Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

Abstract

Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which an activation energy as well as spin–orbit coupling between excited states with different spin multiplicities, were simultaneously controlled. An extremely fast exciton lifetime of 750 ns was realized in a donor–acceptor-type molecular structure without heavy metal elements. An OLED utilizing this TADF emitter displayed deep-blue electroluminescence (EL) with CIE chromaticity coordinates of (0.14, 0.18) and a high maximum EL quantum efficiency of 20.7%. Further, the high maximum efficiency were retained to be 20.2% and 17.4% even at high luminance.

Deep-blue emitting organic materials with low exciton lifetime are required to realize efficient organic light-emitting diodes (OLEDs) at high brightness. Here, the authors report deep-blue OLEDs featuring thermally activated delayed fluorescence molecules with subnano-second exciton lifetime.

Introduction

Organic light-emitting diodes (OLEDs) have been commercialized in flat panel displays and solid-state lighting applications, and significant efforts are still devoted to enhancing OLED performance. In OLEDs, the most important parameter is the internal quantum efficiency (η_int), which is theoretically limited to 25% in traditional fluorescence-based OLEDs, as only singlet excitons can be harvested under electrical excitation^1,2. On the other hand, the utilization of phosphorescent emitters containing heavy metals such as Ir, Pt, Os, and Au enhanced intersystem crossing by the strong spin–orbit coupling (SOC), and these phosphorescent emitters can harvest not only singlet excitons but also triplet excitons, leading to an ideal η_int of nearly 100% in OLEDs^3,4. However, the weak metal–ligand coordination bonds result in a limited device lifetime in blue OLEDs^5–7. As an alternative approach, highly efficient thermally activated delayed fluorescence (TADF)-based OLEDs have recently been realized using simple aromatic compounds as an emitter⁸. In this system, triplet excitons are efficiently upconverted from a lowest triplet state (T₁) to the lowest excited singlet state (S₁) by a reverse intersystem crossing (RISC) process, governed by a small energy gap (ΔE_ST) between the S₁ and T₁ states, resulting in a maximum η_int of close to 100%. Eventually, high external quantum efficiencies (η_ext) of over 20% have been achieved for TADF-OLEDs^9–31.

Nevertheless, blue TADF-OLEDs suffer from severe efficiency rolloff compared to their green and red counterparts because relatively long-lived triplet excitons in blue TADF molecules directly affect the operational stability and efficiency rolloff characteristics of TADF-OLEDs. These effects are observed because of the increase of exciton deactivation processes at high current density, including triplet–triplet annihilation (TTA) and singlet–triplet annihilation (STA)^32,33. Advanced blue TADF molecules with ideally short exciton lifetimes (<1 μs) are thus essentially required for future OLED applications. To realize short exciton lifetimes in pure organic TADF molecules, a rate constant of RISC (k_RISC, T₁ → S₁) is the most critical parameter because TADF molecules emit light primarily via the S₁ → T₁ → S₁ → S₀ and T₁ → S₁ → S₀ delayed processes with the S₁ → S₀ prompt process under electrical excitation. According to the first-order perturbation theory, that is, Fermi’s golden rule, k_RISC between the two states is proportional to <S|Ĥ_SOC|T>/ΔE_ST^34–36:

$$k_{\mathrm{RISC}}\propto {∣⟨\mathrm{S\mid }{Ĥ}_{\mathrm{SOC}}\mathrm{\mid T}⟩∣}^2\exp \left(\frac{-\mathrm{\Delta }{E}_{\mathrm{ST}}}{k_{\mathrm{B}}T}\right).$$ 1

