Strategic modification of the quinoxaline acceptor to induce broad-range orange to red thermally activated delayed fluorescence

Abstract

Thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) offer high external quantum efficiencies without rare metals, but efficient orange–red emitters remain limited due to difficulty in achieving high photoluminescence quantum yields (PLQYs) at longer wavelengths. In this study, we report the design, synthesis, and comprehensive characterization of three novel donor–π–acceptor (D–π–A) TADF emitters Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN based on a cyanophenyl-substituted quinoxaline acceptor unit. Strategic donor modulation and cyano-functionalized quinoxaline acceptor design enabled broad range orange-to-deep red emission (603–700 nm) by enhancing intramolecular charge transfer and minimizing the singlet–triplet energy gap (ΔE_ST). Photophysical, electrochemical, and theoretical studies confirmed strong ICT character and efficient RISC across the series. Cz-PhQx4CN showed pronounced AIE, a high PLQY (37.8%), a small ΔE_ST (34 meV), and the best device performance with an EQE of 9.9% in vacuum-deposited and 3.6% in solution-processed OLEDs. PXZ-PhQx4CN and Ac-PhQx4CN also exhibited TADF behaviour with red-shifted emissions but lower efficiencies. These results highlight how molecular rigidity, extended conjugation, and careful donor–acceptor design enable efficient long-wavelength TADF emission, offering valuable guidance for developing high-performance orange–red OLEDs.

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Introduction

Over the past decades, metal-free and purely organic thermally activated delayed fluorescence (TADF) emitters have gained attention for cost-effective, energy-efficient, and sustainable organic light emitting diode (OLED) applications. Unlike first-generation fluorescent OLEDs, which can only harvest 25% of excitons due to the spin-statistics limitation,^1–8 or second-generation phosphorescent – OLEDs (PhOLEDs) that rely on expensive and scarce heavy metals like iridium and platinum to achieve near 100% internal quantum efficiency (IQE),^9–11 TADF materials offer a promising third-generation alternative. They utilize reverse intersystem crossing (RISC) to upconvert triplet to singlet excitons, enabling efficient emission without heavy metals. A small singlet–triplet energy gap (ΔE_ST) is critical for RISC and is achieved by spatial HOMO–LUMO separation to promote intramolecular charge transfer (ICT) and minimize orbital overlap.^9–17 Numerous strategies have been adopted to maximize radiative outcoupling from all organic emitters focusing largely on the balance between structural rigidity and spin–orbit coupling to enhance photoluminescence quantum yields while maintaining small ΔE_ST.¹⁸ Although blue,^19,20 green²¹ and yellow²² TADF emitters with external quantum efficiencies (EQEs) of up to 20–30% have been reported, efficient orange-red/NIR TADF systems remain scarce. Despite having wide applications in sensors, communication, night vision, bioimaging for targeted drug delivery,^1–3,23,24 it is intrinsically difficult to design highly efficient long wavelength TADF emitters because achieving higher photoluminescence quantum yields (PLQY) in that wavelength range is challenging.