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

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.

Keywords: deep-red to near-infrared (NIR) emission; donor-acceptor chromophores; hot-exciton; hybridized local and charge-transfer state (HLCT); organic light-emitting diodes.

Scheme 1

Synthetic route of the investigated molecules.

Figure 1

Optimized 3D structure and frontier molecular orbital distributions of BTDF-TPA, BTDF-TTPA, and BTDF-TtTPA.

Figure 2

The energy landscape of singlet and triplet excited states for the BTDF-based compounds.

Figure 3

UV-vis absorption (solid lines) and photoluminescence (PL) (dashed lines) spectra of the investigated molecules in toluene solution (10⁻⁵ M) at room temperature.

Figure 4

Linear fitting of the Lippert–Mataga model for BTDF-TPA, BTDF-TPA, and BTDF-TtTPA (f, orientation polarization of solvent media; v_a − v_f, Stokes shift).

Figure 5

(A) Schematic energy level diagram of the doped and non-doped devices based on BTDF-TTPA and BTDF-TtTPA; (B) CIE coordinates of the doped and non-doped devices using BTDF-TTPA and BTDF-TtTPA as the emitter at the current density of 1 mA cm⁻² (the inset photograph is the device of BTDF-TtTPA 1 wt.% in CBP); (C) Current density-voltage-luminance and (D) external quantum efficiency-current density characteristics of the doped devices (Inset: EL spectra of the doped devices at the current density of 1 mA cm⁻²); (E) Current density-voltage-luminance and (F) external quantum efficiency-current density characteristics of the non-doped devices (Inset: EL spectra of the non-doped devices at the current density of 1 mA cm⁻²).