Deep blue thermally activated delayed fluorescence emitters with a 9,9′-spirobifluorene-fused xanthone acceptor for efficient OLEDs

Abstract

Efficient and stable blue luminescent materials are highly required for organic light-emitting diodes (OLEDs), but are still challenging to develop. Herein, we report two deep blue thermally activated delayed fluorescence (TADF) small molecules, mSBFXT-TC and mSBFXT-PC, consisting of a 9,9′-spirobifluorene-fused xanthone acceptor and carbazole-based donors. mSBFXT-TC and mSBFXT-PC exhibit deep blue emissions peaking at 434 and 436 nm in toluene solution, and at 456–471 nm in doped films with photoluminescence quantum yields of 87–98% and good horizontal dipole ratios of 77.5% for mSBFXT-TC and 84.0% for mSBFXT-PC. Consequently, their doped OLEDs show excellent electroluminescence (EL) performances, with EL peaks at around 452–481 nm and maximum external quantum efficiencies (EQE_maxs) of 21.8% and 26.0%. Based on their good EL properties, high-efficiency blue and green-sensitized OLEDs are fabricated by using mSBFXT-TC and mSBFXT-PC as sensitizers and the narrow spectral multi-resonance materials as emitters, achieving high EQE_maxs of 26.0% and 32.4% with small full width at half maxima of 18 and 29 nm. These findings demonstrate that both deep blue TADF small molecules are promising emitters and sensitizers for fabricating efficient OLEDs.

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1. Introduction

Organic light-emitting diodes (OLEDs) have emerged as the core devices for next-generation flexible displays because of various advantages, such as low power consumption, wide color gamut, ultrafast response and flexibility. Whereas green and red materials and OLEDs have achieved rapid progress among the three primary display colors, the advancement of efficient blue electroluminescence (EL) materials is still facing significant challenges.^1–11 Traditional blue fluorescent materials usually have high stability and high color purity, but the exciton utilization efficiencies are normally limited to 25%, resulting in inferior external quantum efficiencies (EQEs, the ratio of the number of total photons emitted by the device to the number of injected charge carriers) typically below 5%.^12,13 Blue noble metal-containing phosphorescent materials can theoretically achieve 100% exciton utilization via the spin–orbit coupling (SOC) effect, but their inherent metal-to-ligand charge transfer (CT) characteristics often induce spectral red shifts and lower blue color quality. Besides, the blue phosphorescent materials often have long lifetimes of triplet excitons, which is harmful to the operational stability of OLEDs.^14–17 Therefore, blue phosphorescent materials with satisfactory stability are still rare.

Recently, purely organic thermally activated delayed fluorescence (TADF) materials have emerged as the third-generation luminescent materials for OLEDs.^18–22 They can theoretically achieve 100% exciton utilization by harvesting triplet excitons via reverse intersystem crossing (RISC). The commonly used way for designing blue TADF small molecules is employing weak electron donors (D) and acceptors (A) to weaken intramolecular charge transfer (CT), and reducing the D–A torsion angle is also favored to suppress CT-induced spectral red-shifts.^23–28 However, TADF materials practically need large D–A torsion angles to acquire small energy splitting (ΔE_ST) between the lowest singlet and triplet states to ensure a RISC process. Therefore, the reduction of the D–A torsion angle for designing deep blue molecules will lead to a large ΔE_ST, which results in a slow RISC rate of the triplet exciton and thus compromises device efficiency. To solve this problem, researchers have proposed numerous solutions. For example, Adachi et al. introduced methyl groups onto the phenyl bridge between the triazine acceptor and carbazole donor to obtain high-performance deep blue TADF materials with a small ΔE_ST, realizing a high maximum EQE (EQE_max) of 19.2% with Commission Internationale de I’Eclairage coordinates (CIE_x,y) of (0.148, 0.098) for the OLED.²⁹ Similarly, Lee et al. prepared efficient deep blue TADF materials by linking a rigid donor (oxygen-bridged triarylboron) to a diindolocarbazole acceptor, and their OLED exhibited an EL peak at 448 nm and a high EQE_max of 21.50%.³⁰ However, the EQEs and operational lifetimes of most reported blue TADF materials are still worse than those of green TADF materials, and cannot fulfill the requirement of OLED industrial applications.^31–36

