Carbazole Dimers Research Articles

Stilbene like carbazole dimer-based electroluminescent materials

Two carbazole dimers ( 1 and 10) were synthesized from 9-ethyl-9 H-carbazole-3-carbaldehyde and 6-bromo-9-ethyl-9 H-carbazole-3-carbaldehyde by McMurry C–C coupling reaction. Palladium(0)-catalyzed C–N coupling reactions of 10 and various diarylamines result in the formation of stable carbazole derivatives in good yields. These compounds are fluorescent in blue to yellow region with moderate to good quantum yields. Also, they are thermally stable and capable of hole-transporting due to the presence of the carbazole moieties. The electroluminescent devices fabricated using 1, 2, and 3 as hole-transporters/emitters with a bilayer structure ITO/ Cpd/TPBI or Alq 3/LiF/Al ( Cpd= 1, 2, and 3) exhibit good performance (e.g., η ext=1.0–2.1%; η p=0.9–1.9 lm/W; η c=2.4–4.8 cd/A at a current density of 100 mA/cm 2).

Interplanar interactions in the excited triplet states of carbazole dimers by means of time-resolved EPR spectroscopy

The interplanar interactions for fully overlapped (FO) and partially overlapped (PO) carbazole dimers were investigated using zero field splitting (ZFS) parameters D and E in the excited triplet (T1) states. The observed D value decreases from the monomer to the dimers due to electron delocalization in the T1 dimers. The highest electron delocalization was observed within the two planes of the FO dimer. The interplanar interactions were analysed in terms of exciton (EX) and charge resonance (CR) types interactions by comparing the D values of monomer and the dimers. It is found that about 90% of excitation is delocalized over the two carbazole units in the T1 state of the FO dimer whereas for PO this contribution is 40%. In a model system, DCR and ECR were calculated using the two approximations, point charge and half point charge, for the evaluation of the ZFSs obtained.

Conjugated carbazole dimer as fluorescence carrier for preparation of iodine-sensitive chemical sensor

The carbazole derivative, 9-ethyl-3-carbazylidene carbazole hydrazone (ECCH) with two conjugated carbazole rings have been applied as a fluorescence carrier for preparation of an iodine sensitive optical chemical sensor. The response of the sensor is based on quenching of the fluorescence of ECCH by iodine. The conjugated carbazole dimer based sensor shows a linear response toward iodine in the concentration range 1.0 × 10 −6 to 1.0 × 10 −4 mol L −1, with a detection limit of 8.0 × 10 −7 mol L −1 at pH of 7.0. The effect of composition of the sensor membrane was studied, and the experimental conditions were optimized. Most commonly coexisting ions do not interfer with the iodine assay. The sensor shows sufficient repeatability, selectivity, operational lifetime of two months and a fast response of less then 50 s. The sensor has been used for determination of iodine in water samples.

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: tuning the HOMO level without influencing the triplet energy in small molecules.

A series of novel carbazole compounds was synthesized and tested for their suitability as host for triplet emitters in an organic-light emitting diode (OLED). In these compounds, a carbazole unit is either connected to other carbazole units to form carbazole dimers and trimers or to fluorene and oxadiazole derivatives to form mixed compounds. The HOMO level of carbazole compounds can be tuned by substitution at the 3, 6, and/or 9 positions. Making oligomers by connecting carbazole molecules via their 3 (3') positions shifts the HOMO level to higher energy, while replacing alkyl groups at the 9 (9') positions by aryl groups shifts the HOMO level to lower energy. Furthermore, it has been found that the triplet energy of these compounds is determined by the presence of poly(p-phenyl) chains in the molecular structure. By identifying the longest poly(p-phenyl) chain, one can predict whether a compound will be a suitable host for a high-energy triplet emitter. An overview of HOMO levels, singlet and triplet levels, and exchange energies is given for all carbazole compounds synthesized. Finally, OLEDs employing two selected carbazole compounds as host and fac-tris(2-phenylpyridine)-iridium (Ir(ppy)(3)) as guest were constructed and characterized electrically.

Mechanical Strain Sensing in a SIS-Type Elastomer with Single Site Strain Probes Based on Carbazole

Four fluorescent strain probes with one or two carbazole units placed in the middle of an elastomeric triblock of polystyrene-polyisoprene-polystyrene (SIS) have been prepared and their fluorescence behaviors investigated under strain. The probes were dissolved in commercial SIS type rubbers at low levels of 0.1 wt %. Films cast from this blend were subjected to tensile elongation while the fluorescence spectra were obtained. A rather large change from ca. 0.7 to 1.1 in the relative intensity at two wavelengths (I 350 /I 370 ) was observed on elongations up to 400%. The spectroscopic change is strongest for small strain levels around 0-200% and leveled out above 300-400%. This is attributed to changes in the intensities of the vibronic bands of the substituted carbazole units rather than changes in molecular overlap of carbazole dimers.

