Integrating molecular rigidity and chirality into thermally activated delayed fluorescence emitters for highly efficient sky-blue and orange circularly polarized electroluminescence

Abstract

By integrating high molecular rigidity and stable chirality, two pairs of D*–A type circularly polarized thermally activated delayed fluorescence (CP-TADF) emitters with an almost absolute quasi-equatorial conformer geometry and excellent photoluminescence quantum efficiencies (PLQYs) are developed, achieving state-of-the-art electroluminescence performance among blue and orange circularly polarized organic light-emitting diodes (CP-OLEDs).

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New concepts

This work reveals for the first time the importance of a rigid chirality element and fixed molecular configuration in designing thermally activated delayed fluorescence (TADF) emitters with high-performance circularly polarized electroluminescence (CPEL). The concept greatly differs from the previous reports, in which flexible chiral elements are incorporated in TADF emitters to induce circularly polarized luminescence, leading to low photoluminescence quantum efficiencies (PLQYs) and unsatisfactory CPEL performance (i.e. the best external quantum efficiency (EQE) is only 12.7% for blue CP-OLEDs and 1.8% for the orange devices). In contrast, the rigid CP-TADF framework in this work provides decent PLQYs (89% for sky-blue and 86% for orange emitters) and an obvious chiral excited singlet state for the emitters, ultimately leading to the most efficient sky-blue and orange CP-OLEDs hitherto reported (EQEs of 20.3% and 23.7%, respectively), while simultaneously featuring intense CPEL signals. The state-of-the-art device performances suggest that the design principle proposed herein is a powerful one for the development of CP-TADF compounds and CP-OLEDs.

Introduction

Circularly polarized organic light-emitting diodes (CP-OLEDs) have gained considerable interest because of their potential application in decreasing light loss arising from the use of a polarizer and a quarter-wave plate in high-contrast OLED displays.^1,2 Despite the rapid enrichment of the chiral luminogenic material library, the development of CP-OLEDs with a satisfactory external quantum efficiency (EQE) and circularly polarized electroluminescence (CPEL) dissymmetry factor (g_EL) still remains a formidable challenge.^3–8

Introduction of the thermally activated delayed fluorescence (TADF) mechanism into circularly polarized luminescent materials turned out to be one successful tactic to boost the efficiency of CP-OLEDs by harnessing both singlet and triplet excitons via the thermally activated reverse intersystem crossing (RISC) process.⁹ These circularly polarized thermally activated delayed fluorescence (CP-TADF) emitters are typically obtained by installing a chiral element into a twisted D–A-type skeleton.¹⁰ The pioneering research was carried out by Pieters's group and Tang's group by introducing enantiopure 1,1′-bi-2-naphthol (BINOL) into the acceptor, affording green CP-OLEDs with EQEs of up to 9.3% and considerable |g_EL| values of up to 9.2 × 10⁻².^10b,d,p Chen and coworkers designed a chiral 1,2-diaminocyclohexane-based acceptor for CP-TADF emitters and the corresponding green CP-OLEDs achieved EQEs of up to 19.8% and |g_EL| values of up to 2.3 × 10⁻³.^10c,h Meanwhile, Zheng's group developed an octahydro-BINOL-based acceptor for green CP-TADF emitters, which revealed high EQEs of up to 32.6% and |g_EL| values of 2.3 × 10⁻³ in devices.^10e,g In a more recent study, Chen's and Zheng's groups reported axially chiral biphenyl unit-based CP-TADF emitters, revealing circularly polarized electroluminescence (CPEL) with EQEs of up to 17.8% and |g_EL| values of up to 1.4 × 10⁻².^10l,n,o

Considerable progress has been made in developing green CP-TADF emitters^{10a–e,g,i,n,o} and their corresponding CP-OLEDs have achieved state-of-the-art device performance with EQEs of over 32%.^10e However, at present, there exist only a few reports on blue^10j–l and orange^10d,f,h,m CP-TADF emitters, and the best EQEs of the fabricated CP-OLEDs are 12.7% for blue^10l and 1.8% for orange,^10f which are significantly lower than those of their green counterparts. The bottleneck for developing high-performance blue and orange CP-TADF OLEDs is the low PLQYs of the reported CP-TADF emitters (<70% for blue emitters, and <40% for orange emitters), typically caused by the activation of nonradiative transition processes via integrating a flexible chirality element into the skeleton. Introducing a chirality element with high rigidity is therefore a prerequisite for constructing excellent blue and orange CP-TADF emitters.

