Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States.
Department of Chemistry, University of California-Riverside, Riverside, California 92521, United States.
Acc Chem Res. 2021 Jan 5;54(1):70-80. doi: 10.1021/acs.accounts.0c00526. Epub 2020 Nov 3.
The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet-triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet-triplet energy losses compared to metal-ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step's efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems.This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 and TET2 efficiencies. The agreement between calculated and measured quantum efficiencies suggests a firm understanding of QD-sensitized photon upconversion. We believe the above conclusions are general and should be widely applicable to similar systems, including singlet fission in hybrid organic-nanocrystal materials.
半导体-纳米晶敏化的三组分上转换系统在过去 5 年中取得了重大进展。这三个组件（即三重态光敏剂、介体和发射器）各自在确定输入和输出光子能量以及整体量子效率（QE）方面发挥着关键作用。纳米晶敏化剂通过系间窜越将吸收的光子转化为单重态激子，然后转化为三重态激子。介体通过直接德克斯特型三重态能量转移（TET）或顺序电荷转移（CT）接受三重态激子，同时延长激子寿命。通过从中介体到发射器的第二个三重态能量转移步骤，后者被填充到其最低激发三重态。两个三重态发射器之间的三重态-三重态湮灭（TTA）产生处于明亮单重态的发射器，然后该发射器发出上转换光子。与金属配体电荷转移配合物相比，量子点（QD）具有可调谐的能带隙、大消光系数和较小的单重态-三重态能量损失。这种高三重态激子产率使 QD 成为光敏剂的良好候选者。就驱动三重态能量转移而言，对于具有最小能量损失的下坡能景观，介体的三重态能量应略低于 QD 敏化剂的三重态激子能量。从中介体到发射器的能量级联也需要相同的能量。最后，发射器的三重态能量必须略大于其单线态能量的一半，以确保 TTA 是放热的。敏化剂、介体和发射器的优化将导致反斯托克斯位移和总量子效率的增加。为了理解现有系统中的限制因素，有必要评估每个单独步骤的效率和动力学。本综述总结了基于硫属化物量子点的光上转换系统，重点介绍了 Tang 和 Lian 小组进行的三重态能量转移的机制方面。通过时间分辨光谱，确定了与两个三重态能量转移步骤相关的速率和主要损耗途径。这些研究集中在近红外（NIR）到可见（VIS）PbS-并五苯基系统上，因为它们允许系统地控制 QD、介体和发射器。我们的结果表明，介体三重态主要是由 QD 的直接 TET 形成的，并且转移速率受结合介体分子的密度影响。电荷转移是一种损耗途径，不会产生三重态激子，可以通过向 QD 添加惰性壳来最小化。该转移速率随 QD 和介体分子之间的距离呈指数下降。发现第二个 TET 速率比扩散限制的碰撞速率慢得多，这导致介体的三重态寿命成为限制其效率的主要因素。最后，使用这些测量量（包括 TET1 和 TET2 效率）可以计算总量子效率。测量和计算量子效率之间的一致性表明对 QD 敏化的光子上转换有了牢固的理解。我们相信上述结论是普遍的，应该广泛适用于包括混合有机-纳米晶材料中的单线态裂变在内的类似系统。
