Designing spiro-fused chiral frameworks toward high-efficiency deep-blue circularly polarized thermally activated delayed fluorescence emission

Abstract

A spiro-functionalized indolocarbazole donor was employed to construct the deep-blue thermally activated delayed fluorescence emitter ICZFBO. This emitter exhibits deep-blue emission at 437 nm in solution, a photoluminescence quantum yield of 91%, and an external quantum efficiency of 15.0% with excellent Commission Internationale de L'Eclairage coordinates (CIE) of (0.149, 0.072). The circularly polarized device shows electroluminescence with a dissymmetry factor of 2.1 × 10⁻³.

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Organic light-emitting diodes (OLEDs) have advanced rapidly over the past few decades and are now established as a leading technology for high-definition displays and solid-state lighting.^1–4 Their performance is critically determined by the emissive layer, prompting intensive efforts toward highly efficient luminescent materials. Among these, thermally activated delayed fluorescence (TADF) has emerged as a powerful strategy, enabling metal-free emitters with a theoretical 100% exciton utilization through efficient reverse intersystem crossing (RISC).^1,5–7

Despite these advances, conventional OLED displays still suffer from severe optical losses, since external polarizing filters—used to suppress ambient reflections and enhance contrast—absorb up to 50% of the emitted light.⁸ A promising route to circumvent this limitation is the direct generation of polarized emissions.^9–12 In particular, the integration of circularly polarized luminescence (CPL) with TADF (CP-TADF) offers dual benefits: improved device efficiency and unique functionalities such as 3D imaging, optical communication, and quantum encryption.^13–17

Substantial progress has been achieved for CP-TADF emitters in the red and green spectral regions, yielding high photoluminescence quantum yields (PLQYs) and distinct CPL signatures.^13,18–20 However, efficient deep-blue CP-TADF materials remain elusive. The difficulty lies in balancing four stringent requirements: a wide bandgap for blue emission, a small singlet–triplet energy gap (ΔE_ST) for TADF, high PLQY, and a robust chiral framework capable of sustaining CPL activity.^21,22 Existing designs—often based on conjugated or axially chiral motifs—show promising g_EL values but rarely meet deep-blue emission standards National Television System Committee (NTSC) or display diminished chiroptical activity, particularly in through-space charge transfer (TSCT) architectures.^23–31

Here, we report a design principle that overcomes these challenges by combining a non-conjugated chiral scaffold with a TSCT core.^32,33 Our emitter, ICZFBO, incorporates a σ-type linker to spatially decouple the chiral center from the donor–acceptor electronic framework, thereby preserving photophysical integrity while maintaining chirality. The weak electron acceptor 2,12-dimethyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BO) expands the optical gap, promoting efficient deep-blue emission. The resulting enantiomers, (R)- and (S)-ICZFBO, deliver emission at 437 nm with a PLQY of 91% and a |g_EL| of 2.7 × 10⁻³ in solution.

OLED devices fabricated with ICZFBO achieve an external quantum efficiency (EQE) of 15.0% and Commission Internationale de L'Eclairage coordinates (CIE) of (0.150, 0.072), fully meeting NTSC requirements for pure-blue emission. Crucially, the devices retain strong chiroptical activity in the electroluminescent state, with a |g_EL| of 2.1 × 10⁻³. These results establish spatially decoupled chiral TSCT emitters as a powerful strategy for realizing high-performance deep-blue CP-TADF materials with simultaneous efficiency, color purity, and CPL activity.

