A multiple spiro donor design strategy for horizontally oriented TADF emitters enabling high-performance solution-processed OLEDs

Abstract

Solution-processed organic light-emitting diodes (OLEDs) are attractive for cost-effective optoelectronic devices, but their performance is constrained by poor control over transition dipole moment orientation, which limits light out-coupling efficiency. Here, we present a rational molecular design strategy to address these challenges by developing a novel thermally activated delayed fluorescence (TADF) emitter, 3SFAc-TRZ, featuring a rigid triazine core, multiple spiro-acridine donor units, and peripheral fluorene substituents with solubilizing alkyl chains. This design affords a planar molecular framework with degenerate frontier orbitals, enabling strong charge-transfer character and an exceptionally small singlet–triplet energy gap. Therefore, efficient TADF with a high photoluminescence quantum yield of 76% and a rapid reverse intersystem crossing is achieved in films. Importantly, 3SFAc-TRZ exhibits remarkable horizontal dipole ratios of 80% and 76% in spin-coated neat and doped films. When employed in solution-processed OLEDs, the emitter achieves a high maximum external quantum efficiency of 30.03% with well suppressed efficiency roll-off. These results highlight the effectiveness of the molecular design strategy in enhancing horizontal dipole orientation, showing great promise for advancing high-performance solution processable OLEDs.

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Introduction

Organic light-emitting diodes (OLEDs) have emerged as a pivotal technology for next-generation displays and solid-state lighting owing to their advantages including high efficiency, wide color gamut, flexibility and compatibility with large-area fabrication.^1,2 Over the past decades, substantial efforts have been devoted to improving OLEDs' performance through precise molecular design of emitters and engineering of device architectures. Among various strategies, thermally activated delayed fluorescence (TADF) emitters have attracted tremendous attention due to their capability to harvest both singlet and triplet excitons without relying on heavy metals, thus enabling 100% internal quantum efficiency (IQE).^3–7

While achieving high IQE represents a major milestone, the external quantum efficiency (EQE) of OLEDs is further dictated by the light out-coupling efficiency (η_out). With the development of highly efficient emitters driving IQE close to its theoretical maximum, further improvements in device performance are predominantly constrained by η_out, which is strongly influenced by the orientation of the transition dipole moments within the emissive layer.^8–12 Specifically, preferential horizontal alignment of transition dipole moments relative to the substrate can significantly enhance light extraction, as horizontally oriented dipoles minimize waveguide modes and improve out-coupling.^13–15 Optical analyses have revealed that a predominantly horizontal transition dipole moment alignment can raise η_out above 40%, significantly surpassing the ∼20–30% typical of isotropic emitters.^16,17 Indeed, recent advances in controlling molecular orientation have been successfully demonstrated in vacuum-deposited OLEDs, leading to external quantum efficiencies (EQEs) exceeding 40%.^18–22 Among the various approaches, spiro-configured molecular architectures have been proven particularly effective. Their rod-like, orthogonal frameworks suppress detrimental intermolecular aggregation while favoring horizontal dipole orientation, thereby improving device efficiency.^23,24 Nevertheless, establishing reliable orientation control through rational molecular design in solution-processed OLEDs remains a formidable challenge due to the inherently rapid solvent evaporation process, which generally drives molecules toward isotropic and thermodynamically stable packing.

Recent efforts to induce horizontal orientation in solution-processed OLEDs have focused on molecular design strategies such as mesogenic units, peripheral anchoring and the cascade effect.^25–27 For example, Chen et al. demonstrated that introducing mesogenic groups into a multi-resonance TADF emitter enabled self-assembly-driven horizontal dipole orientation with an anisotropy factor of 0.28 in solution-processed films, contributing to a green OLED with an EQE of 13.6%.²⁵ Zhao et al. employed a peripheral anchoring strategy by attaching flexible chains terminated with 9,9′-spirobi[fluorene] units, which enhanced molecular planarity and horizontal orientation.²⁶ This anchoring strategy allowed non-doped solution-processed OLEDs to surpass 30% EQE. More recently, building on a cascade design, Zhao et al. further developed a dimerized TADF dendrimer featuring extended π-delocalization, which raised the horizontal dipole ratio to 78% and delivered a high photoluminescence quantum yield (PLQY) of 95%.²⁷ In addition to guest molecular design, host–guest electrostatic interactions have emerged as an effective strategy for orientation control. Zhang et al. introduced a dendritic host with strong positive electrostatic potential to pair with a negatively charged MR-TADF emitter, which enables an exceptional horizontal dipole ratio of 83% in the doped film.²⁸ This synergistic alignment strategy enabled solution-processed blue multi-resonance TADF OLEDs to reach an exceptional EQE of 35.3% with narrowband emission. Despite recent progress, solution-processable TADF emitters with a well-defined horizontal dipole moment orientation remain rare. Developing solution-processable TADF emitters that simultaneously combine high PLQY, excellent solubility, and strong horizontal dipole orientation is thus essential and of practical significance for the continued advancement of OLED performance.