^{6–8,25–29} To address this, researchers have explored several strategies. A common one involves increasing π-conjugation in donor–acceptor frameworks to red-shift emission.^9,16,17 One of the earliest examples of orange-emitting TADF compound is 4CzTPN-Ph, which was reported to exhibit a maximum EQE of 11.2% in the fabricated device. Its molecular design features a highly twisted donor–acceptor architecture that effectively facilitates intramolecular charge transfer (ICT) while simultaneously minimizing ΔE_ST.¹¹ A effective approach for developing red TADF OLEDs involves constructing donor–spacer–acceptor (D–S–A) molecular structures, using a π-spacer like phenyl group as a bridge to link the donor (D) and acceptor (A) units.¹⁶ Hui Wang et al. constructed donor–π–acceptor type red TADF emitter, where they inserted bulky spirobifluorene unit in between a triphenylamine donor and a heterocyclic acceptor. Durai Karthik and coworkers synthesised acceptor–donor–acceptor type of molecules featuring an oxaborole-based heterocyclic acceptor linked to dihydrophenazine donor on either side. These compounds exhibited a remarkably high EQE of 30.3% and 21.8%. The rigid boron-containing acceptor plays a key role in achieving a small ΔE_ST of 0.05–0.06 eV in the red TADF emitters.¹⁶ Jie Xue et al. reported a acenaphthopyrazine and acenaphthoquinoline-based donor–acceptor type molecules that were designed to emit from J-aggregates with strong ICT characteristics in the solid state.¹⁰ They exhibited EQEs of 15.8% for red emission and 14.1% for NIR emission. Many reported long range TADF emitters incorporate 2,3-dicyanopyrazino phenanthroline as the acceptor, owing to its ability to form intramolecular hydrogen bonds and its strong electron withdrawing character, which affectively reduces HOMO and LUMO gap.²⁶ In some other reports, dicyanodibenzo[a,c]phenazine (CNBPz) was used as a π accepting unit for long range emitters with colour coordinates of (0.66, 0.34), a centre at 670 nm, and EQE of 15.0%.¹² In another innovative strategy, a benzoyl group was incorporated in dibenzo[a,c]phenazin-11-yl(phenyl)methanone acceptor to develop orange-red TADF emitters. The presence of benzoyl group was shown to enhance the rate of RISC, reduce delayed fluorescence lifetime and increase photoluminescence quantum yield (PLQY).¹⁵ Baoyan Liang et al. reported an increase in EQE from 4.4% to 13.6% upon replacing the pyrazine unit with a quinoxaline moiety in the acceptor design of long-wavelength TADF materials. In a related structural modification, Chuluo Yang et al. introduced a fluorine substituted quinoxaline in a phenoxazine based donor–acceptor TADF emitter. The introduction of fluorine atom was found to modulate electron density of the acceptor, thereby promoting ICT and enhancing the rate of fluorescence in long-wavelength TADF emitters.¹⁴

