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Angel E Kaifer

Publications and source records attributed to Angel E Kaifer.

At least 19 recordsLinked to original sources
PubMed2006

New dendrimers containing a single cobaltocenium unit covalently attached to the apical position of Newkome dendrons: electrochemistry and guest binding interactions with cucurbit[7]uril.

Two new dendrimer series were prepared and characterized. These dendrimers contain a single bis(cyclopentadienyl)cobalt(III) (cobaltocenium, Cob+) unit covalently attached to the apical (focal) position of Newkome-type dendrons, ranging in size from first to third generation. The dendrimers in the first series (1ECob+-3ECob+) are hydrophobic and have 3, 9, and 27 tert-butyl esters on their peripheries, whereas the dendrimers in the second series (1Cob+-3Cob+) are hydrophilic with 3, 9, and 27 carboxylic acid groups on their surfaces, respectively. In voltammetric experiments, all dendrimers showed the expected one-electron reversible reduction of the cobaltocenium center, and the heterogeneous rate of electron transfer decreased with generation in both dendrimer series. The host-guest binding interactions between water-soluble dendrimers 1Cob+-3Cob+ and the cucurbit[7]uril (CB7) host were investigated using 1H NMR spectroscopy, MALDI-TOF mass spectrometry, and electrochemical techniques. The association equilibrium constants (K) for all dendrimer guests were significantly lower than that measured for the inclusion complex between underivatized Cob+ and CB7 (K = 5.7 x 10(9) M(-1)). Nonetheless, among the three dendrimers surveyed, the second-generation dendrimer, 2Cob+, afforded optimum stabilization for the CB7 inclusion complex.

Journal Article↗
PubMed2006

Efficient electronic communication between two identical ferrocene centers in a hydrogen-bonded dimer.

A novel ferrocene derivative that contains a donor-donor-acceptor-acceptor (DDAA) hydrogen bonding motif forms highly stable, noncovalent dimers in chloroform and dichloromethane solutions. Its voltammetric behavior and the observation of an intervalence charge-transfer band reveal that the two equivalent ferrocene centers in the hydrogen-bonded dimer exhibit a surprisingly efficient level of electronic communication.

Journal Article↗
PubMed2005

Supramolecular assembly of 2,7-dimethyldiazapyrenium and cucurbit[8]uril: a new fluorescent host for detection of catechol and dopamine.

The formation of a highly stable inclusion complex between 2,7-dimethyldiazapyrenium (Me(2)DAP(2+)) and the cucurbit[8]uril host (CB8) was demonstrated by X-ray crystallography; MALDI-TOF mass spectrometry; and (1)H NMR, electronic absorption, and emission spectroscopy. The equilibrium association constant was determined to be 8.9(+/-0.2)x10(5) L mol(-1) from UV-visible data and 8.4(+/-1.5) x 10(5) L mol(-1) from fluorescence data. The Me(2)DAP(2+).CB8 inclusion complex acted as a host to bind compounds containing aromatic pi-donor moieties (D), such as catechol and dopamine. This point was demonstrated by (1)H NMR spectroscopy, and electrochemical and emission measurements. Fluorescence detection of the Me(2)DAP(2+).D.CB8 ternary complexes was evident in aqueous solution and on the surface of silica particles, to which fluorescent diazapyrenium units had been covalently immobilized.

Bridged-Ring Compounds↗
PubMed2005

Complexation of ferrocene derivatives by the cucurbit[7]uril host: a comparative study of the cucurbituril and cyclodextrin host families.

The formation of inclusion complexes between cucurbit[7]uril (CB[7]) and ferrocene and its derivatives has been investigated. The X-ray crystal structure of the 1:1 inclusion complex between ferrocene and CB[7] revealed that the guest molecule resides in the host cavity with two different orientations. Inclusion of a set of five water-soluble ferrocene derivatives in CB[7] was investigated by 1H NMR spectroscopy and calorimetric and voltammetric techniques. Our data indicate that all neutral and cationic guests form highly stable inclusion complexes with CB[7], with binding constants in the 10(9)-10(10) M(-)(1) and 10(12)-10(13) M(-1) ranges, respectively. However, the anionic ferrocenecarboxylate, the only negatively charged guest among those surveyed, was not bound by CB[7] at all. These results are in sharp contrast to the known binding behavior of the same guests to beta-cyclodextrin (beta-CD), since all the guests form stable inclusion complexes with beta-CD, with binding constants in the range 10(3)-10(4) M(-1). The electrostatic surface potentials of CB[6], CB[7], and CB[8] and their size-equivalent CDs were calculated and compared. The CD portals and cavities exhibit low surface potential values, whereas the regions around the carbonyl oxygens in CBs are significantly negative, which explains the strong affinity of CBs for positively charged guests and also provides a rationalization for the rejection of anionic guests. Taken together, our data suggest that cucurbiturils may form very stable complexes. However, the host-guest interactions are very sensitive to some structural features, such as a negatively charged carboxylate group attached to the ferrocene residue, which may completely disrupt the stability of the complexes.

Bridged-Ring Compounds↗
PubMed2005

Unsymmetric dendrimers containing a single ureidopyrimidine unit: generation-dependent dimerization via hydrogen bonding.

A new series of unsymmetric Newkome-type dendrimers have been prepared and characterized. These dendrimers contain a single ureidopyrimidine residue covalently attached to their apical positions. In low polarity solvents, the first and second generation dendrimers form highly stable dimers via hydrogen bonding of their AADD ureidopyrimidine units, whereas the third generation dendrimer dimerizes to a very low extent. [structure: see text]

Journal Article↗
PubMed2005

Electrochemical and guest binding properties of Frechet- and Newkome-type dendrimers with a single viologen unit located at their apical positions.

