High-efficiency nondoped near-ultraviolet organic light-emitting diodes based on spiro luminogens with high-lying reverse intersystem crossing

Abstract

Near-ultraviolet (NUV) organic light-emitting diodes (OLEDs) have drawn abundant attention due to their practical applications in diverse frontier fields. However, high-efficiency NUV luminogens with Commission Internationale de L’Eclairage (CIE) coordinate y values (CIE_ys) of <0.06 and electroluminescence (EL) peaks of ≈400 nm are rarely reported. In this work, three robust NUV luminogens based on spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] donor and benzonitrile acceptor (SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3) are designed and synthesized. These spiro luminogens in neat films emit strong NUV light with photoluminescence (PL) peaks in the range of 412–421 nm and high PL quantum yields of 68–90%. As evidenced by excitation-dependent transient PL decay spectra and theoretical calculations, these luminogens are able to utilize triplet excitons via high-lying reverse intersystem crossing processes, and the nondoped OLEDs using them as emitters radiate strong NUV light with EL peaks at 402–414 nm, narrow full widths at half maximum of 43–49 nm, and low CIE_y values of 0.029–0.039, and furnish impressive EL performances with maximum luminance and peak external quantum efficiencies of up to 7176 cd m⁻² and 7.90%, respectively. These EL performances are among the state-of-the-art nondoped NUV OLEDs.

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New concepts

In this manuscript, a new molecular design strategy based on a spiro substituent for high-efficiency organic near-ultraviolet (NUV) emitters is demonstrated. These luminogens possess hybridized locally excited and charge transfer properties due to the combination with the weak donor and acceptor, which enable reverse intersystem crossing processes from high-lying triplet states (hRISC) and thus a high exciton utilization ratio in electroluminescence devices. Unlike traditional luminogens, the nondoped OLEDs using the newly developed spiro molecules as emitters radiate strong NUV light with narrow full width at half maximum, low CIE_y values and high efficiency, thanks to the hRISC processes. Therefore, these results manifest the high potential of these spiro luminogens as emitters to fabricate high-performance nondoped NUV OLEDs.

Introduction

In recent decades, organic light-emitting diodes (OLEDs), with primary colors of blue,^1–5 green^6–11 and red,^12–15 have been widely utilized in modern flat-panel display and solid-state lighting products. Given the tremendous commercial value of OLEDs, the pursuit of their further applications in new scenarios never stops. For instance, near-ultraviolet (NUV) OLEDs have drawn abundant attention owing to their promising applications in biological and chemical sensors,¹⁶ ultraviolet communication,¹⁷ high-density information storage,¹⁸ excitation lighting source,^19,20 and so on. To practically construct high-performance NUV OLEDs, robust luminogens with NUV emissions are extremely desired. In OLEDs, the excitons are generated under an electric field, and according to spin statistics, the ratios of singlet excitons and triplet ones are 25% and 75%, respectively. For conventional NUV fluorophores, they can only utilize 25% of electrogenerated excitons (i.e. singlet excitons) for emission in OLEDs, while the other 75% of excitons (i.e. triplet excitons) are lost in non-radiative processes, which results in low external quantum efficiencies (EQEs) of normally below 5%.^21–31 To date, efficient organic NUV luminogens with Commission Internationale de L’Eclairage (CIE) coordinate y values (CIE_ys) of <0.06 and electroluminescence (EL) peaks of ≈400 nm have been rarely reported,^28–45 leaving much room for further development.

Converting triplet excitons to singlet ones in OLEDs is an effective strategy to overcome the limitation of exciton utilization and thus improve EQEs. For example, thermally activated delayed fluorescence (TADF) luminogens with twisted donor–acceptor (D–A) structures are able to convert triplet excitons to singlet ones via a reverse intersystem crossing (RISC) process, realizing efficient blue, green red OLEDs with very high EQEs of over 30%.^46–50 However, the strong charge-transfer (CT) states in TADF luminogens normally cause bathochromic shifts in emission spectra and inferior color purity, which are unfavorable for the construction of NUV OLEDs. Currently, the luminogens with hybridized locally excited (LE) states and CT states (HLCT) are considered as promising candidates for achieving efficient exciton utilization and NUV emission simultaneously. In HLCT luminogens, the CT states enable RISC processes from high-lying triplet states (hRISC), while the low-lying LE-emissive states ensure short-wavelength NUV emissions.

