Elsevier

Dyes and Pigments

Volume 184, January 2021, 108810
Dyes and Pigments

Orange-red thermally activated delay fluorescence emitters based on asymmetric difluoroboron chelated enaminone: Impact of donor position on luminescent properties

https://doi.org/10.1016/j.dyepig.2020.108810Get rights and content

Highlights

  • Two enaminone difluoroboron complex acceptor-based TADF emitters are synthesized.

  • The position of the donor unit influences their fluorescent properties.

  • The PLQYs of DMAC-BF (52%) is higher than that of BF-DMAC (29%).

  • DMAC-BF exhibits orange-red EL emission with a higher EQE of 11.3% than BF-DMAC.

Abstract

Asymmetric enaminone difluoroboron complexes (BFs) exhibit highly solid emission and appropriated electron-accepting properties, whereas the position of the substituents has a significant influence on their optical properties due to the structural asymmetry. In this work, we employed BF as acceptor and 9,9-dimethylacridine (DMAC) as donor to construct two thermally activated delay fluorescence (TADF) molecules DMAC-BF and BF-DMAC, in which DMAC unit was set on the B-O and B-N sides of BF, respectively. DMAC-BF and BF-DMAC exhibited the broad orange-red emission bands in toluene, while they emitted yellow and red fluorescence in solid state, respectively. The photoluminescence quantum yield (PLQY) of DMAC-BF (52%) is higher than that of BF-DMAC (29%), owing to its lower nonradiative decay rate. Solution-processed organic light-emitting diode (OLED) devices based on both of DMAC-BF and BF-DMAC exhibited orange-red electroluminicence spectra. Owing to its higher photoluminescence quantum yield, DMAC-BF-based OLED exhibited better electroluminicence performance with a maximum current efficiency of 24.9 cd/A, a maximum power efficiency of 23.0 lm/W, and a maximum EQE of 11.3%.

Introduction

Organic light-emitting diodes (OLEDs), owing to their unique merits such as self-luminescence, fast response and flexibility have been developed rapidly for flexible displays and lighting applications [[1], [2], [3], [4]]. As the most important part of OLED device, three generations of organic electroluminescent (EL) materials have been reported, namely fluorescence materials, phosphorescent materials and thermally activated delayed fluorescence (TADF) materials [[5], [6], [7], [8], [9], [10]]. Among these materials, TADF emitters, which could achieve 100% internal quantum efficiency (IQE) without of the assistance heavy metals, have received great attention in recent years, because they can utilize both singlet and triplet excitons through efficient reverse intersystem crossing (RISC) from nonradiative triplet excitons to radiative singlet excitons [[11], [12], [13], [14], [15], [16], [17]]. To obtain efficient TADF emitters, twisted donor (D)-acceptor (A) structures with small singlet-triplet energy splitting (ΔEST) value have been emerged in large numbers [[18], [19], [20], [21], [22], [23]].

The TADF materials based on boron-contained acceptor units have already established their status as one of the important classes of OLED materials [[24], [25], [26]]. TADF materials based on tricoordinate boron acceptors containing the vacant pz orbital exhibit excellent EL properties. However, they are limited to blue and green TADF materials owing to moderate electron-accepting properties of tricoordinate boron [[27], [28], [29], [30], [31]]. Comparatively, tetracoordinate boron acceptors with tunable ligand backbone and assisted by the fluorine substituents on the boron center, e.g. N,N-, O,O- and N,O-chelated difluoroboron complexes, could adjust the spectra in a wide range [[32], [33], [34], [35]]. In addition, chelated six-membered ring rigidified the ligand backbone and diminished the non-radiative relaxation process. For example, Wang's group reported sky-blue TADF emitter based on N,O-chelated difluoroboron fused with oxadiazole exhibits better EL performances than the acceptor non-chelated difluoroboron [36]. At the same time, Zhang et al. connected the N,O-chelated difluoroboron acceptor with different donors to obtain a series of air-stable TADF materials with high photoluminescence quantum yields (PLQYs), which shifted the emission peak from 460 nm to 530 nm [37]. As reported by Adachi and co-works, the incorporation of a O,O- difluoroboron acceptor with two triphenylamine groups produced near-infrared TADF emission (700–800 nm) with a maximum EQE of 10% [38]. Nevertheless, orange-red TADF emitters based on boron difluoride complexes are still relatively unexplored.

Enaminone derivatives composed of the electron-donating enamine and electron-withdrawing enone show charge transfer and aggregation induced emission (AIE) characteristics as proved by our previous works [39,40]. When chelated with the boron sources to form the enaminone difluoroboron complexes (BFs), the rigidified enaminone skeletons lead to the intense fluorescence in solid state [[41], [42], [43]]. The introduction of difluoroboron structure also lowers the LUMO energy effectively, and thus the BFs can be used as an effective acceptor block to create TADF molecules. The substituents at different positions of the BF ring may have a significant influence on their fluorescence properties, due to the electronic asymmetry and the existence of enaminone and enolimine tautomer [[44], [45], [46], [47]]. However, the effect of donor position for TADF molecules based on asymmetric BF acceptors on the optical properties have not been investigated yet. In this work, we design and synthesis two orange-red TADF molecules which attached the 9,9-dimethylacridine (DMAC) donor at the B-O and B-N sides of asymmetric BF acceptor, respectively, namely DMAC-BF and BF-DMAC (Scheme 1). DMAC-BF exhibited blue-shifted and enhanced emission compared with BF-DMAC. Owing to its higher fluorescence efficiency, solution-processed OLED device based on DMAC-BF obtained an orange-red EL peak at 598 nm with a maximum EQE of 11.3%, which is much higher than that of BF-DMAC.

Section snippets

General information

The 1H NMR and 13C NMR spectra were recorded at 500 MHz (Bruker AV) with TMS as the internal standard. 11B{1H} and 19F{1H} NMR spectra were recorded at 400 MHz (Bruker AV) at 298 K. All chemical shifts are given in ppm and all coupling constants (J values) are reported in Hertz (Hz). High-resolution mass spectra were obtained by using LTQ Orbitrap Velos Pro. Thermal gravimetric analysis (TGA) was performed with TA-TGA55 under nitrogen flow at the heating rate of 10 °C/min. Differential scanning

Synthesis and characterization

The synthetic routes and molecular structures of DMAC-BF and BF-DMAC are shown in Scheme 1. The synthesis of bromine-substituted enaminone derivatives were reported in our previous works [39,40]. Buchwald-Harding amination of bromine-substituted enaminones with DMAC provided the intermediates DMAC-BE and BE-DMAC with excellent yields. Then these two intermediates reacted with the boron trifluoride diethyl ether complex in the presence of triethylamine in anhydrous dichloromethane to afford the

Conclusions

In summary, we have designed and synthesized two orange-red TADF emitters based on the new asymmetric enaminone difluoroboron complex acceptor coupled with DMAC donor unit on its B-O and B-N sides, respectively. Two positional isomers displayed distinctly different photoluminescence behavior. DMAC-BF and BF-DMAC emitted yellow and red fluorescence in solid state, respectively. Through analysis of the photophysical properties and potential surfaces scan, DMAC-BF with donor attached at the phenyl

Author contributions

Under supervision by Hui Tong and Lixiang Wang, Hua Li performed sample preparation, testing and experimental data analysis. Haiyang Shu and Xin wang performed sample characterization. Hongkun Tian performed theoretical calculations. Xiaofu Wu revised the manuscript. All authors read and contributed to the manuscript.

Funding information

This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12010200) and the National Natural Science Foundation of China (Grant Nos. 51833009, 21674111, 51973211 and 21322403).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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