Gintautas Bagdziunas

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.

2,4-Bis(4-aryl-1,2,3-triazol-1-yl)pyrrolo[2,3-d]pyrimidines: synthesis and tuning of optical properties by polar substituents

2,4-Bis(4-aryl-1,2,3-triazol-1-yl)pyrrolo[2,3-d]pyrimidines as D–π–A–π–D chromophores were successfully prepared by CuAAC reaction of 2,4-diazido-7-methylpyrrolo[2,3-d]pyrimidine with ethynylarenes in dichloromethane in the presence CuI/DIPEA/AcOH as a catalyst system. The incorporation of small polar substituents enabled tuning of the energy of frontier orbitals and thus the FMOs energy gap by up to 0.9 eV, while the incorporation of bulky steric substituents resulted in narrowing of the energy gap by up to 0.4 eV. Owing to electron-accepting properties of pyrrolo[2,3-d]pyrimidine core extending to triazole moieties the compounds with electron-donating groups showed expressed intramolecular charge transfer character (ICT) of the excited states which was proved by solvatochromic dynamics and supported by DFT calculations. The optimization of ICT reduced radiative and non-radiative deactivation pathways resulted in enhancement of fluorescence quantum yield up to 73%.

Diastereoselective Strategies towards Thia[n]helicenes

In the present study, we have investigated different strategies for diastereoselective synthesis of thia[n]helicenes. We describe the introduction of different chiral auxiliaries at various positions and investigated their effect in the photocyclization reaction. Different chiral groups were placed at the sterically hindered position of the helical core and their interactions with various solvents and metals like copper were investigated. The use of Cu(I) salts has led to high diastereoselectivity in the photocyclization process and we were successful in obtaining the thia[5]helicene in enantiomerically pure form in good yield. The single diastereomer obtained was characterized by X-ray crystallography. From the study of the barrier of racemization of these thia[5]helicenes, the stability was found to be comparable to unsubstituted tetrathia[7]helicenes and substituted diazadithia[7]helicenes. This approach provides an easy access to enantiopure helicenes.

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.

Crystal structure of 1,3,6,8-tetra­bromo-9-ethyl-9H-carbazole

In the title compound, C14H9Br4N, the tricyclic ring system is almost planar (r.m.s. deviation for the 13 non-H atoms = 0.017 Å) and the methyl C atom deviates from the mean plane of the ring system by 1.072 (17) Å. In the crystal, Br⋯Br contacts [3.636 (3) and 3.660 (3) Å] slightly shorter than the van der Waals contact distance of 3.70 Å are seen.

Pyrenyl substituted 1,8-naphthalimide as a new material for weak efficiency-roll-off red OLEDs: a theoretical and experimental study

Based on the theoretical calculations of excited states and semiconducting properties, a new 1,8-naphthalimide derivative having an electron-donating 1-pyrenyl group at the C-4 position was designed and synthesized. This derivative exhibited an excellent thermal stability and bipolar charge carrier transport ability. It was successfully utilized as a host in red phosphorescent organic light-emitting diodes showing an efficient energy transfer from the host to the phosphorescent emitter. The derivative may be a single material electroplex-forming host for PhOLEDs. The best fabricated red emitting device demonstrated maximum current, power, and external quantum efficiencies of 10.8 cd A−1, 7 lm W−1, and 13.6%, respectively. The best device exhibited a high brightness of 15 300 cd m−2 and weak efficiency-roll-off.

Crystal structure of 3-bromo-9-ethyl-9H-carbazole.

In the title compound, C14H12BrN, the tricyclic ring system is essentially planar (r.m.s. deviation 0.026 Å). The carbon atoms of the ethyl group deviate from the mean plane by 0.148 (9) (CH2) and 1.59 (1) Å (CH3). In the crystal, H⋯π contacts [2.698–2.898 Å] shorter than the van der Waals contact distance of 3.70 Å are observed. A scalable to gram quantities selective synthesis of mono-bromine-substituted carbazole derivatives was developed.

Experimental and Theoretical Investigations of an Electrochromic Azobenzene and 3,4-Ethylenedioxythiophene-based Electrochemically Formed Polymeric Semiconductor

An electrochromic material based on azobenzene and 3,4-ethylenedioxythiophene (EDOT) semiconducting layer was electrochemically deposited on an indium tin oxide coated glass electrode. Chemical synthesis of the azobenzene and EDOT-based chromophore (DAE) and electrochemical formation of its corresponding polymer (pDAE) are reported. The electrochromic properties of the synthesized polymer pDAE were investigated by electrochemical and spectroelectrochemical methods. pDAE exhibited an optical bandgap of 1.82 eV and three distinct colored states in its reduced, neutral, and oxidized forms. The pDAE polymer showed 44 % optical contrast at 710 nm between its reduced and oxidized states and a fast electrochromic switching time of 1.0 s. The frontier molecular orbitals, Raman shifts, and semiconducting properties of this electrochromic polymer were evaluated by density functional theory calculations. The optical absorption bands of the polymer charged states were assigned and investigated.

1,3,6-Tri­bromo-9-ethyl-9H-carbazole

In the title compound, C14H10Br3N, the carbazole ring system is almost planar, with an r.m.s. deviation of 0.023 A from the best fit mean plane of the 13 non-H atoms of the three rings. The methyl C atom lies 1.232 (3) A out of this plane. No hydrogen bonds are found in the crystal structure but weak C—Br⋯π contacts at approximately 3.721 A may stabilize the structure.