# Carbazolyl Electron Donor and Pyridinyl Electron Acceptor Containing Derivatives as Potential Host Materials for Green Organic Light-Emitting Diodes

## TL;DR

This paper introduces new electroactive compounds for green organic light-emitting diodes, showing high thermal stability and efficient light emission.

## Contribution

The synthesis and characterization of novel carbazolyl and pyridinyl derivatives as host materials for OLEDs with improved performance.

## Key findings

- New derivatives RB71 and RB74 were synthesized via Ullmann coupling and showed high thermal stability.
- PhOLEDs using RB71 and RB70 hosts achieved high efficiencies, outperforming existing CBP hosts in some cases.

## Abstract

Here, we present two series of new electroactive compounds containing electron donors (carbazolyl) and electron acceptor (pyridinyl) fragments as potential host materials. The objective compounds 9-(2-ethylhexyl)-3,6-di [3-(methoxypyridin-3-yl)carbazol-9-yl]carbazoles RB71 and RB74 were synthesized by an Ullmann coupling reaction between the intermediate derivatives: 9-(2-ethylhexyl)-3,6-diiodocarbazole and corresponding 3-(methoxypyridin-3-yl)-9H-carbazole. Other target derivatives, 9-alkyl-3-[N-(9-alkylcarbazol-3-yl)-N-(4-methylpyridin-2-yl)amino]carbazoles RB70 and RB75, were also prepared, according to the Ullmann reaction method, from 2-amino-4-methylpyridine and the corresponding 3-iodo-9-alkylcarbazole. Thermogravimetric analysis confirmed that the new derivatives are highly thermally stable compounds, with 5% weight loss in the temperature range of 349 °C to 488 °C. According to differential scanning calorimetry results, some amorphous materials exhibit very high glass transition temperatures exceeding 150 °C in some cases, which is a significant advantage for compounds with potential applications in organic light-emitting devices. The electroluminescent properties of devices utilizing the new hosts RB71 or RB70 with 5.0, 10.0, 15.0, and 20.0 wt.% concentrations of the dopant tris(2-phenylpyridine)iridium(III), Ir(ppy)3, were demonstrated. All the PhOLEDs emitted light at approximately 515 nm with CIE coordinates of (0.30, 0.61) due to Ir(ppy)3 emissions. The most efficient device with RB71 host demonstrated a maximum power efficacy of 8.0 lm/W, maximum current efficiency of 12.7 cd/A, and maximal external quantum efficiency of 5.4% with a relatively low turn-on voltage of 4.3 eV, as well as luminance exceeding 4000 cd/m2. Additionally, 15 wt.% Ir(ppy)3 emitter-based PhOLED with RB70 host outperformed the other devices by displaying a maximum power efficacy of 9.6 lm/W, maximum current efficiency of 16.0 cd/A, and maximal external quantum efficiency of 6.7% with a relatively low turn-on voltage of 3.7 eV, as well as luminance reaching 11,200 cd/m2. Some devices seem to exhibit higher efficiencies than those previously reported for OLEDs that utilize a 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) host.