2,7-Carbazole-Based Conjugated Polymers for Blue, Green, and Red Light Emission

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.

Experimental and theoretical study of three new benzothiazole-fused carbazole derivatives

Three new D-π-A type compounds, each containing one benzothiazole ring as an electron acceptor and one N-ethylcarbazole group as electron donor, were synthesized and characterized by elemental analysis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of three compounds were experimentally determined in several solvents and were simultaneously computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated reorganization energy for hole and electron indicates that three compounds are in favor of hole transport than electron transport. The calculated absorption and emission wavelengths are well coincident with the measured data. The calculated lowest-lying absorption spectra can be mainly attributed to intramolecular charge transfer (ICT). And the calculated fluorescence spectra can be mainly described as originating from an excited state with intramolecular charge transfer (ICT) character. The results show that three compounds exhibited excellent thermal stability and high fluorescence quantum yields, indicating their potential applications as excellent optoelectronic material in optical field.

Intercalative poly(carbazole) precursor electropolymerization within hybrid nanostructured titanium oxide ultrathin films

A protocol for nanostructuring and electropolymerization of a hybrid semiconductor polycarbazole-titanium oxide ultrathin film is described. Ultrathin (<100 nm) films based on polycarbazole precursor polyelectrolytes and titanium oxide (TiOx) have been fabricated by combining the layer-by-layer (LbL) and surface sol-gel layering techniques. Film growth was followed and confirmed through UV-vis spectroscopy, ellipsometry and quartz crystal microbalance with dissipation (QCM-D). Subsequent anodic electrochemical oxidation of the carbazole pendant units afforded a conjugated polymer network (CPN) film within intercalating TiOx layers of cross-linked and π-conjugated carbazole units. Cyclic voltammetry (CV), UV-vis, and fluorescence spectroscopy measurements confirmed this process. The LbL-driven polyelectrolyte deposition process resulted in a quantified electrochemical response, proportional to the number of layers, while the TiOx acted as a dielectric spacer limiting electron transfer kinetics and attenuating energy transfer in fluorescence. Electro-optical properties were compared with other polycarbazole thin film materials with respect to bandgap energy (Eg). The straightforward protocol in film nanostructuring and barrier/dielectric properties of the inorganic oxide slab (denoted here as, TiOx) should enable applications in organic light-emitting diodes (OLEDs), dielectric mirrors, planar waveguides, and photovoltaic devices for these hybrid ultrathin films.

Solar-energy production and energy-efficient lighting: photovoltaic devices and white-light-emitting diodes using poly(2,7-fluorene), poly(2,7-carbazole), and poly(2,7-dibenzosilole) derivatives

World energy needs grow each year. To address global warming and climate changes the search for renewable energy sources with limited greenhouse gas emissions and the development of energy-efficient lighting devices are underway. This Review reports recent progress made in the synthesis and characterization of conjugated polymers based on bridged phenylenes, namely, poly(2,7-fluorene)s, poly(2,7-carbazole)s, and poly(2,7-dibenzosilole)s, for applications in solar cells and white-light-emitting diodes. The main strategies and remaining challenges in the development of reliable and low-cost renewable sources of energy and energy-saving lighting devices are discussed.

Experimental and theoretical study of a new carbazole derivative having terminal benzimidazole rings

A novel intramolecular donor-acceptor compound has been synthesized and characterized. This compound was a symmetrical A-pi-D-pi-A type molecule containing two benzimidazole rings as two electron acceptors (A) and an N-ethylcarbazole group as electron donors (D). The absorption and emission spectra of the compound were determined by experimental methods in solution and were computed by using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) in gas phase and in chloroform solution. The calculated absorption and emission wavelengths were in good agreement with the experimental ones. The fluorescence quantum yields and fluorescence lifetimes of the compound in several solvents have been studied by means of steady state and time resolved fluorescence. The results showed the compound had high quantum yield. The cross-section of two-photon absorption (TPA) of the compound was measured by using femtosecond laser in dichloromethane solution. The result indicated the cross-section maximum of two-photon absorption was 430 GM at 600 nm. These results made the compound of great interest as a new fluorescent probe and photoluminescence material.

2008 Macromolecular Science and Engineering Division Award Lecture — Conjugated polymers: From micro-electronics to genomics

Conjugated polymers have received a lot of attention, since they combine the best features of metals or semiconductors with those of synthetic polymers. For instance, solar cells based on poly(2,7-carbazole) derivatives have revealed power-conversion efficiencies up to 6%. This class of materials could lead to printable and flexible photovoltaic devices. Moreover, water-soluble luminescent polythiophenes have allowed the specific, rapid, and ultra-sensitive detection of unlabelled DNA, RNA, or proteins. This new optical detection mechanism is based on electrostatic interactions between a cationic polythiophene derivative and negatively charged oligonucleotides. This method makes now possible the rapid assessment of the identity of single nucleotide polymorphisms (SNPs), genes, and pathogens without the need for nucleic acid amplification.

