The team of Xu Haibing and Zeng Minghua of Hubei University have made important progress in the study of high efficient upconversion luminescence of rare earth complexes.

The directional design of up-conversion luminescent (UCL) rare earth complex molecular-based materials will probably overcome the surface quenching effect of rare earth nanoparticles, and help to achieve controllable preparation in the application, and then contribute to the future application in deep biological imaging and even accurate diagnosis and treatment. Among them, how to obtain discrete rare earth low nuclear clusters with good solubility and high luminous quantum yield (UC) is one of the frontiers of this kind of research.

The key to the realization of high efficient rare earth UCL system depends on the energy transfer efficiency (η) between sensitizer (S) and activator (A). Therefore, not only to match the energy levels of S and A, but also to control their distance, ratio and the coordination number of saturated rare earth ions, but also to fine tune the excited photon transition pathway and the competition between UC and DL, which is obviously a comprehensive challenge based on the precise correlation between structure-activity and energy transfer. Due to the high energy XH (X = C, N, O) vibrations of organic ligands and solvent molecules directly coordinated with LnIII, it is easy to quench the excited state energy of LnIII, and the absorption cross section (σ abs) of rare earths is low, which leads to the slow progress in the study of UCL rare earth complexes which can be used in solution systems. Since it was first reported in 2011, even under harsh conditions such as deuterated solvent, low temperature and high P (definition), more than a dozen known rare earth complexes UC are still less than 0.1%.

A few days ago, Professor Xu Haibing and Professor Zeng Minghua of the School of Chemistry and Chemical Engineering of Hubei University, through different rare earth photosensitizers (S) and activator (A) blocks matched by energy levels, used the multiple coordination modes of bridging ligands to shorten the distance between them, adjust the ratio of them, and then reduce the non-radiative transition through fluoride ions and regulate the competition of excited photons in the up-conversion (UC) and down-conversion (DL) process. At low power density (P = 2 W/cm2), the UC of discrete rare earth complexes in non-deuterated solution at room temperature is 20 times higher than that recorded in the literature. The related research results, entitled "Discrete Heteropolynuclear Yb/Er Assemblies: Switching on Molecular Upconversion under Mild Conditions", were published in Angew., one of the top international chemical journals, on August 12, 2021, Beijing time. Chem. Int. Ed. Go up

In this study, the team designed and used the multiple coordination modes of tridentate H2hmq ligands to assemble the hetero-rare earth blocks with energy level matching, and controlled the distance between them within 3.7.Then the ratio of them was adjusted, the η of S was increased, and the σ abs of An in the near infrared region was enhanced by multiple levels of S matching. Furthermore, fluoride ions were introduced to limit the low-energy non-radiative energy consumption around LnIII. The UCL signal of 1 ([Yb2Er] +) in organic solvents at room temperature was observed for the first time under low excitation of P = 0.288 W/cm2. With the help of P = 2W C7H8 cm2 and fluoride ion, the amount of potassium UC in room temperature C7H8 reached a record high of 2.29% (Red-PM) / 2.78% (NIR-PMT).

To sum up, through the assembly of different rare earth blocks matched by energy levels using multiple coordination modes of bridged ligands, hetero-rare earth complexes with clear structure, adjustable S and A distance and adjustable molar ratio can be obtained. The comprehensive optimization conditions adjust the competition between linear attenuation and upconversion, which provides a new idea for the design of high efficiency rare earth complex molecular UCL materials with mild conditions, conventional solution and low power excitation.

Wang Jie and Jiang Yue, graduate students from the School of Chemical Engineering, Hubei University, are the first authors of this article.