The design of a rigid electron donor with inherently weak donating ability is a central requirement in this work. Classical donors such as triphenylamine have been widely applied in blue TADF systems (Fig. 1a), but their relatively strong donating character and conformational flexibility make them unsuitable for deep-blue emission.³⁴ To overcome this, structural modification through internal covalent connections within the heteroaromatic skeleton—for example in 9-phenylcarbazole—has proven effective. In this case, the covalent bond between the phenyl and acridine rings enhances molecular rigidity, thereby reducing the Stokes shift and avoiding the introduction of empty orbitals or lone pairs. By contrast, chemical modification through heteroatom incorporation, as in 10-phenylphenothiazine, often reconstructs the frontier molecular orbitals and induces undesirable redshifts.³⁰ However, simple carbazole fusion, such as in 9-phenyl-9H-carbazole, can strongly impair the TADF character even though deep-blue emission is maintained.³¹

Fig. 1 (a) Molecular design scheme for DP; (b) synthetic routes of DP and ICZFBO.

As shown in Fig. 1b, the target chiral deep-blue emitter ICZFBO and its reference fragment DP were synthesized on a gram-scale through a straightforward sequence involving carbanion nucleophilic attack and dehydrative cyclization from commercially available precursors. The enantiomers (R)- and (S)-ICZFBO were separated by chiral high performance liquid chromatography (HPLC) and subjected to comprehensive characterization, including ¹H and ¹³C NMR spectroscopy (see the SI, Section S2). Single-crystal X-ray diffraction (SI, Section S3) confirmed the molecular geometry and revealed a π-stacked arrangement consistent with a TSCT pathway.

The crystal packing shows that the rigid fluorene skeleton enforces close contacts between the BO acceptor and the ICZ donor (Fig. 2a). Weak intramolecular interactions (cyan dotted lines) with an average separation of 3.20 Å (Table S2) are shorter than the π–π spacing in graphite (3.35 Å),³⁵ indicating a coplanar D–A configuration and a π–π stacked arrangement. These stabilizing contacts were further visualized by independent gradient model analysis based on the Hirshfeld partition (IGMH),^36,37 which displays a broad green isosurface between the ICZ donor and BO acceptor (Fig. 2b). Such close π-stacking provides a structural basis for intramolecular charge transfer.

Fig. 2 (a) Crystal structure of ICZFBO (CCDC 2332254); (b) IGMH isosurface of ICZFBO; (c) HOMO and (d) LUMO distribution of ICZFBO based on the B3LYP-D3BJ/def2-SVP level.

Density functional theory (DFT) calculations support this interpretation. As shown in Fig. 2c and d, the highest occupied molecular orbit (HOMO) is localized on the π-extended ICZ donor while the lowest unoccupied molecular orbit (LUMO) resides on the electron-deficient BO acceptor. This spatial separation, combined with short donor–acceptor distance, ensures effective electronic coupling—a prerequisite for efficient TSCT-type TADF. Importantly, the spiro-fluorene framework maintains rigidity while promoting this favorable orbital arrangement.

The thermal properties of ICZFBO were assessed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The material shows excellent thermal stability, with a decomposition temperature (5 wt% loss) of 435 °C and a clear glass-transition temperature at 190 °C (Fig. S7a, S7b and Table S3). These features confirm that ICZFBO possesses the structural robustness required for OLED device fabrication.

Cyclic voltammetry (Fig. S7c) revealed an oxidation onset corresponding to a HOMO energy level of −5.30 eV, calculated using E_HOMO = −(E_ox + 4.8 − E_Fc/Fc+) eV. This relatively deep HOMO, arising from the weak electron-donating strength of the ICZ unit, is advantageous for maintaining deep-blue emission.

The UV-Vis absorption and photoluminescence (PL) spectra of ICZFBO and its reference fragment DP in toluene are shown in Fig. 3a. The high-energy absorption band (≤385 nm) originates from local π–π* transitions of the DP scaffold, whereas a weaker tail extending to 440 nm is attributed to direct intramolecular charge transfer (ICT) between the ICZ donor and BO acceptor. Owing to its wide optical bandgap, ICZFBO exhibits deep-blue emission with a PL maximum at 437 nm. Solvatochromic studies further confirm the charge-transfer character: the emission peak shifts from 413 nm in hexane to 466 nm in dichloromethane (Fig. 3b), consistent with a TSCT excited state.

