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)

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