The Construction And Detection Imaging Properties Of β-Diketone-Based Rare Earth Coordination Molecular Cages

Rare earth coordination molecular cages（RECMCs）are formed by coordination-directed self-assembly between multi-rare earth ions(RE³⁺)and organic ligands（L）,showing cage-like three-dimensional structures.As one of the hotspots in supramolecular coordination chemistry,how to obtain superior functional RECMCs with high stability through the design and synthesis of ligands and complexes is still the key issue for us.It is very challenging for the synthesis of highly stable and multifunctional RECMCs due to the high coordination numbers,oxygen affinity of RE³⁺ions,and the steric hindrance of ligands.Considering the effectively sensitizing ability ofβ-diketone ligands toward RE³⁺ions,we select them as the main ligands of RE³⁺ions to construct RECMCs for the investigation of assembling conditions,structure,and properties,especially for the applications of RECMCs as bioimaging and sensing probes.The detail studies are as follows.Firstly,based on the"face-capped"assembly strategy,a C₃-symmetrical tri-β-diketone ligand L¹ was synthesized and surface-oriented coordinated with RE³⁺ions as a triangular“inlay”,forming a series of RECMCs of RE₄L₄¹,RE³⁺=Y³⁺,Eu³⁺and Gd³⁺.In light of the optical and magnetic advantages of Eu³⁺and Gd³⁺ions,the obtained RECMCs achieved both good cell optical imaging（COI）and magnetic resonance imaging（MRI）abilities.It was found that the luminescent Eu₄L₄¹ cage showed a high quantum yield（QY）of 29%in DMSO and good cell imaging ability;The paramagnetic Gd₄L₄¹ cage achieved a high longitudinal relaxation rate r₁ value of 11.4 m M^-1S^-1 in an aqueous solution containing 1%（v%）DMSO,which is higher than that of the commercial contrast agents Gd-DOTA due to the strong paramagnetism of Gd³⁺ions,large molecular weight,the strong structural rigidity,and the ability to accept outer-sphere water molecules of the cage.Further,we have successfully achieved Eu³⁺/Gd³⁺heteronuclear tetrahedral cages with strong COI and MRI signals by tuning the mole ratios of Eu³⁺/Gd³⁺.This is the firstβ-diketone-based tetrahedral cage RECMCs with good COI-MRI dual mode.Secondly,to improve the optical properties and structural stability,（RE₄L₄¹）（phen）₄ and（RE₄L₄¹）（+/-tpy）₄ cages with tetrahedral structural and enhancing luminescent performance were obtained by introducing multi-dentate auxiliary ligands of 1,10-phenanthroline（phen）and terpyridine derivative chiral ligands（（+/-）-tpy）to the RE₄L₄¹ cages above.A higher QY value of 68%is achieved for（Eu₄L₄¹）（+/-tpy）₄ in CHCl₃.Besides,the chiral tetrahedral cage of（Eu₄L₄¹）（+/-tpy）₄ had circular dichroism（CD）absorption and circular polarization luminescence（CPL）activity.Notably,（Eu₄L₄¹）（phen）₄ is the first example of tetrahedral RECMCs with high structural stability and good optical properties with a high QY value of 41%in toluene.The enhancement of QY in（Eu₄L₄¹）（phen）₄ is contributed to the energy transfer process from the ligands to Eu³⁺ions promoted by phen as an auxiliary ligand.The strongπ-πstacking interactions between（RE₄L₄¹）（phen）₄ cages are beneficial to improve structural stability,avoiding structural collapse after soaking in air,water,and even harsh conditions like acid/base solutions.Besides,the composite material（Eu₄L₄¹）（phen）₄@F127obtained by coating（Eu₄L₄¹）（phen）₄ with amphiphilic polymer F127 was demonstrated to be low cytotoxicity and significant cell imaging abilities.Thirdly,based on the“edge-directed”assembly strategy,a C₂-symmetrical bi-β-diketone ligand L² with flexible spiral chain was synthesized and edge-directed coordinated to RE³⁺ions with auxiliary ligand phen.A series of stable helicate RECMCs(RE³⁺=La³⁺,Eu^3+,and Gd³⁺)were constructed.The excellent luminescence of（Eu₂L₃²）（phen）₂ in solution could be quenched selectively by Fe³⁺and Cu²⁺ions with a low detection limit and good anti-interference ability due to its open cavities.However,only the quenched luminescence of（Eu₂L₃²）（phen）₂ by Fe³⁺can be restored by adding triethylamine（TEA）,which can be used as a fluorescent sensing probe both for metal ions and organic amines.This is the first example of luminescent RECMCs used to recognize organic amines such as TEA in solution through the"on-off-on"mode using Fe³⁺as a quenching medium.Besides,the composite material（Eu₂L₃²）（phen）₂@F127 obtained by coating（Eu₂L₃²）（phen）₂ with amphiphilic polymer F127 showed improving dispersion and luminescence properties in water and also can be used as a fluorescent sensing probe for Fe³⁺ions.In summary,based on the self-assembly of multi-β-diketone ligands with RE³⁺ions,the presupposed tetrahedral and helicate cages can be constructed.Owing to the steric hindrance effect of ligands,the high coordination numbers of RE³⁺ions of RECMCs need to be satisfied by solvent molecules.Meanwhile,the rigidity and high molecule weight of RECMCs could synergistically contribute to a high longitudinal relaxation rate.Therefore,multifunctional imaging RECMCs with both high luminescence and high relaxation efficiency can be obtained.The introduction of a second strong ligand such as phen can replace the weak coordination solvent molecules in RECMCs,and further improve the stability of the cage structure and the luminescence efficiency of RE³⁺ions.Besides,the polymer material coating strategy can be an effective way to improve structural rigidity and optical properties and expand their applications in the bioimaging and detection field.These positive results provide a scientific basis for the structural design and functional strengthening of RECMCs and have important theoretical and practical application values.