Chiral metal-organic frameworks (CMOFs) represent ideal platforms for circularly polarized luminescence (CPL). Incorporating rare-earth ions into MOFs enhances their luminescence via the antenna effect. Chiral imidazolium
carboxylic ligands serve as excellent chiral building blocks for CMOFs. These ligands not only sensitize lanthanide ion emission through the antenna effect but also transfer chirality to the framework through coordination, enabling CPL. Their cationic nature imparts a positive charge to the framework channels, facilitating the immobilization of anionic guests through host-guest interactions. This enables modulation and optimization of CPL performance via energy transfer between the chiral donor and luminescent acceptor.
A team led by Jian Zhang at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, synthesized enantiopure terbium-based CMOFs through the direct assembly of homochiral imidazolium carboxylic acids with Tb3+ ions. The resulting frameworks exhibited significant CPL activity, with a luminescence dissymmetry factor (|glum|) of 0.016. This performance is attributed to efficient chirality transfer and sensitization achieved via the antenna effect. Remarkable enhancement of the CPL performance was observed following the incorporation of luminescent MnCl42- anions as guests within this chiral framework. This yielded a |glum| value of 0.071—representing one of the highest values reported to date for rare-earth-containing CMOF-based CPL systems. This work presents a rare example demonstrating the amplification of CPL in rare-earth-based CMOFs through the combined approach of framework design and a host-guest strategy. Furthermore, it establishes a general strategy for modulating and enhancing chiral optical properties via modular molecular engineering.
The team published their research article in the journal Polyoxometalates on July 11, 2025.
“In this work, we obtained colorless rod-like chiral crystals L-Tb-1 and D-Tb-1 through the coordination of homochiral ligands—1,3-bis((S)- and (R)-1-carboxyethyl)-1H-imidazol-3-ium chlorides (L- and D-[(H2DPA)+Cl⁻]) with Tb(NO3)3·5H2O. The introduction of MnCl2 into the reaction system led to the formation of a new pair of enantiomeric host-guest materials—L-Tb-2∙(MnCl42-) and D-Tb-2∙(MnCl42-). The CD signals of L-Tb-1 and D-Tb-1 are consistent with the absorption regions of the chiral L/D-[(H2DPA)+Cl⁻] ligands, indicating that the chirality of the ligands is transferred through metal coordination, gradually forming chiral helical chains, two-dimensional layers, and three-dimensional frameworks. As a result, the metal centers are endowed with a rich chiral environment. An additional Cotton signal at 225 nm observed in the host-guest systems is presumably attributed to the chiral transfer between the MOF frameworks and MnCl42- guests via hydrogen bonding, which confirms the encapsulation and host-guest interactions at the chiral optical level,” said Professor Jian Zhang, the corresponding author of the study. He serves as Deputy Secretary of the Party Committee at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and concurrently holds the position of Deputy Director of the State Key Laboratory of Structural Chemistry.
The team measured clear CPL signals for L-Tb-1 under 370 nm excitation, with corresponding |glum| values—0.016 at 489 nm, 0.013 at 544 nm, 0.011 at 586 nm, and 0.006 at 621 nm—demonstrating the effective transfer of molecular chirality from the homochiral ligand to the extended framework. L-Tb-2∙(MnCl42−) exhibits a notably higher CPL activity, featuring |glum| values (0.071 at 519 nm and 0.030 at 545 nm) more than four times higher than those of L-Tb-1. “This dramatic amplification is attributed to synergistic interactions between the chiral Tb3+ framework and MnCl42− guest species, which facilitate energy transfer and promote the generation of circularly polarized emission. These findings underscore the effectiveness of integrating chirality transfer and host–guest energy transfer within a single MOF platform as a powerful strategy for tuning and enhancing the CPL performance of rare-earth-based CMOFs,” Professor Jian Zhang said.
To further elucidate the origin of CPL enhancement in L-Tb-2∙(MnCl42−) and verify the role of host–guest energy transfer, the team synthesized and characterized two isostructural control compounds, namely L-Tb-2∙(ZnCl42−) and L-Gd-2∙(MnCl42−). The emission spectrum of L-Tb-2∙(ZnCl42−) strongly overlaps with the absorption profile of L-Gd-2∙(MnCl42−), which indicates the feasibility of energy transfer from the chiral Tb3+ host to the MnCl42− guest in L-Tb-2∙(MnCl42−). The donor–acceptor distance of < 10 nm satisfies the requirements for the Förster resonance energy transfer mechanism. “This work not only provides a rare example of CPL amplification in rare-earth-based CMOFs through modular host–guest engineering but also offers a generalizable design principle for tuning the chiroptical performance of porous materials.” Professor Jian Zhang said.
Other contributors include Tong Hao, Bei Xu, Xinchao Wang, Hongrui Zheng, Shangda Li, and Fei Wang from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.
This work was supported by the National Key R&D Program of China (No. 2022YFA1503300) and the National Natural Science Foundation of China (Nos. 21971241 and 22471273).
About the Authors
Dr. Wang Fei obtained his Ph.D. in Physical Chemistry in 2009 from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Supervisor: Prof. Canzhong Lu), and subsequently joined Prof. Jian Zhang's research group. His research focuses on the design and synthesis of metal-organic frameworks (MOFs). He has made a series of significant advances in areas including Chiral metal-organic frameworks; Titanium-based metal-organic frameworks (Ti-MOFs); Zeolite-type metal-organic frameworks (ZMOFs); Applications of these materials in chiral resolution, asymmetric catalysis, photoelectrocatalysis, and gas separation. He has published over 100 papers as first author and corresponding author. Research Group Website: https://www.fjirsm.ac.cn/zhangjian/
About Polyoxometalates
Polyoxometalates (ISSN 2957-9821) is a peer-reviewed (single-blind), open-access and interdisciplinary journal, sponsored by Tsinghua University. Polyoxometalates publishes original high-quality research papers and significant review articles that focus on cutting-edge advancements in Polyoxometalates, and clusters of metals, metal oxides and chalcogenides. Rapid review to ensure quick publication is a key feature of Polyoxometalates. It is indexed by ESCI, Scopus (CiteScore 2024 = 14.7), Ei Compendex, CAS, and DOAJ.
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