In this work, we wish to report two new deep blue TADF emitters, mSBFXT-TC and mSBFXT-PC. They are composed of a new acceptor created by fusing 9,9′-spirobifluorene (SBF) to xanthone (XT) and weak electron donors of 3,6-di-tert-butylcarbazole (TC) and 3,6-diphenyl-9H-carbazole (PC). Their structures possess the following advantages: (1) the carbonyl group of XT can enhance the SOC and accelerate the reverse intersystem crossing (RISC) process because of its n–π* transition.^37–43 (2) the rigidity and steric hindrance of the SBF group are beneficial for suppressing non-radiative transitions and excessive molecular stacking.^44,45 (3) the carbazole functional group donor can also expand the molecular plane, which may promote horizontal orientation of the emitting dipoles.^46,47 mSBFXT-TC and mSBFXT-PC show blue emissions in toluene solutions and doped films, and can perform as emitters efficiently in OLEDs, providing blue EL emissions and high EQE_maxs of up to 26.0%. Moreover, they can be used as sensitizers for sky-blue and green multi-resonance (MR) emitters, furnishing narrow EL spectra and excellent EQE_maxs of up to 32.4%. These results demonstrate that both new deep blue TADF emitters are good candidates for fabricating efficient OLEDs as emitters and sensitizers.

2. Results and discussion

2.1. Synthesis and characterization

mSBFXT-TC and mSBFXT-PC are prepared based on the synthetic routes as illustrated in Scheme S1, and their structures are verified by NMR and high-resolution mass spectrometry. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results suggest the brilliant thermal stabilities of mSBFXT-TC and mSBFXT-PC, with high decomposition temperatures of 438 °C and 487 °C, respectively. A high glass-transition temperature at 218 °C is detected for mSBFXT-TC (Fig. S1), indicative of its good morphological stability. The cyclic voltammetry (CV) experiment uncovers reversible oxidation and reduction processes of mSBFXT-TC and mSBFXT-PC, indicative of their good electrochemical stability (Fig. S2). The energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are calculated to be −5.49 and −2.90 eV for mSBFXT-TC, and −5.52 and −2.93 eV for mSBFXT-PC, according to the CV data. The geometrical and electronic structures of mSBFXT-TC and mSBFXT-PC are shown in Fig. 1. The HOMOs are mainly located on the carbazole donors, and the LUMOs are primarily concentrated on the XT acceptor and its conjugated SBF part. The small HOMO–LUMO overlap is favorable for the occurrence of a RISC process. And the torsion angle between the weak acceptor and donor is only about 48°, which is beneficial for lowering the CT effect and thus realizing blue emission.

Fig. 1 (A) Molecular structures of mSBFXT-TC and mSBFXT-PC (the green part represents the acceptor and the blue part represents the donor). (B) HOMO and LUMO distributions and energy levels of mSBFXT-TC and mSBFXT-PC (white and purple denote the distinct phases of HOMO and LUMO).

2.2. Photophysical properties

mSBFXT-TC and mSBFXT-PC show similar absorption behaviours in toluene solutions (10⁻⁵ mol L⁻¹), with strong absorption maxima locating at approximately 382 nm, and exhibit deep blue PL emissions peaking at 436 and 434 nm with PL quantum yields (Φ_PLs) of 26% and 27%, respectively. mSBFXT-TC and mSBFXT-PC exhibit small Stokes shifts of 3242 cm⁻¹ and 3137 cm⁻¹, respectively. The small Stokes shifts are attributed to the planar central rings and single rotatable substituent of mSBFXT-TC and mSBFXT-PC, and the intramolecular motions that may enlarge the Stokes shifts, such as the puckering of the central xanthene rings, significant variations in exocyclic double-bond length on the xanthene rings and substituent rotation, are suppressed to a large degree in both molecules.^48–51 The doped films of these molecules in the low-polar host of 1,3-di(carbazol-9-yl)benzene (mCP) at 10 wt% concentration show deep blue PL emission peaks at 456 and 460 nm (Fig. S8), accompanied by enhanced Φ_PLs of 87% and 90%, respectively. While in the high-polarity 2,8-bis(diphenylphosphoryl) dibenzo[b,d]-furan (PPF) host at 10 wt% concentration, mSBFXT-TC and mSBFXT-PC exhibit red-shifted PL emission peaks at 469 and 471 nm, respectively, due to the enhanced CT effect (Fig. 2 and Fig. S3). And their Φ_PLs are increased to 95% and 98%, respectively. The Φ_PLs in the doped films of mCP or PPF are much higher than those in toluene solutions owing to the enhanced TADF property in the aggregated state.^52–57 In doped films with mCP or PPF as a host, mSBFXT-TC and mSBFXT-PC display prominent delayed fluorescence with lifetimes (τ_delayeds) of 31.8–39.5 µs and 17.4–17.8 µs, respectively. Temperature-dependent transient PL decay curves of these molecules in the mCP and PPF hosts show the prolonged lifetimes and increased proportion of delayed fluorescence as the temperature increases from 77 to 300 K, confirming their TADF characteristics (Fig. 2 and Fig. S4 and Tables S1 and S2). Fluorescence and phosphorescence spectra measured at 77 K reveal that the experimental ΔE_ST values for these doped films are all around 0.2 eV (Fig. S6 and S7), indicating that the TADF process can occur. mSBFXT-TC and mSBFXT-PC have large RISC rates (k_RISCs) of about 3.5 × 10⁵ s⁻¹ in the doped films with PPF as the host, which are conducive to suppressing the annihilation of triplet excitons and reducing efficiency roll-offs at high brightness (Table 1).

