English

• Near-infrared (NIR) organic light emitting diodes (OLEDs) holds great importance owing to their diverse applications in biosensing, optical communication, medical diagnostics, environmental detection, night-vision displays and photo-therapeutic devices [1–5]. The increasing demand for NIR-OLEDs across various fields has driven the tremendous research efforts focused on improving the efficiency of OLEDs emitting beyond 700 nm, utilizing various materials such as small donor-acceptor fluorophores, π-conjugated polymers, phosphorescent transition-metal complexes and thermally-activated delayed fluorescent (TADF) emitters [2,6–9]. Despite these advances, the efficiency of NIR-OLEDs still lags behind the visible counterparts, primarily due to intrinsically low luminescence efficiency of NIR emitters, as governed by the energy gap law which states that the non-radiative decay rate increase exponentially as the energy gap decreases [5,10–12].

  To date, noble-metal based NIR phosphorescent emitters have been extensively explored for OLEDs due to their ability to harness nearly unity excitons (internal quantum efficiency, IQE ≈100%), achieving remarkable performance milestones [13–16]. But high-cost, instability and significant external quantum efficiency (EQE) roll-off at higher current densities are the major obstacles towards their commercialization and mass production [14,17,18]. In contrast only few pure organic emitters, including acenaphthopyrazine, benzothiadiazole and boron-curcuminoid derivatives have demonstrated high EQE approaching to 10% with electroluminescence peak beyond 700 nm [19–21]. Nonetheless, these TADF motif typically suffer from the ultra-broad emission, reduced operation stability and efficiency roll-off, posing a barrier for their implementation in the application requiring narrowband pure NIR emission [22–24]. Additionally, the rigid structures of NIR-TADF emitters, often result in poor solubility, rendering them incompatible with solution processing approach, which is highly desirable for scalable OLED fabrication [25–27].

  Recently, a new class of TADF motif based on boron and nitrogen embedded framework, known as multi-resonance (MR)-TADF, with intrinsic narrow FWHM has been proposed to achieve high color-purity in OLED [28,29]. However, the involvement of short-range charge transfer (SRCT) in such covalently bonded network also makes it difficult to shift emission to longer wavelength region [30–32]. Beyond direct TADF electroluminescence, employing small organic fluorescent end-emitter in combination with a suitable TADF host offers economical alternative for large-scale narrowband OLEDs production [33], [34]. However, most of reported studies focus to red, blue or green emission, the research on NIR hyperfluorescent (HF) OLEDs remaining limited due to lack of efficient narrowband emitter [35], [36]. In 2017, Xu et al. demonstrated the potential of this approach in NIR-OLEDs by combining TPANSeD fluorescent emitter with the DMAC-PN TADF sensitizer, achieving a notable EQE of 2.65% with EL peak at 730 nm [37]. Subsequently, Brodeur et al. developed NIR-TADF sensitized OLED with relatively narrow NIR emission [38]. More recently, Shahalizad et al. realized the pure NIR emission (λ_em = 840 nm) with a FWHM below 40 nm and EQE of 3.8% by sensitizing a pyrrolopyrrole cyanine dye with NIR TADF emitter TPAM-BF₂ [25]. These studies manifest the potential of HF approach in realizing high efficiency through harvesting triplet excitons, however, the current results are still far from optimal and there is need to explore new sensitizer/emitter pair to fully leverage this approach.

