3 nm-Scale Molecular Switching between Fluorescent Coordination Capsule and Nonfluorescent Cage

Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly

During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions, or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc. This perspective focuses on the recent advances in the construction of a variety of fluorescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluorescence properties and the applications of these organometallic architectures have also been discussed.

Metal–organic molecular cages: applications of biochemical implications

New well-designed materials are highly demanded with the prospect of versatile properties, offering successful applications as alternates to conventional materials.

Structure modeling, synthesis and X-ray diffraction determination of an extra-large calixarene-based coordination cage and its application in drug delivery

An extra-large octahedral calixarene-based coordination cage was modeled by the isomorphic replacement approach from a known structure of a smaller size. And then it was successfully synthesized and determined.

Polyaromatic nanocapsules displaying aggregation-induced enhanced emissions in water

V-shaped polyaromatic amphiphiles with phenanthrene or naphthalene rings spontaneously and quantitatively formed micelle-like nanocapsules in water at room temperature. In contrast to usual polyaromatic aggregates with weak fluorescent properties, the new capsules providing spherical polyaromatic shells with diameters of ∼2 nm show strong fluorescent emissions due to an aggregation-induced enhanced emission (AIEE) effect and moreover encapsulate a fluorescent coumarin dye to generate highly emissive host-guest composites.

Stoichiometrically controlled revocable self-assembled "spiro" versus quadruple-stranded "double-decker" type coordination cages

The simple combination of Pd(II) with the tris-monodentate ligand bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate, L, at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the "spiro-type" Pd1L2 macrocycle, 1, and the quadruple-stranded Pd3L4 cage, 2, respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.

Cerium-based M4L4 tetrahedra as molecular flasks for selective reaction prompting and luminescent reaction tracing

The application of metal-organic polyhedra as "molecular flasks" has precipitated a surge of interest in the reactivity and property of molecules within well-defined spaces. Inspired by the structures of the natural enzymatic pockets, three metal-organic neutral molecular tetrahedral, Ce-TTS, Ce-TNS and Ce-TBS (H6TTS: N',N'',N'''-nitrilotris-4,4',4''-(2-hydroxybenzylidene)-benzohydrazide; H6TNS: N',N'',N'''-nitrilotris-6,6',6''-(2-hydroxybenzylidene)-2-naphthohydrazide; H6TBS: 1,3,5-phenyltris-4,4',4''-(2-hydroxybenzylidene)benzohydrazide), which exhibit different size of the edges and cavities, were achieved through self-assembly by incorporating robust amide-containing tridentate chelating sites into the fragments of the ligands. They acted as molecular flasks to prompt the cyanosilylation of aldehydes with excellent selectivity towards the substrates size. The amide groups worked as trigger sites and catalytic driven forces to achieve efficient guest interactions, enforcing the substrates proximity within the cavity. Experiments on catalysts with the different cavity radii and substrates with the different molecular size demonstrated that the catalytic performance exhibited enzymatical catalytic mechanism and occurred in the molecular flask. These amides were also able to amplify guest-bonding events into the measurable outputs for the detection of concentration variations of the substrates, providing the possibility for metal-organic hosts to work as smart molecular flasks for the luminescent tracing of catalytic reactions.

Fluorophore incorporation allows nanomolar guest sensing and white-light emission in M4L6 cage complexes

M₄L₆ cages built from BODIPY- and pyrene-containing subcomponents perform multiple functions: sensing anions and amino acids, and forming a white-light emitting complex with perylene.

Trace mercury ion determination based on the highly selective redox reaction between stannous ion and mercury ion enhanced by gold nanoparticles

A novel resonance light scattering (RLS) spectrometric method for mercury ions (Hg(2+)) determination has been established in this article. Mercury (Hg) nanoparticle formed from the highly selective redox reaction between citrate-stabilized stannous ions (Sn(2+)) and Hg(2+). As a result, the RLS intensities of the system can be enhanced and it can be sensitized in the presence of very little amount of gold nanoparticles (AuNPs). According to this phenomenon, trace Hg(2+) in real water sample has been determined directly by RLS spectrometry. It has been found that the enhanced RLS intensities (ΔI(RLS)) characterized at 395 nm are proportional to the concentration of Hg(2+) in the range of 0.1-30 μmol L(-1) with a detection limit (3σ) of 0.051 μmol L(-1). The method described herein has good sensitivity, selectivity, and without complicated sample pretreatment. Moreover, the feasibility for the analysis of Hg(2+) in a wastewater sample was identified with a good recovery (100.2-106.3%).