In this work, we designed a point chiral electron donor, namely 12b-methyl-5,12b-dihydroindeno[1,2,3-kl]acridine (MeIAc), by integrating acridine and indene subunits. MeIAc revealed a suitable electron-donating ability, high-lying triplet state energy level and high structural rigidity. MeIAc-derived D*–A type systems, namely 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12b-methyl-5,12b-dihydr-oindeno[1,2,3-kl]acridine (TRZ-MeIAc) and 2-(4-(tert-butyl)phenyl)-6-(12b-methylindeno[1,2,3-kl]acridin-5(12bH)-yl)-1H-benzo[de]isoq-uinoline-1,3(2H)-dione (NID-MeIAc), demonstrated a very rigid molecular architecture, almost absolute quasi-equatorial conformer geometry, obvious TADF behaviour and high PLQYs exceeding 85%. It is worth noting that CP-OLEDs based on the TRZ-MeIAc and NID-MeIAc enantiomers exhibited highly efficient blue and orange CPEL with excellent performance of maximum EQEs of 20.3% and 23.7%, PEs of 49.6 and 61.3 lm W⁻¹ and CEs of 47.8 and 74.5 cd A⁻¹, respectively. To the best of our knowledge, all these parameters represent the highest device performances for all reported blue and orange CP-OLEDs, respectively. Importantly, the high-performance CP-OLEDs also revealed noticeable CPEL signals with |g_EL| values of up to 2.4 × 10⁻³, indicating efficient chirality transfer of TRZ-MeIAc and NID-MeIAc in electroluminescence (EL).

Results and discussion

Synthesis and characterization

The newly designed donor of MeIAc was synthesized via a three-step route, including Wittig reaction, Pd-catalyzed C–N coupling, and intramolecular cyclization reaction.¹¹ The target molecules of TRZ-MeIAc and NID-MeIAc were afforded through Pd-catalyzed C–N coupling reactions between MeIAc and the acceptor 2-(2-bromophenyl)-4,6-diphenyl-1,3,5-triazine or 4-bromo-9-(4-tert-butylphenyl)-1,8-naphthalimide (Scheme 1).

Scheme 1 Synthetic routes of TRZ-MeIAc and NID-MeIAc.

The resolution of the two pairs of enantiomers ((R/S)-TRZ-MeIAc and (R/S)-NID-MeIAc) was conducted using a chiral high-performance liquid chromatography column (Fig. S1–S4, ESI†). The enantiomeric excess (e.e.) values of (R)-TRZ-MeIAc, (S)-TRZ-MeIAc, (R)-NID-MeIAc and (S)-NID-MeIAc enantiomers are 98.2%, 99.6%, 99.2% and 99.6%, respectively. The structure and the absolute configuration of (R)-TRZ-MeIAc were confirmed using X-ray single crystal analysis (Fig. S5, S6 and Table S1, ESI†). A highly twisted configuration with a nearly orthogonal arrangement was observed between the chiral donor and the phenyl linkage.

The thermal decomposition temperature with 5 wt% loss was found to be 409 °C for TRZ-MeIAc and 379 °C for NID-MeIAc (Fig. S7 and Table S2, ESI†). The sufficiently high thermal stability makes them suitable candidates for the vacuum deposition process. Cyclic voltammetry (CV) curves of TRZ-MeIAc and NID-MeIAc in CH₂Cl₂ solution revealed chiral acridine unit-based reversible oxidation behavior (Fig. S9 and Table S2, ESI†). The experimentally obtained highest occupied molecular orbital (HOMO) levels of TRZ-MeIAc and NID-MeIAc are −5.43 and −5.55 eV, respectively.

Photophysical properties

The basic photophysical properties of the donor unit MeIAc were firstly studied (Fig. S10 and Table S3, ESI†). Attributed to its conjugation-confined architecture, MeIAc revealed a π–π* electronic transition with an absorbance peak centered at 326 nm and ultraviolet emission with a peak around 378 nm in toluene solution. The phosphorescence spectrum of MeIAc in toluene solution at 77 K revealed a typical local-excited feature with multi-peak emission around 439, 469 and 498 nm. The lowest excited triplet (T₁) state energy level determined by the onset of the phosphorescent spectrum is high-lying at 2.91 eV, which can enable the design of CP-TADF emitters with widely tunable emissive colors.

