Carbazole Oligomers Research Articles

(Invited) Synthesis and Optical Properties of Circularly Accumulated Carbazole Oligomers

Carbazole oligomers were synthesized to elucidate the physical properties arising from the interaction between circularly accumulated carbazoles. Aromatic nucleophilic substitution reactions were used to synthesize the target compounds, in which carbazoles were substituted for benzene, naphthalene, and anthracene. Structure-property relationships were evaluated by single-crystal structure analysis, theoretical calculations, and optical measurements including ultrafast time-resolved spectroscopy.

Organic Heterostructures with Indium‐Free Transparent Conductor Electrode for Optoelectronic Applications

Herein, the properties of the organic heterostructures with triple‐layer ZnO/Ag/ZnO as a replacement for ITO and mixed layer containing arylenevinylene oligomer (based on triphenylamine or carbazole) donor and nonfullerene (perylene diimide) acceptor mixed in the ratio 1:2 and the effect of a buffer layer of PEDOT‐PSS intercalated between triple layer and mixed organic layer are discussed. The UV−vis transmission and photoluminescence (PL) properties are investigated in correlation with the surface topography and reveal a good match between the absorption and emission domain, which can favor the generation of the charge carriers. The heterostructure with the mixed layer based on triphenylamine oligomer shows the widest absorption domain, and the PL spectra of the heterostructures realized with either triphenylamine or carbazole oligomer show peaks corresponding to the radiative decay of the donor and acceptor. The I−V characteristics in the dark indicate a slightly nonlinear behavior and the current is affected by the charge carriers recombination on the defects present in the thick mixed layer deposited by matrix‐assisted pulsed laser evaporation. The effect of the PEDOT‐PSS buffer layer on the electrical properties of the organic heterostructure with ZnO/Ag/ZnO electrode is also investigated.

Synthesis and fluorescence properties of butadiyne-linked linear and cyclic carbazole oligomers.

Butadiyne-linked linear and cyclic carbazole oligomers were successfully synthesized. The intensity of the emission band in the 0–0 band of the highly planar macrocyclics decreased compared to that of the 0–1 band. Contrary to this, for larger macrocycles having reduced planarity, the intensity of the emission of the 0–0 band increased as in the cases of the linear compounds. This suggests that the emission color of the π-conjugated molecule can be controlled not only by the difference between the cyclic and chain structures but also by the control of the planarity, and is expected to be a new principle for molecular design in the development of fluorescent materials.

Interpenetrated Binary Supramolecular Nanofibers for Sensitive Fluorescence Detection of Six Classes of Explosives.

In this work, we develop a sequential self-assembly approach to fabricate interpenetrated binary supramolecular nanofibers consisting of carbazole oligomer 1-cobalt(II) (1-Co2+) coordination nanofibers and oligomer 2 nanofibers for the sensitive detection of six classes of explosives. When exposed to peroxide explosives (e.g., H2O2), Co2+ in 1-Co2+ coordination nanofibers can be reduced to Co+ that can transfer an electron to the excited 2 nanofibers and thereby quench their fluorescence. On the other hand, when exposed to the other five classes of explosives, the excited 2 nanofibers can transfer an electron to explosives to quench their fluorescence. On the basis of the distinct fluorescence quenching mechanisms, six classes of explosives can be sensitively detected. Herein, we provide a new strategy to design broad-band fluorescence sensors for a rich identification of threats.

Single-molecule rectifiers based on voltage-dependent deformation of molecular orbitals in carbazole oligomers.

Current-voltage characteristics of single molecule junctions are governed both by the energy level alignment of molecular orbitals with respect to the Fermi level of the electrodes and by the hybridization of electronic structures at the interface between the molecule and the electrodes. While there have been many studies on tuning the former, only a few works intended to control the latter. In the present study, we demonstrate that molecular junctions based on carbazole oligomers showed a current rectification behavior due to asymmetric-symmetric control of electronic hybridization between the molecule and electrodes at the both terminals. The carbazole oligomers originally showed an asymmetric molecular orbital and, hence, electronic hybridization with the electrodes because of the electric dipole moment. Symmetric electronic hybridization was achieved when the applied electric field between electrodes deformed molecular orbital to be symmetric. This is a novel way to control charge transport in single-molecule junctions.