Here, <S|Ĥ_SOC|T> is the SOC matrix element between the excited singlet (S) and triplet (T) states, k_B is the Boltzmann constant, and T is temperature. Using this relationship, minimization of ΔE_ST is a widely adopted strategy to achieve efficient intramolecular charge-transfer (CT) of TADF molecules^8,11–30,32. However, the spin-flip processes, that is, intersystem crossing ISC and RISC, between excited CT singlet (¹CT) and triplet (³CT) states are very inefficient according to the El-Sayed rule because of the independent electric dipole moment with an electron spin, resulting from a weak SOC matrix element^37,38. In contrast, remarkably strong SOC can be expected when the spin-flip processes arise between the CT and energetically close-lying locally excited (LE) states with different spin multiplicities owing to the orbital angular momentum change between the two states^27,39. Several groups have reported that k_RISC is accelerated by the strong SOC matrix element between the ¹CT (S₁) and ³LE (T₂) states, caused by non-adiabatic vibronic coupling (Ĥ_VC) between the ³CT (T₁) and ³LE (T₂) states^40–42. In this study, we demonstrate that a small modulation in excited states of an aromatic organic molecule, mediated by SOC between ¹CT and ³LE states, greatly affects the rate constants of a TADF molecule. As a result, we attained an ideal nano-second-scale exciton lifetime of 750 ns in a deep-blue TADF molecule, which can drastically minimize efficiency rolloff in TADF-OLEDs.

Results

Molecular design and synthesis

To validate our concept, we designed a donor–acceptor (D–A)-type TADF molecule, TMCz-BO (Fig. 1), by introducing 1,3,6,8-tetramethyl-9H-carbazole (TMCz) as a donor unit and 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BO) as an acceptor unit. Both units have ideally high T₁ energies (TMCz and BO⁴³: E_T = 3.00 eV, Supplementary Fig. 1), which are similar to ¹CT of previously reported D–A-type deep-blue TADF emitters^{14,16,17,21–24,28–30,32}; therefore, we expected to attain ideal ¹CT ≈ ³LE. Moreover, the small spatial overlap of this molecule between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in TMCz and BO, respectively, induced the CT character in both S₁ and T₁ states, resulting in a small calculated ΔE_ST of 0.01 eV (i.e., ¹CT ≈ ³CT). This analysis was performed using time-dependent density functional theory (TDDFT) at the PBE0/6-31G(d) level in the gas phase (Supplementary Table 1). To control the ³LE state of an acceptor unit, we also designed a model molecule, TMCz-3P, consisting of 3,11-diphenyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (3P) as an acceptor, which had a lower E_T (2.76 eV) than that of BO because additional phenylenes lead to longer π-conjugation, thereby decreasing E_T. Although this molecule has a similarly small frontier orbital overlap, relatively strong LE character was observed rather than the CT character in its T₁ state according to triplet spin density distribution (TSDD) simulations using the optimized ground-state geometry. In addition, TSDD simulations revealed that TMCz-BO has a strong CT rather than LE character in its T₁ state because the TSDD of TMCz-BO resided throughout the entire D–A molecule, whereas TSDD was only displayed in TMCz-3P on its acceptor unit (3P, Fig. 1).

Fig. 1. Molecular structures and energy levels.

HOMO and LUMO of TMCz-BO and TMCz-3P characterized by DFT calculations at the PBE0/6-31G(d) level of theory and TSDDs in T₁ state.

Figure 2 shows the synthetic routes used to produce the boron-based compounds. The intermediates 3 and 4 were prepared from 1 and 2 using the Buchwald–Hartwig amination with high yields of 80%. The final compounds were obtained using the cyclization reactions of 3 (for TMCz-BO) or 4 (for TMCz-3P) in the presence of n-butyllithium (n-BuLi) and boron tribromide (BBr₃). These final products were purified using temperature-gradient vacuum sublimation. The chemical structures of these compounds were characterized using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), and elemental analysis. The detailed synthetic procedures and characterization data are provided in the Methods section and Supplementary Methods.

Fig. 2. Synthetic scheme.

Synthetic routes for TMCz-BO and TMCz-3P.

