【JACS】南开庞代文、黄灵|1064nm下效率跃升1.9%！新型BODIPY代交联分子，彻底摆脱传统并四苯依赖
文章标题：Triplet-Mediated Photon Upconversion via Near-Infrared-II Excitation without Tetracene Derivatives
通讯作者：Dai-Wen Pang, Ling Huang
文章概要
南开大学庞代文教授与黄苓教授团队在近红外二区（NIR-II）光子上转换领域取得突破性进展。研究团队设计并合成了一种全新的双苯乙烯取代修饰的BODIPY（DS-BDP）衍生物，该分子展现出极佳的稳定性和光谱可调性，成功在量子点上转换体系中兼任表面配体与湮灭剂。这一成果彻底克服了传统并四苯类衍生物易光漂白、能级难调的固有缺陷，在1064 nm激光激发下实现了颜色可调的高效红光发射，为纳米光子学和生物光子学应用开辟了新道路。
引言
低能量光子的有效捕获与转换一直是能源科学与生物医学成像领域的重大挑战。基于三线态-三线态湮灭的光子上转换技术（TTA-UC）能够将低能量的近红外光转化为高能量的可见光，因而备受关注。然而，现有的近红外上转换体系长期依赖并四苯类衍生物（如红荧烯）作为配体或湮灭剂，这些化合物在微量氧气下极易发生光氧化和光漂白，导致体系稳定性骤降。此外，传统体系的发射波长往往受限于红荧烯而无法自由调节。如何在提升稳定性的同时打破波长限制，是该领域亟待解决的科学难题。
Scheme 1. (a) Schematic Illustration of the Mechanism of Quantum Dot-Based Triplet–Triplet Annihilation Photon Upconversion (QD-Based TTA-UC), TET1 Denotes Triplet Exciton Transfer from Quantum Dots to Surface Ligands, and TET2 Denotes Triplet Energy Transfer from Surface Ligands to the Annihilator; These Labels are Used to Clearly Differentiate the Two Sequential Processes; The Abbreviation for Triplet–Triplet Annihilation is TTA; (b) Representative a Tetracene-Based Surface Ligand (5-CT) and an Annihilator (Rubrene) Reported in the Literature, along with Their Limitations; (c) Distyryl-Substituted BODIPY (DS-BDP) Derivatives as Surface Ligands and Annihilators and Their Advantages; (d) Molecular Structures of Surface Ligands DS-BDP-1 to DS-BDP-3, and Annihilators DS-BDP-4 to DS-BDP-7
主要实验及结论
研究团队首先通过理论计算设计了具有匹配三线态能级（约1.18 eV）的DS-BDP分子，并将其作为配体与PbS量子点结合。如图1所示，吸附了单羧酸修饰配体DS-BDP-2的量子点体系成功实现了由三线态能量转移驱动的黄色上转换荧光，首次证明了DS-BDP scaffold作为表面配体的可行性。
Figure 1. Performance of TTA-UC (PbS-3.05/DS-BDP-2/rubrene). (a) Top panel shows the normalized absorption (normalized the first excitonic absorption peak of PbS-3.05 QDs to 1) and fluorescence emission spectra of PbS-3.05 QDs in toluene, 1 μM, λex = 808 nm; bottom panel is normalized absorption and fluorescence emission spectra of DS-BDP-2 in CHCl3, 10 μM, λex = 405 nm. (b) Normalized absorption spectra of PbS-3.05 QDs (normalized the first excitonic absorption peak of PbS-3.05 QDs to 1) after ligand exchange with DS-BDP-2 at different concentrations (0, 100, 200, and 400 μM) in toluene, and the concentration of PbS-3.05 QDs is 10 μM. The inset shows the average number of DS-BDP-2 per PbS-3.05 QD (⟨_N_DS-BDP-2⟩) after ligand exchange with DS-BDP-2 at different concentrations (0, 100, 200, and 400 μM). (c) Upconversion spectra in the presence of PbS-3.05/DS-BDP-2 (photosensitizer), rubrene (annihilator), ⟨_N_DS-BDP-2⟩ (0, 1, 4, 7), λex = 980 nm. The inset shows the ηUC′ of PbS-3.05/DS-BDP-2/rubrene with different ⟨_N_DS-BDP-2⟩. (d) Time-resolved upconversion spectrum of PbS-3.05/DS-BDP-2/rubrene under pulsed 980 nm laser excitation.
