Carbazole Molecules Research Articles

Poled Sol–Gel Materials With Heterocycle Push–Pull Chromophores that Confer Enhanced Second‐Order Optical Nonlinearity

AbstractHybrid organic–inorganic materials doped with zwitterionic push–pull chromophores with high hyperpolarizability have been synthesized by a sol–gel procedure. A large chromophore concentration was reached by using N‐(hydroxyethyl)carbazole as a physical spacer (preventing the dye aggregation). Spin‐coated doped films were electrically poled and second harmonic generation measurements performed in situ. During the thermally assisted poling under a N2 atmosphere, only the carbazole molecules degraded. Second‐harmonic generation measurements gave an estimation of the nonlinear coefficient, r33, of 38 pm V–1 at 1064 nm.

Polarized Fluorescence of Jet-Cooled indole and Carbazole and Their Complexes with Water

The spectra of the fluorescence excitation within the rotational contours of the bands of the pure electronic long-wavelength S 0-S 1 transitions of jet-cooled indole and carbazole molecules and their complexes with water are measured. For the carbazole-water complex, a contour with three maxima is registered, which is possibly related to the occurrence of two isomers, differing in a slight displacement of hydrogen between the nitrogen atom of the imine group of carbazole and the oxygen atom of the water molecule. The degrees of polarization of integral fluorescence upon excitation within the rotational contours of the S 0-S 1 electronic transition bands of the above molecules and their complexes with water are determined for the first time. The coincidence of the calculated (7.7%) and measured (7.3%) values of the degree of polarization upon excitation in the rotational Q branch of the b L 1-A electronic transition of indole confirms the accepted intramolecular orientation of the transition dipole moment at an angle of 38.3° with respect to the principal axis of inertia A. Upon excitation of indole, its complex with water, and carbazole into the P and R branches, the measured and calculated degrees of polarization are also close to each other and amount to 2–3%. This confirms the occurrence of contributions to the fluorescence polarization due to the rotations of the indole molecules around the principal axes of inertia A and C.

Vibronic-spin-orbit interactions in heterocyclic analogues of fluorene with the N and O heteroatoms

The transition dipole moments P 0 s for the transitions from the electronic triplet state 3 B 2(ππ*) to vibrational sublevels of the vibrational out-of-plane modes n of the carbazole and dibenzofuran molecules are calculated. The values of the radiative deactivation rate constant k rad s of the triplet sublevels T s are determined along with the components k SO s and k VSO s of this constant, which depend on the intramolecular spin-orbit (SO) and vibronic-spin-orbit (VSO) interaction. It is ascertained that k rad z > k rad y . For different structural units of the molecules (the heteroatom and the carbon atoms of the dibenzene fragment), the effect of the SO coupling on the constant k VSO∼Σs, n (P 0 s )2 is studied. A competition between the effects on k VSO from the SO coupling in the carbon atoms and in the light N and O heteroatoms is revealed. This competition accounts for the weak influence of the heteroatom on this component of the rate constant k rad in these molecules. It is ascertained that the intensity distribution among the vibronic lines in the phosphorescence spectra of carbazole and dibenzofuran I 0n ∼Σs (P 0 s )2 is different due to the substantially different influence of the N and O heteroatoms on the deactivation of the triplet sublevel T y .

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.

Epoxy-carbazole compositions. Part I. Compositions with 9-(2,3-epoxypropyl)carbazole

Compositions of low-molecular weight epoxy resin Ruetapox 0162 mixed with various amounts of 9-(2,3-epoxypropyl)carbazole (EPK) have been prepared and cured by isophorone diamine. Mechanical, thermal and photoluminescence properties of obtained materials were determined. It was found that low- molecular weight epoxy resin is an excellent dispersing matrix for carbazole groups, which secures suitable mechanical properties, low combustibility and high thermal stability of prepared materials. Moreover, it was observed that the addition of 10 wt. % of EPK improves compression strength by 15%, flexural strength by 33% and also slightly decreases water absorption. Thin film forming ability and optic properties, characteristic for isolated carbazole molecules (photoluminescence maxima at 352 and 368 nm), point at the possibility to use studied materials for light-emitting diodes (LED) construction.

