Thiophene Disubstituted Benzothiadiazole Derivatives: An Effective Planarization Strategy Toward Deep-Red to Near-Infrared (NIR) Organic Light-Emitting Diodes.

Wentao Xie,Shi-Jian Su,Mengke Li,Kunkun Liu,Yuguang Ma,Xinyi Cai,Cheng Gu,Xiaohui Tang,Binbin Li,Zhenyang Qiao

doi:10.3389/fchem.2019.00276

Abstract

As one of the three primary colors that are indispensable in full-color displays, the development of red emitters is far behind the blue and green ones. Here, three novel orange-yellow to near-infrared (NIR) emitters based on 5,6-difluorobenzo[c][1,2,5]thiadiazole (BTDF) namely BTDF-TPA, BTDF-TTPA, and BTDF-TtTPA were designed and synthesized. Density functional theory analysis and photophysical characterization reveal that these three materials possess hybridized local and charge-transfer (HLCT) state feature and a feasible reverse intersystem crossing (RISC) from the high-lying triplet state to the singlet state may conduce to an exciton utilization exceeding the limit of 25% of traditional fluorescence materials under electrical excitation. The insertion of thiophene with small steric hindrance as π-bridge between the electron-donating (D) moiety triphenylamine (TPA) and the electron-accepting (A) moiety BTDF not only results in a remarkable 67 nm red-shift of the emission peak but also brings about a large overlap of frontier molecular orbitals to guarantee high radiative transition rate that is of great significance to obtain high photoluminescence quantum yield (PLQY) in the “energy-gap law” dominated long-wavelength emission region. Consequently, an attractive high maximum external quantum efficiency (EQE) of 5.75% was achieved for the doped devices based on these thiophene π-bridged emitters, giving a deep-red emission with small efficiency roll-off. Remarkably, NIR emission could be obtained for the non-doped devices, achieving an excellent maximum EQE of 1.44% and Commission Internationale de l'Éclairage (CIE) coordinates of (0.71, 0.29). These results are among the highest efficiencies in the reported deep-red to NIR fluorescent OLEDs and offer a new π-bridge design strategy in D-π-A and D-π-A-π-D red emitter design.

Highlights

Since the creative invention of organic light-emitting diodes (OLEDs) by Tang and VanSlyke (1987), OLEDs have been receiving intense research for more than 30 years for the potential applications in flat panel display (Pfeiffer et al, 2002), solid state lighting (Kido et al, 1995) and other applications outside the visible range (Tessler et al, 2002)
In order to explore the relationship between the material property and molecular structure of these compounds, density functional theory (DFT) simulation was performed firstly using the Gaussian suite of programs (Gaussian 09-B01 package)
The highest occupied molecular orbitals (HOMOs) of these compounds are distributed throughout the whole molecular skeleton, but the lowest unoccupied molecular orbitals (LUMOs) are mainly located on the BTDF acceptor and slightly extended to the benzene or thiophene π-bridges

Summary

Introduction

Since the creative invention of organic light-emitting diodes (OLEDs) by Tang and VanSlyke (1987), OLEDs have been receiving intense research for more than 30 years for the potential applications in flat panel display (Pfeiffer et al, 2002), solid state lighting (Kido et al, 1995) and other applications outside the visible range (Tessler et al, 2002). Despite the introduction of the thiophene bridge results in about 67 nm red shift of the emission spectra, the molar extinction coefficients (ε) of the ICT transitions in toluene solution still remain a relatively high level (about 2–4 × 104 L mol−1 cm−1), which should most likely be ascribed to the effective planarization molecular design strategy (Jiang et al, 2017).

Results

Conclusion