Properties of Dual Emissive Dendrimers Based on ThermallyActivated Delayed Fluorescence Dendrons and a Phosphorescent Ir(ppy)₃ Core

Abstract

Dual emission light‐emitting dendrimers composed of phosphorescent fac‐tris[2‐phenylpyridyl]iridium(III) [Ir(ppy)3] cores and thermally activated delayed fluorescence‐based (TADF‐based) dendrons have been prepared. The TADF‐based dendrons are designed to have green or blue emission. The dendrimers show solvatochromism, with the emission switching between phosphorescence and TADF depending on the medium in which the measurement is undertaken. Time‐dependent photoluminescence (PL) spectra measurements show that the TADF dendrons can act as an energy pool for emission from the phosphorescent core. The PL quantum yields (PLQYs) are found to be strongly dependent on the dielectric constant of the solvent, ranging from as high as 76% to as low as 0.3%. Neat films of the dendrimers are found to have relatively balanced hole and electron mobilities of order 10–6 cm2 V–1 s–1, with bilayer organic light‐emitting diodes (OLEDs) containing neat emissive layers having a maximum external quantum efficiency (EQE) of 4.7% for a film having a PLQY of 12%. Finally, the solution processed OLEDs fabricated using 0.4 mol% of the dendrimers blended with 9‐[3‐(9H‐carbazol‐9‐yl)phenyl]‐9H‐carbazole‐3‐carbonitrile result in green emission with maximum EQEs of 9.8% and 15.1% for the dendrimers with green and blue emissive TADF dendrons, respectively.

Understanding the emission from dendrimers composed of thermally activated delayed fluorescence-based dendrons and a phosphorescent fac-tris[2-(thiophen-2-yl)-4-(phenyl)quinoline]iridium(iii) core

“…The homoleptic iridium( iii ) complex TQIr was synthesised by a one-step complexation reaction between iridium ( iii ) chloride hydrate and 1 in glycerol and in the presence of silver trifluoroacetate, yielding the desired product in a 21% yield. For the synthesis of the dendrimers, the bromine functionalised blue, 2 , or green, 3 , dendrons 37 were borylated under palladium catalysed conditions to form their respective boronate esters, 4 or 5 , which were then immediately coupled with the bromine functionalised core complex 6 . 18 Standard Suzuki palladium cross-coupling conditions gave the target dendrimers BR and GR in 27% and 13% yields, respectively.…”

“…The lifetimes of the blue and green dendron emission from the dendrimers were consistent with that previously reported. 37 However, the key point is that core emission was observed out to milliseconds for both dendrimers, which provides strong evidence that the dendrons can act as an energy pool that feeds emission from the core.…”

“…The short wavelength emission has a solvent polarity dependence, which is the same as has been previously reported for BD and GD. 37 The effect can be most clearly seen Fig. 2 Cyclic voltammograms of the dendrimers (BR, GR) and the dendrons (BD, GD) versus Fc/Fc + redox couple.…”

“…36 One strategy to overcome this issue is to attach TADF chromophore-based dendrons to a phosphorescent core. 37 In this manuscript, we introduce two new solutionprocessable phosphorescent dendrimers designed to emit red light that are composed of either green or blue emitting TADFbased dendrons (Fig. 1).…”

“…36 One strategy to overcome this issue is to attach TADF chromophore-based dendrons to a phosphorescent core. 37…”