The photophysical properties of TRZ-MeIAc and NID-MeIAc were then studied in toluene solution (Fig. 1 and Table S4, ESI†). As shown in the absorption spectra, both molecules demonstrated a strong π–π* electronic transition band of the MeIAc subunit and a relatively weak intramolecular charge transfer absorption band (Fig. 1a and b). TRZ-MeIAc and NID-MeIAc emitted structureless sky-blue and orange light with a distinct enhancement of emission intensity after degassing (inset of Fig. 1c and d). Meanwhile, the transient photoluminescence decay curves in toluene solution of both emitters revealed obvious short-lived (ns-scale) and long-lived (μs-scale) emission in an inert atmosphere, while the long-lived portion was totally quenched after exposure to air (Fig. 1c, d and Table S4, ESI†). The above results suggested the involvement of an excited triplet state in the emissive process of the two emitters.

Fig. 1 The absorption and steady-state emission spectra of TRZ-MeIAc (a) and NID-MeIAc (b) in toluene solution (1.0 × 10⁻⁵ M) at room temperature. The transient photoluminescence decay curves of TRZ-MeIAc (c) and NID-MeIAc (d) in toluene solution (1.0 × 10⁻⁵ M) at room temperature before and after degassing. Inset depicts the emission intensity changes of TRZ-MeIAc and NID-MeIAc in toluene solution at room temperature before and after degassing.

To simulate the emissive behaviour in OLEDs, the basic photophysical properties of TRZ-MeIAc and NID-MeIAc in 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN) doped films were further measured (Fig. S12, S13, ESI† and Table 1). The 12 wt% TRZ-MeIAc doped film emitted structureless blue light with the peak centered at 473 nm (Fig. S12a, ESI†) and the 6 wt% NID-MeIAc doped film emitted structureless yellow light with the peak centered at 565 nm (Fig. S13a, ESI†), revealing a charge transfer lowest excited singlet (S₁) state feature. Both of the doped films demonstrated highly efficient emission with PLQYs of 89% for TRZ-MeIAc and 86% for NID-MeIAc, which are among the highest values for all reported CP-TADF emitters. The ΔE_ST values estimated from the fluorescence and phosphorescence spectra of the doped films for TRZ-MeIAc and NID-MeIAc are 0.19 and 0.22 eV respectively, implying an efficient RISC process. The steady-state emission and transient photoluminescence decay curves of the doped films at different temperatures were further studied (Fig. S12b, c and S13b, c, ESI†). The intensity of the steady-state emission and the ratios of the delayed components increased with increasing temperature, further proving the TADF mechanism.

Table 1 Photoluminescence properties of TRZ-MeIAc (12 wt%) and NID-MeIAc (6 wt%) doped into mCPCN films at room temperature

Compounds	τ p (ns)	τ d (μs)	ϕ (%)	ϕ p (%)	ϕ d (%)	k p (×10^7 s^−1)	k d (×10^4 s^−1)	k r,S (×10^7 s^−1)	k ISC (×10^7 s^−1)	k RISC (×10^4 s^−1)	k ^SIC (× 10^6 s^−1)
a The lifetimes of prompt and delayed fluorescence. b The total fluorescence PLQY under oxygen-free conditions. c The prompt and delayed fluorescence PLQY under oxygen-free conditions. d The rate constants of prompt and delayed radiation. e The rate constants of singlet radiative decay, intersystem crossing (ISC), reverse intersystem crossing (RISC), and singlet internal conversion (IC) processes.
TRZ-MeIAc	18.7	82.3	89	69	20	5.35	1.22	3.69	1.20	1.57	4.56
NID-MeIAc	26.3	235.4	86	42	44	3.80	0.42	1.60	1.95	0.87	2.60

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Based on the basic photophysical data of the doped films at room temperature, the relevant rate constants of the S₁ and T₁ states were then estimated according to a reported method (Table 1 and eqn (S1)–(S12), ESI†).¹² The singlet radiative rates (k_r,Ss) of TRZ-MeIAc and NID-MeIAc in the doped films were found to be 3.69 × 10⁷ and 1.60 × 10⁷ s⁻¹, respectively, both of which are in the same order of magnitude as that of typical conventional fluorescent emitters (k_r ranging from 10⁷ to 10⁸ s⁻¹) and much higher than the reported best values of blue (1.18 × 10⁷ s⁻¹)^10l and orange (4.5 × 10⁶ s⁻¹)^10f CP-TADF emitters, respectively. The internal conversion decay rates (k^S_IC) of the two emitters were 4.56 × 10⁶ and 2.60 × 10⁶ s⁻¹, respectively. The much slower k^S_ICs than k_r,Ss in the two emitters demonstrate the limited nonradiative decay processes.