Poly(2,7-carbazole)s: structure-property relationships

Conjugated polymers combine the interesting optical and electrical properties of metals with the processing advantages and mechanical properties of traditional synthetic polymers. With clever use of a variety of synthetic tools, researchers have prepared highly pure polymers with optimized physical properties during the past 30 years. For example, the synthesis of well-defined polyacetylenes, polyphenylenes, polythiophenes, polyfluorenes, and other conjugated polymers have significantly improved the performance of these polymeric materials. However, one important class of conjugated polymers was missing from this chemical inventory: easy access to well-defined poly(2,7-carbazole)s and related polymers. This Account highlights advances in the synthesis of poly(2,7-carbazole) derivatives since they were first reported in 2001. Starting from 2-nitro-biphenyl derivatives, 2,7-functionalized carbazoles are typically obtained from Cadogan ring-closure reactions. In a second step, Yamamoto, Stille, Suzuki, or Horner-Emmons coupling polymerization leads to various poly(2,7-carbazole) derivatives. We discuss the characterization of their optical and electrical properties with a strong emphasis on the structure-property relationships. In addition, we carefully evaluate these polymers as active components in light-emitting diodes, transistors, and photovoltaic cells. In particular, several low band gap poly(2,7-carbazole) derivatives have revealed highly promising features for solar cell applications with hole mobilities of about 3 x 10(-3) cm2 V(-1) s(-1) and power conversion efficiencies up to 4.8%. Finally, we show how these new synthetic strategies have led to the preparation of novel poly(heterofluorene) derivatives and ladder-type conjugated polymers.

Self-assembly and electrochemical oxidation of polyamidoamine-carbazole dendron surfmer complexes: nanoring formation

We report a detailed and quantitative study on the supramolecular complexation of amine-functionalized polyamidoamine (PAMAM) dendrimer G(4)-NH(2) with carboxylic acid terminal dendrons containing peripheral electroactive carbazole groups of different generations (G(0)COOH, G(1)COOH, and G(2)COOH). While the focus is on a detailed understanding and mechanism of complex formation, subsequent electrochemical oxidation of the dendron surfmers resulted in the formation of nanoring structures electrodeposited on the conducting substrate. Complexation was confirmed by NMR, UV-vis, and IR measurements. Critical micelle concentration (CMC), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) studies revealed that the ringlike structures were formed during the equilibrium-decomplexation stage and that the electrochemical process did not destroy the complex but rather stabilized it. The different generations of the dendrons provided various structures and complex formation efficacy. This type of template polymerization combined with electrochemically anodic oxidation has not been previously reported.

Expanding benzene to giant graphenes: towards molecular devices

Phenylene-based conjugated materials provide versatile platforms for the development of molecular devices. Architectures of one- and two-dimensional polyphenylenes, which self-assemble into three-dimensional objects with advantageous electronic properties, have been investigated. Systematic relations between the size, substitution and shape with function were found, which enabled the further optimization of the materials. Hand in hand with the development of suitable methods for visualization and processing, promising results were obtained for performance of nanoscale electronic devices.

Synthesis, Characterization, and Fluorescence Behavior of Twisted and Planar B2N2-Quaterphenyl Analogues

A series of planar and twisted heteroaromatic quaterphenyl analogues containing BN ring linkages has been synthesized using primarily difunctional Lewis acidic diborabiphenyl moieties as molecular cores. Crystal structure analyses indicated the presence of large twist angles between adjacent aromatic rings in 1 and 3, which were also observed to possess nonfluorescent behavior due to a lack of molecular rigidity and insufficient B=N character in the excited state. In contrast, the incorporation of one or two bridging ethylene groups between the adjacent rings (installed via an ethynyl cycloisomerization) was found to afford planar phenanthrene or pyrene moieties, which resulted in weak fluorescence behavior (Phi F = 0.02-0.16) for the n-Bu and Ph derivatives 5-12. Emission colors ranged from green (lambda em = 521 nm) to red (lambda em = 630 nm) and depended primarily on the conformation (2,2'- vs 4,4'-), the extent of chromophore conjugation (phenanthrene vs pyrene), and the type of exocyclic substituent present (n-Bu vs Ph). Communication between the two phenanthrene or pyrene moieties was observed in some cases, which was characterized by bathochromically shifted emission bands relative to that of monomeric phenanthrene or pyrene species. Unique excited-state dimer (excimer) fluorescence was observed for the 2,2'-isomer 8, which was characterized by broad, low-energy emission bands bathochromically shifted from that of the corresponding monomer.