J.V. Grazulevicius

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.

N-annelated perylenes as effective green emitters for OLEDs

New carbazole-substituted N-annelated perylenes with different linking topologies of the chromophores were synthesized and their physical properties were studied with various experimental techniques including thermogravimetric analysis, differential scanning calorimetry, UV-vis, fluorescence and photoelectron emission spectroscopies and cyclic voltammetry. The synthesized materials exhibit extremely high thermal stability with 5% weight loss temperatures ranging from 400 to 457 °C. Photoluminescence quantum yields of the solid films of the compounds range from 33 to 51%. The ionization potentials of the solid samples established by the electron photoemission technique were found to be in the range of 5.14–5.53 eV. Using the newly synthesized compounds as emissive materials, efficient organic light-emitting diodes (OLEDs) were fabricated with the emission pattern covering the broad visible region from 475 to 675 nm. Bright green OLEDs exhibited luminance exceeding 62 000 cd m−2 and external quantum efficiency reaching 4.2%.

The thermal degradation of copolymers of carbazolylethyl(meth)acrylates and methacrylic acid

The thermal degradation of the (co)polymers of 2-(9-carbazolyl)ethyl methacrylate and 2-(9-carbazolyl)ethyl acrylate with methacrylic acid was studied by thermogravimetry and differential thermal analysis. It was established that the thermal stability of the copolymers is higher than that of the corresponding homopolymers. The degradation of copolymers in which the content of methacrylic acid units is high was found to occur in two stages. One-stage degradation was observed for copolymers in which the amount of methacrylic acid units is lower than 80 mol percent. The formation of intramolecular and intermolecular anhydrides was observed in the thermal degradation of all of the copolymers.

Fluorescence properties of carbazolyl-containing amphiphilic copolymers

Abstract Amphiphilic random copolymers of 2-(9-carbazolyl)-1-methylethyl methacrylate, 2-(9-carbazolyl)ethyl methacrylate or 2-(9-carbazolyl)ethyl acrylate with methacrylic acid were synthesized and their fluorescence properties in aqueous solution were examined. The fluorescence spectra of the copolymers showed monomeric emission. The fluorescence intensity decreased with increase of the content of carbazolyl groups in the copolymers, indicating that some excited-state quenching occurs in these systems. The fluorescence intensity of the copolymers also changed with the change of pH of the solutions. The character of these changes was dependent on the content of carbazole chromophores in the copolymer. Solubilization of 8-anilino-1-naphthalenesulfonic acid ammonium salt and perylene by the copolymers synthesized has been studied. The energy transfer to hydrophobically-bound perylene was facilitated for carbazolylalkylmethacrylate copolymers when compared to that observed for carbazolylalkylacrylate copolymers.

N,N′-Diphenyl-1,4-phenylenediamine-Based Enamines Containing Reactive Functional Groups as Building Blocks for Electro-Active Polymers

N,N′-Diphenyl-1,4-phenylenediamine-based enamines containing reactive functional groups were synthesized by a multi-step synthesis route. The materials were examined by various techniques including thermogravimetry, differential scanning calorimetry, UV and fluorescence spectrometry, electron photoemission and time-of-flight techniques. The electron photoemission spectra of the layers of the amorphous materials showed the ionization potentials of ca. 5.3 eV. Hole drift mobilities in the charge transport layers prepared using one of the synthesized compounds exceeded 10−5 cm2/Vs at high electric fields.

3,3′-Bicarbazolyl-Containing Oxirane and its Reaction Products as New Glass-Forming Electroactive Materials

Low-molar-mass oxiranyl-substituted 3,3′-bicarbazolyl derivatives were synthesized by the reaction of 3,3′-bicarbazolyl-containing oxirane with dimercapto compounds as linking agents. The cationic ring opening polymerization of oxirane monomers was performed using ytterbium (III) trifluoromethanesulfonate [Yb(OTf)3] as a cationic initiator to obtain polyether. The full characterization of the compounds by mass spectrometry, IR, 1H NMR, and 13C NMR is presented. All the compounds represent amorphous materials with glass transition temperatures ranging from 28°C to 112°C and with 5%-weight-loss temperatures exceeding 308°C. The electron photoemission spectra of the materials were recorded, and the ionization potentials of ca. 5.5 eV were established. Time-of-flight hole drift mobilities of the amorphous films of some synthesized compounds exceed 10−5 cm2/Vs at high electric fields.

