Protecting Benzylic CH Bonds by Deuteration Doubles the Operational Lifetime of Deep‐Blue Ir‐Phenylimidazole Dopants in Phosphorescent OLEDs

Hye Jin Bae,Virendra Kumar Rai,Jong Soo Kim,Alexander Yakubovich,Satoko Ishibe,Won‐Joon Son,Yongsik Jung,Jun Chwae,Mu‐Hyun Baik,Hyeonho Choi,Sangho Park,Jinhoon Jeong,Sunghan Kim

doi:10.1002/adom.202100630

Abstract

AbstractMuch effort has been dedicated to increase the operational lifetime of blue phosphorescent materials in organic light‐emitting diodes (OLEDs), but the reported device lifetimes are still too short for the industrial applications. An attractive method for increasing the lifetime of a given emitter without making any chemical change is exploiting the kinetic isotope effect, where key CH bonds are deuterated. A computer model identifies that the most vulnerable molecular site in an Ir‐phenylimidazole dopant is the benzylic CH bond and predicts that deuteration may hamper the deactivation pathway involving CH/D bond cleavage notably. Experiments show that the device lifetime until the initial luminance diminishes to 70% (LT70) of a prototype phosphorescent OLED device can be doubled to 355 hours with a maximum external quantum efficiency of 25.1% at 1000 cd m−2. This is one of the best operational performances of blue phosphorescent OLEDs observed to date in a single stacked cell.

Highlights

IntroductionPhosphorescent transition metal complexes have attracted much attention as emitting materials in organic light-emitting diodes (OLEDs) due to their short excited state lifetime, and because their internal quantum efficiency can in principle reach 100%.1–5 Cyclometalated Ir(III) complexes are interesting, as their color can be tuned over the entire visible range from blue to red.[6,7,8,9,10,11,12,13,14,15,16] But finding blue phosphorescent materials that are robust enough for industrial applications proved challenging, which may be rationalized by the large band gap[17,18,19] compared to red or green emitters, leading to high-energy excited states that are more reactive and are expected to degrade more
We demonstrate that kinetic isotope effect (KIE) can significantly increase the operational lifetime of blue phosphorescent materials
We examined four homoleptic iridium(III)-complex carrying phenylimidazole ligands and their deuterated analogues to investigate whether the kinetic isotope effect can be employed to protect C–H bonds and extend the lifetime of deep blue emitters in phosphorescent organic light-emitting diodes (OLEDs) devices

Summary

Introduction

Phosphorescent transition metal complexes have attracted much attention as emitting materials in organic light-emitting diodes (OLEDs) due to their short excited state lifetime, and because their internal quantum efficiency can in principle reach 100%.1–5 Cyclometalated Ir(III) complexes are interesting, as their color can be tuned over the entire visible range from blue to red.[6,7,8,9,10,11,12,13,14,15,16] But finding blue phosphorescent materials that are robust enough for industrial applications proved challenging, which may be rationalized by the large band gap[17,18,19] compared to red or green emitters, leading to high-energy excited states that are more reactive and are expected to degrade more . Cyclometalated Ir(III) complexes are interesting, as their color can be tuned over the entire visible range from blue to red.[6,7,8,9,10,11,12,13,14,15,16] But finding blue phosphorescent materials that are robust enough for industrial applications proved challenging, which may be rationalized by the large band gap[17,18,19] compared to red or green emitters, leading to high-energy excited states that are more reactive and are expected to degrade more Such degradation may give rise to short device lifetime and low efficiency. Fluorine-free, homoleptic phenylimidazolebased Ir(III) dopants showed promising performance,[24,41] but the improvement in OLED device lifetimes were only moderate

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Results

Conclusion