Triarylborane-based thermally activated delayed fluorescence materials with an efficient reverse intersystem crossing

Efficient reverse intersystem crossing (RISC) is an important process for thermally activated delayed fluorescence (TADF) to suppress efficiency roll-off in organic LEDs (OLEDs). Enhancing spin–orbit coupling is effective for fast RISC and is achieved by mediating a locally excited triplet state when RISC occurs between charge transfer states. Here, we experimentally confirmed that efficient RISC occurred in triarylborane-based TADF emitters named Phox- Me π , Phox- MeO π , and MeO3 Ph- FMe π . The three emitters showed large RISC rate constants exceeding 106 s−1. The Phox- Me π -based OLED exhibited higher maximum external quantum efficiency (EQEmax = 10.0%) compared to the Phox- MeO π -based OLED (EQEmax = 6.7%).

Efficient reverse intersystem crossing (RISC) is an important process for thermally activated delayed fluorescence (TADF) to suppress efficiency roll-off in organic LEDs (OLEDs).Enhancing spin-orbit coupling is effective for fast RISC and is achieved by mediating a locally excited triplet state when RISC occurs between charge transfer states.Here, we experimentally confirmed that efficient RISC occurred in triarylborane-based TADF emitters named Phox-Me π, Phox-MeO π, and MeO3 Ph-FMe π.The three emitters showed large RISC rate constants exceeding 10 6 s −1 .The Phox-Me π-based OLED exhibited higher maximum external quantum efficiency (EQE max = 10.0%) compared to the Phox-MeO π-based OLED (EQE max = 6.7%).© 2024 The Author(s).][3][4] The recombination of holes and electrons generates 25% of singlet and 75% of triplet excitons in the emitting layer of OLEDs, 5,6) of which only 25% are available as light in conventional fluorescent materials.Therefore, the theoretical limit of internal quantum efficiency (IQE) had been 25%. 7,8)[18] TADF molecules can harvest triplet excitons as well as singlet excitons as light via reverse intersystem crossing (RISC) and subsequent radiative decay, realizing an IQE of 100% without the use of metallic elements.][21] The rate constant of RISC (k RISC ) is described by Eq. (1) 22) Ĥere, T, and ΔE ST represent the spin-orbit coupling (SOC) matrix element value between singlet (S) and triplet (T) states, Boltzmann's constant, temperature, and energy gap between the lowest excited singlet (S 1 ) and lowest triplet (T 1 ) states, respectively.Equation (1) indicates that minimizing ΔE ST and enhancing SOC play important roles in accelerating RISC.Conventionally, the strategy of separating the HOMO and LUMO has been adopted to achieve small ΔE ST for TADF molecules composed of electron donor (D) and acceptor (A) segments.However, in this case, both S 1 and T 1 tend to have charge transfer (CT) type character (denoted as 1 CT and 3 CT, respectively), 23,24) resulting in negligibly small SOC between them as per El-Sayed's rule. 25)One effective solution to enhance SOC without employing the heavy atom effect is to mediate a locally excited triplet state ( 3 LE) in RISC between 1 CT and 3 CT. 23,26,27)ased on our quantum chemical calculations, and our extensive experience in developing three-coordinate boron compounds for a wide range of applications, [28][29][30][31][32][33][34] we recently designed, synthesized, and characterized three triarylboranebased TADF emitters, namely Phox-Me π, Phox-MeO π, and