  Given the ongoing challenges in achieving narrowband electroluminescence for OLEDs, boron-dipyrromethene (BODIPY), a class of organic dyes, has emerged as a promising candidate due to its intrinsic narrow emission and ease of structural modification, enabling the precise tuning of emission wavelength [39–41]. Furthermore, BODIPY dyes ensure efficient Förster resonance energy transfer (FRET) within host-guest pair even at ultra-low doping concentration, owing to their inherently strong visible light harvesting ability, which may cause the suppression of the dexter energy loss (DET), a critical factor to achieve high efficiency in HF-OLED [42–44]. Significant efforts have been directed to develop long-wavelength narrowband BODIPY emitters by modifying the core with weak donors via C—C linkage, leading to the red-shifted emission [45,46]. For instance, You et al. developed a narrowband red BODIPY emitter, named, "MeBF-PZ" (λ_PL = 617 nm, FWHM = 21 nm) via α-position modification of the BODIPY core, while Hyuk Kwon research team realized red emission by decorating the meso–position with methoxy benzene donor [47,48]. Despite these developments, no BODIPY-based narrowband emitters with wavelengths greater than 700 nm have been reported so far. Additionally, the structural relationship between BODIPY and its emissive behavior, particularly in terms of FWHM, remains unclear, posing a challenge in designing efficient NIR-BODIPY emitters. Therefore, developing a molecular design strategy for efficient narrowband NIR emitters is crucial for advancing BODIPY-based OLED technology.

  In this work, we demonstrated intramolecular conformational interlocked strategy to develop efficient solution-processable narrowband NIR emitters for high performance hyperfluorescent OLEDs (Schemes 1 and 2) [49,50]. Two proof-of-concept molecules, namely Ph-BDP-PY and Ph-BDP-Cz, with fixed donor-acceptor conformation, were prepared by connecting the donor moieties, pyrene and carbazole, respectively, using vinyl bridge to the α-position of BODIPY core (Schemes 1 and 2). The incorporation of vinyl bridge not only increase the electron density of core by extending the conjugation but also induce hydrogen-fluoride (H⋯F) interaction within BODIPY framework, resulting in the enhanced molecular rigidity and constriction of vibrational loss, thereby improving the photoluminescence efficiency [51–53]. Notable, compared to carbon-carbon single bond (C—C), the sp² hybridized C=C double bond is with relatively short bond length and has weaker stretching vibration, thereby can also promise narrow emission [54,55]. Through donor-engineering, we precisely tune the emission wavelength and suppress the scissoring vibration, achieving efficient NIR luminescence (λ_em = 692 nm, Φ_F = 68%) without sacrificing the FWHM (29 nm). Further, the meso–position of the BODIPY was decorated with mesitylene group to avoid the aggregation and suppress DET loss [40,48,49,56]. Using the devised BODIPY dyes as a terminal emitter, we developed the solution processed TADF sensitized NIR-OLEDs, showing a high EQE of 6.9% with emission band at 702 nm and narrow FWHM of 41 nm, that is the state-of-art device performance among reported TADF sensitized HF-OLEDs and BODIPY OLEDs.

  Scheme 1

  

  Scheme 1.  Previously reported emitters for near-infrared OLEDs versus this work.

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  Scheme 2

  

  Scheme 2.  Conventional approach and molecular design strategy adopted in this study for the development of BDOIPY emitters.

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  The designed NIR BODIPY emitters was prepared through the simple synthetic route, as presented in Scheme S1 (Supporting information) [57,58]. A well matured procedure is adopted to synthesize BODIPY precursor involving condensation of pyrrole with aromatic aldehyde, subsequent its oxidation and complexation with boron-trifluoride etherate (BF₃Et₂O) [57,58]. The target compounds Ph-BDP-PY and Ph-BDP-Cz, were prepared through simple condensation of donor moieties [58], pyrene and carbazole, respectively, to the BODIPY scaffold, and purified through column chromatography, obtaining the desired product in high yield. The devised product was fully characterized using nuclear magnetic resonance (¹H NMR and ¹³C NMR) and high-resolution mass spectroscopy to validate the proposed molecular structure. The detailed characterization and single-crystal data of BODIPY emitters are presented in Supporting information.