To explore robust organic NUV luminogens for OLEDs, herein, we wish to report the design and synthesis of three isomeric spiro molecules with weak intramolecular D–A interactions, which are built with a benzonitrile (PCN) acceptor and a spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] (SFIAC) donor (Fig. 1A). The rigid spiro configurations of these luminogens are imaged to provide high thermal stability and hinder close π–π stacking in the aggregated state for reducing non-radiative decay and preventing red-shifts in emission spectra. In these luminogens, the PCN acceptor is connected to the different positions of SFIAC to tune photophysical properties. These tailored spiro luminogens exhibit strong NUV emissions with photoluminescence (PL) peaks (λ_PLs) at 377–391 nm and excellent PL quantum yields (Φ_PLs) of 68–94% in toluene solutions. The experimental and theoretical calculation results demonstrate that they possess HLCT characteristics and can harness triplet excitons via hRISC processes. Consequently, the nondoped OLEDs using them as emitters yield narrow-spectrum NUV light (CIE_y < 0.04) and remarkable EQEs of up to 7.9% with small efficiency roll-offs. To the best of our knowledge, the performances of these nondoped NUV OLEDs (CIE_y < 0.04) are among the best results reported so far.

Fig. 1 (A) Molecular structures, (B) single crystal structures and (C) optimized electronic geometries of SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3.

Results and discussion

The target luminogens, SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3, are prepared according to the synthetic routes illustrated in Scheme S1. Their molecular structures are determined via NMR and high-resolution mass spectra. Furthermore, the single crystals of these luminogens are cultured in dichloromethane/hexane mixed solution via slow diffusion at room temperature, and subject to X-ray diffraction analysis. In these luminogens, the dihedral angles between SFIAC donors and PCN acceptors are as small as 19.0–28.6°, while the SFIAC donors display orthogonal conformations (Fig. 1B), leading to rigid space-demanded molecular structures. They show high decomposition temperatures (T_d = 356–390 °C), suggestive of good thermal stability (Fig. S1). Moreover, SFIAC-PCN-1 exhibits a high glass-transition temperature (T_g) of 151 °C, implying high morphological stability. The electrochemical properties of these luminogens are tested via cyclic voltammetry (CV), and reversible oxidation waves are recorded. Their practical highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are calculated as –5.48 to –5.52 eV and –2.06 to –2.13 eV, respectively (Fig. S2). The thermal and electrochemical properties of these luminogens suggest that they are suitable for applications in OLEDs.

To theoretically analyze the excited-state dynamics, the optimal geometric conformations and electronic structures of these luminogens are simulated via density functional theory (DFT) calculations using the Gaussian 09 program package on the B3LYP/6-31G(d,p) level.⁵¹ As shown in Fig. 1C, these luminogens share similar electronic distributions that the HOMOs and LUMOs are mainly distributed on SFIAC donors and PCN acceptors, respectively. The HOMOs and LUMOs show apparent overlaps, indicative of HLCT characteristics. These luminogens display similar excited-state energy levels as illustrated in Fig. 2. And the energy gaps (ΔE_ST) between the lowest excited triplet (T₁) and singlet (S₁) states are as large as 0.69–0.73 eV, which are too large to allow the RISC process from T₁ to S₁ states,⁵² but hRISC processes from high-lying triplet (e.g. T₂, T₃ and T₄) states to S₁ state is possible. Taking SFIAC-PCN-2 as an example, the energy gap between T₁ and T₂ states is as large as 0.64 eV, while T₂, T₃ and T₄ locate approximately at the same energy level (3.05–3.11 eV). The energy gap between T₂ and S₁ states is only 0.05 eV, indicating potential hRISC processes from T₂–T₄ states to the S₁ state.³⁹ Furthermore, relatively large spin–orbit coupling (SOC) strengths between S₁ and T₂ states are observed (0.14–0.21 cm⁻¹), which are conducive to the hRISC process as well as excitation wavelength-dependent phenomenon of transient PL decay spectra, which will be discussed later.^53–55

Fig. 2 Singlet and triplet energy levels and SOC matrix elements of (A) SFIAC-PCN-1, (B) SFIAC-PCN-3 and (C) SFIAC-PCN-3 in the gas phase, calculated using the B3LYP functional at the basis set level of 6-31G (d,p).