Synthesis and characterizations of carbazole–isoindigo–carbazole oligomers for photovoltaic application

6,6′-Dibromo-N,N′-(2-octyldodecanyl)isoindigo was coupled to either 4-(N-carbazolyl)phenyl boronic acid or 9-phenyl-9H-carbazole-3-boronic acid to give two different conjugated molecules of P-II-P and C-II-C, respectively. The optical band gap/HOMO levels of P-II-P and C-II-C were 1.90/−5.60eV and 1.85/−5.39eV, respectively. Based on the experimental and calculation data, C-II-C is more highly conjugated than P-II-P. The performances of C-II-C-based photovoltaic devices were better than those of P-II-P-based devices, with the best power conversion efficiency of 0.44%.

Morphology and dispersion of polycarbazole wrapped carbon nanotubes

Using MM3 force-field simulations in combination with optical measurements, we investigate the morphology and dispersion of a variety of single-walled carbon nanotubes (SWNTs) non-covalently functionalized by carbazole polymers. Our results elucidate that isomer types of poly-carbazoles together with their length govern the morphology of carbazole–SWNT hybrids and, hence, their dispersion and bundling properties. The π–π stacking between the carbazole and the SWNT results in a stable carbazole–SWNT hybrid complex with the SWNT–carbazole interaction increasing with a decrease in the polymer length. All oligocarbazoles preferentially bind to larger diameter SWNTs with the strongest interactions observed for the hybrids with 2,7-isomers. While short oligocarbazoles cover only one side of the tube, long 2,7-isomers tend to gradually coil around the entire tube. In contrast, 3,6- and 2,7–3,6-oligocarbazoles irregularly bunch together on a small area of the tube. The small size of carbazole oligomers and their localization on one side of the nanotube prevents tube unbundling, and leads to photoluminescence quenching in SWNTs functionalized by 2,7–3,6-oligocarbazoles.

Quantum Chemical Study of the Ground and Excited State Electronic Structures of Carbazole Oligomers with and without Triarylborane Substitutes

Recent experimental investigation (Reitzenstein and Lambert,Macromolecules, 2009, 42, 773) indicated that the quite different optical properties of 2,7- and 3,6-linkage triarylboryl carbazole oligomers may arise from the different nature of their low-lying excited states: a low-lying delocalized within-backbone excitation in longer 2,7-linked oligomers vs a backbone-to-sidechain charge-transfer (CT) excitation independent of the polymerization length in 3,6-linked oligomers. Here in this paper, two long-range corrected functionals, CAM-B3LYP and ωB97X, are applied together with the traditional B3LYP functional in time-dependent density functional theory (TDDFT) calculations to systematically investigate the low-lying electronic excitations in both oligomers. Our calculations indicate that an extensive conjugation exists between monomer molecular orbitals in 2,7-linked oligomers, which is absent in those of 3,6-linked structures, resulting in a considerable narrowing of the HOMO-LUMO gap of their backbone moiety, while having little effect on the side-chains. CAM-B3LYP and ωB97x calculations confirm that the lowest-energy absorption is a within-backbone excitation in longer 2,7-linked oligomers as opposed to a backbone to side-chain charge transfer excitation in 2,7-linked oligmers of shorter length and 3,6-linked oligomers of any length. All these findings are consistent with the experimental findings and the qualitative energy diagram proposed by Reitzenstein and Lambert.

Carbazole oligomers revisited: new additions at the carbazole 1- and 8-positions

In recent years low molecular weight carbazole-based materials have attracted much interest as hole-transporting and host materials for organic light emitting diode (OLED) applications. Herein we report the tri- and tetra-bromination of carbazole and the subsequent synthesis of novel carbazole oligomers substituted at the 1- and 8-positions. The oligomer series range from mono-carbazole (Cz-1) to quinque-carbazole (Cz-5) and we have carried out a comparative study of their thermal, optical, electrochemical and optoelectronic properties. The glass transition temperature increases as the carbazole core is enlarged (0 °C to 54 °C) and all oligomers exhibit excellent thermal stability with high thermal decomposition temperatures in the range of 442 °C to 494 °C. From optical data and Raman spectroscopy we observe negligible changes in the effective conjugation length as the number of peripheral carbazole units increases. However, the oxidation potential decreases as the number of carbazole units increase, in which Cz-5 exhibits the highest HOMO level (−5.19 eV) compared to other carbazole oligomers in the series. All carbazole materials emit deep blue photoluminescence ranging from 416 to 432 nm in the solid-state. Finally, electrophosphorescence was demonstrated from solution processable guest-host organic light- emitting diodes (OLEDs) fabricated using Cz(2–5) as the host materials with device configuration ITO/PEDOT/Cz1–5 : 10% Ir(mppy)3/TPBi/LiF/Al. The device efficiency decreases from Cz-3 to Cz-5. The best OLED device that incorporates Cz-3 as the host exhibits a turn-on voltage of 2.8 V, maximum brightness of 2310 cd m−2, maximum power efficiency of 33.6 lm W−1, maximum current efficiency 35.5 cd A−1 and EQE of 10.6%.

