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Fritz Vögtle

Publications and source records attributed to Fritz Vögtle.

33 records · Page 2Linked to original sources
PubMed2004

Designing systems for a multiple use of light signals.

The photochemical and photophysical behavior of two dendrimers consisting of a benzophenone core and branches that contain dimethoxybenzene units has been investigated. Such dendrimers can undergo a variety of photochemical and photophysical processes, namely: 1) quenching of the fluorescence and phosphorescence of the dimethoxybenzene units by energy transfer to the benzophenone core (antenna effect), 2) direct and sensitized phosphorescence (and delayed fluorescence) of the benzophenone core, 3) hydrogen abstraction by the triplet excited state of the benzophenone core from solvent molecules, 4) intramolecular hydrogen abstraction by the triplet excited state of the benzophenone core from the dendrimer branches, 5) quenching of the phosphorescence and hydrogen abstraction reaction of the benzophenone core by energy transfer to terbium ions and dioxygen; 6) conversion of the UV light absorbed by the dendrimer branches into visible (Tb3+) or near infrared (O2) emission via the sequence of processes 1) and 5). The results obtained emphasize the great potential of suitably designed dendrimers for a multiple use of light signals.

Journal Article↗
PubMed2004

Dendrimers as ligands: an investigation into the stability and kinetics of Zn2+ complexation by dendrimers with 1,4,8,11-tetraazacyclotetradecane (cyclam) cores.

We have investigated the complexation of Zn(2+) with 1,4,8,11-tetrakis(naphthylmethyl) cyclam (1; cyclam=1,4,8,11-tetraazacyclotetradecane) and with two dendrimers consisting of a cyclam core with four dimethoxybenzene and eight naphthyl appendages (2), and twelve dimethoxybenzene and sixteen naphthyl appendages (3). An important, common feature of model compound 1 and dendrimers 2 and 3 is that their potentially fluorescent naphthyl units are quenched by exciplex formation with the cyclam nitrogen atoms. Complexation with Zn(2+), however, prevents exciplex formation and results in the appearance of an intense naphthyl fluorescence signal that can be used for monitoring the complexation process. Luminescence titration, together with competition experiments and (1)H NMR titration, have shown that 1:1 and 1:2 (metal/ligand) complexes are formed in the cases of 2 and 3, whereas model compound 1 gives only a 1:1 complex. We have also investigated the 1:1 complexation kinetics by the stopped-flow technique. In the case of 1, a second-order process (k(1)=44x10(5) M(-1) s(-1)) is followed by two consecutive first-order steps (k(2)=0.53 s(-1) and k(3)=0.10 s(-1)). For 2, a slower second-order process (k(1)=4.9x10(5) M(-1) s(-1)) is followed by a slow first-order step (k(2)=0.40 s(-1)). In the case of 3, only a very slow second-order process was observed (k(1)=1.2x10(5) M(-1) s(-1)). The different metal-ion incorporation rates for model compound 1 and dendrimers 2 and 3 have been discussed in terms of conformational changes of the dendron subunits affecting the chelating properties of the cyclam core. This work reports the first kinetic study on metal-ion coordination by dendrimers with a well-defined coordination site.

Journal Article↗
PubMed2003

Covalent chemistry and conformational dynamics of topologically chiral amide-based molecular knots.

The readily available in gram quantities tris(allyloxy)knot of the amide-type 5 (knotane) can be completely and partially deprotected with nBu(3)SnH in the presence of a palladium catalyst resulting in hydroxyknotanes 7-9. These, in turn, react with diethylchlorophosphate giving rise to knotanes equipped with between one and three phosphoryl groups. Sulfonylation of bis(allyloxy)monohydroxyknotane 8 with p-toluenesulfonyl chloride and, following removal of one or two allyl groups from the intermediate monosulfonate 13, give rise to sulfonyloxy-allyloxy-hydroxy- and sulfonyloxy-dihydroxy-knotanes 15 and 14, respectively. This provides a convenient method for the preparation of knotanes with any substitution pattern. All new knotanes have been isolated in preparative amounts and as highly pure substances with an exception of allyloxy-dihydroxyknotane 9. This compound could only be obtained as a mixture with the corresponding monohydroxy-derivative 8. The structures of all synthesized compounds were established by means of FAB and MALDI TOF mass spectrometry, (1)H and (31)P NMR spectroscopy. The triphosphorylated knotane 10 exhibits high solubility in alcohols, allowing its complete enantiomeric resolution with a commercially available chiral HPLC column. (1)H,(1)H DQF-COSY correlation spectroscopy along with H/D exchange experiments and ab initio calculations provided the first detailed (1)H NMR signal assignments of knotanes in [D(6)]DMSO solution. The combination of variable temperature (1)H and (31)P NMR spectroscopy and molecular modeling has been applied to study the conformational behavior of the new knotanes in different solvents. It has been shown that in DMSO solution at room temperature knotanes exist in a relatively rigid nonsymmetrical conformation similar to that found in the solid state while faster conformational exchange leading to the average D(3) symmetrical structure was detected in a number of other solvents.

Amides↗
PubMed2003

Dendrimers as ligands. Formation of a 2:1 luminescent complex between a dendrimer with a 1,4,8,11-tetraazacyclotetradecane (cyclam) core and Zn2+.

