Fluolab【JACS】1/3自旋统计优势：单分子双重态-三重态湮灭开启固态上转换新路径
文章标题： Unimolecular Doublet-Triplet Annihilation Upconversion in Iron(III) Coordination Compounds
通讯作者： Florian Doettinger, Oliver S. Wenger
文章概要
光子上转换技术能够将低能量的光转化为高能量光，在光伏、光催化以及生物成像等领域具有巨大的应用潜力。传统的敏化三重态-三重态湮灭上转换（sTTA-UC）通常依赖于敏化剂和湮灭剂分子之间的扩散与碰撞，这使得该技术在固态介质中的效率大打折扣。巴塞尔大学的 Oliver S. Wenger 团队在《美国化学会志》（JACS）上发表了突破性研究，提出了一种基于铁(III)配合物的新型单分子双重态-三重态湮灭（DTA） 机制。这种方法彻底摆脱了对分子扩散的依赖，不仅在液体溶液中表现优异，甚至在固态聚合物和低温玻璃态中也能实现高效的光子上转换。
Scheme 1. Schematic Representation of the Simplified Mechanisms of Heteromolecular TTA and the Proposed DTA
引言
目前主流的sTTA-UC机制涉及极其复杂的多个双分子步骤，包括系间窜越（ISC）以及敏化剂与湮灭剂之间的三重态能量转移（TTET）。由于这些过程本质上是受扩散控制的，因此在大型固态器件的应用中面临易燃、易挥发和机械稳定性差等挑战。为了攻克这一难题，研究人员开始探索将敏化剂与湮灭剂共价连接在同一个分子内。然而，传统的闭壳层体系即便实现了共价连接，往往仍需两个分子碰撞才能完成湮灭过程。本文研究者独辟蹊径，利用具有未成对电子的铁(III)配合物作为核心，通过其独特的开壳层电子结构引入了双重态（Doublet） 参与湮灭，从而在基础原理上为实现真正的单分子上转换提供了可能。
Figure 1. Molecular structures of the complexes [FeIII(ImPPer)2]+ (FePer2) and [FeIII(ImPP)2]+ (FeRef) which were reported previously. In this work, the perylene-decorated iron(III) complexes [FeIII(ImPPer2)2]+ (FePer4) and [FeIII(ImPP)(ImPPer)]+ (FePer1) are studied. As schematically illustrated, intramolecular TTA is more likely to occur in FePer4 compared to FePer2. In FePer1, intramolecular TTA is excluded by design. The corresponding counterions are omitted.
主要实验及结论
研究团队精心设计并合成了一系列含有不同数量苝（Perylene）取代基的铁(III)卡宾配合物，分别命名为FePer1、FePer2和FePer4。实验结果显示，这些分子在635 nm的红光激发下，通过配体到金属的电荷转移（2LMCT）态吸收能量，随后经过极快的内层双重态-三重态能量转移，将能量传递至外围的苝单元，使其处于长寿命的三重态。关键的创新点在于，当分子内的铁中心再次被激发并处于双重态时，它能与同一个分子内已处于三重态的苝单元发生双重态-三重态湮灭（DTA）。这种湮灭过程不仅是非扩散依赖的，而且在统计学上具有显著优势：DTA的自旋统计因子为1/3，远高于传统TTA机制的1/9。
Figure 2. Mechanistic comparison between (a) the excited state reactivity of doublet-excited CrIII complex with triplet ground state dioxygen via Dexter-type energy transfer (DET) which is well studied (94−99) and (b) the proposed doublet-triplet annihilation (DTA) between a doublet-excited FeIII and triplet-excited perylene (Per.). In both processes, the total spins before and after electron exchange are conserved (_S_tot = 3/2 or 1/2), rendering them spin-allowed.
Figure 3. (a) UV–vis absorption spectra of FePer1–4 recorded in DCM at room temperature shown in blue, green, and red, respectively. (b) Steady state photoluminescence spectra of FePer1–4 recorded in dry, deaerated DCM at room temperature. Left: 1LC fluorescence of FePer1–4 after excitation at 422 nm. Right: 2LMCT luminescence of FePer1–4 (scaled by a factor of 5 relative to FeRef) after excitation at 635 nm. The earlier reported spectra of FePer2, FeRef (black) and perylene (gray, scaled by a factor of 0.02) are shown as references.
