文章概要#

本研究成功合成了全球首个五硼多重共振热激活延迟荧光（MR-TADF）发光材料M-DABNA-Mes。该分子突破了奇数硼原子共振骨架的合成瓶颈，显著缩小了单重态与三重态的能级差，将反系间跨越速率（$k_{\text{RISC}}$）提升至$2.6 \times 10^6 \text{ s}^{-1}$。基于该材料制备的绿光OLED器件实现了34.3%的最大外量子效率以及极低的高亮度效率滚降。

引言#

有机发光半导体在显示与照明领域展现出巨大应用价值。为了实现高色彩饱和度的超高清显示，开发具有窄带发射特征的高效发光材料至关重要。多重共振热激活延迟荧光（MR-TADF）材料凭借刚性骨架中硼、氮原子的电子诱导效应，能够在保持极窄半峰宽的同时实现高量子效率。

尽管扩展骨架的π共轭体系已被广泛用于调控激发态性能，但在分子中嵌入更多硼原子始终面临严峻的合成挑战。此前学术界已开发出含一个、两个及四个硼原子的偶数硼体系，但奇数硼原子的多硼共振骨架一直未能突破，导致包含五个硼原子的DABNA家族衍生物长期处于空白状态。探索五硼骨架的激发态电子特性与光电性能，成为解决MR-TADF材料反系间跨越速率较慢这一瓶颈的关键科学问题。

Fig.1 Molecular design of boron-enriched MR-TADF emitters. Increasing the number of embedded boron atoms from one (DABNA-1) to five (M-DABNA-Mes) expands the multiple-resonance framework.

主要实验及结论#

研究团队首先通过双混合含时密度泛函理论（DH-TDDFT） 对单硼、三硼及五硼分子的激发态特性进行了系统理论计算。计算结果如图1和图2所示，随着骨架中嵌入硼原子数量从一个增加至五个，单重态与三重态之间的能级差（$\Delta E_{\text{ST}}$）呈现出显著递减趋势，五硼骨架M-DABNA-core的理论$\Delta E_{\text{ST}}$大幅降至26 meV。同时，五硼骨架依然保持了良好的自旋-轨道耦合作用，且总重组能显著降低，这表明五硼引入能够在保留窄带发射特性的前提下，极大地促进反系间跨越过程。

Fig.2 (a) Molecular structures of DABNA-1, ω-DABNA-core, and M-DABNA-core. (b–d) Energy level diagrams of the singlet and triplet excited states at the S1 geometry and corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) distributions (isovalue = 0.02) obtained from the S0-optimized structures. Vertical transition energies for S1, T1, and T2 were calculated at the TDA-B2PLYP (cx = 0.40, cc = 0.23)/cc-pVDZ//M062X/6-31G(d) level of theory. SOC matrix elements between the Tn and Sn states were calculated at the TDA-B2LYP (cx = 0.40, cc = 0.23)/cc-pVDZ//M062X/6-31G(d) level of theory.

在理论指导下，研究人员设计并合成了目标分子M-DABNA-Mes。合成路线如Scheme 1所示，通过多步胺化反应构建前驱体后，在三碘化硼与碱的配合下实现一步引入四个硼原子，随后在氯化铝催化下完成脱烷基与分子内环脱水反应，成功获得了结构确证的五硼多重共振骨架。单晶结构分析清晰展示了其高度融合且带有立体阻蔽的分子构象。

Scheme.1 Synthesis of M-DABNA-Mes and single-crystal structure of M-DABNA-Mes [35]. The molecular structure is shown as a thermal ellipsoid plot with 50% probability displacement ellipsoids (gray, carbon; white, hydrogen; pink, boron; blue, nitrogen).

光物理性能测试证实了理论预测的准确性。如图3和图4所示，在掺杂聚苯乙烯薄膜中，M-DABNA-Mes展现出512 nm的纯绿光发射，其半峰宽仅为19 nm，斯托克斯位移降低至9 nm。得益于五硼骨架对激发态离域的增强，分子的延迟荧光寿命缩短至微秒以下（0.93 $\mu$s），反系间跨越速率达到$2.6 \times 10^6 \text{ s}^{-1}$，相比三硼参照分子提升了近20倍，一举突破了传统MR材料在光谱纯度与自旋转化速率之间的权衡局限。

Fig.3 (a–c) Simulated absorption (black dashed lines) and emission (solid lines) spectra based on Franck–Condon analysis for DABNA-1 (blue), ω-DABNA-core (green), and M-DABNA-core (purple). (d–f) Huang-Rhys factors and (g–i) mode-resolved reorganization energies associated with the S1−S0 transition for DABNA-1 (d, g), ω-DABNA-core (e, h), and M-DABNA-core (f, i). The total reorganization energy (mathematical equation) and average bond length change (ΔLavg) between the optimized S0 and S1 geometries are also shown.

Fig.4 Photophysical properties of ω-DABNA (a,c; excited at 340 nm), M-DABNA-Mes (b,d; excited at 365 nm) in 1 wt% PS films. Steady-state photoluminescence (PL) spectra at 300 K (blue) and 77 K (green), and phosphorescence spectra at 77 K (red) with 3 ms (a) and 25 ms (b) delay. Transient PL decay curves at 300 K, along with their relevant parameters (c,d). The gray curves indicate the instrument response function (IRF). The red curves represent single-exponential fitting results (background = 1 and 4). (e) CIE (x, y) coordinates of the PL spectra of ω-DABNA and M-DABNA-Mes. (f) Comparison of kRISC and FWHM for reported green MR-TADF compounds in the solid state. In both panels, ω-DABNA and M-DABNA-Mes are represented by a black circle and a red star, respectively. The remaining compounds in (f) are classified according to the period of the incorporated heteroatoms: second-period (blue), third-period (green), and fourth-period (yellow).

研究团队进一步将其制备为电致发光器件并评估其应用性能。如图5所示，基于M-DABNA-Mes的OLED器件表现出优异的电致发光特性，发光峰值位于508 nm，半峰宽为23 nm，CIE色坐标为(0.132, 0.692)。器件的最大外量子效率高达34.3%，且在$10000 \text{ cd m}^{-2}$的高亮度下效率仍保持在32.3%，效率滚降极其轻微，最大亮度达到了$275800 \text{ cd m}^{-2}$。

Fig.5 OLED characteristics for the devices using M-DABNA-Mes. (a) Device structures with the estimated energy levels in eV for each component. (b) Normalized EL spectra. (c) CIE (x, y) coordinates. (d) Current density–voltage (J–V) characteristics (blue) and Luminance–voltage (L–V) characteristics (green). (e) EQE–luminance (EQE–L) characteristics. (f) Comparison of the external quantum efficiency at 10,000 cd m−2 (EQE10000) and CIE y coordinate for reported green MR-TADF OLEDs. M-DABNA-Mes is highlighted by the red star, while previously reported compounds are shown as blue circles.

总结及展望#

该研究首次合成了具有五硼多重共振骨架的TADF发光材料，成功打破了奇数多硼骨架的合成限制。实验与理论计算共同证明，高密度硼原子的富集能够有效加速反系间跨越过程，在维持高色彩纯度的同时大幅降低器件的效率滚降。这一成果不仅刷新了绿光MR-TADF材料的性能纪录，更为设计下一代高清显示用超窄带有机发光材料开辟了全新的分子设计维度。