Herein, we report three new D–π–A type orange red TADF emitters: Cz-PhQx4CN, Ac-PhQx4CN and PXZ-PhQx4CN-based on a quinoxaline acceptor featuring cyano and cyanophenyl substitution. This acceptor design enables emission in the 603–700 nm range with high thermal and electrochemical stability. Cz-PhQx4CN delivered the best EQE of 9.9%. These results demonstrate that strategic acceptor engineering and donor modulation offer a pathway to efficient orange red TADF OLEDs. In this work, we employed a planar 6,7-cyano substituted quinoxaline acceptor core, for its strong electron-withdrawing character. To further strengthen this core and red-shift emission, two additional cyanophenyl groups were incorporated at positions 5 and 8. This design increases acceptor strength and facilitates broad range emission tuning from orange to deep-red region. Twisted D–A geometry promotes effective ICT, reduced ΔE_ST and enhanced radiative rate.

Results and discussion

Molecular design strategy

Previous studies have extensively explored quinoxaline-based cores, modifying them with diverse π-extensions such as pyrazino-, phenazine-, and phenanthrene-fused systems to achieve tunable long-wavelength emissions (λ_em = 504–682 nm).^7,30–34 These modifications (Fig. 1) demonstrate how structural fusion at different positions and incorporation of various donor units influence the overall performance of TADF OLEDs. Quinoxaline dicarbonitrile-based acceptors have demonstrated moderate red-shifted emission peaks in the range of 545–620 nm. Introduction of cyano groups at the 6,7 or 5,8 positions of the quinoxaline core significantly altered the energies of FMO leading to a characteristic red shift in emission.³⁵ Inspired by these findings, we strategically designed a novel quinoxaline-derived acceptor by integrating strong electron-withdrawing cyano and cyanophenyl substituents at the 6,7- and 5,8-positions, respectively. An extended π-conjugation is likely to enhance the electron-withdrawing character of the acceptor unit. This dual substitution was also intended to maximize intramolecular charge transfer, enabling red-shifted emission while preserving structural rigidity. The strategic modification not only stabilises the LUMO level but also strengthens intramolecular charge transfer (ICT), resulting in a narrower ΔE_ST and more efficient RISC. As a result, our emitters Cz-PhQx4CN and Ac-PhQx4CN achieved red and deep-red TADF emissions at 603 nm and 688 nm, respectively; with a substantial red shift compared to previous designs. This molecular engineering approach enables precise colour tuning across the orange-red to deep-red spectrum, demonstrating a clear advancement in acceptor design for high-performance TADF materials. The resulting acceptor core was further combined with electron-rich donors to construct a new family of orange-red TADF emitters. This design rationally extends the established quinoxaline framework suitable for next-generation OLEDs.

Fig. 1 Quinoxaline acceptor core modifications for orange-red-NIR TADF emitters.

Theoretical investigation

Three quinoxaline-based donor–acceptor compounds were theoretically investigated using density functional theory (DFT) on Gaussian 16³⁷ QM package. Ground state geometries were optimized at the B3LYP/6-31G level of theory Table S1. The convergence of the geometries to an energy saddle point in the potential energy surface was confirmed by a frequency analysis. Time dependent density functional theory (TDDFT) studies were subsequently performed on the optimized geometries using a range-separated hybrid functional CAM-B3LYP, which is well-suited for systems exhibiting long range charge transfer transitions. The predicted gas phase absorption spectra (Simulation S1) are in close agreement with the experimental data, with the tert-carbazole derivative exhibiting the most blue-shifted absorption, followed by the acridine and phenoxazine analogues. The analysis of electron density distribution over the FMOs, i.e., HOMO and LUMO of all studied compounds are shown in Fig. 2 and Table S2. From this table it is clearly evident that in HOMO orbital, the electron density is mainly localized on the donor segment and in LUMO orbital the electron density is shifted towards the cyano-substituted quinoxaline acceptor segment which can be ascribed to the better intramolecular charge transfer (ICT) in these compounds. Natural transition orbital (NTO) analysis was employed to further elucidate the nature of the electronic transitions. In the tert-carbazole, acridine and phenoxazine derivatives, the transitions were confirmed to have strong intramolecular charge transfer character, with the hole and electron densities localized predominantly on the donor and acceptor units respectively with a minimal spatial overlap between them [Table S3]. These theoretical results support the experimentally observed trends and affirm the CT nature of the excited states, which is critical for the design and performance of efficient optoelectronic materials.

Fig. 2 Emitter FMO plots and their energies.

Synthesis

Scheme 1 presents the schematic route for the synthesis of Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN TADF emitters. Important intermediates (1), (2), and (3) were obtained from the Buchwald coupling of 1,2-bis(4-bromophenyl)ethane-1,2-dione with 3,6-di-tert-butyl-9H-carbazole, 9,9-dimethyl-9,10-dihydroacridine and 10H-phenoxazine, respectively. Further, 4,5-diamino-3,6-dibromophthalonitrile was obtained from the bromination of 4,5-diaminophthalonitrile by the reported procedure.³⁶ The condensation of the modified diamine with intermediates 1, 2, and 3 yielded the intermediates 4, 5, and 6. Finally, the intermediates 4, 5 and 6 were coupled with (4-cyanophenyl)boronic acid by the Suzuki method to obtain final compounds Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN in 62%, 72%, and 64% yields, respectively. This modular synthetic route efficiently integrates electron-donating groups via Buchwald coupling and introduces strong acceptor units through strategic condensation followed by Suzuki coupling reactions. Detailed synthesis has been mentioned in the SI (Schemes S1a and S3c). All the products were purified by column chromatography and characterized by ¹H NMR, ¹³C NMR and mass spectrometry to confirm their chemical structures (Fig. S5–S25).