The absorption and PL spectra of these new luminogens in dilute toluene solutions (10⁻⁵ M) are displayed in Fig. 3A. They show absorption bands at 320–390 nm, which are attributed to the π–π* transitions of the molecular conjugated skeleton. SFIAC-PCN-2 and SFIAC-PCN-3 exhibit structureless PL spectra with peaks at 388 and 391 nm, respectively, while SFIAC-PCN-1 shows a PL spectrum with a fine vibrational structure, and a main peak at 377 nm and a shoulder peak at 392 nm, indicating its higher ratio of LE state probably due to the smaller molecular torsion angle as revealed by the crystal structure of SFIAC-PCN-1 (Fig. 1B). These luminogens also present narrow FWHMs of 40–48 nm, benefiting from the rigid molecular structures. SFIAC-PCN-2 and SFIAC-PCN-3 have excellent Φ_PLs values of 91% and 94%, respectively, slightly better than that of SFIAC-PCN-1 (89%).

Fig. 3 (A) Absorption and PL spectra of the new luminogens in toluene solutions (10⁻⁵ M) and PL spectra of their neat films under excitation wavelength of 300 nm. (B) Transient PL decay spectra of SFIAC-PCN-2 in toluene solution under excitation wavelength of 280 nm. (C) Temperature-dependent transient PL decay spectra of SFIAC-PCN-2 in neat film, measured under nitrogen (IRF = instrument response function) under excitation wavelength of 280 nm. (D) Excitation-wavelength-dependent transient PL decay spectra of SFIAC-PCN-2 in toluene solution (10⁻⁵ M) measured under a nitrogen atmosphere.

To experimentally confirm the HLCT characteristics, the absorption and PL spectra in solvents with various polarities are examined by taking SFIAC-PCN-2 as an example (Fig. S3 and Table S1). Obvious bathochromic shifts in PL peaks of SFIAC-PCN-2 are observed from low-polar hexane (369 nm) to high-polar acetonitrile (440 nm). According to the Lippert–Mataga model, the linear relationship between Stokes shift (v_a–v_f) and solvent polarity (f) with constant μ_e value of 18.6 D reflects the equivalent hybridization between LE and CT initial states (i.e. HLCT), which is beneficial for obtaining high PL efficiency and high exciton utilization simultaneously.^56–58

To further investigate the excited-state properties of these luminogens, their transient PL decay spectra are measured. In dilute toluene solutions, they show apparent double exponential decay processes with short mean PL lifetimes of 5.9–9.3 ns under a nitrogen atmosphere, but single exponential decay processes under ambient conditions, indicating that triplet states are involved in the emission process (Fig. 3B and Table S2). These luminogens have nanosecond-order delayed lifetimes of 26.0–29.7 ns, which are significantly shorter than the microsecond-order ones of normal TADF emitters. Such short, delayed lifetimes should stem from the hRISC process, and may significantly reduce triplet–triplet annihilation and decrease efficiency roll-offs in OLEDs.

These luminogens in neat films exhibit similar PL spectra with peaks at 412–421 nm (Fig. 3A, Table 1 and Fig. S4). Among them, SFIAC-PCN-1 shows relatively large bathochromic shift of 40 nm in PL peak from dilute solution to neat films, which is probably caused by the relatively strong intermolecular interactions of SFIAC-PCN-1 in neat films due to its small dihedral angle between SFIAC donor and PCN acceptor (19.0°). The neat films of SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3 also show good Φ_PLs of 91%, 78% and 90%, respectively.

Table 1 Photophysical properties of the new spiro luminogens

	Solution^a						Neat film^b
	λ abs (nm)	λ PL (nm)	Φ PL (%)	τ (ns)	τ p (ns)	τ d (ns)	λ PL (nm)	Φ PL (%)	τ (ns)	τ p (ns)	τ d (ns)	ΔEST^e (eV)
a Measured in toluene solution at room temperature. b The thickness of the neat film is 60 nm, which is vacuum-deposited on a quartz substrate. c Determined by a calibrated integrating sphere under nitrogen at room temperature under excitation at 300 nm. d τ = mean PL lifetimes; τp = prompt lifetime; τd = delayed lifetime; the excitation wavelength is 280 nm, evaluated at 300 K under nitrogen. e Estimated from the high-energy onsets of fluorescence andphosphorescence spectra at 77 K. f The solutions are excited under 280 nm. g The solutions are excited under 340 nm UV light.
SFIAC-PCN-1	335	377/391	89	9.3^f/5.4^g	5.6^f	29.7^f	417	92	2.6	2.4	5.3	0.64
SFIAC-PCN-2	337	388	91	8.1^f/3.3^g	3.5^f	29.0^f	412	78	1.6	1.4	4.9	0.63
SFIAC-PCN-3	359	391	94	6.5^f/1.7^g	1.8^f	26.2^f	421	90	1.7	0.7	2.1	0.63