Photophysical properties

The fundamental photophysical properties of TMCz-BO and TMCz-3P were first studied in toluene solution at a concentration of 10^–5 M (Table 1 and Supplementary Figs. 2, 3). As shown in Fig. 3a, two clear absorption bands in the ultraviolet–visible (UV–vis) absorption spectra were observed for both materials. The shorter wavelength is attributed to the π–π* transition of the acceptor and donor units, whereas the other one at longer wavelength (>370 nm) corresponds to an intramolecular CT transition from the donor to acceptor units. To attain a deeper understanding of their excited-state properties, we also investigated the solvatochromic effects of TMCz-BO and TMCz-3P in various solvents (Supplementary Fig. 2). Fluorescence spectra with large bathochromic shifts were observed when the solvent polarity was changed from non-polar cyclohexane to polar dichloromethane. The maximum peak wavelength (λ_PL) of TMCz-BO was ~407 nm in cyclohexane and 503 nm in dichloromethane, whereas λ_PL of TMCz-3P was 416 nm in cyclohexane and 517 nm in dichloromethane. Owing to increased solvent polarity, there was a large difference in the emission peak wavelength (Δλ_max = 96 nm for TMCz-BO and Δλ_max = 101 nm for TMCz-3P), indicating pronounced positive solvatochromism, which confirmed the strong CT character of TMCz-BO and TMCz-3P in their S₁ states.

Table 1.

Photophysical characteristics of TMCz-BO and TMCz-3P.

Compound	λabs (nm)	λPL (nm)	ΦPL (%)c	τp (ns)d/τd (μs)d	HOMO (eV)e	LUMO (eV)f	ES/ET (eV)g	ΔEST (meV)h	Ea (meV)i
	Sola	Sola/filmb	Sola/filmb
TMCz-BO	282,377	446/467	81/98	38/0.75	−5.93	−2.77	2.95/2.93	20.0	13.4
TMCz-3P	298,386	455/477	56/76	29/14.5	−5.97	−2.89	2.88/2.74	134	39.8

^aMeasured in oxygen-free toluene at room temperature (298 K).

^b30 wt% doped thin film in a host matrix (host = PPF).

^cAbsolute PLQY evaluated using an integrating sphere under a nitrogen atmosphere.

^dPL lifetimes of prompt (τ_p) and delayed (τ_d) decay components for 30 wt% doped film at 298 K.

^eDetermined by photoelectron yield spectroscopy in pure neat films.

^fDeduced from the HOMO and optical energy gap (E_g).

^gSinglet (E_S) and triplet (E_T) energies estimated from onsets of the emission spectra at 298 and 77 K in 30 wt% doped films, respectively.

^hΔE_ST = E_S − E_T.

ⁱActivation energies of RISC in 30 wt% doped films.

Fig. 3. Absorption spectra and PL transient decay.

a Absorption spectra of TMCz-BO and TMCz-3P in 10⁻⁵ M toluene. b PL emission spectra of TMCz-BO and TMCz-3P in 10⁻⁵ M toluene and 30 wt% doped films in a PPF host matrix. Transient PL decay curves of c 30 wt% TMCz-BO [inset, τ_p (ns)] and d TMCz-3P [inset, τ_p (ns)] doped films in a PPF host matrix.

For further investigation of the photophysical and TADF properties, doped films of both emitters in an amorphous host matrix (i.e., PPF: 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan, E_T = 3.1 eV)⁴⁴ were prepared to avoid concentration quenching. Here, we selected PPF as a host since it can provide both efficient triplet confinement and electron transport abilities. As observed in Fig. 3, the 30 wt% doped films of TMCz-BO and TMCz-3P exhibited blue emission with peaks at λ_PL = 467 and 477 nm, corresponding to Commission Internationale de l’Éclairage (CIE) chromaticity coordinates of (0.15, 0.18) and (0.16, 0.25), respectively. We note that the red-shifted emission maxima of the doped films were observed compared to those of their toluene solution (λ_PL = 446 nm for TMCz-BO and λ_PL = 455 nm for TMCz-3P), which should be primarily associated with a stronger dipole moment of PPF (5.8 D) than that of toluene (0.36 D)²⁷. Such a phenomenon is an additional evidence that these molecules possess a strong CT character.