Scheme 1 Synthesis route of Cz-PhQx4CN, Ac-PhQx4CN and PXZ-PhQx4CN TADF emitters.

Photophysical study

The synthesized compounds were subjected to photophysical characterization to determine excitation/emission wavelengths, solvatochromism and aggregation induced emission (AIE) phenomenon under ambient conditions. To investigate the effect of donor strength on photophysical properties, UV-Vis absorption and photoluminescence (PL) spectra of the emitters were recorded in toluene (10⁻⁵ M) (Fig. 3a). All three compounds displayed intense high-energy absorption bands (250–350 nm), corresponding to localized π–π* transitions from the donor moieties. PXZ-PhQx4CN exhibited slightly broader absorption in this region from the 350–450 nm region, moderate shoulders were attributed to n–π* and extended π–π* transitions. A broad low-energy ICT band was observed around 480–500 nm in Ac and PXZ analogues, consistent with strong ground-state donor–acceptor coupling. Cz-PhQx4CN showed a weaker tail in this region, suggesting limited ground-state ICT based on absorption alone. The PL spectra provided further insight. Cz-PhQx4CN emitted at 603 nm with weak vibronic features, indicative of mixed LE–CT character. Ac-PhQx4CN emitted more broadly at 688 nm, while PXZ-PhQx4CN showed a broad, featureless band at 700 nm, characteristic of a relaxed ICT excited state. Notably, Cz-PhQx4CN exhibited the strongest positive Solvatochromism from cyclohexane 495 nm to chloroform 685 nm (Fig. 3b). A substantial red shift in emission with increasing solvent polarity highlights the development of a strongly polar ICT state upon excitation. This indicates a decoupling of ground- and excited-state electronic character, with significant charge redistribution upon excitation. Additionally, Cz-PhQx4CN exhibited aggregation-induced emission (AIE), (Fig. 3c and d) contributing to its strong solid-state performance. Cz-PhQx4CN molecule exhibited enhanced emission in the aggregated state when measured in the THF:H₂O system. An enhancement in aggregate’s PLQY and fluorescence was observed at 60% water fraction (Fig. 3d). AIE is observed in some molecules due to aggregate formation, as they tend to pack very tightly. Since the donor segment is orthogonal to the acceptor segment, direct π–π interactions. The low-temperature fluorescence (LT-FL) and phosphorescence (LT-Phos) spectra (Fig. S4) for Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN reveal small ΔE_ST, a critical requirement for efficient thermally activated delayed fluorescence. The spectral onsets of fluorescence and phosphorescence for all emitters show minimal separation, indicating ΔE_ST values on the order of tens of meV consistent with the calculated values (Cz: 34 meV, Ac: 13 meV, PXZ: 50 meV). This near-degeneracy enables effective reverse intersystem crossing (RISC) from T₁ to S₁, facilitating delayed emission. Time-resolved photoluminescence decay profiles (Fig. 4a) further confirm TADF behaviour: Cz-PhQx4CN shows both a fast prompt component in the sub-100 ns range and a significant delayed fluorescence tail extending into the microsecond regime, characteristic of triplet harvesting via RISC. PXZ-PhQx4CN and Ac-PhQx4CN also exhibit delayed components, though with faster decay and lower intensities, reflecting their relatively larger ΔE_STs.

Fig. 3 (a) Normalised UV-vis and photoluminescence (PL) spectra of Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN emitters, (b) Solvatochromism studies of Cz-PhQx4CN (10 µM) in cyclohexane, xylene, toluene, 1,4-dioxane, chloroform solvents, (c) PL spectra of Cz-PhQx4CN (10 µM) with increasing water fraction in THF showed aggregation induced emission (d) Photoluminescence quantum yield (PLQY) of Cz-PhQx4CN as a function of increasing water fraction.