通过对不同浓度下上转换发光强度的定量分析，研究人员证实了发光强度与浓度呈线性关系，这有力地证明了湮灭过程发生在单个分子内部，而非分子间的碰撞。为了进一步验证其环境适应性，研究者将这些配合物掺杂进PMMA薄膜以及77 K的低温玻璃态溶剂中。实验观察到，在这些分子运动几乎完全被锁死的固体介质中，依然能够清晰地观测到由红光激发的蓝绿色上转换荧光。这一现象确证了DTA机制在固态环境中的可行性。此外，对比实验发现，随着苝单元数量的增加，虽然分子内的能量传递效率有所提升，但单分子DTA机制在仅含一个苝单元的FePer1中表现得最为典型且纯粹。
Figure 4. (a) Nanosecond transient absorption spectra of FePer1–4. The 3LC spectrum of perylene (73) shifted manually by + 20 nm is included as reference. The comparison confirms the population of a long-lived perylene-localized 3LC state. (b) Kinetic traces corresponding to the intramolecular DTET process obtained from global fit analysis of the femtosecond transient absorption spectra of FePer1–4 recorded in dry aerated DCM (for details concerning these measurements incl. the respective spectra see SI Section 10). (c) Nanosecond transient absorption kinetic traces of the 3LC states of FePer1–4 recorded at 510 nm. (d) Time-correlated single photon counting (TCSPC) data of the 2LMCT luminescence recorded at 720 nm. Due to low signal intensities, the time point of the excitation was set to −0.5 ns, outside of the detection frame. The spectra of FePer2 were reported earlier. All spectra were collected in deaerated, dry DCM at room temperature after pulsed 635 nm excitation, unless specified.
Figure 5. Upconverted 1LC emission of (a) FePer1 and (b) FePer4 recorded after excitation at 635 nm with a continuous wave laser in dry, deaerated DCM solution at room temperature. The gray spectra correspond to the prompt 1LC fluorescence spectra of perylene measured after direct 1LC excitation at 422 nm. (c) Corresponding double-logarithmic plots of the integrated 1LC luminescence intensity of the FePer1–4 compounds (blue triangles, green circles, red squares, respectively). The respective dashed lines represent linear fits revealing slopes of ∼2 which indicates a biphotonic (homomolecular) upconversion process in all cases. Integration wavelength range: 490–540 nm. (d) Double-logarithmic plots of the integrated prompt 2LMCT emissions of FePer1–4 (for data see SI Figures S11.1). The respective dashed lines represent linear fits revealing slopes of 1 which indicates monophotonic processes in all cases.
- Figure 6. (a) Normalized integrated 1LC upconversion luminescence intensity recorded at various sample concentrations after excitation at 635 nm with a constant excitation power density of P/A of 160 W × cm–2 in deaerated, dry DCM at room temperature. The plots indicate a linear relation between concentration and upconversion luminescence intensity and were arbitrarily normalized at a sample concentration of 100 μM. To minimize inner filter effects, the integrated intensities were corrected for the sample absorbances in the integration range between 490 and 540 nm. (b) Double-logarithmic plot of the integrated 1LC upconversion luminescence intensity of FePer1–4 in a frozen DCM/MeOH = 1/1 matrix at 77 K (_c_FePer1 = 130 μM, _c_FePer2 = 100 μM, _c_FePer4 = 90 μM).
总结及展望
这项工作不仅展示了铁(III)配合物在光物理领域的巨大潜力，更重要的是开创了单分子双重态-三重态湮灭（DTA） 这一全新领域。该机制通过消除系间窜越和双分子能量转移带来的能量损耗，极大地精简了上转换的物理过程，并显著提升了自旋统计上限。这种扩散无关的特性为未来开发高性能的固态上转换器件、防伪油墨以及深层生物组织成像提供了全新的设计思路。未来，研究者将致力于进一步优化这类开壳层体系的激发态寿命和吸收截面，以期在太阳光辐照条件下实现更高效的单分子能量上转换。