Optimizing through-space charge transfer in thermally activated delayed fluorescence emitters for enhanced OLED efficiency

Abstract

The intramolecular through-space charge transfer (TSCT) excited state has been utilized to develop thermally activated delayed fluorescence (TADF) emitters. However, excessive TSCT can lead to complete electron–hole separation, which diminishes the transition dipole moment, resulting in non-radiative losses and reduced device efficiency. In this study, three TADF emitters (2TPA, 3TPA and 2PhTPA) were synthesized by tuning donor–acceptor spatial configurations and conjugation lengths to modulate TSCT. Strong TSCT in 2TPA and 3TPA induced severe non-radiative decay, yielding OLEDs with low external quantum efficiencies (EQE_max < 8%). In contrast, 2PhTPA optimized exciton dynamics via moderate TSCT and multi-channel reverse intersystem crossing enabled by extended donor conjugation, suppressing non-radiative losses. This design conferred 2PhTPA a high photoluminescent quantum yield, reduced ΔE_ST, and superior EQE_max of 17.9%. The work underscores TSCT regulation as pivotal for balancing radiative and non-radiative pathways in TADF systems. By structurally controlling TSCT intensity to mitigate exciton separation, this strategy advances OLED efficiency, demonstrating molecular engineering's critical role in enhancing optoelectronic device performance.

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Introduction

Thermally activated delayed fluorescence (TADF) emitters are considered the cornerstone of the third generation of organic light-emitting diodes (OLEDs) due to their unique exciton utilization mechanism.^1–3 TADF emitters convert triplet excitons into singlet excitons through the reverse intersystem crossing (RISC) process, enabling nearly 100% exciton utilization and overcoming the efficiency bottleneck.^4–6 This phenomenon relies on the effective modulation of the singlet–triplet energy level difference (ΔE_ST): when ΔE_ST is sufficiently small (typically < 0.3 eV), thermal energy can drive the RISC process, efficiently converting triplet excitons into singlet excitons that emit light.^4,7,8 However, TADF emitter performance is not solely determined by ΔE_ST but is also strongly influenced by charge transfer (CT) characteristics.^9,10 The conventional molecular design of TADF emitters typically utilizes donor–acceptor (D–A) or donor–π–acceptor (D–π–A) structures, with charge transport mechanisms involving three primary pathways: intramolecular through-bond charge transfer (TBCT), intermolecular charge transfer, and intramolecular through-space charge transfer (TSCT).^11–14

To leverage the benefits of TBCT and TSCT in TADF emitter design, achieving a balance between these two charge transfer modes is crucial.^15,16 Recent studies have demonstrated that combining TBCT and TSCT in a single molecule can simultaneously enhance the RISC rate and reduce ΔE_ST, which helps mitigate efficiency roll-off and improve device stability.¹⁷ However, achieving this synergistic effect necessitates precise control over the strength of the TSCT contribution. While moderate TSCT promotes triplet exciton utilization through spatial orbital overlap, excessive spatial charge transfer often leads to intramolecular exciplex formation. This exciplex state disrupts the radiative recombination of excitons, resulting in non-radiative energy loss, an imbalance in carrier transport, and a significant reduction in device efficiency.^18–21 For instance, TADF emitters dominated by TSCT typically exhibit fully separated electrons and holes, resembling an intramolecular exciplex. While this reduces ΔE_ST, it also diminishes the transition dipole moment, significantly lowers the radiative decay rate (k_r), and enhances non-radiative decay, ultimately resulting in reduced luminous efficiency.^22–25 Therefore, systematically regulating the TSCT strength and understanding its impact on the photoluminescent (PL) and electroluminescent (EL) properties of TADF emitters are crucial for optimizing exciton dynamics (such as reducing non-radiative decay and balancing singlet and triplet exciton utilization), which ultimately enhances device performance. This approach has significant implications for the molecular design of high-performance TADF emitters.