Fig. 4 (a) Time-resolved Fluorescence spectra of the emitters. (b) Cyclic voltammogram of Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN emitters recorded in DCM with tetrabutylammonium perchlorate (TBAP, 0.1 M) as the supporting electrolyte.

Together, these data firmly establish the presence of efficient TADF mechanisms in these emitters, with Cz-PhQx4CN showing the most favourable combination of long-lived delayed fluorescence and minimal ΔE_ST.

Thermal and electrochemical characteristics

Thermal stability was evaluated by thermogravimetric analysis (TGA), as shown in Fig. S3. The 5% weight loss temperatures (T_d) were found to be 315 °C for Cz-PhQx4CN, 277 °C for Ac-PhQx4CN, and 301 °C for PXZ-PhQx4CN indicating excellent thermal robustness across all three materials. These high decomposition thresholds can be attributed to the rigid molecular backbone, high molecular weight, and the presence of the strongly electron-withdrawing acceptor unit bearing multiple cyano and aromatic groups. The extended conjugation in the D–π–A framework further contributes to morphological and thermal resilience, making these compounds promising candidates for stable operation in their respective OLEDs. The electrochemical profiles of Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN reveal clear trends in donor strength and energy levels. Cyclic voltammetry (CV) (Fig. 4b) was employed to probe the electrochemical properties in a standard 3-electrode cell comprising of silver/silver chloride (Ag/AgCl), a platinum wire, and a glassy carbon electrode as the reference, counter, and working electrodes, respectively. Tetrabutylammonium perchlorate (TBAP, 0.1 M) in DCM was chosen as the supporting electrolyte for the scan. The energies of the HOMO levels were determined from the first oxidation potentials by taking the known E_HOMO of ferrocene (Fc) (−4.80 eV) as the reference value. The E_ox of Fc/ferrocenium (Fc⁺) versus Ag/Ag⁺ as internal standard was measured to be −0.44 V and E_HOMO values of all measured compounds were calculated according to the equation: E_HOMO [eV] = −[E_ox − 0.44] − 4.80 and E_LUMO = E_HOMO + E_g. Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN exhibit reversible redox processes with first oxidation potentials of 1.04, 0.63 and 1.10 V, corresponding to HOMO levels of −5.55, −5.52 and −5.46 eV respectively. Similarly, the first reduction potentials were found to be −0.75, −0.72 and −0.77 V, and the LUMO levels of these emitters were hence calculated as −3.32, −3.36 and −3.44 eV, respectively. A summary of photophysical, electrochemical and thermal characteristics of the emitters is presented in Table 1.

Table 1 Summary of photophysical, electrochemical and thermal properties of the emitters

Comp	λ abs ^a (nm)	λ em ^a (nm)	HOMO^b (eV)	LUMO^c (eV)	E g ^d (eV)	T d ^e (°C)	PLQY^f (%)	ΔEST^g (meV)
a UV-vis absorption and Photoluminescence max measured in toluene solution. b Calculated using the equation EHOMO = −[Eox − 0.44] − 4.80 eV. c Calculated using the equation ELUMO = EHOMO + optical band gap from UV. d E g = LUMO–HOMO. e Calculated from TG curve as 5% weight loss temperature. f Doped CBP films at 4% concentration. g Estimated from low temperature fluorescence phosphorescence spectra.
Cz-Ph Qx4CN	297, 484	603	−5.55	−3.32	2.23	315	37.8	34
Ac-Ph Qx4CN	284, 487	688	−5.52	−3.36	2.16	277	11.4	13
PXZ-Ph Qx4CN	375, 503	700	−5.46	−3.44	2.02	301	13.2	50

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These thermal, electrochemical, and photophysical characteristics reflect the materials’ excellent stability, efficient exciton utilization, and balanced energy levels, making them suitable for reliable and high-performance OLED applications.

