Photoinduced host-to-guest electron transfer in a self-assembled coordination cage.

A [Pd2L4] coordination cage, assembled from electron-rich phenothiazine-based ligands and encapsulating an electron-deficient anthraquinone-based disulfonate guest, is reported. Upon excitation at 400 nm, transient absorption spectroscopy unveils photoinduced electron transfer from the host's chromophores to the guest, as indicated by characteristic spectral features assigned to the oxidized donor and reduced acceptor. The structure of the host-guest complex was characterized by NMR spectroscopy, mass spectrometry and single-crystal X-ray analysis. Spectroelectrochemical experiments and DFT calculations both agree with the proposed light-induced charge separation. A kinetic analysis of the involved charge transfer channels reveals, besides a guest-independent LMCT path, 44% efficiency for the host-guest charge transfer (HGCT).

Nonaqueous Emulsion Polycondensation Enabled by a Self-Assembled Cage-like Surfactant.

Nonaqueous emulsions are crucial for a range of applications based on water-sensitive systems such as controlled polymerizations requiring anhydrous reaction conditions and the stabilization of readily hydrolyzable reagents or pharmacologically active components. However, defined molecular surfactants to stabilize such nonaqueous emulsions are scarce. We introduce a self-assembled coordination cage, decorated with cholesterol functionalities, to serve as a molecular surfactant for various oil-in-oil emulsions of immiscible organic solvents. While the positively charged cage forms the amphiphile's polar moiety, the non-polar cholesterol appendices can bend in a common direction to stabilize the emulsion. Templated by the droplets, polycondensation reactions were carried out to produce microstructured polyurethane and polyurea materials of different particle sizes and morphologies. Further, the amphiphilic cage can encapsulate a guest molecule and the resulting host-guest assembly was also examined as a surfactant. In addition, the aggregation behavior of the amphiphilic cage in an aqueous medium was examined.

Coordination Cage-Based Emulsifiers: Templated Formation of Metal Oxide Microcapsules Monitored by In Situ LC-TEM.

Metallo-supramolecular self-assembly has yielded a plethora of discrete nanosystems, many of which show competence in capturing guests and catalyzing chemical reactions. However, the potential of low-molecular bottom-up self-assemblies in the development of structured inorganic materials has rarely been methodically explored so far. Herein, we present a new type of metallo-supramolecular surfactant with the ability to stabilize non-aqueous emulsions for a significant period. The molecular design of the surfactant is based on a heteroleptic coordination cage (CGA-3; CGA=Cage-based Gemini Amphiphile), assembled from two pairs of organic building blocks, grouped around two Pd(II) cations. Shape-complementarity between the differently functionalized components generates discrete amphiphiles with a tailor-made polarity profile, able to stabilize non-aqueous emulsions, such as hexadecane-in-DMSO. These emulsions were used as a medium for the synthesis of spherical metal oxide microcapsules (titanium oxide, zirconium oxide, and niobium oxide) from soluble, water-sensitive alkoxide precursors by allowing a controlled dosage of water to the liquid-liquid phase boundary. Synthesized materials were analyzed by a combination of electron microscopic techniques. In situ liquid cell transmission electron microscopy (LC-TEM) was utilized for the first time to visualize the dynamics of the emulsion-templated formation of hollow inorganic titanium oxide and zirconium oxide microspheres.

Discrete and Polymeric Self-Assembled Palladium(II) Complexes as Supramolecular Gelators.

Supramolecular gels prepared from low-molecular-weight gelators have been extensively explored. However, the exploitation of discrete or polymeric metal complexes as gelators is a relatively recent trend. The synthesis of self-assembled coordination complexes from palladium(II) and selected ligands is well established, but the potential of these complexes as gelators is a less explored treasure. Herein we focus on the gelation abilities of some self-assembled palladium(II) complexes and the resulting unique properties. First, discrete complexes with PdL, PdL2 , Pd2 L, Pd2 L2 , Pd2 L4 , and Pd3 L6 compositions are discussed. Second, gelation behavior promoted by coordination-polymer-like gelators formed in situ is explored. These gel samples have been employed in catalysis and the uptake of organic and dye molecules from the solution and gas phases. It is concluded that untapped unique properties can be realized by further exploration of designer palladium(II) complexes.

Molecular Recombination Phenomena in Palladium(II)-Based Self-Assembled Complexes.