Chiroptical properties

The chiroptical properties of the (R/S)-TRZ-MeIAc and (R/S)-NID-MeIAc enantiomers in the ground and excited states were investigated using circular dichroism (CD) and circularly polarized photoluminescence (CPPL) spectra, respectively. The CD spectra in the π–π* transition region (290 to 355 nm for (R/S)-TRZ-MeIAc and 290 to 370 nm for (R/S)-NID-MeIAc) displayed a bisignate Cotton effect (Fig. 2a and d). The absorption peak at 330 nm can be ascribed to the characteristic absorption of the chiral donor (Fig. S10, ESI†). The CD spectra in the charge transfer region (355 to 425 nm for (R/S)-TRZ-MeIAc and 370 to 495 nm for (R/S)-NID-MeIAc) showed a monosignate Cotton effect, indicating that the chirality is successfully transferred from the point chiral donor to the D*–A systems. The CPPL spectra of the two pairs of enantiomers were found to be structureless mirror-symmetric images (Fig. 2b and e), further manifesting the chirality transfer feature in the S₁ states. The luminescence dissymmetry factors (g_PLs) are +5.9 × 10⁻⁴ for (R)-TRZ-MeIAc, −5.9 × 10⁻⁴ for (S)-TRZ-MeIAc, +2.0 × 10⁻³ for (R)-NID-MeIAc and −2.0 × 10⁻³ for (S)-NID-MeIAc, respectively (Fig. 2c and f).

Fig. 2 CD spectra (a), CPPL spectra (b) and g_PLs (c) of (R/S)-TRZ-MeIAc in toluene solution (1.0 × 10⁻⁴ M). CD spectra (d), CPPL spectra (e) and g_PLs (f) of (R/S)-NID-MeIAc in toluene solution (1.0 × 10⁻⁴ M).

Theoretical simulation

In order to gain more insight into the structure and emissive information of the designed chiral donor and emitters, detailed theoretical calculations based on density functional theory (DFT) and time-dependent DFT were conducted. Serving as a new chiral electron donor, the ground state (S₀) geometrical configurations, contours of the frontier molecular orbitals (FMOs) and energy levels of MeIAc were firstly simulated (Fig. 3, taking (R)-MeIAc as an example). By inserting the chiral quaternary carbon, a twisted structural arrangement was obtained in (R)-MeIAc (Fig. 3a and b). The acridine unit in (R)-MeIAc was arranged in a “V” shape with a dihedral angle of 141° between the phenyl and fluorenyl units (Fig. 3c). The relatively twisted structural arrangement in (R)-MeIAc not only ensured its appropriate electron-donating ability with a HOMO value of −5.44 eV, but also guaranteed a high-lying T₁ state energy level (2.91 eV) with a limited extent of conjugation (Fig. 3d), which matched well with the experimental result and provided the benefit of designing TADF emitters with widely tunable emissive colors.

Fig. 3 The optimized ground state structure (a), the geometrical configurations (b) and (c), and the energy levels and contours of the frontier molecular orbitals (d) of (R)-MeIAc.

The theoretical simulation of the S₀ geometry and excited-state energy levels of the two designed molecules was then conducted (Fig. 4, taking (R)-TRZ-MeIAc and (R)-NID-MeIAc as examples). Compared with the S₀ state geometry of (R)-MeIAc, the optimized S₀ state arrangement of the MeIAc segment in (R)-TRZ-MeIAc and (R)-NID-MeIAc showed little deviation, demonstrating the sufficient rigidity of the chiral donor (Fig. S15 and S16, ESI†). Owing to the high steric hindrance, large twisting angles between MeIAc and the acceptor moieties (72.0° and 93.2° for (R)-TRZ-MeIAc and 67.1° and 87.2° for (R)-NID-MeIAc, respectively) were obtained. As a result, the HOMOs mainly resided on the acridine component, and the lowest unoccupied molecular orbitals (LUMOs) were localized on the acceptor unit. Such FMO distributions resulted in small ΔE_ST values of 0.05 and 0.30 eV for (R)-TRZ-MeIAc and (R)-NID-MeIAc, respectively, guaranteeing an efficient RISC process from the T₁ state to the S₁ state to access TADF.