Several new series of dendrimers containing a single redox-active 4,4'-bipyridinium (viologen) residue were synthesized and characterized. In these dendrimers, the viologen group is covalently attached to the apical position of a Newkome- or Frechet-type dendron, ranging in size from first to third generation of growth. The half-wave potentials corresponding to the two consecutive one-electron reductions of the viologen residue are affected by the size of the dendritic component. The size effects are more pronounced in the Newkome-type dendrimers and seem to result from the polarity contrast between the microenvironments provided by the solution and the internal phase of the dendrimer. Unlike in many other dendrimers having a redox-active core, the voltammetric behavior remains fast (reversible) even in third generation dendrimers. Pulse gradient stimulated echo NMR diffusion coefficient measurements on the Newkome-type dendrimers reveal that their hydrodynamic radii are relatively invariant in solvents of widely different polarities (dichloromethane to dimethyl sulfoxide). The host-guest binding interactions between the viologen residue in these dendrimers and the crown ether host bis-p-phenylene-34-crown-8 were also investigated. While in Newkome-type dendrimers the growth of the dendron caused a substantial attenuation of the binding constant values, this size effect was not observed in the Frechet-type dendrimers. These electrochemical and binding measurements underscore some of the structural differences between these two common types of dendritic architectures.

Journal Article↗
PubMed2005

Noncovalent encapsulation of cobaltocenium inside resorcinarene molecular capsules.

The encapsulation of cobaltocenium (Cob+) inside hexameric molecular capsules of two different resorcinarenes was investigated in dichloromethane solution. Both 1H NMR spectroscopic and voltammetric experiments clearly reveal that Cob+ experiences encapsulation. Diffusion coefficient measurements obtained from PGSE NMR experiments indicate that the molecular capsules exist in dichloromethane solution in the absence of any cations. Bound and free Cob+ ions undergo slow exchange on the NMR time scale, but the bound Cob+ ions rotate and/or tumble freely inside the molecular capsules. Under experimental conditions suitable for voltammetry the encapsulation of Cob+ depends on the nature of the supporting electrolyte. Tetraalkylammonium hexafluorophosphate, tetrafluoroborate, and perchlorate supporting electrolytes prevent the encapsulation of Cob+, while tetraalkylammonium chloride and bromide salts allow it. The nature of the tetraalkylammonium cation plays a smaller role in the encapsulation. Finally, the structure of the resorcinarene also factors into the overall stability of the molecular assembly.

Journal Article↗
PubMed2004

Sulfide-capped wire-like metallaynes as connectors for Au nanoparticle assemblies.

A new series of Ru2(DMBA)4(oligo(phenyleneethyne))2 compounds bearing sulfide termini was synthesized; structural characterization revealed both the rigid rod nature and extended pi-conjugation in these metallaynes; in the presence of these metallaynes, Au nanoparticles readily assembled into dimers and chains with well-defined inter-dimer distances.

Journal Article↗
PubMed2004

Dimeric molecular capsules under redox control.

A new tetraferrocenylurea calix[4]arene was prepared in order to investigate its dimerization properties. 1H NMR spectroscopic data clearly established that this calixarene forms stable dimeric molecular capsules in chloroform solution. The dimeric capsules are stabilized by the formation of multiple hydrogen bonds between the urea functional groups on the calixarene upper rims. In the dimer, eight ferrocene groups are held in proximity to the "seam" of hydrogen bonds. Upon oxidation of the ferrocene residues, electrochemical, PGSE NMR, and IR spectroscopic data revealed that the hydrogen bonds are broken, leading to the dissociation of the dimeric molecular capsule.

Journal Article↗
PubMed2004

Binding selectivity of cucurbit[7]uril: bis(pyridinium)-1,4-xylylene versus 4,4'-bipyridinium guest sites.

The binding interactions between the host cucurbit[7]uril (CB7) and a series of linear guests containing bis(pyridinium)-1,4-xylylene and/or 4,4'-bipyridinium residues were investigated by (1)H NMR spectroscopy. CB7 was found to exhibit considerable binding selectivity for bis(pyridinium)-1,4-xylylene over 4,4'-bipyridinium sites. New pseudo-rotaxane and rotaxane compounds were synthesized utilizing the host-guest interactions between CB7 and the surveyed guests. [structure: see text]

Journal Article↗
PubMed2004

Dendrimers as guests in molecular recognition phenomena.

Dendrimers are highly branched macromolecules which may engage in host-guest interactions, acting as either hosts or guests; this review is specifically concerned with the binding behavior of dendrimers containing single or multiple guest residues interacting with individual, freely diffusing hosts.

Journal Article↗
PubMed2004

Salt effects on the apparent stability of the cucurbit[7]uril-methyl viologen inclusion complex.

The effects of the medium ionic composition on the apparent equilibrium association constant (K) for the formation of a 1:1 inclusion complex between the guest methyl viologen (MV(2+)) and the host cucurbit[7]uril (CB7) were studied in aqueous solutions. The K values were found to decrease with increasing ionic strength, with more pronounced effects for solutions containing divalent Ca(2+) ions than for solutions containing monovalent Na(+) ions. The competing ion-dipole interactions between Ca(2+) or Na(+) and MV(2+) ions appear to be responsible for the remarkable modulation of the K values observed in this work.

Journal Article↗