Substituent effect on the photophysical properties, electrochemical properties and electroluminescence performance of orange-emitting iridium complexes

Abstract

A series of bis(2-phenylbenzothiozolato-N,C^2′)iridium(acetylacetonate) [(bt)₂Ir(acac)] derivatives, 1–4, were synthesized. Different substituents (CF₃, F, CH₃, OCH₃) were introduced in the benzothiazole ring to study the substituent effect on the photophysical, electrochemical properties and electroluminescent performance of the complexes, and finally to select high-performance phosphors for use in organic light-emitting diodes (OLEDs). All complexes 1–4 and (bt)₂Ir(acac) are orange-emitting with tiny spectral difference, despite the variation of the substituent. However, the phosphorescent quantum yield increases with the electron-withdrawing ability of the substituent. This is in contrast to the previous observation that the substituent in the phenyl ring bonded to the metal center of (bt)₂Ir(acac) not only affected the luminescent quantum efficiency but also greatly tuned the emission color of the complexes. Quantum chemical calculations revealed that the substituents in this position do not make a significant contribution to both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which probably accounts for the fact that they do no strongly influence the bandgap and emission color of the complexes. Orange OLEDs were fabricated using 1–4 as doped emitters. The electron-withdrawing CF₃ and F groups favor improving the electroluminescence efficiency in comparison with that of the parent (bt)₂Ir(acac), while electron-donating CH₃ and OCH₃ are not favorable for light emission. The complex 1 based OLED exhibited a maximum luminance efficiency of 54.1 cd A⁻¹ (a power efficiency of 24 lm W⁻¹ and an external quantum efficiency of 20%), which are among the best results ever reported for vacuum deposited orange OLEDs so far.