In this work, we designed and synthesized three novel TADF molecules 2TPA, 3TPA, and 2PhTPA (Fig. 1a). By systematically adjusting the spatial orientation and connection modes of the D–A units (via modifying the conjugation length), we optimized the TSCT characteristics to enhance exciton utilization efficiency and suppress non-radiative pathways, thereby improving device performance. The results show that 2TPA and 3TPA, with strong TSCT characteristics, form significant intramolecular exciplexes. Although both exhibit a very small ΔE_ST and short delay lifetimes, their non-radiative losses greatly diminish the radiative recombination efficiency, resulting in poor OLED performance (EQE < 8%). In contrast, 2PhTPA reduces the TSCT by increasing the conjugation of the TSCT donors, which inhibits non-radiative inactivation. Further photophysical analysis reveals that although the delayed lifetime of 2PhTPA (3.57 μs) is slightly longer than that of 2TPA (2.57 μs) and 3TPA (3.31 μs), its reverse intersystem crossing rate constants (k_RISC) remains of the same order of magnitude, while emissivity increases and non-emissivity decreases. Consequently, the OLED device based on 2PhTPA achieves an EQE_max of 17.9%. This study provides valuable insights into regulating the role of TSCT in TADF emitters and offers a new direction for molecular engineering in the design of high-performance TADF emitters.

Fig. 1 (a) Molecular design strategy and chemical structures and (b) the HOMO and LUMO distributions of 2TPA (left), 3TPA (middle) and 2PhTPA (right).

Results and discussion

Thioxanthone, with its high triplet exciton yield, serves as an excellent TADF acceptor unit.²⁶ However, its use as a TSCT emitter has been rarely reported. To address this, we designed three TSCT-based emitters—2TPA, 3TPA and 2PhTPA—to systematically investigate their structure–property relationships. These emitters were engineered to explore how donor configuration (dual or multiple TPA units) and steric modulation (via phenyl spacers) influence TSCT characteristics (Fig. 1a), triplet harvesting efficiency, and TADF performance, thereby advancing the molecular design of high-performance TSCT-TADF emitters. The synthetic routes for the target compounds 2TPA, 3TPA and 2PhTPA are illustrated in Schemes S1 and S2 (ESI†). The chemical structures of both the intermediate and final products were confirmed using ¹H NMR, ¹³C NMR and HRMS (Fig. S1–S31, ESI†). To assess the thermal stability of compounds 2TPA, 3TPA and 2PhTPA, thermogravimetric analysis (TGA) was performed. The thermal decomposition temperatures (i.e., the temperature at which a 5% weight loss occurs) for 2TPA, 3TPA and 2PhTPA were found to be 481 °C, 500 °C, and 534 °C, respectively, following a temperature increase of 10 °C per minute from room temperature to 600 °C under a nitrogen atmosphere (Fig. S32, ESI†). These results demonstrate that the three TADF emitters exhibit excellent thermal stability, which provides a solid foundation for the fabrication of OLED devices via vacuum evaporation.

The electrochemical properties of the three compounds were assessed using an electrochemical workstation. The redox potentials of 2TPA, 3TPA and 2PhTPA were determined by cyclic voltammetry with a three-electrode system consisting of a glassy carbon electrode, platinum wire electrode, and an Ag/Ag⁺ reference electrode. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the compounds were subsequently calculated. The initial oxidation potentials (E_ox) of 2TPA, 3TPA and 2PhTPA were found to be 0.53, 0.57 and 0.65 eV, respectively, in a 1.0 × 10⁻³ M tetrabutylammonium phosphate (electrolyte) solution. Using the formula E_HOMO = − [E_ox − E_(Fc/Fc⁺) + 4.8] eV, the HOMO energy levels of the compounds were calculated to be −5.20, −5.24 and −5.32 eV, respectively. Additionally, based on the reduction potentials of −1.42, −1.42 and −1.40 eV for 2TPA, 3TPA and 2PhTPA, their LUMO energy levels (E_LUMO = − [E_red − E_(Fc/Fc⁺) + 4.8] eV, E_red is the reduction potential) were determined to be −3.25, −3.25 and −3.27 eV, respectively (Table 1 and Fig. S33, ESI†).^27–29