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Table 2 EL performances of OLEDs using SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3 as emitters^a

Devices	λ EL (nm)	V on (V)	L max (cd m^−2)	CE (cd A^−1)	PE (lm W^−1)	EQE (%)	CIE (x, y)	FWHM (nm)
				Maximum value/at 1000 cd m^−2
a Abbreviations: EML = emitting layer; Von = turn-on voltage at 1 cd m^−2; Lmax = maximum luminance; CE = current efficiency; PE = power efficiency; EQE = external quantum efficiency; λEL = EL peak; CIE = Commission Internationale de I’Eclairage coordinates; FWHM = full width at half maximum. Device configuration: ITO/HATCN (5 nm)/TAPC (50 nm)/TcTa (5 nm)/mCP (5 nm)/EML (20 nm)/PPF (5 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al.
SFIAC-PCN-1	402	3.1	2018	2.27/0.48	0.77/0.18	6.21/3.78	(0.161, 0.029)	43
SFIAC-PCN-2	406	2.9	7176	1.68/1.41	1.76/0.87	7.90/6.64	(0.161, 0.039)	46
SFIAC-PCN-3	414	3.1	5040	1.44/1.19	1.33/0.64	6.24/5.18	(0.158, 0.035)	49

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The fluorescence and phosphorescence spectra at 77 K of SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3 in neat films are collected to evaluate the ΔE_ST values (Table 1, Fig. 4 and Fig. S5). Interestingly, there exist two phosphorescence peaks, and the short-wavelength phosphorescence peaks are red-shifted by 12–38 nm relative to the fluorescence peaks. Considering the nanosecond range lifetimes of these luminogens, the phosphorescence peaks close to fluorescence peaks are believed to be contributed by the high-lying triplet excited states, which also correspond to the energy level structure of the theoretical calculation results.⁴⁴ Consequently, the ΔE_STs of these luminogens in neat films are calculated as 0.63–0.64 eV from the fluorescence peaks and long-wavelength phosphorescence peaks, which are too large to realize delayed fluorescence via RISC.⁵² Unlike TADF molecules, these neat films do not show obvious changes in transient PL decay spectra at different temperatures from 77 to 300 K either (Fig. 3C and Fig. S6, S7, Tables S3 and S4). In short, the double exponential decay of these luminogens should originate from the hRISC process, which can be verified by wavelength-dependent transient PL decay (Fig. 3D and Fig. S8, Table S5). Under a N₂ atmosphere, these luminogens show single exponential decay properties under the excitation of a 340 nm UV light source, while they exhibit double exponential decay properties under the excitation of a 280 nm UV light source. Taking SFIAC-PCN-2 as an example, the lifetime under 340 nm excitation is as short as 3.4 ns, while its lifetime under 280 nm excitation consists of a similar prompt fluorescence component (3.4 ns) and a delayed fluorescence component (29.1 ns), which is oxygen-sensitive (Fig. 3B). It is suggested that the 280 nm excitation enables high-lying intersystem crossing and hRISC processes, accounting for the above phenomenon.^59,60

Fig. 4 Fluorescence and phosphorescence spectra of the neat films of (A) IDSPAC-PhCN-1, (B) IDSPAC-PhCN-2 and (C) IDSPAC-PhCN-3, measured at 77 K. The excitation wavelengths of fluorescence and phosphorescence are 300 nm and 365 nm respectively.