From the transient PL characteristics of the doped films at 300 K, we observed a clear nano-second-scale prompt component and a microsecond-scale delayed PL component, which were fitted using the biexponential model. As shown in Fig. 3c, TMCz-BO has an extremely fast delayed emission lifetime (τ_d) of 750 ns (fractional delayed PL quantum yield: Φ_d = 32%) with a prompt emission lifetime (τ_p) of 38 ns (fractional prompt PL quantum yield: Φ_p = 66%) in N₂, whereas a relatively long τ_d of 14.5 μs (Φ_d = 11%) with similar τ_p of 29 ns (Φ_p = 65%) was observed in the TMCz-3P-based doped film (Fig. 3d). The rate constants, that is, k_r, k_ISC, and k_RISC, were also estimated for both doped films using the reported method and summarized in Supplementary Table 3²⁷. Although similar k_r and k_ISC values were determined from both doped films (TMCz-BO: k_r = 1.7 × 10⁷ s^–1; k_ISC = 0.9 × 10⁷ s^–1 and TMCz-3P: k_r = 2.3 × 10⁷ s^–1; k_ISC = 1.2 × 10⁷ s^–1), TMCz-BO has a large k_RISC of 1.9 × 10⁶ s^–1, which is two orders of magnitude higher than that of TMCz-3P (3.3 × 10⁴ s^–1).

To understand the exciton dynamics in the excited states of TMCz-BO and TMCz-3P, we first measured the phosphorescence (77 K) spectra of 5–30 wt% doped films in the PPF host matrix with fluorescence (298 K) to determine whether they exhibited CT or LE character in their T₁ states. The phosphorescence spectra of 5–30 wt% TMCz-BO-doped films contained broad and structureless characteristics (i.e., CT character) without the vibrational mode, similar to the ³LE of both the donor (TMCz) and acceptor (3P) units (Fig. 4a). Furthermore, E_T (2.93 eV) of TMCz-BO, which was estimated from the onset of the phosphoresce spectrum of the 30 wt% doped film, was quite close to those of both TMCz and BO (³LE = 3.00 eV). In contrast, TMCz-3P was observed to have a similar E_T (2.76 eV) as that of the acceptor (3 P, ³LE = 2.76 eV) (Fig. 4b). The phosphorescence spectra of TMCz-3P coincided well with that of the acceptor (3P) unit with appreciable redshift to the onset of the fluorescence. In addition, LE character with vibrational mode was observed in the phosphorescence spectra of 5–20 wt% TMCz-3P-based doped films, even though the phosphorescence spectrum appeared to indicate CT character in the 30 wt% doped film. As a next step, we also analyzed the temperature dependences of k_RISC (Fig. 4c) using the 30 wt% doped films. According to the classical Arrhenius equation, k_RISC is given as k_RISC = Aexp(−E_a/k_BT), where A is the frequency factor involving the SOC constant. From the Arrhenius plots of k_RISC, similarly small E_a values were experimentally estimated to be 13.4 meV for TMCz-BO and 39.8 meV for TMCz-3P. Moreover, small E_a values for 5–20 wt% doped films were also analyzed in the range of 13.7–14.5 meV for TMCz-BO and 24.0–34.0 meV for TMCz-3P (Supplementary Fig. 4 and Table 2). These results indicated that the large difference in k_RISC values between TMCz-BO and TMCz-3P is strongly associated with SOC in contrast with in common TADF systems.

Fig. 4. PL spectra at room temperature and 77 K.

a, b Fluorescence (line, 298 K) and phosphorescence (dash dot, 77 K) spectra of doped films for a TMCz-BO and b TMCz-3P. c Arrhenius plots of rate constants of RISC (k_RISC) obtained with the doped films of 30 wt% TMCz-BO and TMCz-3P.

Table 2.

EL performance of blue TADF-OLEDs.

TADF emitter	λEL (nm)	λFWHM (nm)	EFWHM (eV)	Von (V)	Lmax (cd m−2)	ηext (%) Max/@100 cd m−2/@1000 cd m−2	ηc (cd A−1)	ηp (lm W−1)	CIE (x, y)
TMCz-BO	471	59	0.327	3.0	5900	20.7/20.2/17.4	29.8	31.2	(0.14, 0.18)
TMCz-3P	479	61	0.324	2.8	6500	20.4/18.3/12.8	37.8	40.3	(0.14, 0.26)

λ_EL EL emission maximum, λ_FWHM and E_FWHM full-width at half-maximum, V_on turn-on voltage at 1 cd m⁻², L_max maximum luminance, η_ext external EL quantum efficiency, η_c maximum current efficiency, η_p maximum power efficiency, CIE (x, y) Commission Internationale de l’Éclairage color chromaticity coordinates measured at 10 mA cm⁻².