Photoinduced Electron Transfer From Carbazole to Halomethanes in a Gas Phase

Intermolecular photoinduced electron transfer (PET) in a gas phase was studied using carbazole vapor fluorescence quenching by halomethanes (CHCl3, CH2Br2, CCl4, CHBr3). The fluorescence quenching rate constants k q changing from 2.3·105 sec−1·torr−1 in mixtures with CHCl3 to 4.6·106 sec−1·torr−1 in mixtures with CHBr3 at a constant temperature of 403 K were estimated. The dependence of the carbazole fluorescence decay rates in the presence of halomethanes on the free energy change ΔG during transfer of the electron from carbazole to halomethanes is considered. It is suggested to take into account the influence of the vibrational energy of the carbazole molecule E vib and its temperature changes in estimation of the ΔG values. The differences between PET in the gas and liquid phases were analyzed. It is found that for mixtures with CCl4 and CHBr3 the negative temperature dependence of k q is observed, when the decay rates and efficiencies of the intermolecular PET decreased with temperature increase in the range 403–573 K, i.e. these mixtures the electron transfer is not a barrier-restricted process.

Preparation and Photoluminescence Characteristics of Carbazole-Poly(p-phenylenevinylene)s

Organic conjugated polymers for light-emitting diodes (LEDs) attract much attention for good optical applications. Organic conjugated polymers have been synthesized for development of photoluminescence (PL) properties.1–3 Since the discovery of poly(pphenylenevinylene) (PPV) exhibiting photoluminescence a variation of conventional PPV was investigated in which backbone conjugation is partially modified using spacers. In 1993, Karasz et al. demonstrated the block copolymer, which has specified units of PPV copolymer with alkyl spacer.4 As mentioned in a lots of papers, progress for achievement of blue-emitting organic polymers has been concentrated on the introduction of side group or newly designed backbones by insertion of heterocycles. We recently synthesized PPV copolymers that have shorter backbone conjugations and examined their luminescence.5, 6 They showed emission spectra in far-blue region and exhibited specific luminescence effect by conjugation length of main chain and methoxy substituents on the phenyl ring. Although hundreds of studies have been conducted using conventional phenylenevinylene-phenylen-vinylene-phenylene unit, effective works need variation of backbones to find a blueemitting polymer showing low turn-on voltage simultaneously. Recently PPV derivatives containing a carbazole moiety in the backbone or side chain have been reported.7–9 In particular, these carbazole-containing PPV copolymers are good materials for making optical and electronic devices due to high hole-transporting capability of carbazole unit.10–12 The introduction of new organic polymers based on the carbazole molecule and exploration of applications in the fabrication of organic light-emitting devices will further interest on carbazole-PPVs for possible optoelectronic applications. So we synthesized three carbazol-PPV derivatives that contain a carbazole unit as a part of main chain and examined luminescence properties. We herein report photoluminescence characteristics and alkoxy substituent effects of carbazole-containing PPV copolymers.

Steady state and time resolved studies on photophysical properties of carbazole and 9-phenyl carbazole molecules and their quenching reactions with suitable electron acceptors in the excited electronic states both at the ambient temperature and at 77 K