Fig. 4 The geometrical configurations, energy levels and contours of the frontier molecular orbitals of (R)-TRZ-MeIAc (a) and (R)-NID-MeIAc (b).

Both of the S₁ states of the two molecules revealed a typical charge transfer (CT) characteristic (96.6% for (R)-TRZ-MeIAc and 98.4% for (R)-NID-MeIAc) with the “hole” and “particle” localized at the chiral donor and the achiral acceptor, respectively (Fig. 5). Due to the relatively high T₁ state level of the MeIAc and TRZ units, the T₁ state in TRZ-MeIAc was dominant with a charge transfer (³CT) feature (74.1%, Fig. 5a and Fig. S14, ESI†), while owing to the relatively low T₁ state level of the NID unit, the T₁ state of NID-MeIAc presented a more locally-excited (³LE) feature (71.2%, Fig. 5b and Fig. S14, ESI†). The spin-orbital coupling (SOC) elements from S₁ to T₁ are 0.401 cm⁻¹ for (R)-TRZ-MeIAc and 0.781 cm⁻¹ for (R)-NID-MeIAc, revealing efficient intersystem crossing (ISC) channels. At the same time, both molecules exhibited strong oscillator strengths (f_s) of singlet transitions (Fig. 5), revealing sufficiently high S₁ → S₀ radiative rates.

Fig. 5 The “hole” (blue) and “particle” (red) contours, the transition weight of singlet and triplet transitions, the oscillator strength of singlet transitions, and spin–orbit coupling element of (R)-TRZ-MeIAc (a) and (R)-NID-MeIAc (b).

To explain the highly emissive feature of the designed emitters, flexible potential surface scanning of the energy of the S₀ state geometry ((R)-TRZ-MeIAc and (R)-NID-MeIAc, for example) was then conducted. Starting from the initial geometries, molecules were optimized (with Δα = ±5° as a step) to relax the irrational intramolecular interaction force. Due to the asymmetric structure of the donor unit, two local energy maxima exist in (R)-TRZ-MeIAc, which can be obtained with a rotation angle Δα = 75° or 60°. The energy discrepancies between the minimized (quasi-equatorial conformer) and local maximum energy turned out to be 121.5 and 467.9 meV (Fig. 6a). Despite the existence of the minimized local (quasi-axial conformer) and charge transfer state (quasi-equatorial conformer), the torsional energy barrier is only 45 meV (much smaller than 126 meV) so the charge transfer S₀ state with a quasi-equatorial conformer is preferred.¹³ According to the Boltzmann distribution, the relative ratio of the charge transfer state was determined to be 97.4% (Table S6, ESI†), demonstrating the structural rigidity of MeIAc.¹³ Notably, ambient temperature (300 K) with 25 meV energy only allows possible molecular rotation of around Δα = 20° in (R)-TRZ-MeIAc.¹⁴

Fig. 6 Flexible potential surface scanning of the energy of the ground state geometry of (R)-TRZ-MeIAc (a) and (R)-NID-MeIAc (b) at different twisting angles (α); the maximized local states and the minimized charge transfer states for (R)-TRZ-MeIAc are labeled with red and blue rectangles respectively; the maximized local states and the minimized charge transfer states of (R)-NID-MeIAc are labeled with red and blue ellipses, respectively. (c) A comparison of the optimized structures of the S₀ state (light blue) with the S₁ state (blue) of (R)-TRZ-MeIAc (left) and the S₀ state (orange) with the S₁ state (red) of (R)-NID-MeIAc (right).

The torsional energy barrier between the minimized local (quasi-axial conformer) and charge transfer states (quasi-equatorial conformer) in (R)-NID-MeIAc is 115 meV (smaller than 126 meV) so the charge transfer S₀ state is predominant (Fig. 6b).¹³ The relative ratio of the charge transfer state in (R)-NID-MeIAc turned out to be larger (more than 99.9%) than that of (R)-TRZ-MeIAc (97.4%), further demonstrating the rigid structure of MeIAc and the larger steric hindrance leading to a greater extent of charge transfer. In the S₀ state of (R)-NID-MeIAc, the possible molecular rotation is limited to around Δα = 10° at ambient temperature. The potential surface scanning of the energy of the S₁ state geometry was also conducted (Fig. S17, ESI†), revealing a predominant charge transfer feature. The S₁ and S₀ state geometries illustrated little deformation in the two emitters (Fig. 6c and Fig. S15, S16, ESI†) with the root-mean-square deviation (RMSD) of 0.26 Å for (R)-TRZ-MeIAc and 0.39 Å for (R)-NID-MeIAc, further indicating the rigid architecture. In light of the above results, it can be realized that the fixed skeleton of the chiral donor and the rigid connection in the two D*–A type molecules provide benefits to enable access to a stable quasi-equatorial conformer for activating TADF and suppressing nonradiative deactivation processes.