To obtain additional insight into the spin-flip RISC process, we carefully analyzed the relationship between the SOC and E_a values with consideration of their ΔE_ST values. Although TMCz-BO has a similarly small ΔE_ST of 20 meV with its E_a, ΔE_ST of TMCz-3P was estimated to be quite large (134 meV), which is three times larger than its E_a. These experimentally obtained k_RISC, E_a, and ΔE_ST suggest that different spin-flip processes should be involved in TMCz-BO and TMCz-3P. As suggested by the energy level diagram in Fig. 5a, the efficient RISC spin-flip process for TMCz-BO should be involved in ³CT → ³LE → ¹CT because energetically close-lying excited states induce efficient Ĥ_VC from the ³CT to ³LE states, and then RISC can be accelerated from ³LE to ¹CT states mediated by a much larger SOC matrix element (<¹CT|Ĥ_SOC|³LE> = 0.124 cm⁻¹) than <¹CT|Ĥ_SOC|³CT> = 0.001 cm⁻¹, resulting in its extraordinarily fast emission lifetime of 750 ns. In addition, it is difficult for the spin-flip process in the TMCz-3P molecule to occur along the same pathway as that for TMCz-BO, even though it has a similarly large <¹CT|Ĥ_SOC|³LE> = 0.128 cm⁻¹, because its experimental ΔE_ST is much larger than the thermal energy (k_BT ≈ 25.9 meV) at 300 K as well as its E_a (Fig. 5b). Experimental data thus suggest that the ³CT → ¹CT spin-flip process (i.e., hyperfine coupling)^45,46 should occur after efficient Ĥ_VC from ³LE to energetically close-lying higher ³CT states, leading to relatively small k_RISC because of the negligible <¹CT|Ĥ_SOC|³CT> of 0.001 cm⁻¹. Therefore, energetically close-lying excited states (i.e., ¹CT ≈ ³CT ≈ ³LE) with suitably large SOC between ¹CT and ³LE should be realized to ensure extremely fast emission lifetime in TADF molecules.

Fig. 5. Schematic illustration of plausible TADF mechanisms.

a Decay and upconversion processes in TMCz-BO and b TMCz-3P. The energy levels and rate constants were estimated using 30 wt% doped films.

Device characterization and performance

Employing TMCz-BO and TMCz-3P as emitters, two multi-layered OLEDs were fabricated using the following device architecture: indium tin oxide (ITO, 50 nm)/HAT-CN (2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatripheny) (10 nm)/TAPC (4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]) (50 nm)/CCP (9-phenyl-3,9′-bicarbazole) (10 nm)/EML (20 nm)/PPF (10 nm)/B3PyPB (1,3-bis[3,5-di(pyridine-3-yl)phenyl] benzene) (30 nm)/Liq (8-hydroxyquinoline lithium) (1 nm)/Al (100 nm), in which HAT-CN and Liq were used as hole injection and electron injection layers, respectively. The materials TAPC and B3PyPB were used as hole transport and electron transport layers, respectively. Thin layers of CCP and PPF with a high T₁ energy (3.0 and 3.1 eV) were inserted to suppress triplet exciton quenching at the neighboring interfaces and to confine the excitons inside the emitting layers^{14,16,17,21–24,28–30,32,47}.

HOMO, LUMO, and thermal properties of TMCz-BO and TMCz-3P are summarized in Supplementary Figs. 5–7a. The EL characteristics of both devices are depicted in Fig. 6, and the key device parameters are summarized in Table 2. As shown in Fig. 6a, the devices based on TMCz-BO and TMCz-3P exhibited blue EL emission with peaks (λ_EL) at 471 nm (deep blue) and 479 nm (blue) with corresponding CIE chromaticity coordinates of (0.14, 0.18) and (0.14, 0.26), respectively, which were consistent with their corresponding PL spectra. It is noteworthy that the devices displayed a rather narrow full-width at half-maximum (FWHM, λ_FWHM) of 59 nm (E_FWHM = 0.327 eV) for TMCz-BO and 61 nm (E_FWHM = 0.324 eV) for TMCz-3P, resulting from minimizing molecular vibrations using the rigid acceptor structure and minimizing molecular conformation changes in their ground and S₁ states using the peri-position dimethyl groups in TMCz^27,31,47.