Taking advantage of strong NUV emissions with high Φ_PLs of these luminogens in neat films, they are adopted as emitting layers (EMLs) to fabricate nondoped OLEDs with the configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TcTa (5 nm)/mCP (5 nm)/EML (20 nm)/PPF (5 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (Fig. 5A). In these devices, hexaazatriphenylenehexacarbonitrile (HATCN) and 1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) serve as hole-injection and hole-transporting layers, respectively; tris[4-(carbazol-9-yl)phenyl]amine (TcTa) and 1,3-di(carbazol-9-yl)benzene (mCP) work as electron-blocking layers; 2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]furan (PPF) is utilized to block high-energy excitons; 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) is used as electron-transporting layer; LiF/Al that are deposited onto the transparent anode indium tin oxide (ITO) work as a reflective cathode. The nondoped OLEDs based on these luminogens can be turned on at low voltages of 2.9–3.1 V and display strong NUV lights. The EL spectra have EL peaks at 402–414 nm and narrow FWHM of 43–49 nm, which render impressively low CIE_y values of 0.029–0.039. Moreover, high maximum EQEs of 6.21–7.90% are attained by the devices. The nondoped OLED of SFIAC-PCN-2 shows the best performance, with the maximum luminance (L_max) of 7176 cd m⁻² and a peak EQE of 7.90%, which retains 6.64% at 1000 cd m⁻², showing small efficiency roll-off of 15.9% (Table 2). The neat films of these luminogens show moderate horizontal dipole orientation ratios of about 62–78% (Fig. S9). Therefore, the exciton utilization efficiencies of the nondoped OLEDs are estimated to 23–34%, indicating that parts of the triplet excitons have been harvested for light emission. The triplet exciton utilization mechanism is further confirmed to be hRISC via transient EL spectra and magneto-EL spectra (Fig. S10). The spiro configurations of these luminogens enable them to suppress concentration quenching and exciton annihilation in neat films. In consequence, the nondoped OLEDs based on their neat films can even outperform the doped OLEDs using their doped films with varying doping concentration of 10–80 wt% (Fig. S11–S13 and Tables S7–S9). These results are comparable to those of the state-of-the-art nondoped NUV OLEDs (Fig. 5D and Table S10).

Fig. 5 (A) Energy diagram and device configurations of the nondoped OLEDs. (B) EL spectra (collected at 5 V) and (C) external quantum efficiency–luminance of nondoped OLEDs based on SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3 emitters. (D) Summary of the external quantum efficiencies of reported nondoped UV/NUV OLEDs (CIE_y < 0.1). (E) Plots of luminance–voltage–current density and (F) CIE color coordinates of nondoped devices.

Conclusions

In conclusion, three novel spiro NUV luminogens (SFIAC-PCN-1, SFIAC-PCN-2 and SFIAC-PCN-3) consisting of SFIAC donor and PCN acceptor are designed, synthesized and systematically characterized. Their short π-conjugation and weak CT effect ensure the short-wavelength emissions, and their spiro configuration successfully suppresses emission quenching in the aggregated state. The computational and experimental results suggest that they possess HLCT characteristics and hRISC processes can occur in their excited states, which are conducive to enhancing exciton utilization. Therefore, these luminogens in neat films radiate strong NUV emissions with PL peaks at 412–421 nm and high Φ_PLs of 68–90%. They exhibit excellent EL properties in nondoped OLEDs. Among them, the nondoped OLED based on SFIAC-PCN-2 emits NUV light with an EL peak at 406 nm (CIE_x,y = 0.161, 0.039; FWHM = 46 nm) and renders the best EL performance with L_max and maximum EQE of 7176 cd m⁻² and 7.9%, respectively, which is among the state-of-the-art nondoped NUV OLEDs (CIE_y ≤ 0.04) reported so far. These results manifest the high potential of these spiro luminogens as emitters to fabricate high-performance nondoped NUV OLEDs.

Author contributions

J. Z.: investigation, validation, formal analysis, writing – original draft and writing – review & editing; G. H.: investigation; X. W.: investigation; X. D.: investigation; T. G.: investigation; Z. Z.: writing – original draft, writing – review &editing, supervision and funding acquisition; B. Z. T.: supervision and funding acquisition. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data related to this work are presented in the main manuscript and the supplementary information (SI). Supplementary information: compound syntheses and characterization, and other spectra. Additional information such as detailed experimental procedures and supplementary figures and tables. See DOI: https://doi.org/10.1039/d5mh01721a.

Additional relevant data are available from the corresponding author upon reasonable request.

CCDC 2485436–2485438 contain the supplementary crystallographic data for this paper.^61a–c

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22375066, U23A20594, and 22505079), the Guangdong Basic and Applied Basic Research Foundation (2023B1515040003) and the State Key Lab of Luminescent Materials and Devices, South China University of Technology (Skllmd-2024-06). We also thank Dr Yongjiang Yu for assisting in crystallographic studies.

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