Host Materials For Blue OLEDs Research Articles

Intramolecular C-H Activation as an Easy Toolbox to Synthesize Pyridine-Fused Bipolar Hosts for Blue Organic Light-Emitting Diodes.

The development of high-performance blue organic light-emitting diodes (OLEDs) remains a challenging task. While exploiting new blue emitters has attracted great interest, developing host materials that considerably determine device performance obviously lags behind. Herein, we present an ease of access to the structurally diverse benzoheteroaromatic-fused pyridine skeletons by the iridium-catalyzed intramolecular C-H/C-H coupling reaction, which unlocks an unparalleled opportunity to rapidly assemble a library of pyridine-fused bipolar host materials. As an illustrated example, the benzo[4,5]thieno[2,3-b]pyridine skeleton (BTP) has been made to a pseudo-symmetric host (DCz-BTP). The merging of a pyridine fragment enables strong intermolecular interactions, leading to satisfactory bipolar transporting properties. Utilizing DCz-BTP as the host, blue thermally activated delayed fluorescent OLEDs (TADF-OLEDs) exhibit a low turn-on voltage of 2.8 V and a high maximum external quantum efficiency (EQEmax ) of 29.0 % and blue TADF-sensitized florescent OLEDs (TSF-OLEDs) give an EQEmax as high as 20.5 %, revealing the potential of the BTP skeleton for the construction of high-performance host materials.

Dissociative nature of C(sp2)-N(sp3) bonds of carbazole based materials via conical intersection: simple method to predict the exciton stability of host materials for blue OLEDs: a computational study.

In this work, the origin of the singlet and triplet exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds around nitrogen (carbazoles, acridines, etc.), connecting donor and acceptor units, was unravelled using DFT and CASSCF methods. The results reveal that molecules (employed in OLEDs) with basic units containing C(sp2)-N(sp3) bonds (nitrogen connected to carbon in a triangular fashion) have a natural tendency to fragment at the C-N bond through an S1/S0 conical intersection (CI). The calculation of barrier heights, to reach a dissociation point, indicates that degradation via triplet states is kinetically less feasible (ΔGT1-TS* > 25 kcal mol-1) compared to that via the first singlet excited state (ΔGS1-TS* ∼7-30 kcal mol-1). However, the long lifetime of triplets (as compared to singlets) aids in the reverse intersystem crossing from triplet to singlet state for subsequent degradation. From the results and inference, ΔGS1-TS* and ΔES1-T1 are proposed to be the controlling factors for exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds. Furthermore, multiple functionalization of carbazole moieties reveals that polycyclic aromatic systems employed as acceptor units of host materials are best suited for PhOLEDs as they will increase their lifetime due to the larger ΔGS1-TS* and ΔES1-T1. For TADF-based devices, materials with fused ring systems (with N(sp3) at the centre) in the donor unit are the most recommended ones based on the findings of this work, as they avoid the dissociative channel altogether. A negative linear correlation between ΔGS1-TS* and HOMO-LUMO gap is observed, which provides an indirect way to predict the kinetic stability of these materials in excitonic states. These initial results are promising for the future development of the QSAR-type approach for the smart design of host materials for long-life blue OLEDs.

Synthesis, Linear, and Nonlinear Optical Properties of Phosphonato‐Substituted Bithiophenes Derived from 2,2‐Dimethyl‐1,3‐propanediol

The synthesis, linear absorption, emission spectra, and third‐order nonlinear optical (NLO) absorptions at 430 nm of two new series of chalcogenophosphonato‐substituted bithiophenes, pdP(X)T2H and pdP(X)T2P(X)pd [pd = OCH2C(CH3)2CH2O; T = 2,5‐C4H2S; X = O, S, Se] are reported. The X‐ray crystal structures of pdP(X)T2H and pdP(X)T2P(X)pd (X = S, Se) have also been determined. The pdP(X)T2P(X)pd compounds show weak intermolecular aromatic π–π interactions while pdP(S)T2H shows S–π interactions. The compounds exhibit emission at wavelengths ranging from 380–400 nm. Some of the quantum yields are significantly larger than that of 2,2′‐bithiophene. The linear absorption spectra, emission spectra, and emission quantum yields show distinct trends with respect to the chalcogen and the number of substituents attached to the 2,2′‐bithiophene ring. NLO transmission measurements of pdP(X)T2P(X)pd (X = S, Se) indicate that saturated solutions of both compounds show significantly greater NLO absorption than saturated solutions of bpP(X)T2P(X)bp (bp = 2,2′‐biphenol, X = S, Se) do. Notably, the fluence at which the transmittance of a 0.27 m CH2Cl2 solution of pdP(Se)T2P(Se)pd falls to 50 % is 0.2 J/cm2. This rivals the best blue nonlinear absorbers in literature. The excellent emission quantum yields and NLO absorptions make these compounds promising candidates for optical power limiting and as host materials for blue OLEDs.