为了进一步提升配体交换量并抑制配体构象弛豫造成的能量损失，团队进一步开发了双羧酸锚定基团修饰的DS-BDP-3配体。如图2所示，飞秒瞬态吸收光谱数据证实，双齿螯合模式显著增强了配体在量子点表面的结合密度，并大幅加快了三线态激子传输速率（达到1.92×10⁹ s⁻¹）。分子动力学模拟进一步表明，这种双锚定结构有效锁定了分子构象，增长了三线态寿命。
Figure 2. (a) Normalized absorption spectra (normalized the first excitonic absorption peak of PbS-3.05 QDs to 1) of PbS-3.05 QDs after ligand exchange with DS-BDP-3 at different concentrations (0, 20, 40, 100, 200, and 400 μM) in toluene, and the concentration of PbS-3.05 QDs is 10 μM. The inset shows the average number of DS-BDP-3 per PbS-3.05 QD (⟨_N_DS-BDP-3⟩) after ligand exchange with DS-BDP-3 at different concentrations (0, 20, 40, 100, 200, and 400 μM). Femtosecond transient absorption (fs-TA) spectra of PbS-3.05 QDs (b) and PbS-3.05/DS-BDP-3 (c), λex = 750 nm. (d) Normalized exciton bleaching (XB) kinetics of PbS-3.05 and PbS-3.05/DS-BDP-3 at 930 nm.
得益于这一精确的分子工程，如方法对比图3所示，该体系即使在极低的表面覆盖度下也展现出惊人的转化效率。当使用更窄带隙的PbS量子点时，在1064 nm的NIR-II光激发下，体系实现了高达1.9%的校正上转换效率，比传统配体高出数倍。
Figure 3. Photon upconversion performance of PbS-3.05/DS-BDP-3/rubrene and PbS-3.05/5-CT/rubrene. (a) Upconversion spectra of PbS-3.05/DS-BDP-3/rubrene, the ⟨_N_DS-BDP-3⟩ is 0, 1, 2, 8, 14, and 25, respectively, λex = 980 nm. (b) The ηUC′ with the different ⟨_N_DS-BDP-3⟩. (c) Dependence of the upconversion intensity of PbS-3.05/DS-BDP-3/rubrene on the incident power density at 980 nm, _I_th = 8.4 W cm–2. (d–f) Comparison of the upconversion intensities of PbS-3.05/DS-BDP-3/rubrene and PbS-3.05/5-CT/rubrene at surface ligand average numbers of 1, 8, and 25, respectively. The insets show corresponding photographs: left, PbS-3.05/5-CT/rubrene with associated ηUC′; right, PbS-3.05/DS-BDP-3/rubrene with associated ηUC′.
随后，团队利用DS-BDP结构的可调性，开发了系列带有推拉电子基团的衍生物（DS-BDP-4至DS-BDP-7）作为湮灭剂。如图4和图5所示，在单一的1064 nm激光照射下，通过调节波长调谐实现了上转换发射在639 nm至668 nm之间的精准色彩调节。更令人振奋的是，长期光稳定性测试表明，新型DS-BDP材料在空气饱和溶液中持续辐照12天后吸光度保持率仍超过96%，而传统的红荧烯在63小时内便几乎完全降解。
Figure 4. DS-BDP-based annihilators. (a) Normalized UV–vis absorption and fluorescence spectra of DS-BDP-4 to DS-BDP-7, λex = 405 nm, with fluorescence quantum yields (Φf) indicated. (b) HOMO and LUMO orbital distributions of DS-BDP-4 to DS-BDP-7. (c) Calculated energy levels of T1 energy levels, and S1 energy levels were determined by the intersection of absorption and emission, and twice the T1 energy (2T1) for DS-BDP-4 to DS-BDP-7.
Figure 5. (a) Upconversion emission spectra of PbS-3.19/DS-BDP-3/DS-BDP-4 to PbS-3.19/DS-BDP-3/DS-BDP-7 with 6 mM annihilator (DS-BDP-4 to DS-BDP-7), λex = 1064 nm. (b) Time-resolved decay of upconversion emission from PbS-3.19/DS-BDP-3/DS-BDP-4 to PbS-3.19/DS-BDP-3/DS-BDP-7 under pulsed 1064 nm excitation. (c) Photostability comparison of rubrene and DS-BDP annihilators under ambient room light irradiation in air-saturated toluene at room temperature. (d) Photostability comparison of 5-CT and DS-BDP ligands under ambient room light irradiation in air-saturated toluene at room temperature.
总结及展望
该研究成功发展了一类兼具优异光稳定性和光谱调谐能力的多功能DS-BDP小分子材料，攻克了近红外二区光子上转换长期依赖不稳定并四苯类材料的瓶颈。通过精细的化学结构微调，不仅显著优化了量子点与配体间的界面电子耦合，还实现了上转换发射颜色的自由裁量。这一重大突破极大地推进了三线态上转换材料在太阳能转化、红外光电探测以及深层组织生物成像等前沿实用领域的产业化落地进程。