Fig. 6. OLED characteristics.

a Normalized EL spectra, b current density–voltage–luminance (J–V–L) characteristics, and c external EL quantum efficiency (η_ext) versus luminance plots of TMCz-BO- and TMCz-3P-based OLEDs.

Figure 6b, c shows the current density–voltage–luminance (J–V–L) and external EL quantum efficiency versus current density (η_ext–J) plots. Both devices exhibited rather low turn-on voltages (V_on) in the range of 2.8–3.0 V and achieved high maximum η_ext exceeding 20% (20.7% for TMCz-BO and 20.4% for TMCz-3P). This high efficiency of the TMCz-3P-based device drastically decreased with increasing current density and luminance, which is similar to the trend observed for most reported blue TADF-OLEDs. As a result, low η_ext values (18.3% at 100 cd m^–2 and 12.8% at 1000 cd m^–2) were obtained from the TMCz-3P-based device. However, the TMCz-BO-based device retained high η_ext values of 20.2% at 100 cd m^–2 and 17.4% at 1000 cd m^–2, corresponding to a 2.4% and 15.9% decrease in η_ext, respectively. Low-efficiency rolloff is primarily attributed to its nano-second-order emission lifetime, which efficiently suppresses the accumulation of triplet excitons within TMCz-BO; hence, TTA and/or STA could be minimized in the device. To the best of our knowledge, the efficiency decrease ratio of the TMCz-BO-based device is the small of those reported for TADF molecules for deep-blue OLEDs (CIEy ≤ 0.20) to date (Supplementary Table 4)^{14,16,17,21–24,28–30,32,47}. We note that the recently some sophisticated devices showed rather relaxed rolloff behavior even with the TADF emitters with microsecond-order delayed lifetimes. Probably wide recombination and exciton formation width may contribute it. Thus, it should be considered to improve total device performance from the aspect of comprehensive control of delayed lifetime, charge carrier transport, and recombination characteristics⁴⁸.

Discussion

Kaji and co-workers⁴⁹ recently reported a through-space CT molecule, TpAT-tFFO, by introducing face-to-face alignment of the donor and acceptor units (Fig. 7a). Such a through-space CT character in a single molecule induces extremely small spatial overlap between the HOMO and LUMO, resulting in a very small ΔE_ST. Although an exceedingly large k_RISC value of over 10⁷ s^–1 was achieved in the doped film, an unusually slow rate constant of fluorescence radiative decay (k_r = 1.1 × 10⁶ s^–1) was observed. This slow rate constant can be attributed to the negligible spatial overlap of the through-space molecule, leading to a decrease of the transition dipole moment, which should affect its small radiative decay, according to Fermi’s golden rule. As a result, the doped film exhibited a relatively long delayed emission lifetime (τ_d) of 4.1 μs. In 2018, Yasuda and co-workers²⁷ successfully designed a non-metallic TADF molecule, MPAc-BS, with the largely enhanced SOC matrix element of <¹CT|Ĥ_SOC|³LE> = 4.67 cm^–1 (in general, aromatic organic molecules have very small <¹CT|Ĥ_SOC|³LE> ≤ 0.10 cm^–1), thereby attaining a large k_RISC of 3.5 × 10⁶ s^–1 (Fig. 7b). However, this molecule also possessed a very large rate constant of ISC (k_ISC = 9.9 × 10⁷ s^–1), which is over 10 times larger than that of k_RISC, as SOC generally affects the whole spin-flip processes, not only k_RISC but also k_ISC. Consequently, the ISC process occurs much more rapidly than the RISC process in this molecular system, which should interrupt conquest in regard to an ideal exciton lifetime. Therefore, the balance of rate constants k_r, k_ISC, and k_RISC is the decisive factor controlling the exciton lifetimes. In other words, a larger k_r than k_ISC and minimal difference between k_ISC and k_RISC while maintaining ¹CT ≈ ³CT ≈ ³LE are concurrently required for TADF molecules possessing ideally fastest delayed lifetime.