Studies at the ambient temperature by employing both steady state and time resolved techniques reveal that the observed fluorescence quenching phenomena of the present electron donor molecules, carbazole (C) and 9-phenyl carbazole (9PC) in the presence of the well-known electron acceptors 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) in acetonitrile (ACN) fluid solution are due to the combined effect of the static and dynamic processes involved. To dissect the donor fluorescence quenching data into its dynamic and static components, a model in the form of a modified Stern–Volmer (SV) relation has been proposed. By treating the data, obtained from the present investigation when the donor chromophores are excited, by nonlinear least squares curve fitting procedure, static ( V) and dynamic ( K SV) contributions in overall quenching mechanisms were evaluated separately. The contribution of the static component ( V) is observed to be so large that it overwhelms the dynamic process and plays major role in overall quenching mechanisms. In dynamic quenching, photoinduced electron transfer (PET) process, whose occurrence being confirmed by measuring redox potentials of the reacting systems (C and 9PC as electron donors and 9FL and 2N9FL as electron acceptors) in ACN solvent, is found to be operative concurrently with the Förster energy transfer process. However, when the electron acceptor molecules are excited in presence of a ground state donor, the quenching of the acceptor fluorescence appears to be mainly of dynamic nature. In this dynamic process, electron transfer (ET) seems to play the major role in nonradiative deactivation of the lowest excited singlet state (S 1) of the acceptor species. From the observed results at 77 K, it is inferred that donor fluorescence quenching is primarily due to the combined effect of the concurrent occurrences of ET in the excited singlet state and the Förster long range energy transfer (S 1 D→S 1 A). Low temperature studies further demonstrate that the triplet donors are not involved in energy transfer as well as in ET reactions with the acceptors. From the observed room temperature oxidation potential values of the donors, it is apparent that the electron donating capability decreases when substitution is made along the short molecular axis of symmetry of C, i.e., by replacing < NH hydrogen atom of C molecule by a phenyl ring (in case of 9PC).

Usability of Polymers Modified by Changing the Carbazole Group in the LEDs Devices

In order to find proper materials for making electroluminescent diodes electrooptics properties of polymers were investigated. Three polymer groups based on carbazole molecule were the subjects of the studies: PVK modified by substituting the acceptor group or donor group in the carbazole ring, poly-ω-(9-carbazolyl)alkyl methacrylates and copolimer ω-(9-carbazolyl)alkyl - methacrylates with 3-(2-metakryletoxy)carbonyl 7 diethylamincoumarin. Absorption and photoluminescent spectra of polymer in solution and films were measured. Electroluminescent diodes were obtained using the polymers mentioned above. Also, hole mobility was measured according to the distance between the carbazole group and the main chain.

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.

Tissue-specific activities of methylated dibenzo[c,g]carbazoles in mice: Carcinogenicity, DNA adduct formation, and CYP1A induction in liver and skin

The potent multitissue carcinogen 7H-dibenzo[c,g]carbazole and nine methylated derivatives, synthesized on the basis of the positions in the parent compound that are involved in metabolism, were tested for their ability to induce sarcomas and hepatic tumors in XVIInc/Z mice. In addition, the capacity of these compounds to induce DNA adducts in skin and liver was investigated by (32)P-postlabeling analysis after their topical administration. Induction by these compounds of cytochromes P450 of the 1A family in liver and skin was investigated and correlated to their carcinogenic potential. A clear correlation was found between the tissue specificity of DNA binding and the capacity of each compound to induce skin or liver tumors. In contrast, no direct relationship was observed between the capacity of the compounds to induce cytochromes 1A1/1A2 and the tissue specificity of carcinogenesis or DNA binding in liver or skin. The results are discussed with respect to the positions of methyl groups in the 7H-dibenzo[c,g]carbazole molecule.

Tissue-specific activities of methylated dibenzo[c,g]carbazoles in mice: Carcinogenicity, DNA adduct formation, and CYP1A induction in liver and skin

The potent multitissue carcinogen 7H-dibenzo[c,g]carbazole and nine methylated derivatives, synthesized on the basis of the positions in the parent compound that are involved in metabolism, were tested for their ability to induce sarcomas and hepatic tumors in XVIInc/Z mice. In addition, the capacity of these compounds to induce DNA adducts in skin and liver was investigated by 32P-postlabeling analysis after their topical administration. Induction by these compounds of cytochromes P450 of the 1A family in liver and skin was investigated and correlated to their carcinogenic potential. A clear correlation was found between the tissue specificity of DNA binding and the capacity of each compound to induce skin or liver tumors. In contrast, no direct relationship was observed between the capacity of the compounds to induce cytochromes 1A1/1A2 and the tissue specificity of carcinogenesis or DNA binding in liver or skin. The results are discussed with respect to the positions of methyl groups in the 7H-dibenzo[c,g]carbazole molecule. Environ. Mol. Mutagen. 35:139–149, 2000 © 2000 Wiley-Liss, Inc.