Luminescent sub-nanometer clusters for metal ion sensing: a new direction in nanosensors

We describe the application of a recently discovered family of materials called quantum clusters, which are sub-nanometer particles composed of a few atoms with well-defined molecular formulae, exhibiting intense absorption and emission in the visible region in metal ion sensing, taking Ag(25) as an example. The changes in the optical properties of the cluster, in both absorption and emission upon exposure to various metal ions in aqueous medium are explored. The cluster can detect Hg(2+) down to ppb levels. It can also detect 5d block ions (Pt(2+), Au(3+) and Hg(2+)) down to ppm limits. Hg(2+) interacts with the metal core as well as the functional groups of the capping agents and the interaction is concentration-dependent. To understand the mechanism behind this type of specific interaction, we have used spectroscopic and microscopic techniques such as UV-vis spectroscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Specific reasons responsible for the interaction of Hg(2+) have been proposed.

Metallo-responsive switching between hexadecameric and octameric supramolecular G-quadruplexes

We report the metallo-responsive high fidelity switching between hexadecameric and octameric supramolecular G-quadruplexes triggered by a change in the metal cation promoter from potassium to strontium, respectively.

pH-Dependent spectroscopic and luminescent properties, and metal-ion recognition studies of Re(I) complexes containing 2-(2'-pyridyl)benzimidazole and 2-(2'-pyridyl)benzimidazolate

A series of Re(I) complexes, [Re(CO)(3)Cl(HPB)] (1), [Re(CO)(3)(PB)H(2)O] (2), [Re(CO)(3)(NO(3))(PB-AuPPh(3))] (3), and [Re(CO)(3)(NO(3))(PB)Au(dppm-H)Au](2) (4) [HPB = 2-(2'-pyridyl)benzimidazole; dppm = 2,2'-bis(diphenylphosphinomethane)], have been synthesized and characterized by X-ray diffraction. Complex 1, which exhibits interesting pH-dependent spectroscopic and luminescent properties, was prepared by reacting Re(CO)(5)Cl with an equimolar amount of 2-(2'-pyridyl)benzimidazole. The imidazole unit in complex 1 can be deprotonated to form the imidazolate unit to give complex 2. Addition of 1 equiv of AuPPh(3)(NO(3)) to complex 2 led to the formation of a heteronuclear complex 3. Addition of a half an equivalent of dppm(Au(NO(3)))(2) to complex 2 yielded 4. In both 3 and 4, the imidazolate unit acts as a multinuclear bridging ligand. Complex 4 is a rare and remarkable example of a Re(2)Au(4) aggregate in combination with μ(3)-bridging 2-(2'-pyridyl)benzimidazolate. Finally, complex 2 has been used to examine the Hg(2+)-recognition event among group 12 metal ions. Its reversibility and selectivity toward Hg(2+) are also examined.

Label-free emission assay of mercuric ions using DNA duplexes of poly(dT)

In this study, an assay to quantify the presence of mercuric ions and methyl mercury by double-stranded DNA containing a poly(dT) sequence was developed using a light switch compound, Ru(phen)(2)(dppz)(2+) (1), which is known to intercalate into double-stranded DNA. Upon treatment with mercuric ions, the metal-to-ligand charge transfer (MLCT) emission derived from the intercalation of 1 was reduced due to the formation of DNA duplexes containing T-Hg(2+)-T base pairs by the dehybridization of poly(dT)-poly(dA) duplexes at room temperature. As the concentration of Hg(2+) was increased, the emission of 1 gradually decreased. This label-free method had a detection limit of 5 nM. Other metal ions, such as K(+), Ag(+), Ca(2+), Mg(2+), Zn(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+), Cr(3+), Fe(3+), had no significant effect on reducing emission. This emission method can differentiate matched and mismatched poly(dT) sequences based on the dehybridization rate of dsDNA and the rate decreased in the order of T(10)C·A(11)∼ T(10)A·A(11) > T(10)G·A(11) > T(11)·A(11).

Metal-organic cyclohelicates as optical receptors for glutathione: syntheses, structures, and host-guest behaviors

Two trinuclear zinc-based cyclohelicates, Zn-PDB (PDB = [5-(dibenzylamino)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) and Zn-PMB (PMB = [5-(bodipy-oxy)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) containing dibenzylamino and BODIPY groups, respectively, were generated by incorporating two amide-containing tridentate chelators into meta-positions of a substituted phenyl ring. Single-crystal structure analysis and related spectroscopic characterizations demonstrated the formation of macrocyclic helicals both in the solid state and in solution. The host-guest behavior of the cyclohelical hosts towards γ-glutamyl-cysteinyl-glycine (GSH) and its component amino acids was investigated by spectroscopic titrations. UV/Vis absorption titration and NMR titrations of Zn-PDB and Zn-PMB upon addition of the above-mentioned guests suggested that the Glu residue of GSH was positioned within the cavity. The COO(-) groups interacted with metal ions through static interactions. The Cys moiety of GSH interacted with the amide groups sited in host molecules through hydrogen-bonding interactions to produce measurable spectral changes. Fluorescent titrations of Zn-PMB upon the addition of GSH and ESI-MS investigations of the titration solutions confirmed the host-guest interaction modes and revealed the possible 1:1 complexation stoichiometry. These results showed that the recognition of a substrate within the cavity of functionalized metal-organic cage-like receptors could be a useful method to produce supramolecular sensors for biomolecules.