Hierarchical self-assembly of metal-organic supramolecular fibers with lanthanide-derived functionalities

Achieving organized assembly structures with high complexity and adjustable functionalities is a central quest in supramolecular chemistry. In this report, we study what happens when a discotic benzene-1,3,5-tricarboxamide (BTA) ligand containing three dipicolinic acid (DPA) groups is allowed to coordinate with lanthanide (Ln) ions. A multi-BTA coordination cluster forms, which behaves as a type of "supramolecular monomer", stacking into fibers via hydrogen bonds enabled by multiple BTA cores. The fibrous morphology and size, as well as the packing unit and the process by which it grows, were investigated by light scattering measurements, luminescence spectra, TEM images and molecular simulation data. More notably, by selecting the kind of lanthanide or mixture of lanthanides that is incorporated, tunable luminescence and magnetic relaxation properties without compromising the fibrous structure can be realized. This case of hierarchical self-assembly is made possible by the special structure of our BTA-like building block, which makes non-covalent bond types that are different along the radial (coordination bonds) and axial (H-bonds) directions, respectively, each with just the right strength. Moreover, the use of lanthanide coordination leads to materials with metal-derived optical and magnetic properties. Therefore, the established approach demonstrates a novel strategy for designing and fabrication of multi-functional supramolecular materials.

A versatile molecular logic system based on Eu(III) coordination polymer film electrodes combined with multiple properties of NADH

Herein, a new type of lanthanide coordination polymer film made up of europium (Eu(iii)) and poly(N-methacryloylglycine) (Eu(iii)-PMAG) was prepared on an ITO electrode surface driven by the coordination between N-methacryloylglycine (MAG) and Eu(iii) through a single-step polymerization process. The fluorescence signal of Eu(iii)-PMAG films at 617 nm originating from Eu(iii) could be well retained in the buffer solution but was regulated by the concentration of Cu(ii) and the complexing agent EDTA. The switching of fluorescence by Cu(ii) was attributed to the inhibition of the "antenna effect" between Eu(iii) and the MAG ligand in the films. The coexistence of reduced β-nicotinamide adenine dinucleotide (NADH) in the solution can apparently quench the fluorescence of Eu(iii)-PMAG films through the internal filtration effect of UV absorbance overlapping the excitation wavelength, but itself exhibiting a fluorescence emission at 468 nm. In addition, the electrocatalytic oxidation of NADH with the help of the ferrocenedicarboxylic acid (FcDA) probe demonstrated a cyclic voltammetry (CV) signal at 0.45 V (vs. SCE). Based on various reversible stimulus-responsive behaviours, a 4-input/10-output logic network was built using Cu(ii), EDTA, NADH and FcDA as inputs and the signals of fluorescence from Eu(iii)-PMAG (617 nm) and NADH (468 nm), the CV response from FcDA and the UV-vis absorbance from the Cu(ii)-EDTA complex as outputs. Meanwhile, 6 different functional logic devices were constructed based on the same versatile platform, including a 2-to-1 encoder, a 1-to-2 decoder, a 1-to-2 demultiplexer, a parity checker, a transfer gate and a reprogrammable 3-input/2-output keypad lock. Combined with the new type of lanthanide coordination polymer film, NADH played central roles in designing sophisticated computing systems with its fluorescence, UV and electrocatalytic properties. This work might provide a novel avenue to develop intelligent multi-analyte sensing and information processing at the molecular level based on one single platform.

Putting Ink into Polyion Micelles: Full-Color Anticounterfeiting with Water/Organic Solvent Dual Resistance

Anticounterfeiting paintings are usually with limited colors and easy blurring and need to be dispersed in an environmentally unfriendly organic solvent. We report a set of water-based polyion micellar inks to solve all these problems. Upon complexation of reversible coordination polymers formed with rare earth metal ions Eu³⁺ and Tb³⁺ and the aggregation-induced emission ligand tetraphenylethylene-L₂EO₄ with oppositely charged block polyelectrolyte P2MVP₂₉-b-PEO₂₀₅, we are able to generate polyion micelles displaying three elementary emission colors of red (R) (Φ_Eu³⁺ = 24%), green (G) (Φ_Tb³⁺ = 7%), and blue (B) (Φ_TPE = 9%). Full-spectrum emission and white light emission (0.34, 0.34) become possible by simply mixing the R, G, and B micelles at the desired fraction. Strikingly, the micellar inks remain stable even after soaking in water or organic solvents (ethyl acetate, ethanol, etc.) for 24 h. We envision that polyion micelles would open a new paradigm in the field of anticounterfeiting.