In this work, we present a rational molecular design strategy to construct a novel solution-processable TADF emitter 3SFAc-TRZ, which exhibits enhanced horizontal dipole orientation. The molecule employs a rigid π-conjugated triazine core that provides a planar framework, while multiple peripheral spiro-acridine substituents are incorporated to direct favorable molecular alignment (Fig. 1a). In addition, 9,9-dihexylfluorene units with flexible alkyl chains are introduced on the periphery of the molecule to improve solubility while expanding the molecular backbone. This design endows the emitter with a large and rigid conjugated framework that facilitates enhanced horizontal alignment, while the multi-donor configuration creates multiple radiative transition channels, thereby supporting high PLQY. When employed as a solution-processable emitter, the corresponding OLEDs exhibit a maximum EQE exceeding 30%, together with remarkably suppressed efficiency roll-off, maintaining an EQE of 23.82% at a luminance of 1000 cd m⁻². Furthermore, the rigid spiro skeleton effectively mitigates the aggregation-caused quenching (ACQ) effect, ensuring high emission efficiency and low efficiency roll-off even at high doping concentrations. These results highlight the effectiveness of this molecular design strategy in simultaneously optimizing molecular orientation, photophysical properties and solution processability, thereby offering a promising pathway toward advancing high-performance solution-processable OLED technologies.

Fig. 1 (a) Molecular structure and design concept of the investigated compound 3SFAc-TRZ; (b) simulated molecular configuration of the optimized ground state.

Results and discussion

Theoretical investigation

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were carried out to gain insight into the geometric and electronic characteristics of the designed molecule.²⁹ The optimized ground state (S₀) geometry predicts that this molecule adopts a highly rigid and nearly planar molecular configuration. Notably, the introduction of multiple donor units extends the conjugated surface area, which is conducive to achieving preferential horizontal alignment of transition dipole moments (Fig. 1b). Frontier molecular orbital (FMO) analysis further indicates that the highest occupied molecular orbital (HOMO) exhibits threefold degeneracy, while the lowest unoccupied molecular orbital (LUMO) displays twofold degeneracy. The degenerate HOMOs are predominantly localized on the spiro-acridine donor units, and the degenerate LUMOs are concentrated on the central triazine acceptor (Fig. 2). This spatial separation of FMOs manifests a pronounced charge-transfer (CT) character, resulting in an extremely small singlet–triplet energy gap (ΔE_ST) of < 0.01 eV, which is highly favorable for an efficient RISC process. Upon photoexcitation, transitions are expected to occur from any of the degenerate HOMOs to the degenerate LUMOs. Although the spiro-acridine donors adopt an almost orthogonal geometry with respect to the triazine core, minimizing direct orbital overlap, the multiplicity of degenerate orbital channels provides multiradiative channels, which can be further assisted by dynamic conformational fluctuations under ambient conditions.³⁰

Fig. 2 Frontier molecular orbital distribution of 3SFAc-TRZ based on the optimized ground state geometry.