To obtain more insight into the relationship between the chiral structure and circularly polarized luminescence, theoretical estimation of the electric ([small mu, Greek, vector]) and magnetic (m⃑) transition dipole moment was conducted (Table 2).^1a,b,o The simulated |[small mu, Greek, vector]| values of the TRZ-MeIAc and NID-MeIAc enantiomers are 4.26 × 10⁻¹⁹ and 2.80 × 10⁻¹⁹ esu cm, respectively; the simulated |m⃑| values for the two pairs of enantiomers are 2.04 × 10⁻²¹ and 2.17 × 10⁻²¹ erg per gauss, respectively. When the direction of [small mu, Greek, vector] is the same as that of m⃑ (the charge moves along the right hand spiral path during the transition), positive signals in the CPPL spectrum can be found; otherwise, negative signals would be present. For (R)-TRZ-MeIAc and (R)-NID-MeIAc, the vector [small mu, Greek, vector] and m⃑ possessed nearly the same direction (the vector angle θ_μ,m of 14.5° for (R)-TRZ-MeIAc and 13.3° for (R)-NID-MeIAc), in accordance with the positive signals in the CPPL spectra. Meanwhile, (S)-TRZ-MeIAc and (S)-NID-MeIAc with nearly opposing directions between the two vectors (θ_μ,m of 165.5° for (S)-TRZ-MeIAc and 166.7° for (S)-NID-MeIAc) demonstrated negative CPPL signals. The simulated |[small mu, Greek, vector]| values are much higher than the |m⃑| values so the equation for calculating the circularly polarized luminescence dissymmetry factor g can be simplified (eqn (S15), ESI†).^1a,b,k,o

According to eqn (1), a chiral emitter with a small |[small mu, Greek, vector]| value but a large |m⃑| value can obtain an ideal |g| value. In the two pairs of CP-TADF emitters, NID-MeIAc enantiomers revealed slightly higher |m⃑| values but lower |[small mu, Greek, vector]| values than those of the TRZ-MeIAc enantiomers (Table 2). The higher |m⃑| value can be ascribed to the slightly stronger charge-transfer extent (hole → particle, 98.4%) in NID-MeIAc than that (hole → particle, 96.6%) in TRZ-MeIAc (Fig. 5).¹⁵ The lower |[small mu, Greek, vector]| value of NID-MeIAc is limited by the relatively weaker oscillator strength (f = 0.0236) compared to that of TRZ-MeIAc (f = 0.0994) (Fig. 5 and eqn (S17), ESI†).^1p,16 In addition, with the more parallel/antiparallel direction between the vector [small mu, Greek, vector] and m⃑ (|cosθ_μ,m| = 0.968 for TRZ-MeIAc enantiomers and |cosθ_μ,m| = 0.973 for NID-MeIAc enantiomers), the NID-MeIAc enantiomers revealed higher |g| values, which is qualitatively consistent with the experimental results. Based on the above results, designing D*–A type CP-TADF emitters with high molecular rigidity and an enhanced charge transfer feature could simultaneously suppress nonradiative processes and achieve an intense |g| value.

Table 2 The simulated electric ([small mu, Greek, vector]) and magnetic (m⃑) transition dipole moment parameters of the TRZ-MeIAc and NID-MeIAc enantiomers

Emitter	|[small mu, Greek, vector]|^a (×10^−19 esu cm)	|m⃑|^b (×10^−21 erg per gauss)	R (×10^−40 erg esu cm per gauss)	cos θμ,m	θ μ,m (°)	(×10^−3)
a The simulated electric transition dipole moment value. b The simulated magnetic transition dipole moment value. c Rotatory strength, R = |[small mu, Greek, vector]|·|m⃑|·cosθμ,m. d The vector angle between the electric and magnetic transition dipole moments.
(R)-TRZ-MeIAc	4.26	2.04	8.41	0.968	14.5	4.64
(S)-TRZ-MeIAc	4.26	2.04	−8.41	−0.968	165.5	−4.64
(R)-NID-MeIAc	2.80	2.17	5.91	0.973	13.3	7.52
(S)-NID-MeIAc	2.80	2.17	−5.91	−0.973	166.7	−7.52