Electroluminescence study

To evaluate the optoelectronic performance of the quinoxaline-based TADF emitters, OLEDs were fabricated using a multilayer vacuum-deposited architecture (Fig. 5a and b). Devices were initially tested across a dopant concentration range (2–10% by weight) to identify optimal emission characteristics. A doping level of 4 wt% emerged as the most consistent across all the three emitters, balancing efficiency with spectral stability. All comparative analyses were therefore performed under this standard condition. The electroluminescence (EL) spectra revealed emission maxima at 600 nm (Cz-PhQx4CN), 630 nm (Ac-PhQx4CN), and 660 nm (PXZ-PhQx4CN), each blue-shifted relative to their corresponding PL peaks in dilute films (Fig. 5c). This bathochromic shift in solution-processed EL spectra arises from enhanced conformational relaxation and solvent stabilization of the excited state. Conversely, the solid-state EL spectra are governed by host–guest confinement, restricted molecular motions, and intermolecular packing (Fig. 5c). The minimal EL–PL shift in Cz-PhQx4CN highlights its rigid and sterically hindered donor–acceptor geometry, characteristic of aggregation-induced emission (AIE) behaviour. In contrast, the more pronounced shifts for Ac- and PXZ based emitters imply conformational reorganization or intermolecular relaxation upon excitation, indicative of less rigid frameworks. Device turn-on voltages at 40 mA cm⁻² were found to be 10.1 V (Cz), 10.9 V (PXZ), and 14.0 V (Ac) (Fig. 5d), closely correlating with their HOMO energy levels derived from cyclic voltammetry (−5.5 eV,−5.52 eV and −5.46 eV respectively). Cz-PhQx4CN and PXZ-PhQx4CN, with slightly shallower HOMO levels, offer better alignment with the HOMO of the hole-transporting layer (TAPC), resulting in more efficient hole injection and reduced driving voltage. Ac-PhQx4CN's deeper HOMO likely introduces an energetic mismatch, forming hole traps and raising injection barriers. Among the three, Cz-PhQx4CN displayed the highest external quantum efficiency (EQE) of 9.90% and a current efficiency of 20.0 cd A⁻¹, significantly outperforming PXZ (3.68%, 5.00 cd A⁻¹) and Ac (1.50%, 3.33 cd A⁻¹) (Fig. 5e and f). These differences are attributed to the interplay of TADF parameters and excited-state dynamics.

Fig. 5 Optoelectronic characteristics of OLEDs based on Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN emitters. (a) Energy level alignment of the OLED device architecture, including HOMO/LUMO levels of emitters and transport layers, extracted from cyclic voltammetry and literature values. (b) Molecular structures of the host material and charge-transporting layers used in the device stack. (c) Electroluminescence (EL) spectra of the devices at 20 mA cm⁻². (d) Current density–voltage (J–V) characteristics of the OLEDs. (e) External quantum efficiency (EQE) as a function of luminance. (f) Current efficiency versus luminance plots.

Cz-PhQx4CN features a small singlet–triplet energy gap (ΔE_ST), a delayed fluorescence lifetime of ∼2 µs, and the highest photoluminescence quantum yield (PLQY = 37.8%) [Fig. S1]. This profile enables efficient reverse intersystem crossing (RISC) and radiative decay, thereby maximizing exciton utilization. In contrast, PXZ-PhQx4CN shows a short delayed lifetime and lower PLQY (13.2%) [Fig. S1], pointing to inefficient RISC and enhanced non-radiative decay possibly due to extended conjugation in the fused donor increasing internal conversion rates. Ac-PhQx4CN shares structural similarity with Cz but exhibits the poorest performance, likely due to severe carrier trapping from its deeper HOMO and the lowest PLQY (11.4%) [Fig. S1]. A summary of all the Electroluminescent parameters is provided in Table 2. To complement the high device performance observed under vacuum deposition conditions for the Cz-PhQx4CN emitter, we further explored its applicability in solution-processed OLEDs. The presence of solubilizing tert-butyl substituents rendered the molecule amenable to solution processing, enabling fabrication via spin-coating. OLED devices were fabricated using a conventional architecture: ITO/PEDOT:PSS (35 nm)/EML (Cz-PhQx4CN:mCBP, 35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al (120 nm) (Fig. S2). The emitter was doped into the mCBP host at varying concentrations (5–20 wt%) and deposited via spin-coating. The devices exhibited low turn-on voltages (∼6.0–6.4 V) and red electroluminescence with emission peaks ranging from 592 to 612 nm depending on dopant concentration. The device with 10 wt% dopant loading showed the best performance, with a maximum external quantum efficiency (EQE) of 3.6%, a current efficiency of 6.6 cd A⁻¹, and a luminance exceeding 3000 cd m⁻². These results highlight the emitter's potential not only for vacuum deposited devices but also for solution-processable OLEDs.