To further elucidate the role of peripheral solubilizing groups, the molecular dimensions were carefully evaluated. Compared with the emissive core composed of a spiro donor and a triazine acceptor, the incorporation of 9,9-dihexylfluorene substituents induces a pronounced expansion of the molecular size within the principal xy plane (Fig. S7). Specifically, after introducing the substituents, molecular dimensions along the xy plane increase from 29.372 Å × 28.330 Å to 44.451 Å × 41.775 Å, while the extent along the axis perpendicular to the xy plane remains essentially unchanged. This anisotropic enlargement of the conjugated surface is anticipated to promote favorable horizontal molecular packing in the aggregated state, thereby promoting horizontal dipole alignment. Further, triplet spin-density distributions were calculated to evaluate the influence of these substituents on the electronic characteristics.³¹ The calculations confirm that the triplet density remains confined to the central donor–acceptor framework with negligible extension onto the fluorene units, indicating that these groups function as electronically inert solubilizing motifs (Fig. S8). Consequently, the 9,9-dihexylfluorenes extend the molecular backbone without altering the intrinsic electronic transitions of the chromophore. Moreover, their bulky and flexible nature is expected to sterically shield the TADF core, mitigating intermolecular interactions and thus effectively suppressing the ACQ effect.

Photophysical properties

The synthesis route of the target compound is provided in the SI, and the molecular structure was unambiguously verified by NMR spectroscopy, MALDI-TOF mass spectroscopy and element analysis. The above theoretical predictions were further substantiated by comprehensive photophysical investigations. In dilute toluene solution, the UV-vis spectrum displays a weak and broad absorption band centered at ∼405 nm, which is assigned to the lowest-energy intramolecular charge transfer transition. The corresponding photoluminescence (PL) spectrum reveals a structureless and broad green emission profile with an emission peak at 499 nm, consistent with the characteristics of the CT excited state. The obvious CT characteristics of the emission are further corroborated by solvatochromic PL measurements, which reveal a pronounced bathochromic shift with increasing solvent polarity (Fig. S2).

To evaluate the TADF properties in the solid state, 3SFAc-TRZ was doped into a universal host 9-(2-(9-phenyl-9H-carbazol-3-yl) phenyl)-9H-3,9′-bicarbazole (PhCzBCz) at a concentration of 5 wt%. The steady-state PL spectrum displays a bright green emission at 500 nm, and almost superimposed fluorescence and phosphorescence spectra are recorded at 77 K (Fig. 3b). The lowest singlet (S₁) and triplet (T₁) energy levels, estimated from the onsets of the fluorescence and phosphorescence spectra at 77 K, are determined to be 2.74 and 2.73 eV, respectively, yielding an exceptionally small ΔE_ST of only 0.01 eV, in excellent accord with the theoretical prediction. Such a vanishingly small ΔE_ST ensures efficient RISC by enabling effective up-conversion of triplet excitons. Transient PL decay measurements further confirm the TADF mechanism, revealing biexponential dynamics with a prompt component of 23 ns and a delayed component of 4.6 µs (Fig. 3c). Under vacuum, an enhancement of the PL intensity accompanied by a prolonged delayed fluorescence lifetime is observed, attributed to the suppressed oxygen quenching of triplet excitons (Fig. S3). The PLQY of the doped film is determined to be 76%, and the corresponding exciton dynamics parameters are subsequently calculated (Table S1).^32,33 Notably, 3SFAc-TRZ exhibits an extraordinarily fast RISC rate of 1.7 × 10⁷ s⁻¹, underscoring the efficient harvesting of triplet excitons. Such rapid RISC is critical for maximizing exciton utilization, thereby enhancing device efficiency while mitigating efficiency roll-off.

Fig. 3 (a) UV-vis absorption and steady-state PL spectra of 3SFAc-TRZ in toluene solution (1 × 10⁻⁵ M); (b) fluorescence spectra measured at 298 K and 77 K, and phosphorescence spectra measured at 77 K with 500 µs time delay of the spin-coated doped film (5 wt% 3SFAc-TRZ: PhCzBCz); (c) transient decay spectra of the spin-coated doped film recorded at 500 nm with different timescales; (d) temperature-dependent PL spectra of the doped film; and (e) temperature-dependent transient decay spectra of the doped film.

To further elucidate the exciton dynamics, temperature-dependent steady-state and transient PL measurements were performed (Fig. 3d and e). Notably, as the temperature decreases from room temperature to 220 K, the steady-state PL spectra exhibit an anomalous enhancement in emission intensity, contrary to the conventional thermally activated behavior typically observed in TADF emitters. This phenomenon can be rationalized by the extremely small ΔE_ST, which renders the RISC process largely insensitive to temperature in this regime. In addition, the suppression of nonradiative decay channels, arising from reduced molecular vibrations at lower temperatures, further contributes to the observed enhancement in emission intensity. Upon further cooling below 220 K, the RISC process becomes inhibited, leading to a decrease in emission intensity.