Fig. 7. Schematic illustration of plausible TADF mechanism.

a TpAT-tFFO, b MPAc-BS, and c TMCz-BO. k_r, k_ISC, k_RISC, CT, LE, and ΔE_ST represent the rate constants of fluorescence radiative decay, intersystem crossing (ISC), reverse ISC (RISC), charge-transfer and localized excited states, and energy gap between lowest singlet and triplet excited states, respectively.

In summary, we successfully designed and synthesized an advanced deep-blue TADF emitter, TMCz-BO, which displayed deep-blue emission with corresponding CIE chromaticity coordinates of (0.15, 0.18) and (0.14, 0.18) under photo- and electro-excitation, respectively. Owing to its extraordinarily fast emission lifetime of 750 ns, originating from well-valanced k_r, k_ISC, and k_RISC mediated by suitably strong SOC between the ¹CT and ³LE while maintaining a small activation energy, high external EL quantum efficiencies of 20.2% and 17.4% at 100 cd m^–2 for displays and 1000 cd m^–2 for lighting sources were achieved, respectively. We presume that our strategy will be widely employed in TADF molecular design for high-performance deep-blue TADF-OLEDs.

Methods

General

All reagents and anhydrous solvents were purchased from commercial sources and were used without further purification. The detailed synthetic procedures and characterization data for the intermediates are given in Supplementary Figs. 8–11. The two final products investigated in this paper were synthesized by following the procedures described below and then purified using temperature-gradient vacuum sublimation with a P-100 system (ALS Technology). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 500 spectrometer. Chemical shifts of ¹H and ¹³C NMR signals were quoted to tetramethylsilane (δ = 0.00) and CDCl₃ (δ = 77.0), as internal standards. Mass spectra were measured in positive-ion atmospheric-pressure chemical ionization mode on a Waters 3100 mass detector. Elemental analyses were performed using a Yanaco MT-5 analyzer. For thin films and OLED fabrication, CCP¹⁶ and PPF⁴⁴ were synthesized following the reported procedures and were then purified using temperature-gradient vacuum sublimation. Other OLED materials were purchased from Luminescence Technology Corporation.

Synthesis of TMCz-BO

A solution of n-BuLi in hexane (3.11 mL, 2.5 M, 7.77 mmol) was added slowly to a solution of 2 (3.0 g, 6.47 mmol) in tert-butylbenzene (30 mL) at −20 °C under a nitrogen atmosphere. After stirring at 50 °C for 4 h, hexane was distilled off at 100 °C under a continuous flow of nitrogen. BBr₃ in dichloromethane (9.71 mL, 1.0 M, 9.71 mmol) was added slowly at −20 °C. The reaction mixture was then allowed to warm to room temperature for 1 h and then stirred at 40 °C for 1 h. After 10% of the solvent had been removed in vacuo, N,N-diisopropylethylamine (DIPEA, 1.67 g, 12.9 mmol) was added at 0 °C. After stirring at 120 °C for 5 h, methanol was added to the reaction mixture at room temperature. The reaction mixture was then filtered with a pad of Florisil. The crude product was washed with methanol to afford TMCz-BO (0.48 g, 15% yield) as a light-yellow solid. ¹H NMR (500 MHz, CDCl₃, δ): 8.77 (dd, J = 7.8, 1.5 Hz, 2H), 7.77 (td, J = 8.1, 1.5 Hz, 4H), 7.58 (dd, J = 7.8, 0.6 Hz, 2H), 7.46 (td, J = 7.4 Hz, 0.9 Hz, 2H), 7.40 (s, 2H), 6.95 (s, 2H), 2.51 (s, 6H), 1.97 (s, 6H) (Supplementary Fig. 12); ¹³C NMR (125 MHz, CDCl_3, δ): 160.65, 156.88, 148.61, 139.60, 134.66, 133.99, 130.29, 129.30 124.42, 123.22, 121.44, 118.63, 117.79, 112.01, 21.11, 19.33. (Supplementary Fig. 13); MS (ASAP) m/z: [M–1]⁺ calcd 493.39; found, 492.17. Anal. calcd (%) for C₃₄H₂₆BNO₂: C 83.10, H 5.33, N 2.85; found: C 83.09, H 5.36, N 2.85.