Dynamical Formation of Intramolecular Carbazole Dimer Cations in Poly(methyl methacrylate) Solids below the Glass Transition Temperature

Structures and dynamical formation of carbazole (Cz) dimer cations in solid states were investigated by absorption spectroscopy of dicarbazolyl compounds doped in PMMA solids. Charge resonance (CR) bands originating from Cz dimer cations were observed even in PMMA solids at room temperature, which is below the glass transition temperature. Peak wavelengths of the CR bands observed were similar to those previously reported for Cz dimer cations formed in N,N-dimethylformamide (DMF) solutions at room temperature. This result shows that Cz dimer cations formed in PMMA solids have structures similar to those formed in DMF solutions. These Cz dimer cations were still observed even at 150 K, although the ratios of dimer cations to monomer cations were lower than those at room temperature. The ratios were kept almost constant below 170 K and steeply increased in the temperature range 200−290 K. This observed temperature corresponds to the transition temperature of the β-relaxation in PMMA solids. We conclude that...

An electro-active system of immuno-assay (EASI assay) utilising self assembled monolayer modified electrodes

Most immunoassays currently rely on optical methods for signal generation e.g. in ELISA and rapid assay formats. It has become apparent as in the Glucose sensor market that there is a need for simple direct electrical immuno-sensors. We have investigated the novel use of organic conducting monolayers used as a direct electrochemical detection support for an immuno-reaction. It was found that antibodies raised to a carbazole dimer monolayer could increase the charge movement across that monolayer surface. Antibody fragments were taken from a specific anti-carbazole antibody fragment library and combined with an antibody fragment directed to the hormone estrone 3 glucuronide (E3G), the target antigen to form a bispecific antibody fragment. The device utilised these specific antibody fragments and incorporated them on the top plate of a capillary fill format as the immuno-assay components. The immuno-reaction utilised a competition assay. Free E3G analyte in the sample displaced the bispecific antibody fragment from the immuno-surface leaving it free to bind the carbazole monolayer surface. There the binding was detected using amperometric or coulometric methods. By combining all there element it was possible to develop a sensitive immuno-assay that could detect E3G in a reproducible calibrated fashion down to 10 ng/ml.

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.

Photoinduced electron transfer between C 60 and carbazole dimer compounds in a polar solvent

Photoinduced electron transfer processes between carbazole (Cz) dimer compounds and C 60 in benzonitrile have been investigated by observing the transient absorption spectra in the visible and near-IR regions. By excitation of C 60 with 532 nm-laser light, electron transfer occurred via the triplet excited state of C 60 ( T C 60 ∗ ) from the Cz-dimer compounds, which was confirmed by observing characteristic absorption bands due to C 60 − and the Cz-dimer radical cation. The Cz-dimer compounds, forming sandwich-like conformation of radical cation, showed the charge resonance (CR) bands in the near-IR region longer than 1200 nm. Rate constants for electron transfer seem to depend on the conformation of the Cz-dimer radical cation. The Cz-dimer compounds taking sandwich conformation donate the electron easily to T C 60 ∗ , because of the stabilization energies of the sandwich dimer radical cations of the Cz-dimer compounds. By excitation of the Cz-dimer compounds with 355 nm-laser light, the absorption bands due to the Cz-dimer radical cations appeared immediately after the laser irradiation and the absorption of C 60 − is formed at diffusion controlled rate. This observation suggests that direct photoejection from the Cz-dimer compounds occurs in benzonitrile, yielding the Cz radical cations and solvated electron, in which the latter was trapped with C 60.

Improved stability of interfaces in organic light emitting diodes with high Tg materials and self-assembled monolayers

In order to improve the thermal stability of organic light emitting diodes (OLED), films made of classical hole transporters (TPD and NPB) and of new blue emitters (based on carbazole dimers) have been deposited on bare ITO substrates, and on ITO grafted with three different self-assembled molecules. These materials have low (60–70°C) and high (90–100°C) glass transition temperatures ( T g). The surface properties like desorption, surface diffusion and wettability, have been studied by observing the morphology of the films with scanning electron microscopy. High T g films show higher stability and reduced desorption. A similar effect is obtained with the grafted substrates. These tendencies can be explained by considering the polarizability of the molecules. This points towards possible improvements in the running temperature of OLED devices.

Electroluminescence from carbazole dimers

Blue light‐emission from a single‐layer organic LED (light‐emitting diode) based on carbazole dimers is reported. The device fabrication, involving vacuum sublimation of the dimer onto an indium tin oxide covered glass substrate, is described and the source of the electroluminescence elucidated. It is deduced that confined intra‐dimer luminescence excitations and a balanced double‐charge injection result in a device with good luminance and modest external quantum efficiencies. Of the three primary colors required for full‐color display applications, blue has been the most elusive to obtain.