Fig. 2 (A) Absorption and normalized photoluminescence (PL) spectra of mSBFXT-TC and mSBFXT-PC in toluene solutions (10⁻⁵ M) at 300 K. Temperature dependent transient PL decay spectra of the doped films of (B) 10 wt% mSBFXT-TC:PPF, and (C) 10 wt% mSBFXT-PC:PPF, measured under nitrogen. (D) Normalized PL spectra of mSBFXT-TC and mSBFXT-PC in doped films (10 wt% in PPF). Measured p-polarized PL intensity of (E) 10 wt% mSBFXT-TC:PPF and (F) 10 wt% mSBFXT-PC:PPF.

Table 1 Photophysical properties of mSBFXT-TC and mSBFXT-PC

	Toluene solution^a				PPF host/mCP host^b
	λabs (nm)	λem (nm)	ΦPL^c (%)	τdelayed^d (μs)	λem (nm)	ΦPL^c (%)	τdelayed^d (µs)	S1^e (eV)	T1^e (eV)	ΔEST^e (eV)	kRISC^f (× 10^5 s^−1)
a Measured in toluene solution (10^−5 M) at 300 K.b Vacuum-deposited on a quartz substrate with a doping concentration of 10 wt%.c Determined by a calibrated integrating sphere under nitrogen at 300 K.d Delayed fluorescence lifetime (τdelayed) evaluated at 300 K under nitrogen.e Estimated from the fluorescence and phosphorescence spectra at 77 K.f Rate constant of the reverse intersystem crossing process.
mSBFXT-TC	382	436	26	7.6	469/456	95/87	17.4/39.5	2.88/2.93	2.70/2.73	0.18/0.20	3.6/2.4
mSBFXT-PC	382	434	27	8.4	471/460	98/90	17.8/31.8	2.86/2.89	2.70/2.70	0.16/0.19	3.5/3.2

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The ratios of horizontal emitting dipole orientation (Θ_‖s) of both compounds are measured by angle-dependent p-polarization-resolved PL spectra (Fig. 2E and F). The phenyl groups on the carbazole donor extend the molecular plane and increase the molecular planarity, which makes mSBFXT-PC exhibit a larger Θ_// of 84.0% than mSBFXT-TC (77.5%).