To evaluate the efficacy of the molecular design in directing the horizontal dipole orientation, angle-dependent p-polarized PL measurements were carried out on spin-coated films. In neat films, 3SFAc-TRZ exhibits a remarkably high horizontal dipole ratio of 80%, a value that ranks among the highest reported for solution-processable TADF emitters.^25–27 This pronounced orientation is attributed to its rigid π-conjugated backbone and sterically extended planar geometry that collectively favor in-plane transition dipole alignment (Fig. 4a). Importantly, this preferential orientation is not limited to neat films but is largely preserved in a host environment. When dispersed in the PhCzBCz host at a concentration of 5 wt%, the spin-coated film still maintains a horizontal dipole ratio of 76% (Fig. 4b). Consistently, 3SFAc-TRZ also maintains the 76% horizontal dipole ratio in a polymer host PMMA (Fig. S4). These results indicate that the orientation originates intrinsically from the molecular architecture, rather than being largely influenced by the host matrix and processing conditions. For comparison, we further investigated a reference emitter, 1SFAc-TRZ, which incorporates only a single spiro-acridine donor unit. In toluene solution, 1SFAc-TRZ displays a blue-shifted emission with a peak at 481 nm (Fig. S5). Angle-dependent p-polarized PL measurements of 1SFAc-TRZ in the doped film reveal a horizontal dipole ratio of 72%, which, although still substantial, is notably lower than that of 3SFAc-TRZ (Fig. S6). This reduction can be attributed to its smaller steric bulk and less rigid molecular geometry, which diminishes the propensity for the slipped-parallel packing associated with enhanced molecular orientation. Nevertheless, the orthogonal spiro-acridine donor and rod-like molecular framework still promote a considerable degree of horizontal orientation, confirming the effectiveness of the molecular design strategy.

Fig. 4 Angular-dependent PL intensity at the peak emission wavelength of (a) the neat film and (b) the doped film (5 wt% 3SFAc-TRZ: PhCzBCz); (c) and (d) simulated packing modes of two 3SFAc-TRZ molecules; and (e) and (f) simulated packing modes of a mixing system consisting of two 3SFAc-TRZ molecules and ten PhCzBCz host molecules.

To obtain deeper mechanistic insight into the origin of this behavior, the packing modes of 3SFAc-TRZ were analyzed via molecular dynamics simulations. In a dimeric system containing two 3SFAc-TRZ molecules, the low-energy packing configuration adopts a slipped-parallel arrangement of the emissive cores. Such a packing motif is advantageous for enhancing horizontal dipole orientation while simultaneously suppressing detrimental face-to-face aggregation, thereby mitigating ACQ and ensuring efficient solid-state emission^34,35 (Fig. 4c and d). The influence of the host environment was further investigated by simulating a ternary system composed of two guest molecules embedded in PhCzBCz host molecules. Strikingly, the two guest molecules largely preserve their slipped-parallel arrangement even in the presence of the surrounding host, underscoring the resilience of the sterically extended and rigid large-planar molecular backbone against external perturbations (Fig. 4e and f). This packing configuration is favorable for maintaining a high degree of horizontal dipole orientation in doped films, which is critical for achieving efficient light out-coupling in OLED devices. The simulations also reveal that the bulky peripheral fluorenes act as effective spacers, reducing host–guest electronic coupling and thereby preserving both the orientation and intrinsic photophysical characteristics of the TADF core. Taken together, the excellent agreement between experimental orientation measurements and computational modeling confirms that the molecular design establishes a robust foundation for enhanced light outcoupling and, consequently, superior OLED performance.

