Abstract
The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.
Highlights
Organic light emitting diodes (OLEDs) are overtaking the field of display applications due to several favorable characteristics in comparison with classic liquid crystal devices (LCDs) such as lower energy consumption and simpler technology (Endo et al, 2011; Sasabe and Kido, 2013)
We find that the use of a para-phenyl bridge or a meta-phenyl bridge both moves the lowest energy triplet state onto the center of the molecule so that its orbitals expand over more than two rings
This lowers the triplet state energy. This finding is consistent with earlier observations by us made on carbazole-biphenyl type molecules that do not contain any donor-acceptor type interactions, as well as with studies by Brunner and coworkers on carbazole derivatives, who emphasized the strong dependence of the triplet state energy on the number of aromatic rings over which the triplet wavefunction can spread (Brunner et al, 2004; Bagnich et al, 2015a,b)
Summary
Organic light emitting diodes (OLEDs) are overtaking the field of display applications due to several favorable characteristics in comparison with classic liquid crystal devices (LCDs) such as lower energy consumption and simpler technology (Endo et al, 2011; Sasabe and Kido, 2013). Pure emitter layers usually show self quenching, it is necessary to dilute the emitters in suitable host materials These hosts have to fulfill several requirements such as having a high triplet energy level, chemical and thermal robustness, and balanced charge carrier mobilities (Wong and Zysman-Colman, 2017). In order to have a balanced charge carrier transport, bipolar carbazole-based hosts were developed by adding acceptor units such as cyano, triazine, and phosphine oxide groups (An et al, 2011; Ding et al, 2015; Kukhta et al, 2017; Shin et al, 2018) or formation of dimers and trimers (Tomkeviciene et al, 2011) Most of these carbazole-based hosts are limited to the use of blue greenish emitters since their triplet energies are below 3 eV. Host materials remain less developed in comparison with emitters, some standard approaches and guidelines could be helpful to progress this field
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