2.3. Electroluminescence performance

To evaluate the EL performances of mSBFXT-TC and mSBFXT-PC, doped OLEDs are fabricated with the configuration of indium tin oxide (ITO)/hexaazatriphenylenehexacabonitrile (HATCN) (5 nm)/1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (50 nm)/tris[4-(carbazol-9-yl)phenyl]amine (TcTa) (5 nm)/mCP (5 nm)/emitting layer (EML) (20 nm)/PPF (5 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) (40 nm)/lithium fluoride (LiF) (1 nm)/Al. The EMLs are 10 wt% emitters doped in PPF or mCP hosts. HATCN and LiF serve as hole- and electron-injection layers, respectively, and TAPC and TmPyPB perform as hole- and electron-transporting layers, respectively, TcTa functions as an electron-blocking layer, PPF works as a hole-blocking layer, and mCP behaves as an exciton-blocking layer. The transient EL spectra of these devices are further characterized (Fig. S10), and the results confirm that the triplet excitons participate in the EL processes. The EL performances of these devices are summarized and listed in Fig. 3 and Table 2. The devices of mSBFXT-TC and mSBFXT-PC in the mCP host radiate deep blue light with EL peaks at 452 and 456 nm, respectively, and attain maximum current efficiencies (CE_maxs) of 10.8 and 20.5 cd A⁻¹, maximum power efficiencies (PE_maxs) of 10.6 and 20.1 lm W⁻¹, and EQE_maxs of 9.8 and 16.0%, respectively. But the devices of mSBFXT-TC and mSBFXT-PC in the PPF host display sky-blue light with EL peaks at 474 and 481 nm due to the enhanced CT effect, which is consistent with the PL emission peaks in the PPF host. Much better CE_max, PE_max and EQE_max of 28.5 cd A⁻¹, 24.8 lm W⁻¹ and 21.8% from mSBFXT-TC and 35.6 cd A⁻¹, 31.1 lm W⁻¹ and 26.0% from mSBFXT-PC are attained.

Fig. 3 (A) Device structure and chemical structures of the organic functional layers. (B) Normalized EL spectra, (C) plots of luminance–voltage–current density, and (D) external quantum efficiency–luminance of the OLEDs based on the 10 wt% deep blue emitters doped in PPF. (E) Normalized EL spectra, (F) plots of luminance–voltage–current density and (G) external quantum efficiency–luminance of the OLEDs based on mSBFXT-TC and mSBFXT-PC doped in mCP host with a concentration of 10 wt%.

Table 2 EL performances of mSBFXT-TC and mSBFXT-PC^a

Emitter	λEL (nm)	Von (V)	Lmax (cd m^−2)	Maximum value/at 1000 cd m^−2			CIE (x, y)
				CE (cd A^−1)	PE (lm W^−1)	EQE (%)
a Abbreviations: λEL = EL peak; Von = turn-on voltage at 1 cd m^−2; Lmax = maximum luminance; CE = current efficiency; PE = power efficiency; EQE = external quantum efficiency; CIE = Commission Internationale de I’Eclairage coordinates.
10 wt% mSBFXT-TC:PPF	474	3.6	3474	28.5/24.7	24.8/19.4	21.8/18.2	(0.149, 0.182)
10 wt% mSBFXT-PC:PPF	481	3.4	4984	35.6/20.3	31.1/13.3	26.0/13.2	(0.146, 0.210)
10 wt% mSBFXT-TC:mCP	452	3.2	2223	10.8/4.4	10.6/3.4	9.8/4.1	(0.157, 0.137)
10 wt% mSBFXT-PC:mCP	456	3.2	4048	20.5/11.3	20.1/9.3	16.0/9.2	(0.144, 0.151)

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Given the outstanding EL performances of mSBFXT-TC and mSBFXT-PC, they are used as sensitizers for MR emitters v-DABNA and tCzphB-Fl in OLEDs.^58–60 Initially, the potential Förster energy transfer between them is evaluated. The absorption spectra of v-DABNA and tCzphB-Fl show great overlap with the emission spectra of mSBFXT-TC and mSBFXT-PC, which means that effective Förster energy transfer can occur between them (Fig. S9). The transient PL decay spectra additionally corroborate the occurrence of efficient energy transfer from the sensitizers to the MR emitters (Fig. S5 and Table S3). The device structures of the sensitized OLEDs are the same as the previous doped devices, and the EMLs are composed by co-doping 2 wt% v-DABNA with 10 wt% mSBFXT-TC (S1) and 10 wt% mSBFXT-PC (S2), and 2 wt% tCzphB-Fl co-doped with 10 wt% mSBFXT-TC (S3) and 10 wt% mSBFXT-PC (S4) in an mCP host. The control devices are also fabricated, in which the EML is composed of v-DABNA (Col1) or tCzphB-Fl (Col2) in the mCP host with a doping concentration of 2 wt%. Concurrently, the corresponding sensitized and control devices are also fabricated by employing PPF as the host with the same device structure (Fig. S11 and Table S4). These results suggest that mCP is a more suitable host material for these sensitized devices. Devices S1 and S2 exhibit EL peaks at 472 nm with a narrow full width at half maximum (FWHM) of 18 nm, and provide high maximum luminance (L_max) of 11 171 and 10 911 cd m⁻², and good EQE_maxs of 26.0% and 25.2%, respectively (Fig. 4 and Table 3). Devices S3 and S4 radiate strong green light with EL peaks at 538 nm and FWHM of 29 nm, and furnish higher L_max of 22 753 and 27 232 cd m⁻², and better EQE_maxs of 29.4% and 32.4%, respectively. In comparison with control devices without these sensitizers, the EL efficiencies of these sensitized OLEDs are apparently increased, while the light color remains barely changed. The excellent EL efficiencies and high color purity of these OLEDs indicate the great potential of mSBFXT-TC and mSBFXT-PC as sensitizers for MR emitters.