  The single-crystal of Ph-BDP-PY was successfully grown after multiple crystallization attempt, through the slow evaporation of DCM/PE solvent under ambient condition. X-ray crystallographic analysis revealed that the mesitylene group at meso–position adopts an orthogonal conformation relative to BODIPY core due to steric repulsion from methyl substituents (Fig. 1a). In contrast, the perylene donor moieties exhibit a slightly planar alignment positioned nearly parallel to the BODIPY core, probably due to intramolecular hydrogen bonding. Notably we observed hydrogen bonding interaction between the vinyl hydrogens and fluorine atoms of BF₂- group, with an approximated bond distance of 2.4 Å, that is close to the Wander Wall's radii. The crystal packing analysis further revealed the reduced intermolecular distance and hydrogen bonding interaction among the neighboring Ph-BDP-PY molecules, locking the molecular conformation. The combined intramolecular and intermolecular hydrogen bonding (C—H⋯F) in BODIPY emitters effectively hinders the molecular rotation and enhances the rigidity, aiding in the suppression of twisting or stretching vibrational energy loss, thereby anticipating a high luminescence efficiency. Despite extensive efforts using various solvents and crystallization techniques, were unable to grow the single crystal of another Ph-BDP-Cz for direct structural comparison among two BODIPY emitters.

  Figure 1

  

  Figure 1.  Crystal structures and packing patterns of Ph-BDP-PY.

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  For further insight in the geometry and electronic properties of the devised NIR BODIPY emitters, theoretical computations were performed using density functional theory (DFT) at PBE0/def2-SV (P) level. As depicted in Fig. 2, both BODIPY emitters exhibit the rigid optimized molecular geometry at the ground (S₀) state with a varying degree of symmetry. The dihedral angle among the planes of BODIPY and bulky donor moieties, pyrene and carbazole, range from 30.17° to 60.28° for Ph-BDP-PY and 13.60° to 30.44° for Ph-BDP-Cz, respectively, suggesting the different radiative efficiency for the two BODIPY emitters. Note that, the mesitylene group in current BODIPY emitters showed a slight deviation/twisting from the central plane, with a dihedral angle of 87.6° for Ph-BDP-PY and 87.3° for Ph-BDP-Cz, which would beneficial to suppress the intermolecular stacking and aggregation caused quenching (ACQ) [47]. The frontier molecular orbital (FMO) analysis revealed that both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are primarily localized on the BODIPY framework, with a slight variation of the extent of electron density localization (Fig. 2). The HOMO distributed on whole BODIPY chromophore with a leakage of electron density to the connected donor moieties. In contrast, LUMO is largely confined on meso–carbon of the BODIPY core with a minor electron density leakage to the adjacent vinyl bridge and aromatic donor moieties. These results suggest the weak intramolecular electronic interaction among the donor aryl moieties and BODIPY scaffold, thereby, it is likely to preserve the locally excited state (LE) characteristic of originating emission. Additionally, the quantum calculations showed the large single-triplet energy gap (ΔE_ST) (Fig. 2), small spin-orbit coupling (SOC) among singlet and highest triplet states (Fig. S11 in Supporting information) and large oscillator strength of S₁ excited state (f = 1.09 for Ph-BDP-PY and f = 0.67 for Ph-BDP-Cz; Table S5 in Supporting information), which are advantageous for efficient radiative transition and high luminescence quantum yield is expected [59–61]. Furthermore, donor engineering appears to leads a significant change in FMO distribution in the two BODIPY derivative, as a result distinct luminescence properties are anticipated (Fig. 2).

  Figure 2

  

  Figure 2.  The optimized geometry and frontier molecular orbitals of BODIPY derivatives, computed by DFT with a PBE0/def2-SV (P) basis using the Gaussian 09 program.

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  To further scrutinize the electronic properties of the BODIPY emitters, cyclovoltametry was conducted in dichloromethane using the standard three electrode-system (Table 1 and Fig. S8 in Supporting information). The HOMO for Ph-BDP-PY and Ph-BDP-Cz were estimated to be −5.15 eV and −4.98 eV, respectively, is in good agreement with theoretical computation results. The LUMO level of emitters were assessed using the formula, E_LUMO = E_HOMO + E_g and a deeper LUMO was observed for Ph-BDP-PY (−3.45 eV) compared to the Ph-BDP-Cz (−3.20 eV), suggesting the great extent of delocalization and electronic communication among BODIPY scaffold and pyrene moiety [62,63].