Response of metal-coordination-based polyelectrolyte complex micelles to added ligands and metals

Polyelectrolyte complex based micelles have attracted significant attention due to their potential regarding bio-applications. Although the morphology and functions have been studied extensively, dynamic properties, particularly component exchange with other surrounding molecules, have remained elusive to date. Here, we show how micelles based on metal-ligand coordination complex coacervate-core micelles (M-C3Ms) respond to addition of extra ligand and metal ions. The micelles are prepared from a polycationic-neutral diblock copolymer and an anionic coordination polyelectrolyte, which is obtained by coordination between metal ions (lanthanides Ln³⁺ and Zn²⁺) and a bis-ligand (LEO) containing two dipicolinic acid (DPA) groups connected by a tetra-ethylene oxide spacer (4EO). Our findings show that the bis-ligand LEO is essential for the growth of coordination polymers and consequently the formation of micelles, leading to equilibrium structures with the same micellar composition and structure independent of the order of mixing. In other words, adding single DPA has no effect on the formed M-C3Ms. As for metal exchange, we find that added Zn²⁺ can replace some of the Ln³⁺ from Ln-C3Ms, leading to a hybrid coordination structure with both Ln³⁺ and Zn²⁺. We find that component exchange occurs in these coordination polyelectrolyte micelles, but it is more favorable in the direction of replacing the weak binding components with strong ones. Hence, the designed M-C3Ms based on the strong binding components, such as Ln-C3Ms, shall be relatively stable in biological surroundings, paving the way for the application of such particles as bio-imaging probes.

The effect of weighted averages when determining the speciation and structure–property relationships of europium(iii) dipicolinate complexes

Europium(iii) dipicolinate complexes have been a model system in lanthanide solution chemistry for decades, here it is investigated in unprecedented detail.

Kinetic trapping - a strategy for directing the self-assembly of unique functional nanostructures

Supramolecular self-assembly into various nano- or microscopic structures based on non-covalent interactions between molecules has been recognized as a very efficient approach that leads to functional materials. Since most non-covalent interactions are relatively weak and form and break without significant activation barriers, the thermodynamic equilibrium of many supramolecular systems can be easily influenced by processing pathways that allow the system to stay in a kinetically trapped state. Thus far, kinetic traps have been found to be very important in producing more elaborate structural and functional diversity of self-assembled systems. In this review, we try to summarize the approaches that can produce kinetically trapped self-assemblies based on examples made by us. We focus on the following subjects: (1) supramolecular pathway dependent self-assembly, including kinetically trapped self-assemblies facilitated by host-guest chemistry, coordination chemistry, and electrostatic interactions; (2) physical processing pathway dependent self-assembly, including solvent quality controlled self-assembly, evaporation induced self-assembly and crystallization induced self-assembly.

Understanding the Structure of Reversible Coordination Polymers Based on Europium in Electrostatic Assemblies Using Time-Resolved Luminescence

In situ characterization of the structure of reversible coordination polymers remains a challenge because of their dynamic and concentration-responsive nature. It is especially difficult to determine these structures in their self-assemblies where their degree of polymerization responds to the local concentration. In this paper, we report on the structure of reversible lanthanide coordination polymers in electrostatic assemblies using time-resolved luminescence (TRL) measurement. The reversible coordinating system is composed of the bifunctional ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4) and europium ion Eu(3+). Upon mixing with the positively charged diblock copolymer poly(2-vinylpyridine)-b-poly(ethylene oxide) (P2VP41-b-PEO205), electrostatic polyion micelles are formed and the negatively charged L2EO4-Eu coordination complex simultaneously transforms into coordination "polymers" in the micellar core. By virtue of the water-sensitive luminescence of Eu(3+), we are able to obtain the structural information of the L2EO4-Eu coordination polymers before and after the formation of polyion micelles. Upon analyzing the fluorescence decay curves of Eu(3+) before and after micellization, the fraction of Eu(3+) fully coordinated with L2EO4 is found to increase from 32 to 83%, which verifies the occurrence of chain extension of the L2EO4-Eu coordination polymers in the micellar core. Our work provides a qualitative picture for the structure change of reversible coordination polymers, which allows us to look into these "invisible" structures.

Revealing and tuning the core, structure, properties and function of polymer micelles with lanthanide-coordination complexes

Controlling self-assembly processes is of great interest in various fields where multifunctional and tunable materials are designed. We here present the versatility of lanthanide-complex-based micelles (Ln-C3Ms) with tunable coordination structures and corresponding functions (e.g. luminescence and magnetic relaxation enhancement). Micelles are prepared by charge-driven self-assembly of a polycationic-neutral diblock copolymer and anionic coordination complexes formed by Ln(III) ions and the bis-ligand L2EO4, which contains two dipicolinic acid (DPA) ligand groups (L) connected by a tetra-ethylene oxide spacer (EO4). By varying the DPA/Ln ratio, micelles are obtained with similar size but with different stability, different aggregation numbers and different oligomeric and polymeric lanthanide(III) coordination structures in the core. Electron microscopy, light scattering, luminescence spectroscopy and magnetic resonance relaxation experiments provide an unprecedented detailed insight into the core structures of such micelles. Concomitantly, the self-assembly is controlled such that tunable luminescence or magnetic relaxation with Eu-C3Ms, respectively, Gd-C3Ms is achieved, showing potential for applications, e.g. as contrast agents in (pre)clinical imaging. Considering the various lanthanide(III) ions have unique electron configurations with specific physical chemical properties, yet very similar coordination chemistry, the generality of the current coordination-structure based micellar design shows great promise for development of new materials such as, e.g., hypermodal agents.