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CP-OLED performance

Considering the efficient TADF behaviour and obvious CPPL properties, the two pairs of CP-TADF enantiomers were employed as chiral emitters for fabricating OLEDs, with the detailed device structure and related performances displayed in Fig. 7, Table 3 and Fig. S18, S19, Tables S7, S8 (ESI†). The device configuration for TRZ-MeIAc (Fig. S18, ESI†) was optimized as follows: ITO (80 nm)/MoO₃ (1 nm)/TAPC (4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)) (45 nm)/mCP (1,3-di(9H-carbazol-9-yl)benzene) (10 nm)/mCPCN: (R/S)-TRZ-MeIAc (4 or 8 or 12 wt%) (20 nm)/3TPYMB (tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane) (50 nm)/LiF/Al. ITO and Al acted as the anode and cathode, respectively. MoO₃ and LiF were employed as the hole- and electron-injection layers. TAPC/mCP and 3TPYMB served as the hole- and electron-transporting layers, respectively. With an increase in the doping concentration from 4 wt% to 8 wt%, the device efficiencies were significantly enhanced. For the NID-MeIAc enantiomers, the devices adopted a similar architecture except that the thickness of TAPC and 3TPYMB was 70 nm (Fig. S19 and Table S8, ESI†). With an increase in the doping concentration from 1.5 wt% to 3 wt% to 6 wt%, the device efficiencies were gradually boosted.

Fig. 7 The device structure, energy level diagram and the molecular structures of the materials employed in the devices (a), electroluminescence spectra (b), and external quantum efficiency characteristics of the optimized devices based on (R)-TRZ-MeIAc (c) or (S)-NID-MeIAc (d).

Table 3 A summary of EL data for the optimized devices containing the (R/S)-TRZ-MeIAc and (R/S)-NID-MeIAc enantiomers

Emitter	V on [V]	ELpeak^b [nm]	CEmax^c [cd A^−1]	PEmax^d [lm W^−1]	EQEmax^e [%]	CIE^f (x, y)	g EL [×10^−4]
a The turn-on voltage was recorded at a brightness of 1 cd m^−2. b The peak wavelength of the EL spectrum. c The maximum value of current efficiency. d The maximum value of power efficiency. e The maximum value of external quantum efficiency. f The Commission International de I’Eclairage coordinates recorded at the maximum EQE. g g EL value of (R/S)-TRZ-MeIAc or (R/S)-NID-MeIAc-based devices at the maximum emission peaks.
(R)-TRZ-MeIAc	2.9	494	47.8	49.6	20.3	(0.18, 0.38)	+6.4
(S)-TRZ-MeIAc	2.9	494	42.1	44.1	18.7	(0.18, 0.38)	−7.3
(R)-NID-MeIAc	3.1	591	74.5	42.3	19.0	(0.53, 0.47)	+20
(S)-NID-MeIAc	3.1	589	68.4	61.3	23.7	(0.53, 0.47)	−24

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The two sets of devices based on (R/S)-TRZ-MeIAc and (R/S)-NID-MeIAc displayed sky-blue and orange emission, respectively. The sole emission originated from the emitter in the EL spectrum, indicating efficient energy transfer from the mCPCN host to the CP-TADF dopants. The turn-on voltages for the TRZ-MeIAc and NID-MeIAc enantiomers were 2.9 V and 3.1 V (Table 3), respectively, demonstrating the well-aligned energy levels of the materials in the device structure.

The OLEDs based on the TRZ-MeIAc enantiomers demonstrated obvious mirror-image CPEL activities with opposing g_EL values of +6.4 × 10⁻⁴ and −7.4 × 10⁻⁴ for the R and S enantiomer-based devices, respectively; while the devices based on (R)-NID-MeIAc and (S)-NID-MeIAc displayed g_EL values of +2.0 × 10⁻³ and −2.4 × 10⁻³, respectively (Fig. S20 and S21, ESI†). These values are in the same order as the corresponding g_PL values. The above results clearly illuminated that the CPEL behavior was inherited from the two pairs of stable and rigid quasi-equatorial CP-TADF enantiomers.