Table 1 Photophysical and electrochemical properties of 2TPA, 3TPA and 2PhTPA

Compound	λ abs (nm)	λ em (nm)	HOMO/LUMO^b (eV)	ΔEST^c (meV)	Φ PL (%)	τ PF (ns)	τ DF (μs)	k r (10^6 s^−1)	k ^Snr ^d (10^6 s^−1)	k ^Tnr ^d (10^5 s^−1)	k RISC (10^5 s^−1)
a Maximum wavelength of UV-vis absorption (λabs, 298 K) and fluorescence (λem, 298 K) measured in toluene solution (1 × 10^−5 M), excited at 420 nm. b Determined from the onset of the oxidation potentials by cyclic voltammetry (CV). c S1–T1 energy gap (ΔEST) measured in 2-Methylfuran solution (1 × 10^−5 M), excited at 420 nm. d Absolute photoluminescence quantum yield (ΦPL), as well as rate constants of singlet non-radiative decay (k^Snr), triplet non-radiative decay (k^Tnr), radiative decay (kr) and reverse intersystem crossing (kRISC) measured in 5 wt%-doped films of the emitters in 3,5DCzPPy.
2TPA	305, 336, 426	637	−5.23/−3.27	1	50.6	55.18	2.57	6.14	5.99	2.91	2.91
3TPA	305, 337, 425	633	−5.24/−3.25	4	36.1	32.12	3.31	8.86	15.68	2.70	1.13
2PhTPA	309, 348, 425	562	−5.20/−3.25	12	86.7	27.61	3.57	29.09	4.46	1.90	1.13

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To further investigate the spatial configuration and electron cloud distribution of compounds 2TPA, 3TPA and 2PhTPA, density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d,p) level. The calculated distributions and energy levels of the HOMO and LUMO are shown in Fig. 1b and Fig. S34 (ESI†). The HOMO and HOMO−1 of 2TPA, 3TPA and 2PhTPA are primarily localized on the TSCT donor triphenylamine, while the HOMO−2 of 3TPA and 2PhTPA is mainly localized on the TBCT donor triphenylamine. The LUMO is almost entirely located on the acceptor thioxanthone (Fig. 1b). The separation of the HOMO and LUMO energy levels establishes the basis for their small ΔE_ST. Subsequently, the excited state properties of 2TPA, 3TPA and 2PhTPA were analyzed using time-dependent density functional theory (TD-DFT) calculations. The ΔE_S1−T1 values of 2TPA, 3TPA and 2PhTPA were determined to be 25, 22 and 6 meV, respectively (Fig. S35–S37, ESI†). The small ΔE_ST not only reduces electron exchange energy but also suggests that these compounds are potential thermally activated delayed fluorescence (TADF) emitters. Further natural transition orbital (NTO) analysis reveals that the excited states of 2TPA, 3TPA and 2PhTPA exhibit charge transfer (CT)-dominated properties, which aligns with the transition of their molecular orbitals (Fig. S35–S37, ESI†). The presence of multiple intramolecular donors causes their S/T levels to be closely spaced, theoretically supporting the possibility of multi-level transitions. This enables the potential for a multi-channel RISC process, facilitating rapid crossing of the energy gap.