Device performance

In light of the favorable photophysical properties of 3SFAcTRZ, solution processed OLEDs were fabricated with a device architecture of ITO/poly(3,4ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) (40 nm)/poly(N-vinyl carbazole) (PVK) (15 nm)/x wt% 3SFAcTRZ: PhCzBCz (30 nm)/2,8-bis(diphenylphosphoryl) dibenzo[b,d]furan (PPF) (10 nm)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1′“terphenyl]-3,3′”-diyl]bispyridine (TmPyPB) (45 nm)/CsF (1.4 nm)/Al (150 nm) to evaluate the electroluminescence (EL) performance. Within this multilayered architecture, PEDOT:PSS and PVK serve as the hole-injection layer and hole-transporting layer, respectively, while PhCzBCz is selected as the host material. PPF acts as the hole-blocking layer, and TmPyPB serves as the electron-transporting layer.^36,37 The molecular structures of all functional materials are shown in Figure S9, and the corresponding energy-level alignment is illustrated in Fig. 5a. To elucidate the influence of the doping concentration on device performance, OLEDs were fabricated with varying doping concentrations of 5%, 10%, 15%, 20% and 30%. A detailed summary of device performances is provided in Table S2. At the optimized doping concentration of 5 wt%, the device exhibits a broad green EL spectrum centered at ∼500 nm with Commission Internationale de l’Eclairage (CIE) coordinates of (0.20, 0.45) (Fig. 5d). Under these conditions, the device achieves outstanding performance with a maximum EQE of 30.03%, a current efficiency of 77.05 cd A⁻¹ and a power efficiency of 39.02 lm W⁻¹. The EQE of OLEDs can be expressed by the following equation: EQE = γ × η_r × η_out × PLQY, where γ is the carrier balance factor, representing the efficiency of electron–hole recombination; η_r is the exciton utilization efficiency, determined by the spin statistics and the effectiveness of RISC in TADF systems; and η_out is the light out-coupling efficiency, describing the fraction of internally generated photons that can escape from the device into free space.^27,38

Fig. 5 (a) Energy-level diagram and structure of OLED devices; (b) current density–voltage-luminance characteristics; (c) EQE-luminance curves; and (d) electroluminescence spectra of OLEDs.

In the present case, despite the PLQY of the doped film being lower than unity, the device achieves an EQE exceeding 30%, which provides compelling evidence that preferential molecular orientation plays a crucial role in boosting the light out-coupling efficiency. The strong correlation between PLQY and device efficiency thus emphasizes that optimizing molecular orientation is as critical as improving intrinsic emissive properties. Moreover, owing to the rapid RISC process enabled by the extremely small ΔE_ST, the device exhibits significantly suppressed efficiency roll-off, maintaining a high EQE of 23.82% at a luminance of 1000 cd m⁻². As the doping concentration increases, the EL spectra exhibit a slight red-shift, attributed to weak intermolecular interactions.³⁹ Nevertheless, the devices maintain relatively high efficiencies, achieving a maximum EQE of 24.37% at a 30 wt% doping concentration, while exhibiting suppressed efficiency roll-off with an EQE of 21.81% at 1000 cd m⁻². This reflects the steric protection provided by the rigid spiro donors and the electronically inert fluorene substituents. This combined set of attributes establishes 3SFAcTRZ as a highly promising TADF emitter for the development of solution-processed OLEDs with superior efficiency.

Conclusions

In summary, we have developed a novel solution-processable TADF emitter, 3SFAc-TRZ, through a molecular design strategy that integrates a rigid triazine core, multiple spiro-acridine donors, and solubilizing fluorene derivatives. This design simultaneously affords a largely extended planar molecular framework, high PLQY, an extremely small ΔE_ST and a rapid RISC process, collectively enabling highly efficient TADF. Critically, the molecular architecture effectively induces preferential horizontal dipole orientation, achieving high horizontal dipole ratios of 80% in the neat film and 76% in the doped film, which substantially enhance light out-coupling efficiency. Solution-processed OLEDs based on 3SFAc-TRZ achieve a maximum EQE of 30.03%, alongside remarkably suppressed efficiency roll-off, maintaining 23.82% EQE at 1000 cd m⁻². Even at higher doping levels, the devices retain high performance, reflecting effective mitigation of the ACQ effect. These results highlight that the synergistic molecular design, which concurrently optimizes orientation, photophysical characteristics, and solution processability, establishes a promising molecular design paradigm for high-performance solution-processed OLEDs.

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). The supplementary information includes additional experimental details, extended methods, supplementary figures and tables supporting the findings of this study. See DOI: https://doi.org/10.1039/d5tc03295d.

Acknowledgements

The authors greatly appreciate the financial support from the National Natural Science Foundation of China (52273179, 52303228, W2531042 and U23A20594), the Guangdong Basic and Applied Basic Research Foundation (2025A1515011230), and the China Postdoctoral Science Foundation Funded Project (BX20230129).

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