Audrius Bucinskas

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%.

Structure–property relationship of isomeric diphenylethenyl-disubstituted dimethoxycarbazoles

Isomeric 3,6-dimethoxy- and 2,7-dimethoxycarbazoles containing diphenylethenyl moieties were synthesized by condensation of the appropriate dimethoxycarbazoles with diphenylacetaldehyde. The solid-state structures and the molecular order of the compounds were proven by X-ray crystallography. Both compounds were found to be capable of glass formation with comparable glass transition temperatures (70–71 °C). They exhibited high thermal stabilities, with the 5% weight loss temperatures exceeding 375 °C. The isomer having diphenylethenyl groups at C-3 and C-6 positions and methoxy groups at C-2 and C-7 positions (3a) exhibited aggregation-induced emission (AIE), while its counterpart having diphenylethenyl groups at C-2 and C-7 positions and methoxy groups at C-3 and C-6 positions (3b) showed the opposite effect, i.e. aggregation-caused quenching (ACQ). The derivative 3b showed superior charge transporting properties. Time-of-flight hole drift mobilities in its layers approached 10−3 cm2 V−1 s−1 at high electric fields. A comparative theoretical analysis of the compounds was performed using density functional theory (DFT) and time-dependent DFT calculations. They proved more effective π-conjugation in the derivative of 3,6-dimethoxy carbazole (3b), which was also observed by UV and fluorescence spectroscopies. The theoretical study revealed relatively low ground state dipole moment of 0.69 D of the isomer 3b, while its counterpart (3a) showed much higher ground state dipole moment of 5.98 D. The difference in polarity was found to have the crucial effect on the molecular arrangement in the crystals and consequently, on the thermal transitions and charge-transporting properties.

Synthesis, functionalization, and optical properties of chiral carbazole-based diaza[6]helicenes.

In the present study, carbazole-based diaza[6]helicenes were synthesized utilizing versatile quinoline and 9-(2-ethylhexyl)-2,7-dimethoxycarbazole-3-carbaldehyde building blocks via the Wittig reaction-photocyclization strategy. The presence of bifunctional units comprising electrophilic chloroquinoline and electron-rich carbazole has opened up new opportunities. The chloro group was substituted with a chiral amine, allowing diastereomeric separation, and the chiral forms were monofunctionalized via electrophilic substitution on the carbazole unit. Postcyclization functionalization via substituting the carbazole unit provides a platform for the synthesis of chiral functionalized materials with potential application in fields such as asymmetric synthesis and organic electronics. The configuration of the diaza[6]helicene diastereomers was demonstrated by time-dependent density functional theory (TD-DFT) calculations. Furthermore, on the basis of the DFT calculations of the HOMO-LUMO energy levels of the chiral forms, these compounds can be potentially of interest as hole-transporting compounds.

Aggregation-Enhanced Emission and Thermally Activated Delayed Fluorescence of Derivatives of 9-Phenyl-9H-Carbazole: Effects of Methoxy and tert-Butyl Substituents

Derivatives of 9-phenyl-9H-carbazole were synthesized as efficient emitters exhibiting both thermally activated delayed fluorescence and aggregation-induced emission enhancement. Effects of methoxy and tert-butyl substituents at the different positions of carbazolyl groups on the properties of the emitters were studied. Depending on the substitutions, photoluminescence quantum yields (PLQY) of non-doped solid films of the compounds ranged from 17 % to 53 % which were much higher than those observed for the solutions in low-polarity solvent toluene. Compounds substituted at C-3 and C-6 positions of carbazole moiety by methoxy- and tert-butyl- groups showed the highest solid-state PLQY. Ionization potentials of the studied derivatives in solid-state were found to be in the short range of 5.75-5.89 eV. Well-balanced hole and electron mobilities were detected for tert-butyl-substituted compound. They exceeded 10-4  cm2  (V×s)-1 at electric fields higher than 3×105  V cm-1 . Two compounds with the highest solid-state PLQYs showed higher efficiencies in non-doped organic light-emitting diodes than in the doped devices. Maximum external quantum efficiency of 7.2 % and brightness of 15000 cd m-2 were observed for the best device.