Tingting Huang‡
a,
Li Yuan‡c,
Xueying Lua,
Yupei Qua,
Cheng Qua,
Yincai Xu
*a,
You-Xuan Zheng
*c and
Yue Wang
*ab
aState Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: ycxu17@mails.jlu.edu.cn; yuewang@jlu.edu.cn
bJihua Laboratory, 28 Huandao Nan Road, Foshan, 528200, Guangdong Province, P. R. China
cState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China. E-mail: yxzheng@nju.edu.cn
First published on 28th August 2024
Helicene-based circularly polarized multiple resonance thermally activated delayed fluorescence (CP-MR-TADF) materials are promising for ultra-high-definition and 3D displays, but most of them encounter potential problems such as easy racemization during the thermal deposition process, low luminous efficiency, and low luminescence dissymmetry factor (glum), making the development of efficient circularly polarized organic light-emitting diodes (CP-OLEDs) a significant challenge. Here, we report a pair of CP-MR-TADF enantiomers with high-order B,N-embedded hetero[8]helicene, (P/M)-BN-TP-ICz, by fusing two MR chromophores, DtCzB and indolo[3,2,1-jk]carbazole (ICz). BN-TP-ICz exhibits green emission in toluene with a peak of 531 nm and a full-width at half-maximum (FWHM) of 36 nm. The optimized CP-OLEDs with enantiomers (P/M)-BN-TP-ICz exhibit green emission with peaks of 540 nm, FWHMs of 38 nm and Commission Internationale de L'Eclairage coordinates of (0.33, 0.65). Moreover, they showcase maximum external quantum efficiencies (EQEs) of 32.0%, with gELs of +6.49 × 10−4 and −7.74 × 10−4 for devices based on (P)-BN-TP-ICz- and (M)-BN-TP-ICz, respectively.
With the increasing diversification of lifestyle and the continuous evolution of display technology, the development of ultra-high-definition (UHD) displays with a wide color gamut has become the focus of cutting-edge OLED displays.8 Therefore, integrating CPL with high-color-purity multiple resonance thermally activated delayed fluorescence (MR-TADF) materials has become a prevalent trend to create CP-MR-TADF emitters with multiple figures-of-merit.9 CP-MR-TADF materials can directly emit CPL, avoiding energy loss caused by the need for circular polarizers and quarter-wave plates, and they can reduce glare and enhance contrast, clarity, and expressiveness of images in practical applications. Furthermore, they address the issue of low color purity encountered with traditional CPL materials.10 Currently, the actively researched helicene-type CP-MR-TADF materials are mainly focused on [6]helicene or even lower-order helicenes.11 Higher-order helicenes can easily overcome the problem of the low energy barrier of racemization, thereby avoiding the risk of configuration instability during vacuum thermal deposition and significantly improving repeatability.12 Additionally, the helical chiral π-conjugated system can participate in the distribution of frontier molecular orbitals (FMOs) of molecules, favoring π-electron flow, which facilitates enhancement of optical rotation and luminescence dissymmetry factor (glum).13
The general approach for constructing high-order helicenes is to increase the number of benzene rings in the helicene,14 which not only poses challenges to molecular synthetic methodology but also usually accelerates the intersystem crossing (ISC) process from the lowest singlet (S1) to the lowest triplet (T1) excited state, leading to the accumulation of non-radiative triplet exciton transition and a significant reduction in photoluminescence quantum yields (ΦPLs). Additionally, the extended persistent π-conjugated structure may destroy the alternating distribution pattern of FMOs and lead to an unpredictable broadening of the emission spectrum or even an uncontrollable loss of TADF characteristics.15 Therefore, it is an ongoing pursuit to develop CP-MR-TADF materials with narrow full-width at half-maximum (FWHM), high ΦPL, decent glum, and high energy barrier of racemization from large-sized higher-order helicenes.16
Our group has implemented the edge-topology molecular-engineering strategy to expand the sub-resonance skeleton of MR molecules and successfully developed an efficient helicene-type CP-MR-TADF emitter BN-Py.17 This work provides a molecular construction template and synthetic methodology for the subsequent development of helical chiral CP-MR-TADF materials. In this contribution, following the well-defined paradigm, we initially made a tentative work to introduce a pure carbon PAH into the star MR-TADF molecule DtCzB,18 and constructed a hetero[7]helicene-type molecule, BN-DBC, with a moderate energy barrier of racemization (Scheme 1). It exhibits yellow emission with a peak of 548 nm, a broad FWHM of 44 nm, and a low ΦPL of 55% in toluene solution. Unfortunately, it lacks TADF characteristics, limiting its electroluminescence (EL) performance in devices. Subsequently, we further stitched a unique MR chromophore with a rigid planar structure independent of the B,N-containing MR framework, indolo[3,2,1-jk]carbazole (ICz), with the DtCzB fragment to develop a CP-MR-TADF molecule with high-order B,N-embedded hetero[8]helicene, BN-TP-ICz, with a high energy barrier of racemization. It displays vivid green emission with a peak of 531 nm, a narrow FWHM of 36 nm, and a high ΦPL of 93% in toluene solution. The optimized CP-OLEDs with enantiomers (P/M)-BN-TP-ICz exhibit green emission with peaks of 540 nm, FWHMs of 38 nm and Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.65). Moreover, they showcase maximum external quantum efficiencies (EQEs) of 32.0%, with electroluminescence dissymmetry factors (gELs) of +6.49 × 10−4 and −7.74 × 10−4 for devices based on (P)-BN-TP-ICz and (M)-BN-TP-ICz, respectively. The successful construction of BN-TP-ICz effectively provides a new perspective for the future research on high-order helicene-type CP-MR-TADF materials.
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Scheme 1 Schematic diagram of molecular structure evolution of chiral hetero[n]helicene based on the parent molecule DtCzB and the corresponding photophysical properties. |
For comparison, the FMO distribution of BN-DBC was calculated (Fig. S9†), and its fundamental photophysical properties were characterized (Fig. S10, S11 and Table S1†). Compound BN-DBC in toluene solution (1 × 10−5 M) and PMMA (polymethyl methacrylate) film (1 wt% doping concentration) exhibits yellow emission with peaks of 548 and 553 nm, broad FWHMs of 44 and 50 nm, and low ΦPLs of 55% and 60%, respectively. Furthermore, the lowest-excited singlet–triplet energy splittings (ΔESTs) were estimated to be 0.32 and 0.31 eV, respectively, which implies that BN-DBC likely does not exhibit TADF properties. The triplet spin density distribution (TSDD) is primarily on the dibenzo[g,p]chrysene group, which may reveal a substantial decrease in T1 energy (Fig. S12†). Further analysis of the fluorescence lifetimes in toluene solution and PMMA doped film show mono-exponential transient photoluminescence (PL) decay with only nanosecond-scale prompt fluorescence detected (Fig. S13†). The combined theoretical calculations and experimental results indicate the absence of TADF properties in BN-DBC, which is therefore only used as a reference molecule and the properties are not further explored in this work. The associated experimental results of BN-DBC demonstrate that it would be challenging to achieve the desired goals only by embedding a large-sized pure carbon PAH helicene into the MR-TADF molecule DtCzB.
A single crystal of racemic BN-TP-ICz was acquired via vacuum vapor-phase deposition (Fig. S14, S15 and Table S2†). Single-crystal X-ray crystallographic analysis reveals that the crystal BN-TP-ICz belongs to the triclinic crystal system and P space group, and the π-conjugated framework consists of 54 sp2 hybridized carbon atoms, 1 coordinated boron atom and 3 embedded nitrogen atoms (C54BN3), exhibiting an unequivocal non-planar hetero[8]helicene structure (Fig. 1a). Viewed from the side, the dihedral angle between the planes containing the DtCzB moiety and ICz moiety is 32.13°, giving BN-TP-ICz a highly folded and distorted conformation and exhibiting an angular two-dimensional topology. The bulky steric hindrance between ICz and DtCzB pulls and deforms the N–B–N-substituted central phenyl ring in DtCzB, forming a torsion angle of 30.49° (Fig. 1b). In the three-dimensional stacking structure of the crystal, BN-TP-ICz adopts a regular and dense linear strand arrangement along the b-axis, with two adjacent molecules of (P)- and (M)-enantiomers appearing in pairs, placed in an inverted manner (Fig. 1c). As a result, there are abundant π⋯π interactions in the crystal, including the π⋯π interactions between ICz planes of the paired molecules (P)-BN-TP-ICz and (M)-BN-TP-ICz, and the π⋯π interactions between DtCzB planes of molecule (P)-BN-TP-ICz and molecule (M)-BN-TP-ICz of the neighboring pair, with the distances of 3.571 and 3.363 Å, respectively.
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Fig. 2 (a) HOMO and LUMO distribution, HOMO–LUMO energy level gap, and oscillator strength of (P)-BN-TP-ICz. (b) Energy barrier of the racemization of enantiomers (P/M)-BN-TP-ICz. |
Configurational stability is the paramount stereochemical feature of helicenes and their derivatives, which will directly impact their applications in chiroptics and optoelectronic devices. Therefore, we calculated the energy barrier of racemization of BN-TP-ICz (Fig. 2b).23 Analysis of the transition states of (P/M)-BN-TP-ICz reveals an energy barrier of racemization up to 49.21 kcal mol−1, which is higher than that of (P/M)-BN-DBC (37.25 kcal mol−1, Fig. S18†), and is adequate to support chiral resolution and further research on chiroptical performances under thermal evaporation. Chiral resolution of BN-TP-ICz was favorably achieved, and a pair of enantiomers (P/M)-BN-TP-ICz with high optical purity were obtained (enantiomeric excess (ee) value >99.0%, Fig. S19†). The formation of the high energy barrier of racemization can be further elucidated by analyzing the intramolecular non-covalent interactions (NCIs) within (P)-BN-TP-ICz as a function of reduced density gradient (RDG) (Fig. S20†).24 It is visually visible from the RDG isosurface that these NCIs are mainly concentrated in the spatial overlap region of head-to-tail rings of the hetero[8]helicene, demonstrating a robust steric hindrance effect that stems from the highly distorted and folded conformation of (P)-BN-TP-ICz. Consequently, during the configurational transition of (P/M)-BN-TP-ICz, overcoming these crowded steric hindrances becomes crucial, contributing to the high energy barrier of racemization of BN-TP-ICz.
Compound | λabsa [nm] | λemb [nm] | FWHMc [nm] | ES1d [eV] | ET1e [eV] | ΔESTf [eV] | Egg [eV] | HOMOh [eV] | LUMOh [eV] | ϕPLi [%] |
---|---|---|---|---|---|---|---|---|---|---|
a Peak wavelength of the lowest energy absorption band.b Peak wavelength of the PL spectrum in toluene solution (1 × 10−5 M, 298 K).c Full-width at half-maximum.d Singlet energy estimated from the onset of the fluorescence spectrum in toluene solution (1 × 10−5 M, 77 K).e Triplet energy estimated from the onset of the phosphorescence spectrum in a frozen toluene matrix (1 × 10−5 M, 77 K).f ΔEST = ES1 − ET1.g Optical band gap estimated from the absorption edge of the UV-vis spectrum.h Determined from cyclic voltammetry using the formula: EHOMO = −(Eox + 4.8) eV and ELUMO = −(Ered + 4.8) eV.i Absolute photoluminescence quantum yield measured with an integrating sphere in N2-bubbling toluene solution (1 × 10−5 M, 298 K). | ||||||||||
BN-TP-ICz | 510 | 531 | 36 | 2.44 | 2.25 | 0.19 | 2.32 | −5.06 | −2.69 | 93 |
Subsequently, the solid-state thin film of BN-TP-ICz doped in 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-3,9′-bicarbazole (PhCzBCz) with 5 wt% doping concentration was prepared via vacuum thermal evaporation for further investigation of photophysical properties. The doped film of BN-TP-ICz emits green light with a peak of 540 nm and a FWHM of 43 nm (Fig. S25†). The bi-exponential transient PL decay curve, composed of nanosecond and microsecond components, observed at room temperature and under a vacuum atmosphere (Fig. S26a†), exhibits typical TADF features of BN-TP-ICz. It demonstrates prompt fluorescence decay with a lifetime (τp) of 5.8 ns and delayed fluorescence decay with a lifetime (τd) of 138.5 μs. Moreover, it is observed from the temperature-dependent transient PL decay curves that both the intensity and proportion of the delayed fluorescence component significantly increase as the temperature increases from 80 to 320 K (Fig. S26b†), intuitively substantiating TADF properties. Derived from the photophysical data of the doped film, the calculated rate constants show that the rate constant of prompt fluorescence (kF) of BN-TP-ICz is 13.2 × 107 s−1, significantly higher than its rate constant of internal conversion (kIC) of 1.15 × 107 s−1, indicating that the non-radiative energy loss is small; the corresponding rate constant of RISC (kRISC) is 0.61 × 104 s−1 (Table S3†). These characteristics are consistent with the general photophysical behavior of MR-TADF materials.
To assess the chiroptical properties of enantiomers (P/M)-BN-TP-ICz in S0 and S1 states, their CD and CPL spectra were studied. The absolute configurations of the two components separated by chiral high performance liquid chromatography (HPLC) were confirmed by comparing the experimentally tested and theoretically simulated CD spectra of (P)-BN-TP-ICz (Fig. 3b),25 identifying the first and second fractions as (M)-BN-TP-ICz and (P)-BN-TP-ICz, respectively. The enantiomers (P/M)-BN-TP-ICz exhibit distinct mirror symmetry in both CD and circularly polarized photoluminescence (CPPL) spectra in toluene solution (Fig. 3c). In the higher energy absorption bands at 313–362 and 362–489 nm, the CD spectra display intense Cotton effects, which correspond to the intrinsic localized state absorption of B,N-embedded hetero[8]helicene. In the lower energy region between 489 and 541 nm, the weak Cotton effect originates from the chirality transfer from the B,N-embedded hetero[8]helicene to the chromophore DtCzB. The photoluminescence dissymmetry factors (gPLs) of (P)-BN-TP-ICz and (M)-BN-TP-ICz in toluene solution are +5.73 × 10−4 and −5.80 × 10−4, respectively (Fig. 3d). To date, helicene-based CPL emitters have attracted widespread attention. CPL brightness (BCPL), as a comprehensive indicator parameter of CPL performance, can easily compare the CPL activity of different compounds. Here, we calculated the BCPL of (P)-BN-TP-ICz and compared it with the representative single-helicene-based CPL emitters reported previously to fully grasp the overall research progress in this field (Table S6†).26 The results show that (P)-BN-TP-ICz exhibits decent CPL performance. The CPPL spectra of enantiomers (P/M)-BN-TP-ICz in the thermally deposited doped film (temperature: <160 °C, pressure: <5 × 10−4 Pa, thickness: 100 nm) also showcase good mirror symmetry, with gPLs of +8.06 × 10−4 and −6.76 × 10−4, respectively (Fig. 3e and f). Next, the optical purity of the residual enantiomers (P/M)-BN-TP-ICz samples after thermal deposition was further examined. Chiral HPLC analysis reveals that the ee values (>99.0%) are essentially consistent with those of the samples without thermal deposition treatment (Fig. S27†), indicating no racemization during the vacuum thermal deposition process and excellent configurational stability. The above results experimentally validate the rationality of incorporating ICz into DtCzB to form the BN-TP-ICz molecule with B,N-embedded hetero[8]helicene to construct CP-MR-TADF materials, laying a solid foundation for further research on CP-OLEDs.