A series of self-assembled binuclear complexes (1-4) of Pd2L'2L2 composition are prepared by combination of selected cis-protected palladium(II) component cis-PdL' with equimolar amount of the nonchelating bidentate ligand 1,3-bis((pyridine-4-yl)methyl)urea, L. The cis-protecting unit L' crafted in the complexes [Pd2(L')2(L)2](NO3)4, 1-4, are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen), respectively. Complexation of palladium(II) with the ligand L, however, resulted in the self-assembled trinuclear complex [Pd3(L)6](NO3)6, 5, of Pd3L6 composition. A few other mononuclear complexes relevant to this work are the homoleptic (6-9) and the heteroleptic (10-12) of PdL'2 composition, where the term L' in the formula stands for en, tmeda, bpy, and phen. The two units of L' in the complexes 6-12 are en-en, tmeda-tmeda, bpy-bpy, phen-phen, en-tmeda, en-bpy, and en-phen, respectively. Keeping in mind the recombination of fragments in biomolecular systems, it was considered to study the recombination of fragments among assorted self-assembled coordination complexes. In this context, the complexes 1-12 are employed. The recombination was planned on the basis of information already coded in the complexes that we realized from our experience. Designated complexes from this list are subjected to deliberate circumstances, whereupon remarkable "molecular recombination" processes ensued. For instance, combination of [Pd2(en)2(L)2](NO3)4, 1 with [Pd(tmeda)2](NO3)2, 7 at 1:2 molar ratio produced [Pd2(tmeda)2(L)2](NO3)4, 2 and concomitant [Pd(en)(tmeda)](NO3)2, 10 at 1:2 molar ratio. A number of reorganization reactions are studied through selective combinations of two types of complexes from the pool of 1-12. The detailed finding is described in the article so as to conceptualize molecular recombination and other related dynamic processes in self-assembled coordination complexes of a chosen family. Crystal structures of a few complexes (2, 4, and 5) and intermolecular interactions in the crystal packing with particular reference to π-π interactions in 4 are also discussed.

Multi-Stimuli-Responsive Metallogel Molded from a Pd2L4-Type Coordination Cage: Selective Removal of Anionic Dyes.

A self-assembled binuclear coordination cage of the Pd2L4 formulation has been constructed by complexation of Pd(NO3)2 with N, N'-bis(3-pyridylmethyl)naphthalenediimide (L). The cage, i.e., [Pd2(L)4](NO3)4 (1), displayed a further self-assembly phenomenon to afford a gel phase, upon dissolution in either dimethyl sulfoxide or acetonitrile-water (1:1) followed by standing at room temperature. It was observed that a synergy among the metal ion, ligand, counteranion, solvent, and concentration played a vital role for metallogel formation. The morphology of the metallogel as observed from microscopy studies revealed the formation of a rare variety of nanoscale metal-organic particles. Salient features of the gel include the thixotropic (mechanoresponsive) nature, in addition to the reversible chemical-stimuli-responsive behavior. The presence of naphthalenediimide moieties at the backbone of the cage and the cationic nature of the cavity of the cage could be exploited to study the functional aspects of the gel. The porous gel exhibited the abilities to uptake pyrene as a guest and to selectively remove anionic dyes from aqueous solutions. The gel could bind representative anionic dyes like "acid blue 93" and "methyl orange" in the absence or presence of certain cationic dyes, making the material suitable for selective dye removal applications.

Nanoscale metallogel via self-assembly of self-assembled trinuclear coordination rings: multi-stimuli-responsive soft materials.

A rare variety of coordination rings having the M3L6 composition are prepared by the combination of Pd(NO3)2 with an imidazolyl or benzimidazolyl appended bidentate non-chelating ligand (i.e.L1 or L2). The variable concentration 1H NMR spectra of the trinuclear complexes [Pd3(L1)6](NO3)6, 1a and [Pd3(L2)6](NO3)6, 2a in DMSO-d6 provided valuable information on the self-assembly phenomenon of the already self-assembled complexes. Interestingly, the signals of 2a are broadened upon increasing the concentration. The solution of 2a in DMSO formed a supramolecular metallogel above a certain concentration (i.e. 2% w/v), however, complex 1a could not form any gel. This phenomenon is attributed to the presence of an auxiliary π-surface in the benzimidazole moiety in L2, in contrast to the imidazole moiety of L1. The influence of anions in gel formation was studied by preparing several samples using a variety of palladium(II) salts and L2 in DMSO. It was found that oxoanions like nitrate, perchlorate, triflate and tosylate are amicable for gel formation, probably due to their capabilities of forming H-bonds. The counter anions like tetrafluoroborate, hexafluorophosphate, or hexafluoroantimonate could not assist in the formation of gel. The stimuli-responsive nature of the gel and the reversible gel­sol conversion were demonstrated by dis-assembly and re-assembly processes of 2a as controlled by a pair of stimuli such as halide-nitrate, DMAP-HNO3, and ethylenediamine-Pd(NO3)2. No gel could be prepared by the combination of Ni(NO3)2 or Pt(NO3)2 with ligand L2. Thus, a subtle change in the ligand design, metal ion and counter anion is demonstrated as the responsible parameter for the construction of the three component multi-stimuli-responsive supramolecular gel.