Synthesis and Characterization of 2,7‐Linked Carbazole Oligomers

A set of monodisperse 2,7-linked carbazole oligomers (3-mer, 5-mer, 7-mer, and 9-mer) was synthesized, and their photophysical, electrochemical, and thermal properties were investigated. In solutions, these oligomers exhibited bright blue emission with almost quantitative fluorescence quantum yield. The emission spectra of these oligomers in films are quite different. 3-Mer and 5-mer exhibited featureless emission spectra, whereas 7-mer and 9-mer showed well-resolved emission spectra.

Synthesis and photophysical properties of tetrafluorophenyl-modified carbazole oligomers

Abstract A series of tetrafluorophenyl-modified oligo(3,6-carbazole ethynylene)s (CfCzCf, CfCz2Cf, CfCz3Cf, (CfCz)2Cf, and Cf2CzCf2) were synthesized by a Pd/Cu-catalyzed Sonogashira coupling reaction and fully characterized. Their photophysical and electronic properties as well as their thermal stabilities were investigated. It is noteworthy that these tetrafluorophenyl modified compounds showed relative thermal stabilities and low-lying HOMO levels ranging from −5.54 to −5.60 eV, which might be the promising candidates for hole-transporting materials in OLEDs.

White organic light-emitting diodes based on electroplex from polyvinyl carbazole and carbazole oligomers blends

White organic light-emitting diodes with a blue emitting material fluorene-centred ethylene-liked carbazole oligomer (Cz6F) doped into polyvinyl carbazole (PVK) as the single light-emitting layer are reported. The optical properties of Cz6F, PVK, and PVK:Cz6F blends are studied. Single and double layer devices are fabricated by using PVK: Cz6F blends, and the device with the configuration of indium tin oxide (ITO)/PVK:Cz6F/tris(8-hydroxyquinolinate)aluminium (Alq3)/LiF/A1 exhibits white light emission with Commission Internationale de l'Éclairage chromaticity coordinates of (0.30, 0.33) and a brightness of 402 cd/m2. The investigation reveals that the white light is composed of a blue–green emission originating from the excimer of Cz6F molecules and a red emission from an electroplex from the PVK:Cz6F blend films.

Zigzag Molecules from Pyrene-Modified Carbazole Oligomers: Synthesis, Characterization, and Application in OLEDs

A series of monodisperse, pyrene-modified oligocarbazoles were synthesized and fully characterized. Carbazoles were linked by ethynylene through the 3- and 6-positions, forming a zigzag molecular backbone and stable, size-independent absorption, and emission were observed from these oligomers in solutions because their conjugation length was confined. Oligomers with pyrene exhibited much higher fluorescence quantum yields than those of oligomers without it. Different positions of pyrene in the oligocarbazole main chain resulted in a remarkable change in absorption and emission spectra. Moreover, devices with four different architectures (types 1-4) based on these oligomers were fabricated and investigated. Pyrene-modified carbazole oligomers exhibited bifunctional properties (light-emitting and hole-transport). Moreover, these devices showed moderate performances; for example,the device based on Cz3PyCz3 presented an external quantum efficiency of 0.69% and a maximum brightness of 1782 cd/m2 at 13.5 V, which indicated these oligomers were promising optoelectric materials.

Fluorene-Centered, Ethynylene-Linked Carbazole Oligomers: Synthesis, Photoluminescence, and Electroluminescence

Two monodisperse fluorene-centered, ethynylene-linked carbazole oligomers (Cz4F and Cz6F) were synthesized and fully characterized. They were highly fluorescent and emitted bright blue in solutions and in films. Single- and double-layer devices based on them were fabricated to investigate their electroluminescence (EL) as well as their hole-transport properties. Cz4F, doped with PVK, in a double-layer device exhibited a rather broad emission spectrum with a maximum brightness of 926.6 cd m-2 at 17.0 V, which indicated that it might have potential use in white organic light-emitting diodes (WOLEDs). Moreover, Cz6F exhibited a bifunctional property, blue- or white-light-emitting property, and hole-transport ability, which qualified it as a light-emitter as well as hole-transport material.

