The magic methyl effect of thermally activated delayed fluorescent emitters on blue organic light-emitting diodes

Abstract

A methyl group is a common substituent in medicinal chemistry. The introduction of methyl groups always results in a profound enhanced biological activity of pharmaceuticals, known as the magic methyl effect. Meanwhile, a methyl group is also widely used for the construction of organic materials in organic light-emitting diodes (OLEDs). In this study, we systematically study the methyl effect of blue thermally activated delayed fluorescent (TADF) emitters on the photophysical properties and device performance in OLEDs. Three new blue TADF emitters with different numbers of methyl groups, namely 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, have been successfully designed and synthesized. It is found that the methyl group induces steric hindrance and greatly affects their photophysical, thermal, and TADF properties. Their emission maximum is gradually blue-shifted from 464 to 455 nm, simply by increasing the number of methyl groups. However, 3Me-HDT1 resulted in an unexpected low external quantum efficiency (EQE) of only 1%, in which 3Me-HDT1 decomposed upon device fabrication. Contrarily, the devices based on 1Me-HDT1 and 2Me-HDT1 result in high EQEs of up to 21.2% and 19.1%. Furthermore, applying 1Me-HDT1 for hyperfluorescent OLEDs leads to pure-blue electroluminescence at 471 nm, and a higher EQE of 26.2%, together with improved CIE_x,y of (0.13, 0.16).