MeO3
Ph-FMe π, (Fig. 1). 35)Phox-MeO π and MeO3 Ph-FMe π were designed to enhance SOC by minimizing the 1 CT (S 1 )-3 LE (T 2 ) energy gap.As illustrated in Fig. 1, the second (T 2 ) and third (T 3 ) lowest triplet states of Phox-Me π are both LE type ( 3 LE D : LE confined on donor and 3 LE π : LE confined on the πbridge, respectively), but are 0.23 and 0.58 eV higher than the T 1 state, respectively, indicating that they cannot participate in the RISC process.To lower the energy level of the 3 LE state, different substitutions were introduced at the π-bridge, rather than at the donor or acceptor moiety, because we can change the energy level of the LE state without changing those of the CT states.In Phox-MeO π, two electron donating (OMe) groups were introduced into the π-bridge to achieve a small T 1 ( 3 CT )-T 2 ( 3 LE π ) energy gap.MeO3 Ph-FMe π was designed to raise the energy levels of the S 1 ( 1 CT) and T 1 ( 3 CT) states by replacing the donor segment with a weaker 2,4,6trimethoxyphenyl donor, and to reduce the T 1 ( 3 CT )-T 2 ( 3 LE π ) energy gap by introducing electron withdrawing (CF 3 ) groups at the π-bridge.These strategies were expected to provide fast RISC via the 3 LE π (T 2 ) state for both Phox-MeO π and MeO3 Ph-FMe π.
Herein, we investigate the photophysical properties of doped films of these emitters to confirm the design concept mentioned above and then fabricate OLEDs using these emitters.The photophysical measurements revealed that Phox-Me π, Phox-MeO π, and MeO3 Ph-FMe π exhibited short delayed fluorescence lifetimes (τ DF ) of 1.4, 0.9, and 3.2 μs, respectively.Phox-MeO π and MeO3 Ph-FMe π showed large k RISC of 1.6 × 10 6 and 2.0 × 10 6 s −1 as expected.Contrary to our expectation, Phox-Me π also showed a large k RISC of 2.6 × 10 6 s −1 .
We have further investigated the photophysical behavior of
Figure 2(b) displays the experimental transient PL decay curves; the lifetimes of prompt fluorescence (τ PF ) were 60.0, 31.9, and 82.5 ns and τ DF were 1.4, 0.9, and 3.2 μs for Phox-Me π, Phox-MeO π, and MeO3 Ph-FMe π, respectively.All emitters showed relatively short τ DF (∼1-3 μs), originating 041003-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd from fast RISC and the subsequent radiative decay.Table I shows the photophysical properties including the rate constants [see Eqs.(S1-S4) for the derivation]. 27)The k RISC of three emitters were on the order of 10 6 s −1 , which were one or two orders of magnitude larger than those of ordinary TADF emitters (k RISC ∼ 10 4-5 s −1 ). 39,40)The large k RISC of Phox-MeO π and MeO3 Ph-FMe π were consistent with our design concept.In contrast, Phox-Me π also showed an unexpectedly large k RISC in spite of the fact that the 3 LE state of Phox-Me π would not be involved in RISC from the calculated energy levels in Fig. 1.A possible reason is that the actual energy level of the 3 LE state is closer to 1 CT and 3 CT than that expected from the quantum chemical calculation.Finally, we investigated OLED performance using these three molecules as emitters.Their good solubilities encouraged us to fabricate the devices by a solution process.In these devices, indium-tin-oxide (ITO) and aluminum (Al) act as the anode and cathode, respectively.Poly(styrene sulfonic acid)-doped poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) was employed for hole injection, poly(N-vinylcarbazole) (PVK) for hole transport and electron blocking, PPF for hole blocking, 2,2′,2″-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) for electron transport, and 8-hydroxyquinolinato lithium (Liq) for electron injection [Fig.3(a)].The device structure was as follows: ITO (50 nm) / PEDOT:PSS (45 nm) / PVK (15 nm) / 5 wt% emitter: host (40 nm) / PPF (10 nm) / TPBi (45 nm) / Liq (1 nm) / Al (80 nm).The host materials and doping concentrations for Phox-Me π and Phox-MeO π were the same as those used for the photophysical measurements (CBP and 5 wt%), while no host material was used for MeO3 Ph-FMe π (Fig. S2) because of its high Φ PL (79%) in the neat film.
Figures 3(a)-3(c) illustrate the device structure, electroluminescence (EL) spectra, and EQE-luminance curves.The device performances are summarized in Table II.As shown in Fig. 3(b) and Table II, the devices prepared from Phox-Me π and Phox-MeO π exhibited EL spectra with maximum wavelengths (λ EL ) of 569 and 595 nm, respectively.Both EL spectra were blue-shifted compared to the PL spectra, due to the microcavity effect in the devices.The EQE max values were 10.0% for Phox-Me π and 6.7% for Phox-MeO π [Fig.3(c)].
The theoretical maximum EQEs (EQE theo,max ) were calculated as follows: 41)  Here, γΦ out was assumed to be 0.2 following a previous reports for solution-processed OLEDs. 41)Φ p and Φ d represent the prompt and delayed components of Φ PL , respectively.For the OLEDs based on Phox-Me π and Phox-MeO π, the EQE theo,max values are calculated to be 12.7% and 4.2%, respectively, which are in good agreement with the experimental results (Table II).From Eqs. ( 2) and ( 3), we found that the contributions of delayed    In summary, we have investigated photophysical and device performances of Phox-Me π, Phox-MeO π, and MeO3 Ph-FMe π.Phox-MeO π and MeO3 Ph-FMe π were designed to accelerate RISC without using heavy atoms, and their k RISC were determined to be 1.6 × 10 6 and 2.0 × 10 6 s −1 , respectively.The efficient RISC for both emitters was due to mediation by the 3 LE state as designed.Phox-Me πand Phox-MeO π-based devices prepared by a solution process exhibited EQE max values of 10.0% and 6.7%, respectively.Delayed fluorescence largely contributed to the EQEs, indicating that highly efficient RISC occurred in the devices.
η p and η d are the contributions of the prompt and delayed components to EQE theo,max , respectively, and γ and Φ out are charge recombination and light out-coupling factors, respectively.