New Thiophene-Based Glass-Forming Hydrazones for Optoelectronic Applications

A new series of thiophene-containing glass-forming hydrazones is synthesized. The thermal, photoelectrical, and optical properties of these compounds are examined. The synthesized hydrazones exhibit the thermal stability sufficient for their application in optoelectronic devices. The values of ionization potential of amorphous films of the newly synthesized hydrazones are from 4.43 to 5.79 eV. The hole drift mobilities measured by the xerographic time-of-flight method in a molecularly doped polymer containing 50 wt.% of one synthesized hydrazone exceed 10−6 cm2/Vs at an electric field intensity of 6.4 × 10−5 V cm−1.

Synthesis and Properties of Triindole-Based Monomers and Polymers

New star-shaped derivatives of 10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (triindole) with reactive functional groups are synthesized. Triindole monomers were found to have similar profiles of absorption and fluorescence spectra due to the presence of the same chromophore. The ionization potentials of the monomers are found to be comparable (5.37–5.53 eV). Hole drift mobilities of the amorphous layers of oxetane and vinyl ether molecularly dispersed in bisphenol Z polycarbonate (50%) exceed 10−5cm2/Vs at high electric fields. Cationic photocross-linking of triindole containing oxirane, oxetane and vinyl ether initiated with diphenyliodoniun tetrafluorborate is carried out.

Synthesis and photopolymerization of new triphenylamine-based oxiranes

Synthesis and cationic photopolymerization of new triphenylamine-based oxiranes, i.e., 4-[(oxiranylmethoxy) methyl] triphenylamine, 4-hydroxymethyl-4′-(2-oxiranylmethoxymethyl) triphenylamine and 4-[4-(oxiranylmethoxymethyl) phenylphenylamino]-4′-(diphenylamino) biphenyl are reported. Di-(tert-butylphenyl) iodonium tetrafluoroborate (BPIT), diphenyliodonium tetrafluoroborate (DPIT) and (η 5-2,4-cyclopentadien-1-yl) [1,2,3,4,5-η)-(1-methylethyl) benzene]-iron (+)-hexafluorophosphate (−1) (Irgacure 261) were used as photoinitiators. These photopolymerizations give products with a degree of polymerization varying between 4 and 108. The photopolymerization of the triphenylamine based hydroxyl group containing oxirane apparently occurs by activated monomer mechanism in its final stage. Photopolymerization of 4-[(oxiranylmethoxy) methyl] triphenylamine was studied in more detail. The influence of photoinitiator, temperature, and photoinitiator concentration on the rate of polymerization and conversion limit is discussed.

Polyethers containing 3,6-diarylcarbazolyl groups as polymeric materials for hole transporting layers of OLEDs

Several polyethers containing pendent 3,6-diarylcarbazolyl moieties have been synthesized by the multi-step synthetic routes. Full characterization of their structures is presented. The polymers represent materials of high thermal stability with initial thermal degradation temperatures exceeding 419 °C. The glass transition temperatures of the amorphous materials were in the range of 66–108 °C. The electron photoemission spectra of thin layers of the polymers showed ionization potentials in the range of 5.47–5.69 eV. Hole transporting properties of the polymeric materials were tested in the structures of organic light emitting diodes with Alq3 as the green emitter. The device containing hole transporting layers of the polymer with 3,6-di(4-biphenyl)carbazolyl moieties exhibited the best overall performance with a maximum photometric efficiency of about 2.3 cd/A and maximum brightness exceeding 2630 cd/m2.