Table 2 EL parameters of the OLEDs with different emitters at a doping concentration of 4% vacuum-deposited device

Emitters	V on [V]	CE [cd A^−1]	EQE [%]	PE [lm W^−1]	λ EL [nm]	CIE [x, y]	FWHM [nm]
a Maximum efficiency. b Measured at 5 cd m^−2. c Measured at 50 cd m^−2. d Measured at 500 cd m^−2.
Cz-PhQx4CN	3.47	20.0^a	9.90^a	17.4^a	600	0.46, 0.44	152
		16.3^b	8.15^b	13.8^b
		11.4^c	5.68^c	7.59^c
		6.96^d	3.43^d	3.04^d
Ac-PhQx4CN	6.25	3.33^a	1.50^a	1.77^a	630	0.47, 0.45	277
		1.54^b	0.70^b	0.68^b
		1.80^c	0.50^c	0.61^c
		1.97^d	0.89^d	0.47^d
PXZ-PhQx4CN	4.83	5.00^a	3.68^a	2.52^a	660	0.52, 0.43	157
		5.00^b	3.68^b	—^b
		4.46^c	3.29^c	1.73^c
		3.30^d	2.43^d	0.69^d

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Conclusions

In summary, we have strategically designed and synthesized a series of TADF emitters Cz-PhQx4CN, Ac-PhQx4CN, and PXZ-PhQx4CN featuring a newly developed cyanophenyl-substituted quinoxaline acceptor and systematically varied donor units. Among these, Cz-PhQx4CN exhibited superior performance, with a orange-yellow electroluminescence centered at 600 nm with a maximum external quantum efficiency (EQE_max) of 9.90%, a photoluminescence quantum yield (PLQY) of 37.8%, and a minimal singlet–triplet energy gap (ΔE_ST) of 34 meV in a vacuum processed device. The emitter also gave a reasonably good EQE_max of 3.6% in a solution processed device at 10% doping. Its rigid, sterically hindered D–A geometry suppresses non-radiative decay and promotes efficient reverse intersystem crossing, aided by aggregation-induced emission in the solid state. These findings highlight Cz-PhQx4CN as a promising orange-red TADF emitter and reveal that moderate donors can surpass stronger ones when structural rigidity and packing are optimized. These findings elucidate critical structure–property relationships in long-wavelength TADF emitters, emphasising the synergistic role of donor tuning, cyano-functionalized acceptors, and conformational rigidity and this acceptor helps in future to design highly efficient TADF emitters.

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 is available. See DOI: https://doi.org/10.1039/d5tc02803e.

Acknowledgements

S. N. N gratefully acknowledges the Chhatrapati Shahu Maharaj Research Training and Human Development Institute (SARTHI), Pune, India, for the Junior Research Fellowship. Authors are also grateful to DST, Government of India for FIST support (SR/FST/CS-I/2023/291).

Notes and references

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Footnote

† These authors contributed equally to the manuscript.

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