  Table 1

  

  Table 1.  Summary of electrochemical and photophysical data of BODIPY emitters.

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  Compound	λabs (nm)a	λem (nm)a	HOMO/LUMO (eV)b	Stokes shift (nm)	FWHM (nm)a	S1 (eV)c	ΦPL (%)a	τP (ns)d	kr (× 107 s-1)e
  Ph-BDP-Cz	672	694	-4.98/-3.20	22	29	1.84	68	5.20	1.12
  Ph-BDP-PY	692	723	-5.15/-3.45	31	34	1.78	46	2.80	5.26
  a In toluene (c = 1 × 10 -5 mol/L). b Calculated by CV measurement. c Calculated from the emission of solution (c = 1 × 10 -5 mol/L). d PL lifetimes were measured by TCSPC, (λex = 375 nm). e Calculated from the PL lifetimes.

  The photophysical properties of designed BODIPY emitters were thoroughly investigated by the UV–vis absorption, steady-state and time-resolved photoluminescence studies (Figs. 3a and b). The absorption spectra of both compounds exhibit muti-peaks characteristic absorption signatures of the styryl-BODIPY [64]. A pronounced absorption band was observed at 672 nm for Ph-BDP-Cz and 692 nm for Ph-BDP-PY, corresponding to the S₀ → S₁ transitions of BODIPY core, accompanied by a high energy shoulder band at 615 nm for Ph-BDP-Cz and 639 nm for Ph-BDP-PY, respectively, attributed to the 0–1 vibrational transition. Additionally, a broad but relatively weaker absorption band near 420 nm was observed, which is ascribed to the S₀ → S_n (π-π*) transition of the BODIPY core (Figs. 3a and b). Besides that, distinct absorption peaks were also observed in the UV-region, at 334 nm for Ph-BDP-Cz and 347 nm for Ph-BDP-PY, associated to the S₀ → S₁ transitions of carbazole and pyrenyl substituents, respectively. Compared with Ph-BDP-Cz, the absorption profile of Ph-BDP-PY not only showed bathochromic shift of ca. 20 nm, but also exhibit a slight broadening, which may be attributed to the presence of greater extent of delocalization and ground state electronic communication and is in agreement with theoretical computations (Fig. 2). Furthermore, the absorption maxima of both compounds are insensitive to solvent polarity, indicating negligible changes in the ground-state dipole moments of these compounds with varying solvent polarity (Fig. S6 in Supporting information).

  Figure 3

  

  Figure 3.  Photophysical properties of the BODIPY derivatives. (a, b) UV–vis absorption spectra and PL spectra in toluene solvent at room temperature. c ≈ 1 × 10⁻⁵ mol/L. (c) Comparison of PL spectra in toluene solvent at room temperature. c ≈ 1 × 10⁻⁵ mol/L. (d) Transient photo-luminescence decay curves, λ_ex = 375 nm, 20 ℃.

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  The photoluminescence spectra of BODIPY emitters, Ph-BDP-Cz and Ph-BDP-PY, exhibits a clear mirror-image relationship with their respective S₀ → S₁ absorption transitions, displaying a sharp deep red and NIR emission profiles at 694 nm and 723 nm, respectively (Fig. 3c). A moderate Stokes shift of ca. 22–31 nm was observed for these emitters, which is advantageous for improving the luminescence efficiency by minimizing the self-absorption [65,66], in line with their high photoluminescence quantum yield (Φ_PL). Notably, both BODIPY emitters exhibited a high Φ_PL, 46% for Ph-BDP-PY and 68% for Ph-BDP-Cz, with an ultra-narrow emission, having FWHM of 34 nm for Ph-BDP-PY 29 nm for Ph-BDP-Cz (Table 1). The transient photoluminescence decay curves, measured using TCSPC techniques, exhibited the mono-exponential decay profiles with lifetimes of 5.2 ns and 2.8 ns for Ph-BDP-Cz and Ph-BDP-PY respectively, consistent with their high radiative rate (k_r = 1.12 × 10⁵ s⁻¹ for Ph-BDP-Cz and 5.26 × 10⁵ s⁻¹ for Ph-BDP-PY; Fig. 3 and Table 1). These finding suggest that donor-engineering at the α-position of BODIPY core significantly alter the luminescence properties. Additionally, both BODIPY emitters exhibited the slight solvatochromic behavior, indicative of the predominant LE characteristic in their emissions (Fig. S6) [67,68].

