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 and Spectral Properties of Thianthrene and Its Benzoyl-Containing Derivatives

The IR absorption spectra of the recently synthesized series of benzoyl-containing thianthrene derivatives were studied in the context of their structural identification. Geometry optimizitation of the ground singlet state by density functional theory (DFT) calculations with the gradient and Hessian search were performed for thianthrene molecule in the framework of the C2v symmetry restriction. The excited singlet and triplet states of thianthrene were found to be distorted along the b3u vibrational mode of the D2h point group, as well as the ground state, which leads to the non-planar batterfly-like structure (C2v). But the excited states require additional symmetry reduction; they are closer to planarity but have no symmetry elements. Optimized ground states structure for the thianthrene-benzoyle molecule and its four derivatives with fluoro-substituents and different substitution positions were analysed through complete assignment of all their vibrational modes and comparison with experimental infrared absorption spectra. A good agreement between experimental data and DFT calculated IR spectra provides additional structural support to results of the X-ray diffraction analysis of all synthesized compounds. The Hirshfeld surfaces analysis of the crystalline 3-fluorobenzoylthianthrene (T3F) was performed in order to analyze intermolecular interactions in T3F crystal. It indicates the presence of weak CH...F, CH...S and CH...O intermolecular contacts, stabilizing the crystal structure of T3F. The CH...O interactions appear in the IR spectrum of T3F crystal as two vibrational modes with frequencies 3084 and 3078 cm-1. The intermolecular interactions CH...F and CH...S do not affect the IR spectrum of T3F