Photophysical Properties of a Post-Self-Assembly Host/Guest Coordination Cage: Visible Light Driven Core-to-Cage Charge Transfer

Supramolecular systems are capable of unique photophysical properties due to possible interactions between subcomponents, such as between an encapsulated molecule and its cage in a host/guest environment. Here, we report that the encapsulation of a chromophore by a metallacage dramatically enhances its photophysical properties. In the visible region, the encapsulated photosensitizer achieves a 6.5-fold enhancement to its absorptivity. The triplet lifetime of the encapsulated photosensitizer is three times longer than that of its free analogue. These enhancements are attributed to two key factors: (i) encapsulation-induced core-to-cage charge transfer (CCCT) generates new visible-light absorbing states, accounting for the enhanced absorption, and (ii) the microenvironment inside the metallacage inhibits nonradiative decay processes, resulting in prolonged triplet lifetime. The CCCT arises from the electrostatic interaction between the delocalized electrons of the guest coronene and the positive charge associated with the metallacage host. The work herein provides insight into the CCCT phenomenon.

Self-assembly of [3]catenanes and a [4]molecular necklace based on a cryptand/paraquat recognition motif

Hierarchical self-assembly centered on metallacyclic scaffolds greatly facilitates the construction of mechanically interlocked structures. The formation of two [3]catenanes and one [4]molecular necklace is presented by utilizing the orthogonality of coordination-driven self-assembly and crown ether-based cryptand/paraquat derivative complexation. The threaded [3]catenanes and [4]molecular necklace were fabricated by using ten and nine total molecular components, respectively, from four and three unique species in solution, respectively. In all cases single supramolecular ensembles were obtained, attesting to the high degree of structural complexity made possible via self-assembly approaches.

Selective Formation of Heterometallic Ru-Ag Supramolecules via Stoichiometric Control of Multiple Different Tectons

Stoichiometric control of Ru, Ag, and tetrazolyl ligands resulted in the formation of different heterometallic Ru-Ag supramolecular architectures. Although the reaction of Ru and 5-(2-hydroxyphenyl)-1H-tetrazolyl (LH2) in a molar ratio of 2:1 or 6:4 resulted in the formation of dimeric or hexameric Ru complexes, Ag metal ions caused the Ru complexes to form three-dimensional cylindrical Ru6Ag6L6 and double-cone-shaped Ru6Ag8L6 complexes by occupying vacant coordination sites.

Self-Assembly of New Arene-Ruthenium Rectangles Containing Triptycene Building Block and Their Application in Fluorescent Detection of Nitro Aromatics

A suite of two new tetraruthenium metallarectangles 5 and 6 have been obtained from [2 + 2] self-assemblies between dipyridylethynyltriptycene 2 and one of the two dinuclear arene ruthenium clips, [Ru2 (μ-η4-OO∩OO) (η6-p-cymene)2][OTf]2 ; (OO∩OO = oxalate 3; 6,11-dihydroxy-5,12-naphthacenedionato (dotq) 4; OTf = triflate). These molecular rectangles are fully characterized by 1H NMR spectroscopy, electrospray mass spectrometry. A single crystal of 6 was suitable for X-ray diffraction structural characterization. These new metallarectangles showed fluorescence behavior in solution, have been examined for emission quenching effects with various aromatic compounds, and show high quenching selectivity and sensitivity towards nitroaromatics, particularly picric acid and trinitrotoluene. Excited-state charge transfer from the rectangles to nitro aromatic substrates can be used to develop selective fluorescent sensors for nitro aromatics.

Self-assembled supramolecular hetero-bimetallacycles for anticancer potency by intracellular release

Two new tetracationic hetero-bimetallacycles, compounds 4 and 5, have been constructed from an N,N'-bis(4-(pyridin-4-ylethynyl)phenyl)pyridine-2,6-dicarboxamide ligand (1), and cis-blocked complexes [M(dppf)(OTf)2 ] (dppf=1,1'-bis(diphenylphosphino)ferrocene; OTf=trifluoromethanesulfonate; M=Pd (2), Pt (3)) in CH3 NO2 /CH2 Cl2 (1:1) solvent. Both complexes were isolated with adequate yields as triflate salts and were then characterized using (1) H, (13) C, and (31) P NMR spectroscopy, elemental analysis, UV/Vis spectroscopy, and high-resolution electrospray mass spectrometry (HR-ESMS). The molecular structure of 4 was determined by molecular mechanics force-field calculations. The cytotoxic effect of both new complexes were analyzed against T98G (brain tumor), KB (head and neck cancer), SNU-80 (thyroid cancer), and HEK-293 non-malignant cell lines. The cytotoxicity of complexes 4 and 5 were found to be considerably more effective against cancer cells than reference drug cisplatin. Annexin-V/PI staining, caspase-3/7 activity, and the reduction in mitochondrial membrane potential justify a significant level of apoptosis in complex-treated cells.