  To understand the difference in luminescence properties, especially the FWHM of devised BODIPY derivatives, the reorganization energy, Root-mean-square deviation (RMSD) and Huang-Rhys factor (HRf) were computed. Ph-BDP-Cz found to have a smaller reorganization energy (0.129 eV) compared to Ph-BDP-PY, which exhibited noticeably a large reorganization energy of 0.160 eV (Figs. 4a and b), as result Ph-BDP-Cz exhibited a narrow emission [63]. This was further supported by RMSD analysis (0.24 Å for Ph-BDP-Cz and 0.26 Å for Ph-BDP-PY) (Fig. S9 in Supporting information), which revealed reduced structural relaxation in the Ph-BDP-Cz compared to the Ph-BDP-PY. This reduced structural flexibility is likely due to change in electron density around BODIPY skeleton via donor modification at the α-position. The HRf values of both BODIPY emittersvalidate this observation. As illustrated in Figs. 4c and d, the stretching and twisting vibration of the BODIPY are restricted by attachment of the donor moiety at α-position via a vinyl bridge and replaced by the rocking vibration of the donor moiety [63]. This change in vibrational behavior resulted in low HRf value in the low frequency region for both emitters. Specifically, the strong electron donating ability of the carbazole in Ph-BDP-Cz increase the electron density around BODIPY framework, leading to reduce the molecular structural flexibility and corresponding a small HRf (17.89 cm⁻¹, HRf = 2.27 for Ph-BDP-Cz) which results in its narrow emission. In contrast, Ph-BDP-PY which retains greater molecular flexibility (37.65 cm⁻¹, HRf = 2.35 for Ph-BDP-PY) leads to a slight the broadening of emission profile. These results indicate that donor modification at the α-position of BODIPY create distinct electronic environment which significantly impact the vibrations mode and ultimately controls the emission width.

  Figure 4

  

  Figure 4.  Reorganization energy of (a) Ph-BDP-Cz and (b) Ph-BDP-PY between S₀ and S₁. (c, d) The calculated Huang–Rhys factor of Ph-BDP-Cz and Ph-BDP-PY (inset: selected vibration modes with the most significant contribution to the Huang–Rhys factor).

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  The remarkable photophysical properties and excellent solubility of the devised BODIPY NIR emitters provoked us to investigate their potential for solution processed device fabrication. However, the inherent limitation of typical fluorescent emitters in harvesting the triplet excitons, pose a significant challenge to achieve high device efficiency [69,70]. To overcome this, we sensitized the BODIPY emitters using a TADF sensitizer, capable of upconverting triplet excitons and recycle it by transferring them to the BODIPY emitter via FRET mechanism, thereby enhancing the device performance [57]. Note that, in TADF sensitized HF device, the FRET plays a pivotal role in determining the overall device efficiency [42,44]. Considering this, several red TADF emitters were evaluated and 2DPACzBP was selected for to its deep red emission, excellent spectral overlap with the absorption profile of BODIPY emitters and its deep LUMO level, which facilitate efficient FRET while minimizing the likelihood of electron trapping on BODIPY emitters, ultimately leading to the superior device performance [71,72].

