Organic light-emitting diodes (OLEDs) have become integral to modern display technologies, yet the development of high-performance blue emitters continues to lag behind that of their red and green counterparts, particularly in terms of efficiency and operational stability. Thermally activated delayed fluorescence (TADF) materials offer a promising route toward nearly complete exciton utilization by converting non-emissive triplet excitons into emissive singlet excitons through reverse intersystem crossing (RISC). However, the relatively long-lived excited states in TADF systems are susceptible to bimolecular exciton annihilation and aggregation-caused quenching, especially at high doping concentrations, leading to pronounced efficiency roll-off in blue OLEDs. Developing blue TADF emitters that simultaneously combine high efficiency with strong resistance to concentration quenching therefore remains a formidable challenge.
Now, writing in Science China Chemistry, Can-Zhong Lu and Xu-Lin Chen at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, report a highly efficient blue TADF emitter, SAF-BO, based on a multi-spiro molecular design strategy. Through systematic photophysical and device investigations, the researchers demonstrate that the incorporation of multiple spiro units effectively regulates molecular packing and suppresses both aggregation-caused quenching and bimolecular exciton annihilation, enabling excellent luminescence and device performance even at high doping concentrations.
The research team introduced multiple sp 3 -hybridized spiro units into an oxygen-bridged triarylboron acceptor-spiroacridine donor framework, and synthesized two control molecules, SA-BO and PAF-BO, for comparison, ultimately obtaining SAF-BO with excellent aggregation-resistant properties. Theoretical calculations reveal that the multi-spiro modification has a marginal effect on the intrinsic electronic structure of the molecule. Single-crystal structural analysis further indicates that the multi-spiro strategy, combined with the intramolecular cyclization in SAF-BO, effectively enlarges the intermolecular stacking distance (>4.4 Å), suppresses π–π interactions, and attenuates deleterious Dexter exchange interactions ( Figure 1 ). Meanwhile, the rigid spiro skeleton restrains molecular vibrational relaxation, thereby reducing non-radiative losses and enhancing the photostability of the films.
Photophysical measurements demonstrate that SAF-BO retains similar TADF kinetic characteristics to the control compounds SA-BO and PAF-BO. Remarkably, even at a high doping concentration of 50 wt%, the photoluminescence quantum yield (PLQY) of SAF-BO films reaches 99.5%, with a non-radiative decay rate substantially lower than that of the two controls. SAF-BO also maintains a blue emission peak at 477 nm with a full width at half maximum of approximately 50 nm ( Figure 2 ), alongside a fast delayed fluorescence lifetime of 1.29 μs and a reverse intersystem crossing rate constant of 1.78 × 10 6 s -1 . These results confirm that the multi-spiro design does not compromise the intrinsic charge-transfer emission character of the TADF material, but instead effectively suppresses concentration quenching and exciton annihilation by tuning the molecular packing mode.
The authors further fabricated vacuum-deposited OLED devices ( Figure 3 ). The emissive layer based on SAF-BO achieved a maximum external quantum efficiency (EQE max ) of 32.1% with an electroluminescence peak at 476 nm, demonstrating excellent blue emission performance. In terms of efficiency roll-off, the SAF-BO device exhibited a markedly slower decline, retaining 23.0% efficiency at 1000 cd/m 2 . By contrast, the control devices based on SA-BO and PAF-BO, which lack the rigid multi-spiro architecture, retained only 13.6% and 12.0%, respectively, under the same luminance—corresponding to an approximately 1.7- to 1.9-fold improvement in high‑brightness efficiency retention.
The researchers also explored the potential of these materials as sensitizers in a TADF-sensitized fluorescence (TSF) system, using them to sensitize the deep-blue multiple-resonance TADF emitter v-DABNA. SAF-BO enabled ultrafast and highly efficient Förster resonance energy transfer (rate constant 5.38 × 10 8 s -1 , efficiency 90.4%), while effectively suppressing short-range Dexter energy transfer, with a Dexter rate constant as low as 1.86 × 10 4 s -1 —significantly lower than those observed for SA-BO and PAF-BO-based systems. In TSF devices, the SAF-BO-sensitized device (named, SAF-BO-TSF) achieved an EQE max of 35.2% and maintained 27.1% efficiency at 1000 cd/m 2 , along with excellent deep-blue color purity (CIE y = 0.130). This work demonstrates the value of the multi-spiro molecular design strategy in constructing aggregation-resistant, low-roll-off blue TADF materials, and provides new directions for the future development of highly stable and efficient OLED emitters.
Science China Chemistry
Experimental study