Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System

Abstract

Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor-acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.

Figure 1

Chemical structures of QD, a-QD, and TQB and the proton transfer model for TQB. The energy diagram shows the computationally calculated energies in the ground state for TQB-TA and in the excited state for TQB-TA, TQB-TB, TQB-TC, and TQB-TD. S₁-ver: vertical excitation. S₁-adi: adiabatic excitation.

Figure 2

Calculated 2D potential energy surface (PES) plots. (a) Chemical structure of TQB with atom labels. (b) PES of the first excited state S₁. (c) PES of the ground state S₀. Symbols: star = TQB-TA, circle = TQB-TB, triangle = TQB-TC. TQB-TD was not investigated because of its much higher energy at its stationary point.

Figure 3

Distributions of electron density in HOMO–1, HOMO, LUMO, and LUMO+1. The computational calculations were performed at the B3LYP/6-31++G(d,p) level for TQB-TA, TQB-TB, a-QD-TA, and a-QD-TB.

Figure 4

PL and transient PL spectra in solution and solid state. (a) Ultraviolet–visible (UV–vis) absorption (dashed lines) and photoluminescence (PL) spectra (solid lines) for TQB measured in solution and calculated by DFT (for the calculations, the TQB-TA structure was used for absorption and the TQB-TB structure for the emission). (b) UV–vis absorption (dashed lines) and PL spectra (solid lines) for TQB-doped films (ca. 10 wt %), a neat TQB film, and a TQB crystal (excitation spectrum is displayed instead of UV–vis absorption spectrum for the crystal). (c) Temperature dependence of transient PL decay for a TQB-doped film of DPEPO. Inset: PL spectra at 6 K (no delayed component) and at 300 K resolved into prompt and delayed components. (d) PL transient decay spectra at room temperature of TQB-doped films (ca. 10 wt %) of DEPPO, PPT, mCBP, CBP, and CzSi that were encapsulated in a glovebox. IRF is the instrument response function.

Figure 5

Characteristics of OLEDs using TQB as the emitter. (a) Energy level diagram of the devices. Energy levels are in units of eV, and values in parentheses indicate layer thicknesses in nm. (b) External electroluminescence quantum efficiency as a function of current density for OLEDs with different host materials. (c) Current-density–voltage (J–V) characteristics of the OLED devices.