Electron-transfer processes in the reaction of K −, K +(15-crown-5) 2 with carbazole: unexpected formation of metallic potassium

Transfer of one electron from K − to the aromatic ring occurs in the reaction of K −, K +(15-crown-5) 2 with carbazole giving K° and a radical anion. The latter rapidly decomposes with the liberation of hydrogen and formation of carbazylpotassium. It can also undergo an intramolecular proton-exchange reaction followed by a transfer of the second electron from the part of K° product. That leads to a dianion which then reacts with another carbazole molecule. Finally, carbazylpotassium and 1,4-dihydrocarbazylpotassium are formed.

Intramolecular Electron Hopping in Double Carbazole Molecules Studied by the Fluorescence-Detected Magnetic Field Effect

The intramolecular electron hopping in cation dimer radicals of 1,n-bis(N-carbazolyl) alkanes for n = 3,4 that are produced via photoinduced electron transfer to 1,4-dicyanobenzene (DCNB) has been studied by means of the exciplex fluorescence detected magnetic field effect (MFE) in a 18 vol % mixture of dimethylformamide with tetrahydrofuran. It has been found that on passing from a single cation radical of N-methylcarbazole to the cation dimers, the parameter B1/2 decreases from ca. 50 G to ca. 26 G, while saturation values of MFE (at B = 300 G) are almost the same (ca. 3.0% for MeCz and CzB) but strongly decreased for CzP (ca. 0.5%). The experimental results obtained are in fair agreement with the Schulten theory of intramolecular electron exchange. In contrast to the well-known pyrene/N,N-dimethylaniline exciplex system, the carbazole derivatives/DCNB system exhibits a two-exponential decay of exciplex fluorescence and a significantly smaller MFE that might result from the fact that in the latter exciplex system the radical ion pair state is the lowest energy state above the ground state.

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.

Poling behavior of optical absorption spectra in carbazoles with one- and two-dimensional charge-transfer character

Spectral change in optical absorption due to molecular alignment induced by electric poling has been studied experimentally and theoretically, in one- and two-dimensional charge-transfer carbazole molecules. After poling, an increase in absorbance at λmax was observed in 3,6-dinitrocarbazoles which present a two-dimensional charge-transfer character, while the absorbance at λmax decreased in 3-monosubstituted carbazoles which possess a one-dimensional charge-transfer character. From molecular-orbital calculations and evaluations of the molecular polarizability spectra for random and uniaxial orientations of the molecules, an explanation is provided for the poling behavior in terms of the vector directions of the transition and ground-state dipole moments of the molecules. Theoretical investigation of various conformers has been made, resulting in the suggestion that the poling behavior of absorption spectra should be strongly affected by the conformation of the acceptor groups in the case of disubstituted carbazoles.

Electronic coupling energy and solvent reorganization energy in the nonadiabatic intermolecular electron transfer reactions of some carbazole and phenol antioxidant derivatives

The quenching of the first excited singlet states of a series of substituted carbazole molecules and substituted phenol and tocopherol derivatives by methylene bromide has been examined at 298 K in acetonitrile, ethanol and 3-methylpentane. The first-order rate constant of electron transfer (ET) is separated from the diffusion rate constant by applying the Fuoss-Eigen formalism. The first-order rates of the ET are correlated with the free energy changes for ET through the use of the semiquantum mechanical ET theory proposed by Onuchic for outer-sphere electron transfer dynamics. The fluorescence quenching of the phenols and tocopherols is diffusion-influenced, so that the rates are only tentatively analyzed by the nonadiabatic theory. The solvent reorganization energy (λs) obtained in acetonitrile and ethanol are generally higher than that calculated from the Marcus dielectric continuum model. The electronic coupling matrix elements are relatively small (< 50cm -1) and decrease exponentially with the center-to-center intermolecular D-A distances. These results are consistent with the idea that the rate-determining step in the reactions between vitamins and free radicals might involve an electron transfer oxidation.