P and n dopable semi-interpenetrating polymer networks

Electrochemical oxidation of thin films of 9-(2,3-epoxypropyl)carbazole (OEPC) oligomer deposited on an electrode leads to the formation of an in-situ cross-linked structure with biscarbazyl redox sites due to the anodic coupling of pendant carbazole units. Electrochemical, optical, and thermal characterizations confirm the formation of a conducting network containing electroactive biscarbazolyl units. When the electrochemical cross-linking takes place in a starting thin film containing a mixture of OEPC and a perylene bisimide polymer [poly(Pery-EO3)], a semi-interpenetrating polymer network is formed which possesses both electron donor and electron acceptor properties. Indeed, the electrochemical behavior of the semi-interpenetrating polymers network (IPN) is characterized by two reversible oxidations and two reversible reduction waves corresponding to the biscarbazolyl and perylene redox moities, respectively. The energy levels of the highest occupied molecular orbital E HOMO and lowest unoccupied molecular orbital E LUMO of each donor and acceptor in the semi-IPN have been evaluated and indicate a possible electron transfer from biscarbazole to perylene, which could be taken advantage of in the field of solar cells application.

Carbazole oligomers and the dimer-derived DPPH-type diradical

Carbazole dimer and trimer were prepared by Stille coupling reaction. These oligomers showed UV-absorption around 310 nm. Electrochemical studies indicate that both oligomers have lower oxidation potentials than carbazole. A diphenylpicrylhydrazyl (DPPH)-type diradical was derived from the dimer for further functionalzation.

Effect of the polydispersity on electrochemical properties in solution of poly( N-alkylcarbazole)s

Poly( N-alkylcarbazole) synthesized from the corresponding 3,6 dihalide monomer using zerovalent nickel as a catalyst has an average value for its polydispersity of ca. 2. The electrochemical behaviour in solution is characterized by as many signals as weight populations in the crude sample. We have prepared, by selective solubilization in acetone, various samples having a low polydispersity (index lower than 1.4) and studied their electrochemical properties when these compounds are solubilized in dichloromethane. We observed that the electrochemical behaviour of these samples with low polydispersity is much better defined than the crude samples but less well-defined than the only carbazole oligomer having a well-defined structure easily available, i.e. the 9,9′-diethyl 3,3′ dicarbazyle. These results allow the study of the variation of the apparent diffusion coefficient versus the average molecular weight M n. Finally, we show that the decrease of electrochemical activity of poly( N-alkylcarbazole) having a low polydispersity deposited as a thin layer on platinum as a function of the number of electrochemical cycles is less pronounced compared with a film of the crude product. Furthermore, this decrease is also less pronounced with higher molecular weight fractions. Thus, a decrease in the polydispersity as well as an increase in the molecular weight of the polymer may be helpful in the improvement of the life time of electrochromic devices.

The photoluminescent and electroluminescent properties of cyclic carbazole oligomers

The emission properties of cyclic carbazole oligomers are described. The photoluminescent spectra of these cyclic oligomers excited by 350 nm as an excitation wavelength in chloroform have been measured. We found that the emission properties of o-xylene-derived cyclic dimer were different from those of p- and/or m-xylene-derived cyclic dimers although they have the similar molar absorption coefficients and structures. The electroluminescent properties are also described.

Electroluminescent applications of a cyclic carbazole oligomer

S. Maruyama, X. Tao, H. Hokari, T. Noh, Y. Zhang, T. Wada, H. Suzuki, T. Watanabe and S. Miyata, J. Mater. Chem., 1999, 9, 893 DOI: 10.1039/A809313J

Convenient syntheses of cyclic carbazole oligomers by 1-pot Knoevenagel reaction

Syntheses of cyclic carbazole oligomers by a 1-pot Knoevenagel reaction without using high dilution methods are described. Cyclic carbazole oligomers of several ring sizes could be obtained in high yields when 3,6-diformyl-9-docosylcarbazole and appropriate aromatic compounds having a bis(cyanoacetate) group were reacted in THF using DMAP as a base at 40 °C.