Notably, the rationally designed CP-TADF emitters with high PLQYs provided the corresponding devices with very impressive efficiencies. The optimized (R)-TRZ-MeIAc-based device showed sky-blue emission with a maximum EQE of 20.3%, a maximum current efficiency (CE) of 47.8 cd A⁻¹ and a maximum power efficiency (PE) of 49.6 lm W⁻¹, which are the highest values among all reported blue-emissive CP-OLEDs (Fig. 8a and Table S9, ESI†). It is clear that the efficient harvesting of triplet excitons through the TADF mechanism enabled the superior performance of the TRZ-MeIAc-based devices compared to the reported traditional fluorescent CP-OLEDs with blue emission, and the rigid configuration-promoted highly efficient singlet radiative channel in TRZ-MeIAc ensured the greater device performance than that of previous blue CP-TADF OLEDs. Meanwhile, the (S)-NID-MeIAc-based device displayed orange emission, exhibiting a maximum EQE of 23.7%, a maximum CE of 68.4 cd A⁻¹ and a maximum PE of 61.3 lm W⁻¹, which represent the state-of-the-art performance among all reported orange CP-OLEDs (Fig. 8b and Table S9, ESI†). The excellent performance compared to that of traditional fluorescence and TADF-based orange CP-OLEDs can be attributed to the efficient exciton harvesting and the highest PLQY.

Fig. 8 (a) The EQE values of the reported blue CP-OLEDs (traditional fluorescent (ref. 6b and e) and TADF (ref. 10d, i and l) CP-OLEDs) and this work. (b) The EQE values of the reported orange to red CP-OLEDs (traditional fluorescent (ref. 6d and e), phosphorescent (ref. 7b, 8e and g) and TADF (ref. 10d and f) CP-OLEDs) and this work.

The EQE is even comparable to the reported best value (23.7%) of iridium complex-based orange CP-OLEDs.^8e Based on the above results, it can be concluded that designing D*–A type CP-TADF emitters with high rigidity can provide excellent candidates to obtain highly efficient CP-OLEDs.

Conclusions

In conclusion, by integrating acridine and indene subunits, we have ingeniously designed a rigid point chiral TADF donor of MeIAc and derived two pairs of sky-blue and orange CP-TADF enantiomers, (R/S)-TRZ-MeIAc and (R/S)-NID-MeIAc. These emitters with an almost absolute quasi-equatorial conformer geometry, high molecular rigidity and efficient charge transfer character demonstrated the highest PLQYs and k_r,Ss among the reported blue and orange CP-TADF emitters to date. The corresponding devices displayed state-of-the-art efficiencies in blue and orange CP-OLEDs. Moreover, the stable quaternary carbon stereo-center endowed the corresponding CP-OLEDs with intense |g_EL| values of up to 2.4 × 10⁻³. This work proves that the rational design of high rigidity D*–A systems can afford efficient charge and chirality transfer and highly emissive CP-TADF emitters for high-performance CP-OLEDs with an intense electroluminescence dissymmetry factor.

Author contributions

F. Ni, C.-W. Huang, X. Cao, C.-C. Wu and C. Yang wrote the manuscript. F. Ni, Y. Wu and S. Xia synthesized the compounds. F. Ni measured the photophysical, thermal and electrochemical properties of the compounds. C.-W. Huang, Y. Tang, J.-H. Hsu, W.-K. Lee and C.-C. Wu fabricated and characterized the devices. F. Ni and C. Yang conceived the original idea for investigation. K. Zheng and Z. Huang performed the X-ray single crystal diffraction analysis. Z. Chen performed the quantum chemical calculations. All authors discussed the progress of research and reviewed the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51903161, 91833304 and 21721005), the Shenzhen Science and Technology Program (KQTD20170330110107046), and the China Postdoctoral Science Foundation-funded project (No. 2019M653035). C.-C. Wu acknowledges the support from Ministry of Science and Technology of Taiwan (MOST 107-2221-E-002-160-MY3 and 108-2221-E-002-148-MY3). W.-K. Lee acknowledges the post-doctoral fellowship from Ministry of Education (MOE) and Ministry of Science and Technology (MOST) of Taiwan. The numerical calculations in this paper were performed using the supercomputing system in the Supercomputing Center of Wuhan University. We are particularly grateful to Prof. Y. X. Zheng's group (Nanjing University) for providing the support for CPL measurements.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1971785. For ESI and crystallographic data in CIF or other electronic formats, see DOI: 10.1039/d0mh01521k

‡ F. Ni and C. W. Huang contributed equally to this work.

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