Cavendish Laboratory, University of Cambridge, Cambridge, UK.
Institute for Composites Science Innovation, School of Materials Science and Engineering, Zhejiang University, Hangzhou, China.
Nature. 2020 Nov;587(7835):594-599. doi: 10.1038/s41586-020-2932-2. Epub 2020 Nov 25.
The generation, control and transfer of triplet excitons in molecular and hybrid systems is of great interest owing to their long lifetime and diffusion length in both solid-state and solution phase systems, and to their applications in light emission, optoelectronics, photon frequency conversion and photocatalysis. Molecular triplet excitons (bound electron-hole pairs) are 'dark states' because of the forbidden nature of the direct optical transition between the spin-zero ground state and the spin-one triplet levels. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin-orbit coupling or tuning of the singlet-triplet energy splitting via molecular design. Both these methods place constraints on the range of properties that can be modified and the molecular structures that can be used. Here we demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles. This allows the classically forbidden transitions from the ground-state singlet to excited-state triplets to gain oscillator strength, enabling triplets to be directly generated on molecules via photon absorption. Photogenerated singlet excitons can be converted to triplet excitons on sub-10-picosecond timescales with unity efficiency by intersystem crossing. Triplet exciton states of the molecules can undergo energy transfer to the lanthanide ions with unity efficiency, which allows us to achieve luminescent harvesting of the dark triplet excitons. Furthermore, we demonstrate that the triplet excitons generated in the lanthanide nanoparticle-molecule hybrid systems by near-infrared photoexcitation can undergo efficient upconversion via a lanthanide-triplet excitation fusion process: this process enables endothermic upconversion and allows efficient upconversion from near-infrared to visible frequencies in the solid state. These results provide a new way to control triplet excitons, which is essential for many fields of optoelectronic and biomedical research.
在分子和混合体系中，三重态激子的产生、控制和转移引起了人们的极大兴趣，因为它们在固态和溶液相体系中的寿命和扩散长度都很长，并且在发光、光电、光子频率转换和光催化等方面有应用。分子三重态激子（束缚电子-空穴对）是“暗态”，因为在自旋为零的基态和自旋为一的三重态能级之间，直接光跃迁是被禁止的。因此，三重态动力学通常通过基于重金属的自旋轨道耦合或通过分子设计来调节单线态-三重态能量分裂来控制。这两种方法都限制了可以修改的性质范围和可以使用的分子结构。在这里，我们证明通过将有机分子与镧系掺杂的无机绝缘纳米粒子耦合，可以控制三重态动力学。这使得从基态单重态到激发三重态的经典禁戒跃迁获得了振子强度，从而使三重态能够通过光子吸收直接在分子上产生。通过系间窜越，单重态激子可以在亚 10 皮秒的时间尺度内以 100%的效率转化为三重态激子。分子的三重态激子态可以以 100%的效率与镧系离子进行能量转移，这使我们能够实现暗三重态激子的发光收集。此外，我们证明了在镧系纳米粒子-分子杂化体系中，通过近红外光激发产生的三重态激子可以通过镧系-三重态激发融合过程有效地进行上转换：该过程能够进行吸热上转换，并允许在固态中从近红外到可见频率的高效上转换。这些结果为控制三重态激子提供了一种新方法，这对光电和生物医学研究的许多领域都是至关重要的。