To investigate their photophysical properties, the UV absorption, fluorescence, phosphorescence, and transient fluorescence spectra were measured. The UV absorption spectra of 2TPA, 3TPA and 2PhTPA in Fig. 2b–d exhibit similar maximum absorption wavelengths at approximately 305 nm, corresponding to the π → π* transition. The absorption band centered at 356 nm corresponds to the symmetry-allowed n → π* electronic transition. Notably, 3TPA and 2PhTPA exhibit significantly increased absorption intensity at this wavelength compared to the weakly defined shoulder feature observed in 2TPA. This spectral evolution directly correlates with the extended π-conjugation length achieved through strategic donor modifications—specifically, the incorporation of additional triphenylamine units in 3TPA and the π-extended bridging architecture in 2PhTPA. Additionally, a weak absorption peak around 425 nm is observed, which can be attributed to the charge transfer (ICT) characteristics. In contrast to 2TPA, the direct conjugation of triphenylamine donors to the thioxanthone acceptor core in both 3TPA and 2PhTPA significantly enhances ICT absorption. Notably, 2PhTPA exhibits a slight reduction in ICT absorption intensity at 425 nm compared to 3TPA, which is attributed to the weakening of TSCT. This result clearly demonstrates that the strategic incorporation of a bridging benzene ring in 2PhTPA's TSCT donor moiety disrupts spatial donor–acceptor orbital overlap, thereby attenuating TSCT characteristics while maintaining ICT activity.

Fig. 2 (a) Room-temperature fluorescence in toluene (10⁻⁵ M) of five emitters. UV-vis absorption (Abs., 298 K) in toluene (10⁻⁵ M), low-temperature fluorescence (Fl., 77 K) and low-temperature phosphorescence (Phos., 77 K) spectra of (b) 2TPA, (c) 3TPA and (d) 2PhTPA in 2-Methyltetrahydrofuran (10⁻⁵ M).

The optical bandgap energies of 2TPA, 3TPA and 2PhTPA were determined to be 2.42, 2.47 and 2.53 eV, respectively (Fig. 2b–d). This inverse correlation between bandgap magnitude and emission wavelength indicates that 2TPA and 3TPA exhibit red-shifted emission bands compared to 2PhTPA, consistent with their narrower bandgaps. Further verification through PL spectroscopy in toluene revealed distinct emission maxima at 637 nm for 2TPA, 633 nm for 3TPA, and 562 nm for 2PhTPA (Fig. 2a and Table 1). The emission wavelengths of 3TPA and 2TPA remain similar, even though the newly introduced donor triphenylamine in 3TPA is directly linked to the acceptor unit. In contrast, the emission of 2PhTPA is significantly blue-shifted, which is attributed to the introduction of a π-bridge between the triphenylamine donor and the bridging benzene ring. This structural modification increases the distance between the triphenylamine donor and the acceptor unit, thereby weakening the TSCT. The S₁/T₁ energy levels of 2TPA and 3TPA were determined from the maximum emission peaks in the fluorescence (Fl.) and phosphorescence (Phos.) spectra at 77 K (Fig. 2b–d). These energy levels are 2.306/2.305 eV, 2.284/2.280 eV, and 2.296/2.284 eV, respectively. The ΔE_ST of 2TPA and 3TPA were 1 and 4 meV, respectively, which was due to the complete separation of HOMO and LUMO. Compared with 3TPA, the decrease of TSCT significantly increased the ΔE_ST of 2PhTPA to 12 meV. The negligible ΔE_ST value indicates a strong tendency feasibility for the RISC process, which provides a solid foundation for the existence of thermally activated delayed fluorescence (TADF) characteristics in these emitters. Further studies of the fluorescence spectra of 2TPA, 3TPA and 2PhTPA in different polar solvents reveal that, as the solvent polarity increases, the emission peaks of 2TPA and 3TPA show slight red shifts and broadening, indicating similar ICT characteristics (Fig. S38, ESI†). In contrast, the emission peak of 2PhTPA exhibits a more significant red shift and broadening, suggesting that the ICT characteristics of 2PhTPA are more sensitive to changes in solvent polarity. The distinct excited-state dynamics conclusively demonstrate that the weak TSCT in 2PhTPA fundamentally differs from the strong TSCT-dominated luminescence observed in both 2TPA and 3TPA.