A discrete amphiphilic organoplatinum(II) metallacycle with tunable lower critical solution temperature behavior

Oligo(ethylene glycol) (OEG)-decorated supramolecular assemblies are distinguished by their neutral character and macroscopic temperature-sensitive phase transition behavior. OEG functionalization is an emerging strategy to obtain thermoresponsive macrocyclic amphiphiles, although known methods organize the hydrophilic and hydrophobic segments by covalent bonding. Coordination-driven self-assembly offers an alternative route for organizing OEG-functionalized precursors into nanoscopic architectures, resulting in well-defined metallacycle cores surrounded by hydrophilic scaffolds to impart overall amphiphilic character. Here a tri(ethylene glycol)-functionalized thermosensitive amphiphilic metallacycle was prepared with high efficiency by means of the directional-bonding approach. The ensembles thus formed showed good lower critical solution temperature behavior with a highly sensitive phase separation and excellent reversibility. Moreover, the clouding point decreased with increasing metallacycle concentration and addition of K(+).

Photoinduced transformations of stiff-stilbene-based discrete metallacycles to metallosupramolecular polymers

Control over structural transformations in supramolecular entities by external stimuli is critical for the development of adaptable and functional soft materials. Herein, we have designed and synthesized a dipyridyl donor containing a central Z-configured stiff-stilbene unit that self-assembles in the presence of two 180° di-Pt(II) acceptors to produce size-controllable discrete organoplatinum(II) metallacycles with high efficiency by means of the directional-bonding approach. These discrete metallacycles undergo transformation into extended metallosupramolecular polymers upon the conformational switching of the dipyridyl ligand from Z-configured (0°) to E-configured (180°) when photoirradiated. This transformation is accompanied by interesting morphological changes at nanoscopic length scales. The discrete metallacycles aggregate to spherical nanoparticles that evolve into long nanofibers upon polymer formation. These fibers can be reversibly converted to cyclic oligomers by changing the wavelength of irradiation, which reintroduces Z-configured building blocks owing to the reversible nature of stiff-stilbene photoisomerization. The design strategy defined here represents a novel self-assembly pathway to deliver advanced supramolecular assemblies by means of photocontrol.

Molecular self-assembly of arene-Ru based interlocked catenane metalla-cages

Two interlocked and two simple trigonal prisms are formed through the coordination self-assembly of arene-Ru acceptors with a new tridentate donor.

Biomedical and biochemical applications of self-assembled metallacycles and metallacages

Metal ions and metal complexes with organic molecules are ubiquitous in nature. Bulk metal ions of Na, K, Mg, and Ca constitute as much as 1% of human body weight. The remaining trace ions, most commonly of Fe, Ni, Cu, Mn, Zn, Co, Mo, and V, make up ∼0.01% by weight, but their importance in biological processes cannot be overstated. Although nature is limited to the use of bioavailable metal ions, many rarer transition metals can elicit novel biological responses when they interact with biomolecules. For this reason, metal-biomolecule complexes are of interest in medicinal applications. A well-known example is cisplatin, which contains Pt, rare in nature, but highly effective in this context as an anticancer drug in the form of cis-Pt(NH3)2Cl2 and analogous Pt(II) complexes. This and other examples have led to strong interest in discovering new metalloanticancer drugs. In this Account, we describe recent developments in this area, particularly, using coordination-driven self-assembly to form tunable supramolecular coordination complexes (SCCs) with biomedical applications. Coordination-driven self-assembly describes the spontaneous formation of metal-ligand bonds in solution, transforming molecular building blocks into single, 2D metallacycles, or 3D metallacages depending on the directionality of the precursors used. Such SCCs have well-defined internal cavities and simple pre- or post-self-assembly functionalizations. They are highly tunable both spatially and electronically. Metal ions are necessary structural elements for the directional bonding approach, which can be exploited to provide biological activity to an SCC, particularly for Pt- and Ru-based structures. Since these two metals are not only among the most commonly used for coordination-driven self-assembly but are also the basis for a number of small molecule anticancer agents, researchers have evaluated a growing number of SCCs for their antitumor properties. The biological application of SCCs is still an emergent field of study, but the examples discussed in this Account confirm that supramolecular scaffolds have relevance to a wide variety of biochemical and biomedical targets. SCCs can serve as anticancer agents, act as selective sensors for biologically important analytes, or interact with DNA and proteins. The myriad of possible SCCs and their almost limitless modularity and tunability without significant synthetic penalty suggests that the biological applications of such species will continue along this already promising path.