Fig. 3 (a) UV-Vis spectra of ICZFBO and DP and room temperature PL spectra measured of ICZFBO; (b) PL spectra of ICZFBO in different solvents; (c) LTPL and PHOS spectra of ICZFBO and PHOS spectra of DP measured at 77 K in toluene; (d) electron–hole distribution of ICZFBO in the S1 state and T1 state based on the PBE0/def2-SVP level; (e) time-resolved transient PL decay of ICZFBO in a doped film (20 wt% in DPEPO) from 300 K to 100 K.

The energy levels of the singlet (S₁) and triplet (T₁) excited states were determined by the onsets of the low-temperature fluorescence and phosphorescence spectra (Fig. 3c). The vibrational structure and wavelength of the phosphorescence match those of DP, confirming that the donor contributes a ³LE triplet state. By contrast, TD-DFT calculations indicate that the S₁ state possesses a characteristic TSCT nature, with the hole density on the ICZ donor and the electron density on the BO acceptor (Fig. 3d). However, both the electrons and holes are located on the ICZ donor, indicating a ³LE state. This complementary spin–orbital distribution facilitates efficient reverse intersystem crossing (RISC) from ³LE to ¹CT, the hallmark of TADF.

In nitrogen-purged toluene solution (1 × 10⁻⁵ M), ICZFBO shows a PLQY of 31%. Upon embedding in a co-doped bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) film (14 wt%), the PLQY dramatically increases to 91%, attributable to suppressed nonradiative loss and enhanced rigidity in the solid matrix. This behavior indicates that ICZFBO possesses a semi-rigid structure, in which the rotation of the donor–acceptor dihedral angle is restricted while solvent relaxation in solution still enables partial nonradiative decay. Transient PL measurements (Fig. 3e) display multiexponential decay kinetics with a strong temperature dependence, firmly establishing the TADF character. Kinetic fitting (Table S4) yields a prompt radiative rate constant (k_F) of 1.0 × 10⁷ s⁻¹, attributed to the coplanar D–A configuration. Furthermore, the rigid molecular framework, together with the solid film state, suppresses nonradiative relaxation pathways, thereby contributing to the high overall PLQY. A summary of the key photophysical parameters is provided in Table 1.

Table 1 Photophysical parameters of ICZFBO

	λ Abs (nm)	λ PL (nm)	FWHM^a (nm)	E S1 (eV)	E T1 (eV)	ΔEST (eV)	Φ PLQY (%)
a Full-width at half-maximum. b In doped film (14 wt% in DPEPO). c In nitrogen-filled toluene solution (1 × 10^−5 M).
ICZFBO	366, 383	437	56	3.03	2.83	0.20	91^b/31^c

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The chiroptical properties of (R)- and (S)-ICZFBO were examined in both the ground and excited states using circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The CD spectra (Fig. 4a) exhibit clear mirror-image profiles, with a pronounced Cotton effect near 385 nm that originates from π–π* transitions of the ICZ donor. At longer wavelengths (>385 nm), weaker bands appear, corresponding to intramolecular TSCT from the ICZ donor to the BO acceptor, consistent with the absorption features of the racemate.

Fig. 4 (a) CD spectra and (b) CPPL spectra of (R)-ICZFBO/(S)-ICZFBO.

Excited-state chirality was further confirmed by CPL spectroscopy. The enantiomers display mirror-image CPL spectra (Fig. 4b), with (S)-ICZFBO and (R)-ICZFBO reaching g_PL values of +2.7 × 10⁻³ and −2.3 × 10⁻³, respectively (Fig. S9). These values rank among the highest reported for CP-TADF emitters, particularly in the deep-blue region. The results demonstrate that the rigid spiro-fused donor (DP) effectively transmits chirality into the emissive state, enabling strong CPL activity alongside efficient TADF.