Fig. 4 (A) and (D) Normalized EL spectra at 500 cd m⁻², (B) and (E) plots of luminance–voltage–current density and (C) and (F) external quantum efficiency–luminance of these sensitized OLEDs.

Table 3 EL performances of sensitized OLEDs using mSBFXT-TC and mSBFXT-PC as sensitizers^a

	λEL (nm)	Von (V)	Lmax (cd m^−2)	CEmax (cd A^−1)	PEmax (lm W^−1)	EQEmax (%)	CIE (x, y)	FWHM (nm eV^−1 cm^−1)
a Abbreviations: λEL = EL peak; Von = turn-on voltage at 1 cd m^−2; Lmax = maximum luminance; CEmax = maximum current efficiency; PEmax = maximum power efficiency; EQEmax = maximum external quantum efficiency; CIE = Commission Internationale de I’Eclairage coordinates; FWHM = full width at half maximum.
Device S1	472	3.2	11171	26.4	25.9	26.0	(0.12, 0.15)	18/0.101/818
Device S2	472	3.2	10911	29.7	29.1	25.2	(0.13, 0.17)	18/0.099/798
Device Col1	472	4.0	7940	18.7	14.7	20.7	(0.12, 0.12)	18/0.099/799
Device S3	538	3.0	22753	101.4	106.1	29.4	(0.30, 0.66)	29/0.124/1002
Device S4	538	3.0	27232	103.4	108.2	32.4	(0.30, 0.66)	29/0.124/1002
Device Col2	538	3.6	17239	91.0	79.3	23.3	(0.30, 0.67)	29/0.126/1019

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3. Conclusions

In summary, two efficient deep blue TADF emitters mSBFXT-TC and mSBFXT-PC are designed and synthesized by using SBF-fused XT acceptor and carbazole-based donors. The mSBFXT-TC and mSBFXT-PC radiate strong deep blue emissions at 434 and 436 nm in toluene solution, and blue delayed fluorescence emission with a peak at 456–471 nm with high Φ_PLs of 87–98% in the mCP and PPF host. Notably, mSBFXT-TC and mSBFXT-PC show a high Θ_// of 77.5% and 84.0%. Both of them exhibit excellent EL performances with blue emission peaking at 452–481 nm with EQE_maxs of 21.8% and 26.0%, respectively. Based on their excellent EL properties, high-performance sensitized OLEDs are achieved by using them as sensitizers for the blue and green MR-TADF emitters of v-DABNA and tCzphB-Fl. The blue and green sensitized OLEDs exhibit high EQE_maxs of 26.0 and 32.4% with small FWHM of 18 and 29 nm, demonstrating that these deep blue TADF materials are good candidates for developing sensitized OLEDs. These findings provide a constructive way to design efficient deep blue TADF materials with high horizontal dipole ratios as emitters and sensitizers for applications in OLEDs.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: instrumentation, synthesis and characterization data, TGA and DSC thermograms, cyclic voltammograms, fluorescence and phosphorescence spectra, transient PL decay curves, transient EL decay curves and device performance data. See DOI: https://doi.org/10.1039/d5tc03019f.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U23A20594, 22375066 and 22405091), GuangDong Basic and Applied Basic Research Foundation (2023B1515040003 and 2025A1515010011), and the State Key Lab of Luminescent Materials and Devices, South China University of Technology (Skllmd-2024-06).

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Footnote

† These authors contributed equally to this work.

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