The role of carbazole in organic light-emitting devices

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.

Characterization and application of carbazole modified polysiloxanes to electrochromic displays

Carbazole modified polysiloxanes have been electrochemically cross-linked to form composite thin films containing oxidized carbazole dimers (bicarbzolyls) together with associated counter ions. These materials can be reversibly switched between an oxidized (coloured) state and a reduced (bleached) state. Film lifetime is studied by cyclic voltammetry, and is found to be strongly influenced by film deposition rate and choice of switching potential. Lifetimes of 2 × 10 3 cycles are reported. The spectral response of films in the coloured state is presented, films have a contrast ratio of 9.3, sufficient for display applications. Potential step chronoamperometry has shown that the colouration mechanism is essentially independent of counter ion, while the rate of the bleaching process is slower with larger counter ions.

Synthesis and biological activity of new dimers in the 7H-pyrido[4,3-c]carbazole antitumor series

Ditercalinium (NSC 366241) is a 7H-pyrido[4,3-c]carbazole dimer with a diethylbipiperidine rigid chain linking the two heterocyclic rings. Ditercalinium is characterized by a high DNA affinity and bisintercalating ability, associated with potent antitumor properties, involving an original mechanism of action. Unfortunately as ditercalinium is hepatotoxic, its clinical evaluation has been interrupted. In order to eliminate or at least minimize the serious drawbacks related to its toxic effects, several chemical modifications have been made to the structure of ditercalinium, and their influence has been evaluated by measuring the DNA affinities, intercalation properties, and toxicity toward leukemia cells of the newly synthesized dimers. Reduction of the pyridinic moieties of ditercalinium, in order to suppress the permanent charges provided by the quaternizing chain, led to an almost complete loss of activity, although the DNA bisintercalating property of the dimer was preserved. Dimerization of the 7H-pyrido[4,3-c]carbazole rings by introduction of the rigid spacer on the N7- or C6-positions corresponding to the convex face of the pyridocarbazole, instead of the N2-position in ditercalinium, led to DNA bisintercalating dimers practically devoid of antitumor properties. However after quaternarization of the N2 atoms, the dimer linked by the N7 atoms exhibited a very high DNA affinity (greater than 10(9) M-1) and recovered antitumor activity, supporting the requirement of positive charges for the emergence of antitumor activity in these dimers. Introduction on the C6 of the 7H-pyridocarbazole ring of an aminomethyl or carboxyl group, a sugar residue, or C or N free amino acids such as Lys or Glu has also been carried out, in order to increase the hydrophilic properties of the molecules or to enable them to use amino acid transport systems. Although some of these compounds were active, none of them exhibited the pharmacological potency of ditercalinium.

Comparison of the bis‐intercalating complexes formed between either ditercalinium or a flexible analogue and d(CpGpCpG)2 or d(TpTpCpGpCpGpApA)2 minihelices: 1H‐ and 31P‐nmr analyses

The 400-MHz 1H- and 162-MHz 31P-nmr have been used to study complexes constituted by (a) the d(TpTpCpGpCpGpApA)2 or the d(CpGpCpG)2 self-complementary oligonucleotides and (b) two bifunctional 7H-pyrido [4,3-c] carbazole dimer drugs, the antitumoral ditercalinium (NSC 366241), a dimer with a rigid bis-piperidine linking chain and its pharmacologically inactive analogue, a dimer with a flexible spermine-like linking chain. Nearly all proton and phosphorus signals have been assigned by two-dimensional (2D) nmr (correlated spectroscopy, homonuclear Hartmann-Hahn, nuclear Overhauser enhancement spectroscopy, 2D 31P (1H) heteronuclear correlated spectroscopy and 31P-31P chemical exchange experiments). Both drugs bis-intercalate into the two CpG sites. The complexes show small differences in the position of the 7H-pyrido [4,3-c] carbazole ring into the intercalation site and possibly in the ribose-phosphate backbone deformation. However, the inactive analogue exhibits a longer residence lifetime in octanucleotide than the ditercalinium does. All these results are discussed in terms of differences in dimer activities.

