indicates that the coupling of Ni(0)-activated species is responsible for the chain growth.

The anionic bulk polymerization of α-methylstyrene-II. Kinetic study of the propagation

Abstract A kinetic study of the anionic bulk polymerization of α-methylstyrene initiated with tert-butyllithium (t-BuLi) in presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) has been performed. Kinetic measurements have been carried out using dilatometry at 25° and 40° up to 37% conversion. The reactivity of the system is found to be a function of the ratio r = [TMEDA]/[t-BuLi]. The apparent rate constant decreases with increasing values of r for r 1. The results are explained by the presence of three different active species, viz. contact ion-pairs contact ion-pairs stretched solvated with a TMEDA molecule, ion-pairs stretched by nTMEDA molecules. When the temperature is raised to 62°, the active centres become unstable and cyclic oligomers are formed.