Liquid-Crystalline Thermally Activated Delayed Fluorescence: Design, Synthesis, and Application in Solution-Processed Organic Light-Emitting Diodes

Abstract

Realizing both high efficiency and liquid crystallinity in one molecule remains a challenge in thermally activated delayed fluorescence (TADF) emission. Herein, two isomeric compounds─<i>m</i>-DPSAc-LC and <i>p</i>-DPSAc-LC with different connection positions between donor and acceptor moieties─were synthesized and characterized. Diphenylsulfone (DPS) was used as the acceptor, acridine (Ac) was used as the donor, and biphenyl derivatives (LC) were employed as the mesogenic group. Both compounds showed a smectic mesophase evidenced by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and temperature-dependent small-angle X-ray scattering (SAXS). The compound <i>p</i>-DPSAc-LC clearly exhibited thermally activated delayed fluorescence due to the much more distorted geometry, whereas <i>m</i>-DPSAc-LC showed simple fluorescence. Compared to the parent TADF molecules without appended mesogenic groups (DPS-Ac), these liquid-crystalline emitters possessed higher hole mobilities and improved device performance. The OLEDs fabricated via solution processing using the liquid-crystalline compound <i>p</i>-DPSAc showed a maximum external quantum efficiency of ∼15% and as such is the first example of a liquid-crystalline TADF material in an OLED device.