Supramolecular polymers with tunable topologies via hierarchical coordination-driven self-assembly and hydrogen bonding interfaces

A powerful strategy to obtain complex supramolecular materials is the bottom-up construction of noncovalently bound materials by hierarchical self-assembly. This assembly process involves stepwise, uniform increases to the architectural complexity of a substrate, starting from discrete precursors and growing in dimensionality through controlled reactivity to a final product. Herein, two orthogonal processes are exploited: coordination-driven self-assembly and hydrogen bonding. The former relies on the predictable formation of metal-ligand bonds wherein the directionalities of the rigid precursors used determines the structural outcome. The latter uses 2-ureido-4-pyrimidinone interfaces that are structurally robust by virtue of the quadruple hydrogen bonding that can occur between subunits. By combining these two processes into a single system, it is possible to generate hierarchical materials that preserve the attractive tunability associated with discrete supramolecular coordination complexes. For instance, the synthesis of a one-dimensional chain comprising linked metalla-rhomboids is readily adapted to a 2D cross-linked hexagonal network by simply selecting a different metal acceptor precursor as an assembly component. The specific interactions between subunits, in this case platinum(II)-pyridyl bonds and the quadruple H-bonding of ureidopyrimidinone, are unchanged, establishing a unique strategy to obtain supramolecular polymers with marked topological differences with minimal synthetic redesign. In addition, the structural rigidity imposed by the inclusion of the platinum metallacycles serves to minimize the formation of cyclic oligomers, increasing the efficacy of formation and improving the properties of the resultant materials. Furthermore, this study taps the potential of organoplatinum(II) metallacycles in materials science.

Hierarchical self-assembly: well-defined supramolecular nanostructures and metallohydrogels via amphiphilic discrete organoplatinum(II) metallacycles

Metallacyclic cores provide a scaffold upon which pendant functionalities can be organized to direct the formation of dimensionally controllable nanostructures. Because of the modularity of coordination-driven self-assembly, the properties of a given supramolecular core can be readily tuned, which has a significant effect on the resulting nanostructured material. Herein we report the efficient preparation of two amphiphilic rhomboids that can subsequently order into 0D micelles, 1D nanofibers, or 2D nanoribbons. This structural diversity is enforced by three parameters: the nature of the hydrophilic moieties decorating the parent rhomboids, the concentration of precursors during self-assembly, and the reaction duration. These nanoscopic constructs further interact to generate metallohydrogels at high concentrations, driven by intermolecular hydrophobic and π-π interactions, demonstrating the utility of coordination-driven self-assembly as a first-order structural element for the hierarchical design of functional soft materials.

Anticancer potency and multidrug-resistant studies of self-assembled arene-ruthenium metallarectangles

A suite of three tetraruthenium metallacycles have been obtained from [2+2] self-assemblies between N,N'-Di-(4-pyridyl)-1,4,5,8-naphthalenetetracarbo-xydiimide (4) and one of the three dinuclear arene ruthenium clips, (η(6)-p-iPrC6H4Me)2Ru2(OO∩OO)][OTf]2 (OO∩OO = oxalate 1, 2,5-dioxydo-1,4-benzoquinonato (dobq) 2, 5,8-dihydroxy-1,4-naphthaquinonato (donq) 3; OTf = triflate). All complexes were isolated in good yield (>85 %) as triflate salts and were fully characterized by using (1)H NMR and UV/Vis spectroscopies, and high-resolution electrospray mass spectrometry. A single crystal of the metallarectangle 5 was suitable for X-ray diffraction structural characterization. The biological activities of the metallacycles were determined by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays, establishing their in vitro anticancer properties. Our results show that for the AGC (gastric cancer) cell lines, the cytotoxicity of (donq)-containing SCC 7 exceeds that of cisplatin, which was used as a control. For HCT15 (colon cancer) cell lines, the cytotoxicity is comparable to both cisplatin and doxorubicin. An in vivo hollow fiber model was used to show growth-inhibitory activity against HCT15 and image-based cytometry experiments indicated that 7 induced apoptosis as the mode of cell death. Complex 7 also showed significant antitumor activity for multidrug-resistant HCT15/CLO2 cell lines, for which doxorubicin was ineffective.