Subsequently, we further investigated their photophysical properties in the thin film state. A thin film with a doping ratio of 5 wt% was prepared by vacuum evaporation using the matrix 3,5-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (3,5DCzPPy). The corresponding absolute PLQYs were 50.6%, 36.1% and 86.7%, respectively (Table 1). The PLQYs of 2TPA and 3TPA were significantly reduced due to the formation of intramolecular exciplexes, which resulted from substantial non-radiative decay. The transient photoluminescence (PL) decay in 2TPA, 3TPA and 2PhTPA doped films was measured. Transient curve exhibited a double exponential decay, comprising transient fluorescence (PF) and delayed fluorescence (DF) emission components (Fig. 3b). The PF lifetimes (τ_PF) of 2TPA, 3TPA and 2PhTPA were 55.18, 32.12 and 27.61 ns (Fig. 3a and Table 1), respectively. The DF lifetimes (τ_DF) were 2.57, 3.31 and 3.57 μs (Table 1), respectively. As the TSCT decreases, the τ_PF of compounds 2TPA, 3TPA and 2PhTPA gradually decreases, while the τ_DF of 2TPA, 3TPA and 2PhTPA increases. The decrease of τ_PF contributes to the rapid attenuation of radiative emission. Additionally, temperature-dependent transient PL decay curves were obtained. Over the temperature range of 300 K to 100 K, the proportion of delayed components gradually decreased as the temperature decreased, further confirming the intrinsic properties of TADF (Fig. S39, ESI†). Based on the transient PL data and PLQY values, the relevant rate constants for different kinetic processes were calculated (ESI†). The radiative decay rate constants (k_r) for 2TPA, 3TPA and 2PhTPA were calculated to be 6.14 × 10⁶ s⁻¹, 8.86 × 10⁶ s⁻¹ and 2.91 × 10⁷ s⁻¹, respectively. The k_RISC were 2.91 × 10⁵ s⁻¹, 1.13 × 10⁵ s⁻¹ and 1.13 × 10⁵ s⁻¹, respectively. The singlet nonradiative rate constants (k^S_nr) were 5.99 × 10⁶ s⁻¹, 1.57 × 10⁷ s⁻¹ and 4.46 × 10⁶ s⁻¹, respectively. The triplet nonradiative rate constants (k^T_nr) were 2.91 × 10⁵ s⁻¹, 2.70 × 10⁵ s⁻¹ and 1.90 × 10⁵ s⁻¹, respectively (Table 1). Although 2PhTPA exhibits a slightly reduced k_RISC compared to 3TPA, its k_RISC remains within the same order of magnitude range (10⁵ s⁻¹). This preservation is attributed to the synergistic effects of its strategically engineered small ΔE_ST and the multi-donor configuration that facilitates multiple vibronic coupling channels, ensuring sustained spin–flip probability despite reduced TSCT contributions.^30,31 At the same time, it is observed that the non-radiative decay rate constants (k^S_nr and k^T_nr) of compounds 2TPA and 3TPA increase significantly. Consequently, the luminescence efficiency of the intramolecular exciplex, dominated by TSCT, is considerably reduced in this system. The weakening of TSCT in emitter 2PhTPA preserves a fast reverse intersystem crossing, and the significantly increased radiative rate contributes to a high PLQY, thereby providing a foundation for excellent device performance.

Fig. 3 Transient photoluminescence decay curves of 2TPA, 3TPA and 2PhTPA doped films: (a) prompt fluorescence (PF) decay curves at nanosecond scale, (b) delayed fluorescence (DF) curves at microsecond scale.

