Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Hisahiro Sasabe,Satoru Ohisa,Yuki Chikayasu,Ryutaro Komatsu,Junji Kido,Hiroki Arai,Tatsuya Ohsawa,Daisuke Yokoyama,Yuichiro Watanabe

doi:10.3389/fchem.2020.00427

Hisahiro Sasabe, Satoru Ohisa + Show 7 more

Open Access

https://doi.org/10.3389/fchem.2020.00427

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Journal: Frontiers in Chemistry	Publication Date: May 25, 2020
Citations: 25	License type: CC BY 4.0

Affiliation: Yamagata University

Abstract

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

Highlights

We propose that the contributions from multiple CH/π contacts can collaborate with other strong intermolecular interactions to realize significant horizontal orientation
We performed a systematic investigation on the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials
It can be considered that (i) an increase in the π-conjugation leads to stronger π-π stacking, (ii) the introduction of CN group induces CH/N weak hydrogen bond (H-bond), and (iii) the introduction of a strong acceptor P=O and ether groups induces CH/O H-bonds, and all these interactions enhance the horizontal orientation of TADF emitters

Summary

INTRODUCTION

A series of fluorescent emitters exhibiting significant delayed fluorescence, so-called thermally activated delayed fluorescent (TADF) emitters, has attracted much attention due to its potential usefulness in high-performance organic light-emitting devices (OLEDs) that can realize an internal quantum efficiency (ηint) of 100% (Uoyama et al, 2012; Sasabe and Kido, 2013; Adachi, 2014; Kaji et al, 2015; Lin et al, 2016; Im et al, 2017; Wong and Zysman-Colman, 2017; Yang et al, 2017; Komatsu et al, 2018). Recent rapid development in TADF emitters enables OLEDs to achieve an external quantum efficiency (ηext) over 30% (Kaji et al, 2015; Komino et al, 2016; Lin et al, 2016; Liu et al, 2017; Rajamalli et al, 2017; Wu et al, 2018; Ahn et al, 2019; Kondo et al, 2019) To obtain such a high-performance in OLEDs, horizontal orientation of the emission dipole moment (EDM) is absolutely essential. We have already developed a series of pyrimidine-based TADF emitters with different donor group(s) (Figure 1) (Komatsu et al, 2016a,b; Nakao et al, 2017) By comparing these three molecules, we can obtain insight into the effects of two factors: (i) donor number, which reflects the molecular shape anisotropy in these comparisons, and (ii) donor structure.

RESULTS AND DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT