A quasiplanar TADF emitter employing a dual-locking strategy enables efficient solution-processed deep blue OLEDs

Abstract

Quasiplanar thermally activated delayed fluorescence (TADF) emitters are promising for high-efficiency deep blue organic light-emitting diodes (OLEDs), but they can seldom be used for wet processes. In this work, a novel solution-processed molecule, BOAC-OH, was developed by grafting a B-OH group onto the prototype molecule BOAC. Driven by synergistic O–H⋯O intramolecular hydrogen bonding and B–C σ bonds, BOAC-OH undergoes a conformational change from a highly twisted form of BOAC to a quasiplanar form. As a result, it achieves remarkably blue-shifted emission below 440 nm and promotes the improvement of the radiative transition rate compared to BOAC. The solution-processed OLED device based on BOAC-OH affords a maximum external quantum efficiency of 10.3% with deep blue emission peaking at 444 nm, corresponding to CIE coordinates of (0.15, 0.08), matching well with the National television system committee blue standard. This work showcases the great potential of this dual-locked strategy in developing deep blue TADF emitters for solution-processed OLEDs

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Introduction

Thermally activated delayed fluorescence (TADF) emitters are capable of harvesting both singlet and triplet excitons by leveraging a reverse intersystem crossing (RISC) process, showing great potential in efficient organic-light emitting diodes (OLEDs).^1–5 Twisted donor (D)-acceptor (A) structures with significant torsion angles (ϑ_DA) between the D and A segments are a mainstream design strategy for developing TADF emitters. In such a picture, the distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are effectively separated, facilitating an efficient RISC process.^6–10 However, such a twisted D–A design typically induces strong intramolecular charge transfer (ICT) properties, resulting in redshifted emission, and thereby making blue emission with peaks below 450 nm and commission internationale de l’eclairage (CIE) coordinates approaching the National television system committee (NTSC) blue standard, i.e., (0.14, 0.08), difficult to achieve.^11–15

TADF emitters with quasiplanar geometries are favourable for weakening ICT and blue-shifting the emission, thereby offering a promising solution to this predicament.^16–20 For example, Duan et al. developed a quasiplanar molecule, namely pICZ (see Fig. 1), which successfully achieved blue emission at 445 nm with a CIE_y coordinate of 0.10.²¹ Afterwards, Tang et al. reported a quasiplanar TADF emitter based on a σ-bond lock strategy, namely TIC-BO, achieving deep blue emission peaking at 428 nm with a CIE_y coordinate of 0.05.¹⁷ However, the chemical structures of such quasiplanar molecules are generally rigid with poor solubility, hindering their application in solution-processed OLEDs with low-cost, scalable manufacturing.^22,23 Therefore, there is a strong demand for developing quasiplanar TADF emitters that can be used in efficient solution-processed deep blue OLEDs.

Fig. 1 (a) Strategies for designing quasiplanar blue TADF emitters and the chemical structure of BOAC and BOAC-OH. (b) Molecular geometry and packing modes of BOAC-OH.

Herein, a newly developed molecule, 3,6,13,17-tetra-tert-butyl-15,15-dimethyl-9,20-dioxa-10b-aza-4b,19-diboradinaphtho[3,2,1-de:1′,2′,3′-qr]pentacen-19(15H)-ol (BOAC-OH), was thus designed and synthesized by introducing a boron hydroxyl (B-OH) group on the prototype molecule, 2,7-di-tert-butyl-10-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9-dimethyl-9,10-dihydroacridine (BOAC). This motif not only brings boron-carbon (B–C) σ-bonds but also enables O–H⋯O intramolecular hydrogen bonds. Within this design, BOAC-OH exhibits a quasiplanar geometry, resulting in significantly blue-shifted emission compared with BOAC (486 nm), peaking at 438 nm in diluted toluene. In addition, it affords a much-improved photoluminescence (PL) quantum yield (PLQY) of up to 73% in a doped film due to an increased radiative rate (k_r) and sufficient organo-solubility, enabled by multiple flexible units (e.g., tert-butyl and B-OH groups). Consequently, BOAC-OH as an emitter was successfully applied in solution-processed OLEDs, yielding deep blue electroluminescence (EL) with a maximum external quantum efficiency (EQE_max) over 10% and a CIE_y coordinate of 0.08, matching well with the NTSC blue standard. This work demonstrates a promising strategy to construct deep blue TADF emitters toward solution-processed OLED applications.

Results and discussion

The synthesis routes of BOAC and BOAC-OH are shown in Scheme S1. BOAC, the prototype molecule, as well as the precursor for BOAC-OH, was synthesized via a simple Buchwald–Hartwig coupling reaction between 7-bromo-2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BO) and 2,7-di-tert-butyl-9,9-dimethyl-9,10-dihydroacridine (AC). Following a bromination reaction of BOAC and an organolithium-mediated borylation-annulation reaction in sequence, the target molecule BOAC-OH was then obtained. Both compounds were characterised and confirmed by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS).

To examine the thermal stability of BOAC and BOAC-OH, differential scanning calorimetry (DSC) measurements were carried out. As depicted in Fig. S1, the glass transition temperature (T_g) of BOAC was recorded as 69 °C, while no glass transition was observed for BOAC-OH in the range of 25–280 °C, indicating that BOAC-OH exhibits better amorphous film stability, which is favourable for OLED operation. Additionally, thermogravimetric analysis (TGA) under a nitrogen atmosphere showed that both BOAC and BOAC-OH underwent decomposition, reaching 5% weight loss at 356 and 345 °C, respectively. Notably, at lower temperatures, BOAC-OH exhibits a mild decomposition signal, possibly resulting from a dehydration reaction involving B-OH groups. This indicates that BOAC-OH is unsuitable for evaporation. However, the presence of B-OH and tert-butyl groups is favourable for organo-solubility, making BOAC-OH a suitable candidate for wet processes.

Cyclic voltammetry (CV) measurements were then performed to characterise their electrochemical properties (Fig. S2). The HOMO energy levels were calculated to be −5.38 eV for BOAC and −5.70 eV for BOAC-OH from the onsets of their oxidation curves. Accordingly, the LUMO energy levels were estimated as −2.40 eV for BOAC and −2.58 eV for BOAC-OH by summing the HOMO and optical bandgap (E_g) values (vide infra).²⁴

To understand the effect of geometric changes on the frontier molecular orbital (FMO) distributions and excited state levels, density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations were performed. As illustrated in Fig. 2, BOAC displays a perpendicular configuration with a large torsion angle of 89.55°. As a consequence, the HOMO is predominantly localized on the AC segment, whereas the LUMO is mainly concentrated on the BO moiety. In the case of BOAC-OH, the introduction of the B-OH group simultaneously brings about a pair of B–C σ-bonds and a strong intramolecular O–H⋯O hydrogen bond with a length of 1.97 Å, resulting in a quasiplanar geometry. The HOMO is thus delocalised from the AC to the BO moiety, while the LUMO is still restricted on the BO acceptor (Fig. S4). A substantial oscillator strength (f) of 0.14 is thus obtained for BOAC-OH, which is conducive to obtaining a high PLQY value. The calculated E_g values of BOAC and BOAC-OH are 2.95 and 3.47 eV, respectively, implying that BOAC-OH has mitigated ICT intensity due to the enhanced planarity. In addition, the natural transition orbital (NTO) distributions were acquired to figure out their excited state properties. As illustrated in Fig. S3, both their lowest singlet (S₁) and triplet (T₁) excited states show ICT-dominated distributions.²⁵ The S₁/T₁ energy levels of BOAC and BOAC-OH are estimated to be 2.23/2.22 eV and 2.56/2.47 eV, respectively. The higher S₁ of BOAC-OH indicates that it could display a hypsochromic shift compared to BOAC. The energy splitting (ΔE_ST) values between the S₁ and T₁ states are predicted to be 0.01 and 0.09 eV for BOAC and BOAC-OH (Table S2), both of which are able to induce efficient up-conversion processes from T₁ to S₁.^26,27

Fig. 2 HOMO and LUMO distributions and simulated key energy-level diagrams of (a) BOAC and (b) BOAC-OH.

Single crystal samples of BOAC-OH were obtained and characterised via X-ray diffraction (Table S1). As depicted in Fig. 1, due to the incorporation of B–C σ-bonds, the geometry of BOAC-OH is substantially changed compared to that of the prototype BOAC, from highly twisted to slightly distorted. Furthermore, a clear O–H⋯O hydrogen bond is observed inside each BOAC-OH molecule in the crystal pattern, with a bond length of 2.05 Å, which further promotes molecular planarity and thus facilitates a quasiplanar configuration, in good agreement with the estimated results.

Ultraviolet-visible (UV-vis) and PL spectra of BOAC and BOAC-OH were first measured in dilute toluene (10⁻⁵ M) at room temperature to investigate the photophysical properties. As illustrated in Fig. 3a, both emitters display multiple absorption bands. BOAC displays strong absorption bands in the 300–400 nm range attributed to π–π*/n–π* transitions and BOAC-OH with a similar pattern below 370 nm.²⁸ Upon employing the dual-locking strategy, BOAC-OH delivers significantly intensified and broadened absorption bands in the range of 370–425 nm, attributed to ICT transitions, while BOAC shows nearly vanished signals in a similar region. This difference is consistent with their different f values.²⁹ Interestingly, the PL spectrum of BOAC-OH can be tuned from the sky-blue region of BOAC (486 nm) to the deep blue region with an emission peak of 438 nm. The reason for this disparity lies in the fact that BOAC-OH possesses a weaker ICT strength due to the planarized geometry. As illustrated in Fig. S5, BOAC and BOAC-OH in toluene exhibit obvious delayed components after degassing oxygen with nitrogen, suggesting that both compounds are capable of harvesting triplet excitons.

Fig. 3 Normalized UV-vis absorption and fluorescence spectra of (a) BOAC and (b) BOAC-OH in toluene (1 × 10⁻⁵ M) at room temperature, and transient PL decay curves of (c) 5 wt% BOAC and (d) 5 wt% BOAC-OH doped in the mCP host at 298 K (inset: prompt components).

To further characterise the PL properties of both emitters in the amorphous conditions, we prepared their 5 wt% doped thin films using 1,3-di(9H-carbazol-9-yl) benzene (mCP) as the host matrix. As illustrated in Fig. S6, in the range below ∼330 nm, the absorption from the host matrix mCP is significant. Beyond ∼330 nm, mCP barely absorbs, and thus, the absorption signals from the dopants can be clearly detected, which are similar to their behaviours in dilute toluene. Upon photoexcitation, the BOAC-OH-based film exhibits deep blue emission peaking at 443 nm, showing a significant spectral blueshift compared with BOAC in the film (470 nm) owing to the weakened ICT transitions. As summarized in Table 1, the PLQYs of these doped films are measured at 73% for BOAC-OH and 45% for BOAC, respectively, revealing that BOAC-OH permits superior exciton harvesting compared to BOAC. Fig. S6 displays the fluorescence and phosphorescence spectra of their doped films at 77 K. The S₁ and T₁ levels are determined to be 2.95/2.91 eV for BOAC and 3.05/2.91 eV for BOAC-OH, respectively, based on the corresponding spectral onsets. Their ΔE_ST values are thus calculated to be 0.04 and 0.14 eV, respectively, matching well with the theoretical estimation results (Table S2). The larger ΔE_ST value of BOAC-OH can be attributed to its comparatively weaker ICT characteristics. Fig. S7 displays their transient PL decay profiles characterised under a nitrogen atmosphere at various temperatures. The delayed lifetime of BOAC shows obvious declines as the temperature gradually increases from 100 to 300 K, confirming its TADF characteristics. In the case of BOAC-OH, the signals of the delayed components at lower temperatures (e.g., 100 and 200 K) are very weak, in which the detected data are basically IRF, while as the temperature further increases to 300 K, the delayed contributions gradually become obvious, demonstrating an evident TADF contribution.³⁰ At room temperature, the measured prompt lifetimes (τ_P) of BOAC and BOAC-OH are 21.40 and 3.98 ns, and their delayed lifetimes (τ_d) are 0.90 and 12.68 μs, respectively (Fig. 3c and d). The distinctly different lifetimes reveal their different dynamics. Their key kinetic parameters are further calculated according to the reported equations.³¹ As summarised in Table S3, BOAC exhibits a faster RISC rate (k_RISC) of 1.73 × 10⁶ s⁻¹ than BOAC-OH (4.05 × 10⁴ s⁻¹), attributable to its smaller ΔE_ST value. In contrast, BOAC-OH exhibits a significantly enhanced k_r rate of 1.53 × 10⁸ s⁻¹, representing a 17-fold increase compared to BOAC because of its substantially improved f value, which eventually leads to more efficient exciton harvesting.

Table 1 Key photophysical properties of BOAC and BOAC-OH

Emitter	λ abs (nm)	λ PL /λFilm^b (nm)	S1^c (eV)	T1^c (eV)	ΔEST^c (eV)	PLQY^b (%)
a Measured in toluene (1 × 10^−5 M) at room temperature. b Measured from 5 wt%-doped mCP thin films under a nitrogen atmosphere. c Determined from the onsets of the fluorescence and phosphorescence spectra of the 5 wt%-doped mCP thin films at 77 K, ΔEST = S1 – T1.
BOAC	420	486/470	2.95	2.91	0.04	45
BOAC-OH	397	438/443	3.05	2.91	0.14	73

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To reveal the effect of the dual-locking strategy on EL performance, BOAC and BOAC-OH-based solution-processed OLEDs were fabricated with the following device structure: indium tin oxide (ITO)/poly(ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS, 40 nm)/emitting layers (EMLs, 30 nm)/dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) (PPT, 10 nm)/3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1′:3′,1′′-terphenyl]-3,3′′-diyl)dipyridine (TmPyPB, 30 nm)/lithiumfluoride (LiF, 1 nm)/Al (120 nm). In this device architecture, ITO and Al act as the anode and cathode, respectively; PEDOT:PSS and LiF are applied as the hole- and electron-injection layers, respectively; PPT and TmPyPB serve as the hole-blocking and electron-transporting layers, respectively. In the EMLs, BOAC and BOAC-OH work as dopants with a doping ratio of 5 wt% dispersed in the mCP host with a high-triplet-energy level. The device energy-level diagram and chemical structures of the materials used for OLEDs are illustrated in Fig. 4a and 4b, respectively.

Fig. 4 (a) The device energy-level diagram and (b) chemical structures of the materials used for the solution-processed OLEDs. (c) EL spectra at 10 mA cm⁻² (inset: the CIE diagram). (d) EQE versus current density curves and (e) and (f) current density–voltage-luminance (J–V–L) curves based on BOAC and BOAC-OH at a 5 wt% doping concentration.

As shown in Table 2, the BOAC-based device exhibits a higher turn-on voltage (V_on) of 3.9 V than BOAC-OH (3.5 V) due to the relatively mismatched HOMO energy levels, which easily form hole traps for carrier and exciton capture.^32,33 As shown in Fig. 4c, the BOAC-based device only exhibits sky-blue EL peaking at 480 nm, corresponding to CIE coordinates of (0.13, 0.24), whereas the BOAC-OH-based one demonstrates a significantly blue-shifted EL spectrum with a peak at 444 nm. The corresponding CIE coordinates shift to (0.15, 0.08), which is very close to the NTSC blue standard. Noticeably, BOAC-OH achieves a much-improved EQE_max of 10.3% compared with BOAC (6.6%), consistent with its higher PLQY value upon photoexcitation. These results further illustrate that the dual-locking strategy is conducive to simultaneously blue-shifting the EL spectra and improving the device efficiencies. Notably, the BOAC-OH-based device suffers from a more severe efficiency roll-off at high current densities than the BOAC-based devices, because the relatively slow k_RISC induces serious triplet accumulation and quenching. It is expected that this phenomenon can be relieved by further device optimization.

Table 2 EL performance of the solution-processed OLEDs based on BOAC and BOAC-OH

Emitter	V on (V)	λ EL (nm)	L max (cd m^−2)	CEmax^d (cd A^−1)	PEmax^e (lm W^−1)	EQE^f (%)	CIE^g (x,y)
a Measured at 1 cd m^−2. b Peak wavelength of the EL spectrum. c Maximum luminance. d Maximum current efficiency. e Maximum power efficiency. f Maximum external quantum efficiency. g CIE coordinates measured at 10 mA cm^−2.
BOAC	3.9	476	2929	4.4	7.9	6.6	(0.13,0.24)
BOAC-OH	3.5	444	471	10.6	9.5	10.3	(0.15,0.08)

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Conclusions

A novel TADF emitter, BOAC-OH, was synthesised based on a dual-locking strategy via intramolecular O–H⋯O hydrogen bonds and a pair of B–C σ bonds on a prototype molecule BOAC. With this approach, BOAC-OH exhibits a quasiplanar molecular configuration, offering remarkably blue-shifted emission below 440 nm and a much-improved PLQY compared to BOAC. In solution-processed OLEDs, BOAC-OH achieves deep blue EL peaking at 444 nm and an EQE_max of over 10%. The CIE coordinates of (0.15, 0.08) are very close to the NTSC blue standard. This work provides a feasible route to develop a deep blue TADF emitter towards solution-processed OLEDs.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information: General information, photophysical properties, device characterization. See DOI: https://doi.org/10.1039/d5tc02547h.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52422309, 52130304, 52373193), the National Key Research & Development Program of China (Grant No. 2020YFA0714601 and 2020YFA0714604), the Collaborative Innovation Center of Suzhou Nano Science & Technology, and the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Suzhou Key Laboratory of Advanced Photonic Materials (Grant SZS2023010).

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