Lanthanide-Dipicolinic Acid Coordination Driven Micelles with Enhanced Stability and Tunable Function

Lanthanide-incorporated polymer micelles have been prepared driven by the lanthanide-dipicolinic acid (Ln-DPA) coordination. The terdentate DPA ligand is grafted to the PVP block of a diblock copolymer poly(4-vinylpyridine)-b-poly(ethylene oxide) (P4VP48-b-PEO193). Upon addition of Eu(III) ions to a solution of the DPA16-g-P4VP48-b-PEO193 block copolymer, intermolecular cross-links form and the ligand-carrying blocks assemble, leading to the formation of micelles, stabilized by the hydrophilic PEO blocks. The DPA exhibits a dual function in this study. First, the chelate group strongly coordinates to Eu(III) in a three to one ratio, and leads to high stability of the formed micelles, as proven by light scattering and luminescence spectroscopy. Second, DPA acts as an antenna that transfers energy to the Eu(III) ion and dramatically enhances the luminescence emission. The Eu(III) emission is moreover most sensitive for local environment and allows to shine light on the internal structure of this class of self-assembled 36 nm size soft nanoparticles. With the same strategy gadolinium(III) can be incorporated providing micelles which show enhanced magnetic relaxation rates. Micelles capping a mixture of Eu(III) and Gd(III) show both enhanced luminescence emission and magnetic relaxation rates, and the functions can be tuned by regulating the mixing ratio of Eu(III) and Gd(III), showing great potential for developing multimodal imaging agents for diagnostic as well as therapeutic applications.

Enhanced energy transfer efficiency and stability of europium β-diketonate complex in ionic liquid-based lyotropic liquid crystals

The self-assembly of luminescent lyotropic liquid crystals with europium β-diketonate complex confined within via hydrogen bonds between P123 and imidazolium cations is demonstrated.

Supramolecular self-assembly enhanced europium(III) luminescence under visible light

In this paper, we report on the luminescence of europium by directly exciting europium ions with visible light in aqueous medium. Upon replacing all the water molecules that coordinate around a central europium ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4), the quenching from water molecules is efficiently eliminated, offering considerable europium emission. By stoichiometrically mixing with a positively charged block polyelectrolyte, the negatively charged L2EO4-Eu coordinating complex can be transformed into a coordination 'polymer', which simultaneously forms electrostatic micelles with further enhanced europium fluorescence emission, owing to the increased fraction of L2EO4-coordinated Eu(III) as revealed by the fluorescence lifetime measurements. This approach avoids the use of the antenna effect that often utilizes UV light as the irradiation source. We further use those micelles for bio-imaging, and for the first time demonstrate the use of directly excited Eu-containing nano-probes for in vivo fluorescence imaging in small animals under visible excitation. Although literature results have shown that the direct excitation of europium ions in water may lead to emissions in the presence of coordinating ligands, those emissions were too weak to be applied due to the remaining water molecules in the coordination sphere. Our work points out that the direct excitation of europium can generate considerable europium emission given that all the water molecules in the coordination sphere are excluded, which does not only greatly reduce tedious lab work in synthesizing antenna molecules, but also facilitates the application of europium in aqueous medium under visible light.

Hierarchical molecular self-assemblies: construction and advantages

Hierarchical molecular self-assembly offers many exotic and complicated nanostructures which are of interest in nanotechnology and material science. In the past decade, various strategies leading to hierarchical molecular self-assemblies have been developed. In this review we summarize the recent advances in the creation and application of solution-based self-assembled nanostructures that involve more than one level of arrangement of building blocks. The strategies for construction hierarchical self-assembled structures and the advantages brought up by these assemblies are focused on. The following contents are included: (1) general approaches to fabricate hierarchical self-assembly, including self-assemblies based on supramolecules and specially designed block copolymers; (2) the advantages brought about by the hierarchical self-assembly, including the fabrication of special self-assembled structures, rich responsiveness to external stimuli, and the materials' performance.

Five diverse bivalent metal coordination polymers based on benzene dicarboxylate and bent dipyridyl ligands: syntheses, structures, and photoluminescent properties

Five divalent metal coordination polymers based on two different azine based bent N,N′-donor ligand and 1,4-benzene dicarboxylate were synthesized and characterized.