Avoiding Energy Loss on TADF Emitters: Controlling the Dual Conformations of D–A Structure Molecules Based on the Pseudoplanar Segments

Abstract

The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light-emitting devices (OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9-dimethyl-9,10-dihydroacridine, are actually pseudoplanar segments and have two possible conformations-a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

“…There is another smaller amount of TADF species that feature two stable conformations within the single molecular structure containing acridine or phenoxazine units. The famous TADF molecule DMAC‐DPS (Figure was found to have two conformations: a planar conformation with large steric hindrance yet expanded conjugations, and a crooked form with twisted orthogonal conformation that possess the smallest steric hindrance yet also smallest conjugation between D and A . In fact, both nearly‐orthogonal and nearly‐planar conformations can be observed through simple PL measurement in co‐deposited films, with emission from the planar structure observed in the shorter wavelength range, and the orthogonal one in the longer wavelength range, consistent with the theoretical predictions of a potential energy surface study (Figure ).…”

“… Theoretical computations of energy surface with variation of different dihedral angles between D and A. Reproduced with permission from Ref . Copyright 2017, Wiley‐VCH.…”

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

“…Meanwhile, being consistent with the El‐Sayed rule of intersystem crossing (ISC) process, when the natures of 1 CT and 3 CT are different, the SOC between 1 CT and 3 CT might be efficient as well . More importantly, avoiding dual emission phenomenon, which caused by conformational transforming brought by flexible moieties, is considered as the first priority of TADF molecular design strategy . Conformational transforming not only results in energy loss, but also limits the triplet exciton utilization because not all possible conformations have potential to achieve TADF phenomenon …”

A Methodological Study on Tuning the Thermally Activated Delayed Fluorescent Performance by Molecular Constitution in Acridine–Benzophenone Derivatives

“…Notably,b oth TADF emitters have slightly highert riplets than that of TPBi,w hich mayc ause energy loss in EL devices. [14] The OLED device based on BP-Ph-DPAC,o wing to its lack of TADF property,e xhibited the lowest EQE, luminance, and CE values of the three emitters, even with the possible triplet-triplet mechanism.…”