  As shown in Fig. 5a, the luminescence of 2DPACzBP exhibited a significant overlap with absorption profile of Ph-BDP-Cz, suggesting an efficient FRET process [57]. This assumption was further confirmed by the photoluminescence study of ternary blend (DMIC-TRZ: 2DPACzBP: Ph-BDP-Cz/Ph-BDP-PY = 100:10:1 by wt%), which displayed the dominant BODIPY emission around 500–850 nm, with a negligible contribution from 2DPACzBP host at shorter wavelength (640 nm), even at ultra-low doping concentration of BODIPY emitters. Specifically, for Ph-BDP-Cz, more efficient FRET was observed, as indicated by the absence of residual emission from TADF host.

  Figure 5

  

  Figure 5.  (a) Spectral overlap between absorption spectra of BODIPY derivatives and PL spectra of 2DPACzBP sensitizer in toluene solution, c = 1 × 10⁻⁵ mol/L. (b) Normalize emission of Ph-BDP-Cz and Ph-BDP-PY in 1% doped films. (c) The transient photoluminescence decay curves, λ_ex = 375 nm, 20 ℃. (d) Delayed fluorescence lifetime decay curves of Ph-BDP-Cz and Ph-BDP-PY in 1% doped films, λ_ex = 375 nm, 20 ℃.

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  For a more quantitative assessment, the energy transfer rates from 2DPACzBP host to BODIPY emitters (Ph-BDP-Cz/Ph-BDP-PY) were compared (Table S2 in Supporting information). As expected, a higher FRET rate was observed for the Ph-BDP-Cz (5.14 × 10⁷ s⁻¹) than for the Ph-BDP-PY (4.84 × 10⁷ s⁻¹), likely due to larger Förster radius (R₀), indicating the more efficient energy transfer from TADF sensitizer (2DPACzBP) and TADF host (DMIC-TRZ) to Ph-BDP-Cz. It is worth to mention that DMIC-TRZ was employed as an assistant host owing to its TADF nature and charge balance characteristic, which would help to achieve the multiple sensitizing channel and accelerate the exciton dynamics, further boost the device performance [43]. The transient photoluminescence study (Figs. 5c and d), which showed the shorter lifetime of ternary blend compared to the TADF host, further confirm the efficient singlet energy transfer from 2DPACzBP auxiliary host to the doped BODIPY emitters. This efficient FRET process reduces ISC/RISC process cycles, result in shortening the excited state lifetime. Additionally, a large energy gap among triplets of the TADF host and BODIPY emitter suggest. a minimal dexter energy transfer (DET) process, supporting the efficient exciton utilization [48,73].

  To access the electroluminescence (EL) performance of the devised BODIPY derivatives, the solution processed OLEDs with aforesaid ternary blend as emitting layer (EML) and following optimized device configuration; ITO/PEDOT: PSS (40 nm)/PVK: poly-TPD (4:1, 30 nm)/EML (40 nm)/TmPyPB (40 nm)/CsF (1 nm)/Al (120 nm) were fabricated, where PEDOT: PSS (poly(3,4-vinyldioxythiophene)/poly(styrylsulfonate)) serve as hole injecting layer, the blended PVK and poly-TPD (poly[bis(4-phenyl)(4-butylphenyl)amine]) (4:1, by weight) are used as the hole transporting layer. The detailed configuration and corresponding molecular structure used in the device is presented in Figs. 6a and b.

  Figure 6

  

  Figure 6.  (a) Device structures and energy level diagrams. (b) The structural formula of the material used in the device. (c) Electroluminescence spectra. (d) Energy transfer process from the TADF host and TADF sensitizer to the BODIPY emitter. (e) Current density-voltage-radiance characteristics of the devices. (f) External quantum efficiency-radiance (EQE-R) (The doped ratio was: DMIC-TRZ: 2DPACzBP: Ph-BDP-Cz/Ph-BDP-PY = 100:10:1).

