Effect of the Nature of the Core on the Properties of the Star-Shaped Compounds Containing Bicarbazolyl Moieties

Three star-shaped compounds containing bicarbazolyl side arms and various core moieties are described. Bicarbazolyl moiety proves to be a stronger donor than single carbazolyl group, providing lower ionization potential and superior thermal and electrochemical stability. The influence of the central core, that is, 2,4,6-triphenyl-1,3,5-triazine, 1,3,5-triphenylbenzene, and 9-phenylcarbazole fragments, on the properties of the compounds is investigated and supported by DFT calculations. The dependence of photophysical properties on the rigidity and polarity of the media is discussed. Owing to the differences in molecular architecture, tuning of singlet and triplet energy values can be achieved. The synthesized compounds showed good hole-transporting properties with the mobility values exceeding 10^–3 cm² V^–1 s^–1

Strategy Toward Tuning Emission of Star-Shaped Tetraphenylethene-Substituted Truxenes for Sky-Blue and Greenish-White Organic Light-Emitting Diodes

Star-shaped, <i>C</i>₃-symmetric, tetraphenylethene (TPE) and 2,3,3-triphenylacrylonitrile (TPAN)-substituted truxenes <b>7</b>, <b>8</b>, and <b>9</b> were designed and synthesized by the palladium-catalyzed Suzuki and Sonogashira cross-coupling reactions. The TPE-substituted truxenes <b>7</b> and <b>8</b> show aggregation-induced emission behavior, whereas TPAN-substituted truxene <b>9</b> shows aggregation-caused quenching effect in tetrahydrofuran/water medium due to the π–π stacking. The computational calculation on truxenes <b>7–9</b> was performed, which reveals that in truxene <b>9</b>, the electron density transfers from truxene to TPAN. The truxenes <b>7–9</b> showed high thermal stability as the 10% weight loss temperature is more than 400 °C. The ionization potentials close to 6.0 eV were estimated for the solid samples of truxenes <b>7–9</b> by photoelectron emission spectrometry. Solid samples of the studied truxenes exhibited strong emission with high quantum yields (up to 47%). Electroluminescent properties of truxene derivatives <b>7–9</b> were investigated in solution-processed and vacuum-deposited organic light-emitting diodes (OLEDs). Greenish-white nondoped OLEDs with maximum brightness of 7000 cd/m² and maximum external quantum efficiency of 3.8% were fabricated using truxene-cored compound <b>7</b> as the fluorescent emitter

Exciplex-Enhanced Singlet Emission Efficiency of Nondoped Organic Light Emitting Diodes Based on Derivatives of Tetrafluorophenylcarbazole and Tri/Tetraphenylethylene Exhibiting Aggregation-Induced Emission Enhancement

Two derivatives of tetrafluorophenylcarbazole and tri/tetraphenylethylene displaying aggregation-induced emission enhancement were synthesized and investigated by theoretical and experimental tools. The synthesized compounds exhibit efficient emission in solid state with fluorescence intensity maxima at 511 and 502 nm and photoluminescence quantum yields of 57 and 27%. They exhibit high thermal stability with 5% weight loss temperatures of 362 and 314 °C and glass-forming properties with glass-transition temperatures of 112 and 80 °C. Ionization potentials measured by photoelectron emission spectrometry were found to be comparable (5.83 and 5.87 eV). The layers of the compounds showed bipolar charge-transporting properties with balanced electron and hole mobilities reaching 10^–3 cm²/V s at high electric fields. Exciplex-host-based OLEDs containing one or two emitting layers of tetraphenylethenyl-containing emitter were fabricated and showed more than 50% higher external quantum efficiency as compared to that of the corresponding nondoped device. The best nondoped OLED containing the synthesized emitter showed turn-on voltage of 9.1 V, maximum brightness of 11 800 cd/m², maximum current efficiency of 4.5 cd/A, and external quantum efficiency of ca. 1.7%. The best modified device, with exciplex-based host layer showed turn-on voltage of 9.1 V, maximum brightness of 16 300 cd/m², maximum current efficiency of 7.3 cd/A, and external quantum efficiency of ca. 2.6%

Star-Shaped Carbazole Derivatives for Bilayer White Organic Light-Emitting Diodes Combining Emission from Both Excitons and Exciplexes

Star-shaped carbazole-based compounds were synthesized by the Buchwald–Hartvig method. The materials were examined by various experimental and theoretical methods, including differential scanning calorimetry, UV spectrometry, electron photoemission, time-of-flight techniques, and DFT (B3LYP) calculations. The synthesized compounds showed high thermal stability with the initial weight loss temperature higher than 400 °C. The electron photoemission spectra of the layers of the amorphous materials showed ionization potentials of 4.9 eV. Tri­(9-hexylcarbazol-3-yl)­amine showed high hole mobility (μ = 10^–3 cm² V^–1 s^–1 at an electric field of 3.6 × 10⁵ V/cm). The star-shaped compounds were used for the preparation of bilayer white organic light-emitting diodes which combine emission from both excitons and exciplexes. The brightness of the white organic light emitting diode at 7 V is 300 cd/m² with current efficiency 2.3 cd/A and CIE coordinates (0.37, 0.35) which are very close to the equienergy white point (0.33, 0.33)

Deep-Blue High-Efficiency TTA OLED Using <i>Para</i>- and <i>Meta</i>-Conjugated Cyanotriphenylbenzene and Carbazole Derivatives as Emitter and Host

Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a <i>para</i>-junction revealed higher thermal stability (<i>T</i>_ID up to 480 °C), lower ionization potentials (5.51–5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90–0.97). <i>Meta</i>-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the <i>para</i>-position induces intensive delayed fluorescence via a triplet–triplet annihilation up-conversion mechanism. By utilization of the <i>para</i>-substituted derivative as an emitter and the <i>meta</i>-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated

Structure Properties Relationship of Donor–Acceptor Derivatives of Triphenylamine and 1,8-Naphthalimide

Solution-processable donor–acceptor molecules consisting of triphenylamine core and 1,8-naphthalimide arms were designed and synthesized by palladium-catalyzed Heck reaction. Dilute solutions of the synthesized compounds show strong absorption peaks in the visible wavelength range from 400 to 550 nm, which can be ascribed to the intramolecular charge transfer. Fluorescence quantum yields of dilute solutions of the synthesized materials range from 0.45 to 0.70, while those of the solid samples are in the range of 0.09–0.18. The synthesized molecules exhibit high thermal stability with the thermal degradation onset temperatures ranging from 431 to 448 °C. The compounds form glasses with glass-transition temperatures of 55–107 °C. DFT calculations show that HOMO and LUMO orbitals are almost entirely localized on the donor and acceptor moieties, respectively. Consequently, the frontier orbital energies for the three synthesized compounds are similar and practically do not depend on the number of 1,8-naphthalimide moieties. Ionization potentials of the solid samples (5.75–5.80 eV) are comparable. The charge-transporting properties of the synthesized materials were studied using xerographic time-of-flight method. Hole mobilities in the layers of the compounds having one and two 1,8-naphthalimide moieties exceed 10^–3 cm²·V^–1·s^–1 at high electric fields at room temperature. The differences on the hole mobilities between the three synthesized compounds are discussed in the frame of Marcus theory by comparing the reorganization energy and electronic coupling parameters