Thermally Activated Delayed Fluorescence Materials Based on Homoconjugation Effect of Donor–Acceptor Triptycenes

Abstract

Donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2 were synthesized and their emissive properties were studied. They exhibited a blue-green fluorescence with emission lifetimes on the order of a microsecond in cyclohexane at room temperature. The long lifetime emission is quenched by O 2 and is attributed to thermally activated delayed florescence (TADF). Unimolecular TADF is made possible by the separation and weak coupling due to homo-conjugation of the HOMO and LUMO on different arms of the three-dimensional donoracceptor triptycene. Organic light emitting devices (OLEDs) were fabricated using TPA-QNX(CN)2 and TPA-PRZ(CN)2 as emitters which displayed electroluminescence with efficiencies as high as 9.4% EQE.Since the first report by Tang and Van Slyke in 1987 1 , multi-layered organic light emitting diodes (OLEDs) have attracted interest for utilization in high efficiency illumination and flexible displays.2 OLEDs using fluorescent materials 3 have low internal quantum efficiencies (IQEs) of ≈ 25%, 4 due in part to the inherent limitation of electrical excitation, which generates singlets and triplets in a 1:3 ratio. 5High quantum yield OLEDs with Ir or Pt phosphorescent materials have been intensely investigated for the last several decades 5,6 and now achieve 100% IQE. 5aAlthough phosphorescent materials have defined the present state of OLED technology, there are significant issues including cost, stability of blue emitters, and strong triplet-triplet annihilation at high current density.7 As a result of recent efficiency increases, thermally activated delayed fluorescence (TADF) has become a viable alternative for harvesting both singlet and triplet state in OLEDs. 8,9 TADF is based on reversible intersystem crossing from thermally equilibrated triplet and singlet excited states, and competitive luminescence from the singlet states. If non-radiative pathways are negligible then TADF can achieve 100% electroluminescence IQE. 9f An advantage of TADF materials is that they can be purely organic materials and do not Typical TADF designs employ an electron donor and electron acceptor, which are connected directly but have a twisted geometry ( Figure 1a) to minimize the HOMO-LUMO overlap. 9,13An alternative approach is a through-space interaction wherein electronic systems are in communication by homo-conjugation 14 but are sufficiently separated to create a small singlet-triplet ∆E ST (Figure 1b). The design we report herein places the donor and acceptor on the different fins of a triptycene scaffold. These structures display homoconjugation and many triptycene derivatives display intrinsically high thermal stability, which is critical to OLED manufacturing and opperation. We designed the donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2, as novel TADF materials ( Figure 1c). The triphenylamine functions as the donor and dicyanoquinoxaline or dicyanopyrazine as the acceptor. Our designs were guided by time-dependent density functional theory (TD-DFT) calculations, which prov...

Isomeric Thermally Activated Delayed Fluorescence Emitters for Color Purity-Improved Emission in Organic Light-Emitting Devices

“…14 The mixture was stirred in room temperature for 0.5 h then heating to 60 o C for 4 h. The reaction was terminated by adding 1 mL of water and the liquids were washed by saturated ammonium chloride solution and extracted by dichloromethane. The solvent was volatilized by rotary evaporation and gross product was purified by column chromatography on silica gel (eluent:…”

“…12 With an extremely small singlet-triplet splitting (∆E ST ), TADF emitter can realize efficient upconversion of non-radiative triplet excitons to radiative singlet excitons via reverse intersystem crossing (RISC) process. [13][14][15][16] Thus, compared with PhOLEDs, the TADF OLEDs can also deliver a full exciton harvesting theoretically, but without heavy metals. 18-21 3 To realize the key point of extremely small ∆E ST s for TADF emitters, minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecules is essential.…”

Tailoring Optoelectronic Properties of Phenanthroline‐Based Thermally Activated Delayed Fluorescence Emitters through Isomer Engineering

“…[5][6][7][8][9][10][11][12] Generally, a small singlet-triplet energy splitting (ΔE ST ), which can be realized by spatially separating the overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distribution, is required to ensure effective RISC process; [13][14][15][16] meanwhile a certain degree of frontier molecular orbital (FMO) overlap is also necessary to guarantee intrinsically high photoluminescence quantum yield (PLQY). [17][18][19][20] However, the intrinsic tradeoff between these two requirements has to be carefully managed towards designing ideal TADF emitters. [21,22] Numerous attempts to achieve this compromise have revealed that the feasible design strategy is to introduce pre-twisted charge-transfer configuration in donoracceptor (D-A) systems with or without aryl linkage.…”

“…[21,22] Numerous attempts to achieve this compromise have revealed that the feasible design strategy is to introduce pre-twisted charge-transfer configuration in donoracceptor (D-A) systems with or without aryl linkage. [13,18,23] Therefore, the suitable selection of electron D, A units and their linking tactics are of vital importance to achieve excellent TADF emitters with near-zero ΔE ST and high PLQY. [6,7,14,20,24] Although promising results have been achieved on TADF emitters with the state-of-art external quantum efficiencies (η ext s) close to those of phosphorescent OLEDs, the further improvement in the device efficiency and stability is still in demand, and the insight into the relationship between the TADF molecular structures and properties is to be deepened.…”

Red Organic Light‐Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

“…Generally, efficient reverse intersystem crossing (RISC) process and high fluorescence quantum yield ( Φ F ) are two essential requirements for efficient TADF emitters 1, 5. For the former one, an extremely small singlet‐triplet energy split (Δ E ST ) between lowest singlet excited state (S 1 ) and lowest triplet excited state (T 1 ) is highly desired to up‐convert triplet excitons to singlet excitons through thermal excitation 4, 6, 7, 8, 9, 10. Thus, electron‐donor (D)–electron‐acceptor (A) frameworks with a highly twisted way are generally employed to construct TADF emitters, as they can well isolate highest occupied molecular orbitals (HOMOs) in D moieties and lowest unoccupied molecular orbitals (LUMOs) in A moieties, respectively, resulting in small Δ E ST s and thus effective RISC process 11, 12, 13, 14, 15.…”