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  The fabricated devices based on Ph-BDP-Cz and Ph-BDP-PY, exhibited the NIR electroluminescence at 702 nm and 736 nm and a narrow FWHM of 41 nm and 49 nm, respectively, which aligns well with their photoluminescence spectra. By virtue of efficient FRET and high Φ_PL of BODIPY emitter, these NIR solution processed devices demonstrate the remarkable performance, the relevant data is summarized in Fig. 6 and Table 2. Specifically, the NIR device based on Ph-BDP-Cz, achieved a high EQE_max of 6.9% and maximum radiance of 3037 mW sr⁻¹ m⁻² with a narrow FWHM of 41 nm, which according to our knowledge is the best performance among the TADF-sensitized NIR OLEDs and BODIPY-based NIR OLEDs (Fig. 7, Tables S3 and S4 in Supporting information). In comparison, the device based on Ph-BDP-PY exhibited EQE_max of 2.4%. The considerable difference in EQE among the two devices can be attributed to the more efficient energy transfer and minimal charge trapping in the Ph-BDP-Cz-based device. Additionally, the higher molar absorption coefficient of Ph-BDP-Cz facilitates enhanced energy transfer from the TADF assistant host through the FRET process, reducing exciton losses, and further boosting the device efficiency.

  Table 2

  

  Table 2.  Electroluminescence data of HF-OLEDs.

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  Device	TADF	Von (V)	EQEmax (%)a	CIE (x, y)	Peak (nm)	FWHM (nm)
  Ph-BDP-Cz: 0.9wt%	9 wt%	8.4	6.9	(0.59, 0.38)	702	41
  Ph-BDP-PY: 0.9wt%	9 wt%	6.6	2.4	(0.56, 0.40)	736	49
  a EQE maximum. The doped ratio was: DMIC-TRZ: 2DPACzBP: Ph-BDP-Cz/Ph-BDP-PY = 100:10:1.

  Figure 7

  

  Figure 7.  Comparison of the performance of previously reported TADF-sensitized NIR OLEDs and BODIPY-based NIR OLEDs versus this work.

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  In summary, we have successfully developed two novel BODIPY-based near-infrared (NIR) emitters, Ph-BDP-Cz and Ph-BDP-PY, with enhanced molecular rigidity and photoluminescence efficiency, by employing a Conformational Locking Strategy combined with donor-engineering. Integrating these emitters into thermally activated delayed fluorescence (TADF)-sensitized hyperfluorescent OLEDs led to an impressive external quantum efficiency (EQE) of 6.9%, with an emission peak at 702 nm and narrow FWHM of 41 nm, representing the best performance among reported TADF-sensitized and BODIPY-based NIR OLEDs. Our finding shows that the suppression of twisting and scissoring vibrations of BODIPY core through the vinyl bridge, alongside precise-tuning of emission wavelengths via donor engineering were the key to achieving both narrowband emission and efficient NIR device performance. This work not only demonstrates one of the highest efficiencies for solution-processed TADF-sensitized NIR OLEDs but also provide detailed analysis of the relationship between molecular structure and FWHM, paving the way for the development efficient organic emitters for cost-effective NIR optoelectronic devices.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yueyan Zhang: Validation, Methodology, Investigation, Data curation, Conceptualization. Zhihai Yang: Validation, Investigation. Xia Suo: Methodology, Investigation. Ruicheng Wang: Methodology, Investigation. Xuewei Nie: Investigation. Zafar Mahmood: Resources, Methodology. Yanping Huo: Resources, Project administration. Shi-Jian Su: Resources, Project administration. Shaomin Ji: Supervision, Resources, Project administration.

  Acknowledgments

  This work is financially supported by Natural Science Foundation of Guangdong Province (No. 2022B1515020041), National Natural Science Foundation of China (Nos. 22350410384, 52273179, 52303228, U23A20594) and Guangzhou Basic and Applied Basic Research (No. 2023A04J1374). We also thank the Instrumental Analysis Center of Guangdong University of Technology for their support.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111071.