Photoconductive crystals for nonlinear optics: molecular design and crystal structure

Acceptor-substituted carbazole derivatives were newly designed as novel nonlinear optical materials with photoconductive properties. Various 3-, 6- and 9-substituted carbazole derivatives were synthesized systematically and their optical second-harmonic generation (SHG) activity and crystallographic structures were examined. Enhancement of molecular hyperpolarizability ( beta ) was achieved and an intracharge-transfer band was observed by introducing electron-withdrawing groups in 3 and/or 6 positions. The hydrogen bonding in the 9-hydroxyethyl substituent plays an important role in generating non-centrosymmetric packing of carbazole molecules. Single crystals of 9-hydroxyethylcarbazole (HEK), 3-nitro-9-hydroxyethylcarbazole (NHEK) and 3,6-dinitro-9-hydroxyethylcarbazole (DNHEX) can be obtained by the solution growth technique. The HEK and NHEK crystals were determined to have non-centrosymmetric crystallographic structures with tetragonal and monoclinic unit cells. These crystals have unique space groups I41 and P21, and hydrogen bonding was formed in adjacent hydroxy groups. In these crystals each carbazole ring is aligned along the same direction. On the other hand, DNHEK has a centrosymmetric unit cell with space group P1. In this case the hydrogen bond involves a hydrogen on a hydroxy oxygen interacting with the nitro oxygen in an adjacent molecule. The highly aligned carbazole system is attractive in terms of structure related properties such as the carrier transport process and second-order nonlinear optical responses.

Electronic Structure of the Supermolecules and Sup-Ramolecules Constructed from Carbazole and Acceptor Molecules

Abstract Quantum chemical investigations of the electronic structure of the design carbazole (Cz) containing supermolecules, supramolecules, single charge donor and acceptor molecules were performed in the framework of the semiempirical neglect of differential overlap (MNDO) method. The data obtained show that it is a small charge redistribution in Cz-(CH2)4-1,2,4,5-tetracyanobenzene (TCNB) and Cz::TCNB. These designed derivatives are potential molecular photo-diodes. The sequence of the stability of the Cz containing supramolecules is found. The modeling of the photoinduced charge separation in Cz, TCNB, 2,4,7-trinitro-9-diciano-fluorene (TN9(CN)2F) and 2,4,5,7-tetranitro-9-diciano-fluorene (TeN9(CN)2F) molecules was done.

Polymers with pendant carbazolyl groups, 5. Photoconductive properties of some carbazole derivatives with electron donor and acceptor groups

AbstractIn this paper photocurrents of N‐isopropylcarbazole (NIPC) derivatives in polycarbonate hosts are reported and compared to data for a benzotriazole derivative (BTA). The carbazole moiety was substituted with both electron donor and acceptor substituents and the photoconductivity was measured as a function of concentration, electrical field, and temperature using the time‐of‐flight (TOF) technique. The observed increase of the effective mobility of charge carriers at low dopant concentrations upon substitution with a moderate electron acceptor is in contrast to literature data for some other materials. We interpret this increase as being due to an increase in the delocalisation radius of the transport state of the substituted carbazole molecule. The field dependence of the effective mobility shows Poole‐Frenkel behaviour, which is attributed to polaron formation. Comparison of the effective mobility of the NIPC derivatives with BTA shows that the size of the molecule plays a decisive role for the absolute value of the effective mobility.