Approaches to the simultaneous inactivation of metallo- and serine-beta-lactamases.

A series of cephalosporin-derived reverse hydroxamates and oximes were prepared and evaluated as inhibitors of representative metallo- and serine-beta-lactamases. The reverse hydroxamates showed submicromolar inhibition of the GIM-1 metallo-beta-lactamase. With respect to interactions with the classes A, C, and D serine beta-lactamases, as judged by their correspondingly low K(m) values, the reverse hydroxamates were recognized in a manner similar to the non-hydroxylated N-H amide side chains of the natural substrates of these enzymes. This indicates that, with respect to recognition in the active site of the serine beta-lactamases, the OC-NR-OH functionality can function as a structural isostere of the OC-NR-H group, with the N-O-H group presumably replacing the amide N-H group as a hydrogen bond donor to the appropriate backbone carbonyl oxygen of the protein. The reverse hydroxamates, however, displayed k(cat) values up to three orders of magnitude lower than the natural substrates, thus indicating substantial slowing of the hydrolytic action of these serine beta-lactamases. Although the degree of inactivation is not yet enough to be clinically useful, these initial results are promising. The substitution of the amide N-H bond by N-OH may represent a useful strategy for the inhibition of other serine hydrolases.

Correlation of tuned aperture computed tomography with conventional computed tomography for evaluation of osseous healing in calvarial defects.

OBJECTIVE: To compare the diagnostic efficacy of iteratively restored tuned aperture computed tomography (TACT) with conventional computed tomography (CT) for evaluation of osseous healing in induced calvarial defects. STUDY DESIGN: Fifty-six calvarial defects in 14 rabbits received 1 of 4 possible treatments: copolymer membranes with and without bone marrow stromal cells (BMSCs), BMSCs alone, or no treatment (control). Healing was measured after 2, 4, and 8 wks as remaining defect areas measured on TACT and CT images. Histomorphometric analyses were done on the specimens. RESULTS: Bone formation was minimal to none in control defects and those treated with BMSCs or polymer matrices alone. Healthy bone formation was noted in defects treated with polymers impregnated with BMSCs. Unresolved defect area measurements using TACT and CT of osseous healing showed a high positive correlation. CONCLUSIONS: Potential for TACT to accurately detect osseous healing in surgical defects was demonstrated. High resolution of TACT combined with generation of information in 3D yields comparable performance to CT.

Vascularization and tissue infiltration of a biodegradable polyurethane matrix.

Urethanes are frequently used in biomedical applications because of their excellent biocompatibility. However, their use has been limited to bioresistant polyurethanes. The aim of this study was to develop a nontoxic biodegradable polyurethane and to test its potential for tissue compatibility. A matrix was synthesized with pentane diisocyanate (PDI) as a hard segment and sucrose as a hydroxyl group donor to obtain a microtextured spongy urethane matrix. The matrix was biodegradable in an aqueous solution at 37 degrees C in vitro as well as in vivo. The polymer was mechanically stable at body temperatures and exhibited a glass transition temperature (Tg) of 67 degrees C. The porosity of the polymer network was between 10 and 2000 microm, with the majority of pores between 100 and 300 microm in diameter. This porosity was found to be adequate to support the adherence and proliferation of bone-marrow stromal cells (BMSC) and chondrocytes in vitro. The degradation products of the polymer were nontoxic to cells in vitro. Subdermal implants of the PDI-sucrose matrix did not exhibit toxicity in vivo and did not induce an acute inflammatory response in the host. However, some foreign-body giant cells did accumulate around the polymer and in its pores, suggesting its degradation is facilitated by hydrolysis as well as by giant cells. More important, subdermal implants of the polymer allowed marked infiltration of vascular and connective tissue, suggesting the free flow of fluids and nutrients in the implants. Because of the flexibility of the mechanical strength that can be obtained in urethanes and because of the ease with which a porous microtexture can be achieved, this matrix may be useful in many tissue-engineering applications.