1H and 31P nuclear magnetic resonance studies of the differences in DNA deformation induced by anti-tumoral 7H-pyrido[4,3-c]carbazole dimers

Ditercalinium (2,2′-{[4,4′-bipiperidine]-1,1′-diyldi-2,1-ethane-diyl}bis-{10-methoxy-7H pyrido[4,3-c]carbazolium} tetramethane sulfonate (NSC 366241)), a DNA bis-intercalating compound, is a potent anti-tumoral rigid dimer. Previous studies have shown that a reduced flexibility of the linking chain of such a dimer is essential for its biological activity. In order to understand, at the molecular level, the mechanism of action and the structure-activity relationships of this series of DNA intercalators, new dimers with additional methylene groups between the two piperidine rings have been synthesized. Addition of one methylene group in the chain preserved the activity, whereas addition of two methylene groups reduced the cytotoxicity, which finally disappeared when three methylene groups were inserted. Therefore, the study of the interaction of dimers bearing no (202), two (222) and three (232) methylene groups with the self-complementary hexanucleotide d(CGATCG) 2 have been investigated by 1H and 31P nuclear magnetic resonance studies. The results reported here indicate that all dimers bis-intercalate into the minihelix. The intermolecular nuclear Overhauser effects (NOEs) between the dimers and the nucleotide lead to the conclusion that the three dimers intercalate with their rigid bis-ethyl bipiperidine chain fitting the major groove of the helix. Inter-residue nuclear Overhauser effects at the DNA level, as well as induced shifts, are discussed in relation to the conformational changes induced in DNA upon intercalation and to the different activity of the dimers.

Spectroscopy and structure of the jet-cooled carbazole dimer

The fluorescence excitation and emission spectra of the jet-cooled dimer of carbazole have been obtained. The spectra are assigned and low-frequency vibrations due to intermolecular vibrations are observed. The fluorescence arises from the initially excited state and excimer formation is not observed in contrast to the behaviour of the isoelectronic fluorene dimer. The difference can be explained in terms of differing intermolecular interactions which determine the structure of the ground state dimers. AM1 calculations predict that the carbazole dimer is di-hydrogen bonded and exciton splittings are calculated on the basis of the calculated structure using the point-dipole approximation. One-photon transitions to only one of the excitonic states are observed, as is predicted by symmetry considerations.

Near-infrared charge resonance band of intramolecular carbazole dimer radical cations studied by nanosecond laser photolysis

The absorption of the charge resonance (CR) bands of dimer radical cations of meso- and rac-2,4-di(N-carbazolyl)pentanes (m- and r-DCzPe), and trans-1,2-di(N-carbazolyl)cyclobutane (DCzBu) was observed by nanosecond laser photolysis. The band peaks for m-DCzPe, r-DCzPe and DCzBu appeared around 1600, 1800 and 2000 nm, respectively. The peaks were shifted to shorter wavelengths with increasing overlapping of two carbazole chromophores. That is, the band gap of the CR band becomes larger with increasing interaction with the neighboring chromophore.

Asymmetrical bisintercalators as potential antitumor agents.

Ditercalinium and its analogues are dimeric molecules made up of two identical 7H-pyrido[4,3-c]carbazole rings linked by symmetrical linking chains. These dimers elicit antitumor properties through a new mechanism of action. Recently, a relationship was found between their antitumor properties and their cytotoxic effect on the polA Escherichia coli mutant strain, suggesting that 7H-pyrido[4,3-c]carbazole dimers might induce a DNA deformation that could be recognized by the E. coli SOS repair system. Thus, the role of symmetry in ditercalinium analogues for their DNA binding, antitumor properties, and bacterial toxicity is investigated in the present study, by introducing asymmetric parameters in their structures. Dimers were either synthesized with an asymmetrical rigid linking chain or made up of two chemically different chromophores, i.e., acridine and 7H-pyrido[4,3-c]carbazole. The asymmetrical dimers remain able to bisintercalate into DNA with high affinities, but a dramatic loss in their antitumor potency is observed. On the other hand, these asymmetrical dimers are cytotoxic for polA E. coli mutants, like their symmetrical analogues. These results show that the symmetry plays a crucial role for the antitumor potency in the 7H-pyrido[4,3-c]carbazole dimers series.

Kinetics of Geminate Pair Recombination in Polyvinylcarbazole

AbstractRecombination of geminate electron hole pairs (GP) in Polyvinylcarbazole (PVK) has been investigated by (i) monitoring the decay of delayed luminescence (DF) following pulse excitation with 4.02 eV photons at temperatures between 20 K and 294 K and (ii) Monte Carlo computer simulation. It is found that the decay of the GP reservoir obeys a t−s law with s being close to unity and exhibiting small yet systematic variations depending on temperature and time domain. DF extends up to seconds even at 294 K delineating GP lifetimes accordingly long. It is concluded that recombination kinetics is controlled by both the intrinsic energetic disorder of the bulk hopping sites, i.e. the carbazole groups, and by intermediate trapping at presumably incipient carbazole dimers. The Noolandi, Hong and Street theory is found inadequate for data analysis because it ignores the energetic relaxation of carriers subject to random walk within the energy distribution of hopping sites generally present in amorphous solids.