To further investigate the electroluminescence (EL) performance of 2TPA, 3TPA and 2PhTPA, multilayer OLEDs were fabricated. The device structure is ITO/TAPC (35 nm)/mCP (10 nm)/EML (30 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al (100 nm), where the luminescent layer is doped with 5 wt% 3,5DCzPPy as the host. TAPC (4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline]) and mCP (9,9′-(1,3-phenylene)bis-9H-carbazole) were used as hole transport and electron-blocking layers, respectively. The electron transport layer consisted of TmPyPB (3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1′′-terphenyl]-3,3′′-diyl]bispyridine). The related structures, energy levels, and molecular structures of the device are shown in Fig. 4a and Fig. S40a (ESI†). The EL characteristics of the devices based on 2TPA, 3TPA and 2PhTPA are depicted in Fig. 4b, d and Fig. S40b (ESI†), with the corresponding device performances summarized in Table 2. As shown in Fig. 4c, emission peaks at 596, 588, and 580 nm were obtained for 2TPA, 3TPA and 2PhTPA, respectively. Consistent with the PL spectrum trend in toluene solution, the blue shift of these EL spectra can be attributed to the gradual weakening of TSCT. Based on the EL spectra and current density–voltage–brightness (J–V–L) characteristics, the maximum current efficiency (CE), maximum power efficiency (PE), and maximum external quantum efficiency (EQE_max) were calculated as follows: 17.0 cd A⁻¹, 10.7 lm W⁻¹, and 7.9% for device 2TPA; 17.0 cd A⁻¹, 13.3 lm W⁻¹, and 7.5% for device 3TPA; and 52.5 cd A⁻¹, 45.8 lm W⁻¹, and 17.9% for device 2PhTPA. The inferior EL performance of devices 2TPA and 3TPA can be attributed to the lower PLQY and higher k^S_nr and k^T_nr of them. In contrast, the excellent EL performance of the 2PhTPA device can be attributed to its exceptionally PLQY, large radiative rate constant, and reduced non-radiative attenuation (k^S_nr and k^T_nr).

Fig. 4 EL characteristics of devices 2TPA, 3TPA and 2PhTPA at 5 wt% doping concentrations: (a) device structure, (b) current density and luminance versus the voltage (J–V–L) characteristic, (c) EL spectra and (d) EQE–luminance characteristics.

Table 2 Summary of the EL data

Emitters	λ em (nm)	V on (V)	EQEmax/EQE1000^c (%)	CEmax^d (cd A^−1)	PEmax^e (lm W^−1)	CIE^f (x, y)
a Electroluminescence peak wavelength. b Turn-on voltage at 1 cd m^−2. c External quantum efficiency: maximum, values at 1000 cd m^−2. d Maximum current efficiency. e Maximum power efficiency. f Chromaticity coordinate.
2TPA	596	3.6	7.9/4.0	17.0	10.7	(0.53, 0.46)
3TPA	588	3.5	7.5/3.1	17.0	13.3	(0.53, 0.46)
2PhTPA	580	3.5	17.9/9.6	52.5	45.8	(0.51, 0.48)

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Conclusions

The development of three new TADF emitters—2TPA, 3TPA and 2PhTPA—enabled an in-depth investigation of the effect of intramolecular exciplex formation caused by space charge transfer (TSCT) in thioxanthone derivatives on PL and EL properties. In comparison with 2TPA and 3TPA, the reduction in the TSCT in 2PhTPA results in photophysical properties that demonstrate how the system's multi-donor structure and moderate space charge transfer facilitate multichannel charge transfer, significantly enhancing radiative transitions and reducing non-radiative decay associated with unfavorable intramolecular exciplexes. This results in a k_r of up to 2.91 × 10⁷ s⁻¹ and a PLQY of 86.7%. The orange-red TADF-OLED incorporating 2PhTPA as the doped emitter achieves an EQE_max of 17.9%, significantly outperforming the reference devices, 2TPA (7.9%) and 3TPA (7.5%). These findings suggest that regulating the strength/weakness of TSCT offers a viable optimization strategy for designing high-performance TADF emitters.

Author contributions

The authors’ contributions are as follows: Zhi Pang: writing – review & editing, writing – original draft, investigation, formal analysis, data curation; Shaogang She: investigation; Xin Xie: investigation; Xinyi Lv: resources; Yifan Liu: resources; Jianjun Liu: writing – review & editing, formal analysis; Ying Wang: writing – review & editing, supervision, formal analysis. All authors participated collaboratively in experimental implementation, analytical verification, and manuscript finalization, ensuring rigorous interdisciplinary coordination throughout the research process.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the National Key Research and Development Program of China (No. 2022YFA1207600), International Partnership Program of Chinese Academy of Sciences (IPP) (1A1111KYSB20210028) and National Program for Support of Top-notch Young Professionals.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00953g

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