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Lanthanide-doped inorganic nanoparticles turn molecular triplet excitons bright

Abstract

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 emission1, optoelectronics2,3, photon frequency conversion4,5 and photocatalysis6,7. 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 levels8. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin–orbit coupling9,10,11 or tuning of the singlet–triplet energy splitting12,13 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.

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Fig. 1: Lanthanide-nanocrystal-coupled triplet excitation.
Fig. 2: Ultrafast intersystem crossing in organic molecules coupled to lanthanide-doped nanoparticles.
Fig. 3: Triplet energy transfer from molecules to nanoparticles.
Fig. 4: Lanthanide–triplet excitation fusion upconversion in nanoparticle–molecule blends.

Data availability

The data underlying all figures in the main text and Supplementary Information are publicly available from the University of Cambridge repository at https://doi.org/10.17863/CAM.59063.

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Acknowledgements

This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 758826) and from Marie Skłodowska-Curie grant agreements nos 797619 (TET-Lanthanide project), 748042 (MILORD project) and 646176 (EXTMOS project). We acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability, the Singapore Ministry of Education (grant MOE2017-T2-2-110), the Singapore Agency for Science, Technology and Research (grant A1883c0011), and the National Research Foundation, Prime Minister’s Office, Singapore under the NRF Investigatorship programme (award no. NRF-NRFI05-2019-0003). R.D. acknowledges support from the National Natural Science Foundation of China (grant 51872256) and the Zhejiang Provincial Natural Science Foundation of China (grant LR19B010002). Computational resources were provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifique de Belgique (FRS-FNRS) under grant no. 2.5020.11, and by the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, which is infrastructure funded by the Walloon Region under grant agreement no. 1117545. L.N. acknowledges support from the Jardine Foundation. J.Z. thanks the China Scholarship Council for a PhD scholarship (no. 201503170255). S.A. acknowledges financial support from DST-UKIERI (DST/INT/UK/P-167/2017) and SERB-ECRA (ECR/2018/002056).

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Contributions

S.H., R.D. and A.R. designed the experiments. S.H., R.D., Z.Y. and B.Z. performed nanocrystal synthesis and film preparation. S.H., R.D., L.N., U.H., A.S. and S.A. carried out spectroscopic measurements. S.H., Q.G. and J.Z. contributed to transient absorption experiments and data analysis under the supervision of A.B. and A.R. H.T., A.P. and D.B. carried out theoretical calculations. H.X. prepared organic molecules. Z.H. prepared tetracene derivatives under the supervision of M.L.T. M.A.-J. and A.S. performed PDS measurements. S.H., R.D., X.L. and A.R. wrote the manuscript. X.L. and A.R. supervised the project. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Renren Deng, Xiaogang Liu or Akshay Rao.

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Peer review information Nature thanks Jiajia Zhou and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

This file contains Supplementary Figures 1-54 and Supplementary Tables 1-3.

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Han, S., Deng, R., Gu, Q. et al. Lanthanide-doped inorganic nanoparticles turn molecular triplet excitons bright. Nature 587, 594–599 (2020). https://doi.org/10.1038/s41586-020-2932-2

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