Abstract
Carbazole (Cz) is the one of the most popular electron donors to develop thermally activated delayed fluorescence (TADF) emitters, but additional groups are generally required in the molecules to enhance the steric hindrance between Cz and electron acceptor segments. To address this issue, we replaced Cz with its derivative 1,3,6,8-tetramethyl-carbazole (tMCz) to develop TADF emitters. Two novel compounds, 6-(4-(carbazol-9-yl)phenyl)-2,4-diphenylnicotinonitrile (CzPN) and 2,4-diphenyl-6-(4- (1,3,6,8-tetramethyl-carbazol-9-yl)phenyl) nicotinonitrile (tMCzPN) were designed and synthesized accordingly. With the same and simple molecular framework, tMCzPN successfully exhibits TADF behavior, while CzPN is a non-TADF fluorophor, as the additional steric hindrance of methyl groups leads to a more twisted structure of tMCzPN. In the organic light-emitting diodes (OLEDs), tMCzPN exhibits extremely high forward-viewing maximum external quantum efficiency of 26.0%, without any light out-coupling enhancement, which is significantly higher than that of 5.3% for CzPN. These results indicate that tMCzPN is an excellent TADF emitter and proves that tMCz is a more appropriate candidate than Cz to develop TADF emitters.
Highlights
The (E)-1-(4-bromophenyl)-3-phenylprop2-en-1-one was synthesized by aldol reaction between benzaldehyde and 1-(4-bromophenyl)ethan-1-one
The cyanopyridine derivative was synthesized by cyclizing a pyridine ring between 3-oxo-3-phenylpropanenitrile and (E)-1-(4bromophenyl)-3-phenylprop-2-en-1-one with ammonium acetate as the source of nitrogen
Due to the insufficient steric hindrance of Cz, additional groups are generally required to enhance the separation between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for Cz-based thermally activated delayed fluorescence (TADF) emitters, resulting in complicated synthetic procedures and high costs
Summary
Organic light-emitting diodes (OLEDs) have attracted great attention and are considered as next-generation solid-state lighting and displays because of their flexibility, light weight, and lowcost fabrication (Pope et al, 1963; Tang and VanSlyke, 1987; Baldo et al, 1998; Goushi et al, 2012; Uoyama et al, 2012; Zheng et al, 2013; Liu et al, 2015b,c; Li et al, 2018; Shi et al, 2018). Based on spin quantum statistics, electrical excitation generates 25% singlet excitons and 75% triplet excitons in the devices (Segal et al, 2003). OLEDs based on traditional fluorescent emitters can only utilize singlet excitons with a maximum internal quantum efficiency (IQE) of 25% (Baldo et al, 1999; Segal et al, 2003). Phosphorescent OLEDs, using heavy metal complexes as emitters, were developed, and successfully realized with 100% IQE, due to the strong spin-orbit coupling effect of heavy metal irons (Baldo et al, 1998; Adachi et al, 2001; Sajoto et al, 2009; Yersin et al, 2011). Noble metal complexes lead to expensive costs and environmental hazards, which further constrains the development of phosphorescent OLEDs (Méhes et al, 2012; Zhang et al, 2014c, 2015).
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