Synthesis of TMCz-3P

TMCz-3P was synthesized using the same procedure described above for the synthesis of TMCz-BO, except that 4 (3.0 g, 4.72 mmol), n-BuLi (2.26 mL, 5.66 mmol), BBr₃ (7.08 mL, 7.08 mmol), and DIPEA (1.22 g, 9.44 mmol) were used as the reactants, giving TMCz-3P (0.46 g, 15% yield) as a light-yellow solid. ¹H NMR (500 MHz, CDCl₃, δ): 8.74 (d, J = 8.1 Hz, 2H), 7.70 (td, J = 7.6, 1.4 Hz, 8H), 7.64 (dd, J = 8.0, 1.7 Hz, 2H), 7.46(t, J = 7.6 Hz, 4H), 7.38 (tt, J = 7.35, 3.0 Hz, 2H), 7.34 (s, 2H), 6.90 (s, 2H), 6.87 (s, 2H), 2.42 (s, 6H), 1.90 (s, 6H) (Supplementary Fig. 14); ¹³C NMR (125 MHz, CDCl_3, δ): 160.07, 155.98, 147.48, 145.79, 138.52, 134.04, 129.28, 128.26 128.01, 127.36, 126.34, 123.37, 120.39, 116.78, 115.58, 111.10, 20.09, 18.33 (Supplementary Fig. 15); MS (ASAP) m/z: [M]⁺ calcd 643.58; found, 643.23. Anal. calcd (%) for C₄₆H₃₄BNO₂: C 85.85, H 5.32, N 2.18; found: C 85.22, H 5.37, N 2.13.

Quantum chemical calculations

Quantum chemical calculations were performed using the Gaussian 16 program package. The molecular geometries in the ground state were optimized using the PBE0 functional with the 6-31G(d) basis set in the gas phase. The lowest excited singlet and triplet states as well as the TSDD simulation were computed using the optimized structures with TDDFT at the same level. The SOC matrix elements were determined using the ADF2018 program package⁵⁰ following the literature method²⁷.

Photophysical measurements

Thin-film samples (40 nm) were deposited on quartz glass substrates by vacuum evaporation to study their exciton confinement properties in the film state. UV–vis absorption and PL spectra were recorded using a PerkinElmer Lambda 950 KPA spectrophotometer and JASCO FP-6500 fluorescence spectrophotometer, respectively. The absolute PL quantum yields were measured on a Quantaurus-QY measurement system (C11347-11, Hamamatsu Photonics) under nitrogen flow, and all the samples were excited at 360 nm. The transient PL decay characteristics were recorded using a Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics). The HOMO energy levels were determined using the onset of a photoelectron yield spectroscopy (AC-3, Riken-Keiki) in neat films, and then the LUMO energy levels were estimated by subtracting the optical energy gap (E_g) from the measured HOMO energies. In the case of the E_g values were determined from the onset of the PL spectra of neat films.

Device fabrication and measurements

Pre-patterned ITO (50 nm)-coated glass substrates were cleaned with detergent, deionized water, acetone, and isopropanol. The substrates were then exposed to UV–ozone treatment for 15 min before being loaded into an ALS Technology E-200 vacuum evaporation system. The organic layers and cathode Al layer were thermally evaporated on the substrates under vacuum (<6 × 10⁻⁵ Pa) at a deposition rate of <0.2 nm s⁻¹ through a shadow mask, defining a pixel size of 0.04 cm². The thickness and deposition rate were monitored in situ during deposition by an oscillating quartz thickness monitor. The current density–voltage–luminance (J–V–L) characteristics of the fabricated OLEDs were measured using a Keithley 2400 source meter and a CS-2000 spectroradiometer (Konica Minolta).

Author contributions

C.A. initiated and supervised the project. J.U.K. designed, synthesized, and characterized the blue TADF emitters. J.U.K. performed the computational calculation, photophysical, and electrochemical measurements of the TADF emitters. J.U.K., I.S.P., and M.T. fabricated the OLEDs and measured the device performance. Y.T., H.N., and C.A. provided suggestions on experiments and writing manuscript. All authors discussed the progress of the research and reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Supplementary information

Supplementary information is available for this paper at 10.1038/s41467-020-15558-5.