  
  
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  Scheme 1  Previously reported emitters for near-infrared OLEDs versus this work.

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  Scheme 2  Conventional approach and molecular design strategy adopted in this study for the development of BDOIPY emitters.

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  Figure 1  Crystal structures and packing patterns of Ph-BDP-PY.

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  Figure 2  The optimized geometry and frontier molecular orbitals of BODIPY derivatives, computed by DFT with a PBE0/def2-SV (P) basis using the Gaussian 09 program.

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  Figure 3  Photophysical properties of the BODIPY derivatives. (a, b) UV–vis absorption spectra and PL spectra in toluene solvent at room temperature. c ≈ 1 × 10⁻⁵ mol/L. (c) Comparison of PL spectra in toluene solvent at room temperature. c ≈ 1 × 10⁻⁵ mol/L. (d) Transient photo-luminescence decay curves, λ_ex = 375 nm, 20 ℃.

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  Figure 4  Reorganization energy of (a) Ph-BDP-Cz and (b) Ph-BDP-PY between S₀ and S₁. (c, d) The calculated Huang–Rhys factor of Ph-BDP-Cz and Ph-BDP-PY (inset: selected vibration modes with the most significant contribution to the Huang–Rhys factor).

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  Figure 5  (a) Spectral overlap between absorption spectra of BODIPY derivatives and PL spectra of 2DPACzBP sensitizer in toluene solution, c = 1 × 10⁻⁵ mol/L. (b) Normalize emission of Ph-BDP-Cz and Ph-BDP-PY in 1% doped films. (c) The transient photoluminescence decay curves, λ_ex = 375 nm, 20 ℃. (d) Delayed fluorescence lifetime decay curves of Ph-BDP-Cz and Ph-BDP-PY in 1% doped films, λ_ex = 375 nm, 20 ℃.

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  Figure 6  (a) Device structures and energy level diagrams. (b) The structural formula of the material used in the device. (c) Electroluminescence spectra. (d) Energy transfer process from the TADF host and TADF sensitizer to the BODIPY emitter. (e) Current density-voltage-radiance characteristics of the devices. (f) External quantum efficiency-radiance (EQE-R) (The doped ratio was: DMIC-TRZ: 2DPACzBP: Ph-BDP-Cz/Ph-BDP-PY = 100:10:1).

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  Figure 7  Comparison of the performance of previously reported TADF-sensitized NIR OLEDs and BODIPY-based NIR OLEDs versus this work.

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  Table 1.  Summary of electrochemical and photophysical data of BODIPY emitters.

  Compound	λabs (nm)a	λem (nm)a	HOMO/LUMO (eV)b	Stokes shift (nm)	FWHM (nm)a	S1 (eV)c	ΦPL (%)a	τP (ns)d	kr (× 107 s-1)e
  Ph-BDP-Cz	672	694	-4.98/-3.20	22	29	1.84	68	5.20	1.12
  Ph-BDP-PY	692	723	-5.15/-3.45	31	34	1.78	46	2.80	5.26
  a In toluene (c = 1 × 10 -5 mol/L). b Calculated by CV measurement. c Calculated from the emission of solution (c = 1 × 10 -5 mol/L). d PL lifetimes were measured by TCSPC, (λex = 375 nm). e Calculated from the PL lifetimes.

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  Table 2.  Electroluminescence data of HF-OLEDs.

  Device	TADF	Von (V)	EQEmax (%)a	CIE (x, y)	Peak (nm)	FWHM (nm)
  Ph-BDP-Cz: 0.9wt%	9 wt%	8.4	6.9	(0.59, 0.38)	702	41
  Ph-BDP-PY: 0.9wt%	9 wt%	6.6	2.4	(0.56, 0.40)	736	49
  a EQE maximum. The doped ratio was: DMIC-TRZ: 2DPACzBP: Ph-BDP-Cz/Ph-BDP-PY = 100:10:1.

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