Molecular engineering of an electron-transport triarylphosphine oxide-triazine conjugate toward high-performance phosphorescent organic light-emitting diodes with remarkable stability

Through-Space Conjugated Electron Transport Materials for Improving Efficiency and Lifetime of Organic Light-Emitting Diodes

Thermally stable electron transport (ET) materials with high electron mobility and high triplet state energy level are highly desired for the fabrication of efficient and stable organic light-emitting diodes (OLEDs). Herein, a new design strategy of constructing through-space conjugated folded configuration is proposed to explore robust ET materials, opposite to the widely used planar configuration. By bonding two quinolines to the 9,10-positions of phenanthrene, two novel folded molecules with high thermal and morphological stabilities and high triplet state energy levels (>2.7 eV) are created. These folded molecules possess excellent ET ability with electron mobilities of three orders of magnitude higher than those of linear and planar counterparts. Theoretical calculation and crystallography analysis demonstrate the through-space conjugated folded configuration has not only reduced reorganization energy but also enlarged charge transfer integral at various dimensions, bringing about efficient multi-dimensional ET, independent of molecular orientation. By adopting the folded molecule as ET layers, OLEDs with no matter delayed fluorescence or phosphorescence emitters can achieve high external quantum efficiencies and long operational lifetimes simultaneously. This work paves a new avenue towards robust ET materials to improve efficiency and stability of OLEDs.

Room-Temperature Phosphorescence of Nicotinic Acid and Isonicotinic Acid: Efficient Intermolecular Hydrogen-Bond Interaction in Molecular Array

Pure organic room-temperature phosphorescence (RTP) has attracted wide interest due to its unique advantages and promising applications. However, it is still challenging to develop efficient RTP through precise molecular design. In this work, RTP is observed from two simple aromatic acids, nicotinic acid (NA) and isonicotinic acid (INA), in the crystal state. Single crystal structure analysis indicates that an intense hydrogen bond between the pyridine nitrogen atom and the carboxyl group results in zigzag and linear molecular packing modes in NA and INA crystal. From theoretical calculations, the hydrogen bond can effectively promote the intersystem crossing process and stabilize triplet exciton. The identical molecular orientations in the molecular array contribute to the larger dipole moment of INA as compared to that of NA, which should be responsible for the red-shifted photoluminescence and RTP of INA. When the hydrogen bond is destructed by grinding or deprotonation, the RTP decreases sharply, further confirming the crucial role of the hydrogen bond on RTP.

Diversity of Luminescent Metal Complexes in OLEDs: Beyond Traditional Precious Metals

Organic light-emitting diodes (OLED) have attracted increasing attention due to their excellent properties, such as self-luminosity, high color gamut and flexibility, and potential applications in display, wearable devices and lighting. The emitters are the most important composition in OLEDs, mainly classified into fluorescent compounds (first generation), metal phosphorescent complexes (second generation), and thermally activated delayed fluorescence (TADF) materials (third generation). In this review, we summarize the advances of novel emitters of organic metal complexes in the last decade, focusing on coinage metals (Cu, Ag, and Au) and non-precious metals (Al, Zn, W, and alkali metal). Also, the design strategy of d¹⁰ and Au(III) complexes was discussed. We aim to provide guidance for exploring efficient metal complexes beyond traditional phosphorescent complexes.