OLED devices incorporating ICZFBO were fabricated to evaluate electroluminescence (EL) performance. Vacuum-deposited OLEDs were fabricated with a device configuration of ITO/dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 10 nm)/4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC, 40 nm)/tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA, 10 nm)/DPEPO:ICZFBO (5, 10, 14, 20 wt%, 20 nm)/3,3′-(5′-(3-(pyridin-3-yl)phenyl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB, 40 nm)/8-hydroxyquinolinolato-lithium (Liq, 2 nm)/Al (100 nm) where ITO, HAT-CN, TAPC, and TCTA are the anode, hole injection, and hole transport layers, respectively. For the emitting layers, ICZFBO was co-evaporated with DPEPO for device ICZFBO, and TmPyPB was used as the electron transport layer. All devices exhibited stable deep-blue emission at 448 nm across a wide doping range (5–20 wt%; Tables S6, S7 and Fig. 5a). The devices exhibit a narrow full width at half maximum (FWHM) of approximately 55 nm—comparable to the PL spectrum in toluene. The stable EL at different doping concentrations indicates that aggregation-induced quenching is minor, which may be attributed to the rigid, three-dimensional molecular structure. The optimal device, based on 14 wt% ICZFBO in DPEPO, delivered an EQE of 15.0%. Notably, the Commission Internationale de l’Eclairage (CIE) coordinates of (0.149, 0.072) fully satisfy the stringent NTSC requirements for pure deep-blue emission (CIEy < 0.08; Fig. 5b). Encouraged by these results, we further investigated the circularly polarized electroluminescence (CPEL) properties of the ICZFBO enantiomers. Devices based on (S)- and (R)-ICZFBO exhibited distinct mirror-image CPEL spectra (Fig. 5c and Fig. S12), with g_EL values of +1.6 × 10⁻³ and −2.1 × 10⁻³, respectively, at their EL maxima. These results confirm that the intrinsic chirality of ICZFBO is successfully translated into the electroluminescent state, enabling efficient deep-blue CP-TADF OLEDs with high color purity and robust chiroptical activity.

Fig. 5 (a) EL spectra of ICZFBO-based devices; (b) CIE plot of the EL spectra of the ICZFBO-based device; (c) CPEL spectra of (R)-/(S)-ICZFBO.

Conclusions

In summary, we have developed a chiral deep-blue TADF emitter, ICZFBO, featuring a spiro-fused donor framework with an asymmetrically expanded ring. This design enables efficient chirality transfer into the emissive state, yielding pronounced chiroptical activity with |g_PL| up to 2.7 × 10⁻³ in solution and |g_EL| of 2.1 × 10⁻³ in OLED devices. Owing to its wide optical bandgap and high PLQY (91%), ICZFBO exhibits deep-blue emission at 437 nm in solution and 448 nm in devices, while achieving a maximum EQE of 15.0%. Importantly, the device meets the strict NTSC standard for pure deep-blue emission, with excellent CIE coordinates of (0.149, 0.072). These results demonstrate that spiro-fused frameworks represent a powerful strategy for constructing chiral deep-blue TADF emitters and establish ICZFBO as a benchmark system for next-generation CP-OLEDs, combining high efficiency, color purity, and robust chiroptical activity.

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: experimental procedures, characterization data, X-ray crystallography data, and ¹H/¹³C NMR spectra for the new compounds. See DOI: https://doi.org/10.1039/d5tc03265b.

CCDC 2332254 contains the supplementary crystallographic data for this paper.³⁸

Acknowledgements

The authors acknowledge financial funding from the National Natural Science Foundation of China (No. 22175124, 62175171, 52573209 and 22501194). This work is also supported by Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology, the 111 Project, and the Joint International Research Laboratory of Carbon-Based Functional Materials and Devices. We also acknowledge the Science and Technology Development Fund, Macao SAR (grant number: 0072/2024/RIB2).

Notes and references

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Footnote

† These authors contributed equally.

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