x wt% | λema [nm] | FWHMb [nm] | CIE(x, y)c | Vond [V] | Lmaxe [cd m−2] | CEmaxf [cd A−1] | PEmaxg [lm W−1] | EQEh [%] |
---|---|---|---|---|---|---|---|---|
a EL peak wavelength.b Full-width at half-maximum.c Commission Internationale de L'Eclairage coordinates (value taken at 100 cd m−2).d Turn-on voltage at 1 cd m−2.e Maximum luminance.f Maximum current efficiency.g Maximum power efficiency.h Maximum external quantum efficiency, and values at 100 and 1000 cd m−2, respectively. (Note: (A) denotes the device with (P)-BN-TP-ICz, (B) denotes the device with (M)-BN-TP-ICz. | ||||||||
3 (A) | 536 | 38 | (0.32, 0.66) | 3.1 | 25![]() |
130.0 | 131.7 | 31.1/19.5/6.6 |
5 (A) | 540 | 38 | (0.33, 0.65) | 3.0 | 20![]() |
135.0 | 140.7 | 32.0/20.3/6.1 |
8 (A) | 540 | 40 | (0.34, 0.64) | 3.0 | 16![]() |
123.8 | 129.7 | 30.7/18.6/4.5 |
3 (B) | 540 | 38 | (0.32, 0.66) | 3.0 | 27![]() |
121.9 | 119.7 | 30.0/18.5/5.8 |
5 (B) | 540 | 38 | (0.33, 0.65) | 3.0 | 25![]() |
128.7 | 134.8 | 32.0/20.1/5.3 |
8 (B) | 540 | 39 | (0.34, 0.65) | 3.0 | 24![]() |
113.2 | 115.9 | 31.2/19.9/5.2 |
Owing to the similar photophysical properties of enantiomers (P/M)-BN-TP-ICz, they also exhibit almost the same EL performances in devices. All the single host-based devices demonstrate effective carrier injection and transport, with turn-on voltages (Von at 1 cd m−2) of ≤3.1 V. They achieve complete energy transfer at 3 wt% doping concentration, but the optimal EL performances are attained when the doping concentration increases to 5 wt%. At the optimal doping concentration, (P/M)-BN-TP-ICz-based devices show green emission with peaks of 540 nm and narrow FWHMs of 38 nm, and CIE coordinates of (0.33, 0.65). (P)-BN-TP-ICz- and (M)-BN-TP-ICz-based devices achieve maximum EQEs of 32.0% and 32.0%, maximum CEs of 135.0 and 128.7 cd A−1, and maximum PEs of 140.7 and 134.8 lm W−1, respectively, which are much better than those of the BN-DBC-based single-host device with an identical device structure at 5 wt% doping concentration (maximum EQE = 4.7%, CE = 17.2 cd A−1 and PE = 16.4 lm W−1, Fig. S30† and Table S4†). The devices based on emitter (P/M)-BN-TP-ICz have remarkably maximum EQEs, principally because the EML PhCzBCz: 5 wt% BN-TP-ICz simultaneously has a high ϕPL of up to 92% (Table S3†) and dominant horizontal emitting dipole orientation ratio (Θ//) of up to 80% (Fig. S31†). The calculated S0 → S1 transition dipole moment (TDM) of molecule (P)-BN-TP-ICz reveals that its TDM direction is mainly along the x-axis, and the TDM vector has a minor Z component (Fig. S32†), indicating that when anchored on the surface of a thin film, the emitter (P/M)-BN-TP-ICz may have a natural tendency to preferentially align the molecular axis horizontally. When the doping concentration increases to 8 wt%, the EL performances of the devices show an observable decrease, which is potentially attributed to molecular aggregation-induced quenching and confirmed by the appreciable crystal π⋯π stacking. Furthermore, the devices with 5 wt% doping concentration (P/M)-BN-TP-ICz exhibit good spectral stability with the increase in driving voltage (Fig. S33†), suggesting that there is only one stationary exciton recombination zone located in the EML within the device architecture, which is beneficial for its practical applications. More importantly, (P/M)-BN-TP-ICz-based devices with 5 wt% doping concentration shows distinct mirror circularly polarized electroluminescence (CPEL) spectra signal with decent gELs of +6.49 × 10−4 and −7.74 × 10−4 for (P)-BN-TP-ICz and (M)-BN-TP-ICz, respectively. In summary, (P/M)-BN-TP-ICz-based devices not only exhibit excellent EL performance but also show good CPEL characteristics, which verifies the scientificity of fusing ICz into DtCzB to construct CP-MR-TADF materials with B,N-embedded hetero[8]helicene, making their application in UHD CP-OLED displays attractive. In addition, since the efficiency roll-off of BN-TP-ICz-based devices is relatively large, the use of the sensitization strategy can alleviate the device efficiency roll-off to a certain extent.28 Related research is provided in the ESI (Fig. S34–S37 and Table S5).†
Raw and processed data: experimental measurements, including raw data from spectroscopic, NMR, and other characterization techniques.
ESI material†: additional information such as detailed experimental procedures, intermediate data, and supplementary figures.
Requests for data should be directed to [yuewang@jlu.edu.cn]. The data will be made available in a timely manner to ensure transparency and reproducibility of the results presented in this study.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 2361156. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc03854a |
‡ T. Huang and L. Yuan contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2024 |