We have investigated the formation of metal complexes between Zn2+ and two derivatives, 1 and 2, of the well-known 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand. Compound 1 is 1,4,8,11-tetrakis(naphthylmethyl) cyclam, and compound 2 is a dendrimer consisting of a cyclam core with appended 12 dimethoxybenzene and 16 naphthyl units. Compound 1 exhibits an emission band with a maximum around 480 nm, assigned to the formation of exciplexes between amine and excited naphthyl units. Dendrimer 2 exhibits three types of weak emission bands, assigned to naphthyl localized excited states (lambdamax = 337 nm), naphthyl excimers (lambdamax ca. 390 nm), and naphthyl-amine exciplexes (lambdamax = 480 nm). In CH3CN-CH2Cl2 1:1 v/v, titration of ligand 1 with Zn2+ causes the disappearance of the exciplex emission and the appearance of a strong naphthyl localized fluorescence; the titration plot is linear and reaches a plateau for a 1:1 stoichiometry, showing that a highly stable [Zn(1)]2+ complex is formed. In the case of 2, titration with Zn2+ causes the disappearance of the exciplex band, with a concomitant increase in the excimer and naphthyl localized emissions; the titration plot is again linear, but in this case it reaches a plateau for a 2:1 stoichiometric ratio, showing the unexpected formation of a [Zn(2)2]2+ complex. Such an unexpected stoichiometry for the complex of the dendritic ligand has been fully confirmed by 1H NMR titrations. The results obtained show that the dendrimer branches not only do not hinder, but in fact favor coordination of cyclam to Zn2+.

Journal Article↗
PubMed2002

Dendronized molecular knots: selective synthesis of various generations, enantiomer separation, circular dichroism.

For the first time, knot molecules (of the amide type) are synthesized, which bear one to three dendritic units of various generations at their periphery. They were obtained through two different routes: i) attachment of dendritic wedges to new mono-, di- and trihydroxy functionalized dodecaamide knots that have been obtained by selective debenzylation of oligobenzyloxy substituted knots, or ii) cyclization of already dendron substituted pyridine-2,6-dicarbonyl dichlorides with an "extended diamine" to directly yield the "tri-dendroknots". The derivatization of knot molecules by functional substituents and even large dendritic units is an important advance in the synthesis and property variation of molecular knots. This holds true in particular for substitution of the pyridine units of the knots, whereas the isophthalic acid units seem not to tolerate larger substitutents, as reflected in lower knot yields. These syntheses also demonstrate knots to be accessible indirectly by substitution of the corresponding mono-, di- and tri-functionalized knot skeleton. An advantage of dendritic "decoration" is the control of solubility and chromatographic behaviour of the molecular knots (knotanes). Suggestions are made about the threading mechanism by supramolecular template effects leading to the formation of amide-based molecular knots. The topological chirality of the new "dendroknots" is shown by efficient enantioseparations (separation factor alpha between 1.22 and 1.48). For this purpose (commercially unavailable) chiral column material of the Chiralpak type was used, in which the chiral component is covalently bonded to the silica gel support. The racemate splittings provide additional evidence for the knotted structure, as all other conceivable isomers such as macromonocyclic or catenated dodecaamides would not be chiral. The pure enantiomers obtained exhibit pronounced Cotton effects in their circular dichroism spectra. By comparison with the unsubstituted knot, the absolute configuration (Lambda, Delta) of all new knots is derived.

Journal Article↗
PubMed2002

Probing the stereointegrity of Tröger's base--a dynamic electrokinetic chromatographic study.

Under acidic conditions the enantiomers of Tröger's base 1 (2,8-dimethyl-6 H,12 H-5,11-methanodibenzo[b,f][1,5]diazozine) are subject to enantiomerization. During enantioselective dynamic electrokinetic chromatography using 10 mM hydroxypropyl-beta-cyclodextrin as the chiral mobile phase additive in 50 mM tris/phosphate buffer at pH 2.2, enantiomerization of Tröger's base gives rise to characteristic elution profiles featuring plateau formation and peak broadening. Introduction of a permanent positive charge attributed to quaternization in the monobenzylated derivative of Tröger's base 2 (5-benzyl-2,8-dimethyl-6 H,12 H-5,11-methanodibenzo[b,f][1,5]diazozinium bromide) decreases the enantiomerization barrier significantly. To determine the rate constants of enantiomerization the experimental chromatograms were evaluated by a direct calculation method and by using the computer simulation program ChromWin. From temperature-dependent measurements the Eyring activation parameters for 1 and 2 were determined: 1: DeltaG( not equal ) (298 K)=100.9+/-0.5 kJ mol(-1), DeltaH( not equal )=89.5+/-2.0 kJ mol(-1), DeltaS( not equal )=-42+/-10 J K(-1) mol(-1); 2: DeltaG( not equal ) (298 K)=90.2+/-0.5 kJ mol(-1), DeltaH( not equal )=91.4+/-2.0 kJ mol(-1), DeltaS( not equal )=9.8+/-10 J K(-1) mol(-1).

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PubMed2002

Luminescent lanthanide ions hosted in a fluorescent polylysin dendrimer. Antenna-like sensitization of visible and near-infrared emission.

We have investigated the complexation of the luminescent Nd(3+), Eu(3+), Gd(3+), Tb(3+), Er(3+), and Yb(3+) ions by a polylysin dendrimer containing 21 amide groups in the interior and, in the periphery, 24 chromophoric dansyl units which show an intense fluorescence band in the visible region. Most of the experiments were performed in 5:1 acetonitrile/dichloromethane solution at 298 K. On addition of the lanthanide ions to dendrimer solutions, the fluorescence of the dansyl units is quenched; in Nd(3+), Er(3+), and Yb(3+), a sensitized near-infrared emission of the lanthanide ion is observed. At low metal ion concentrations, each dendrimer hosts only one metal ion and when the hosted metal ion is Nd(3+) or Eu(3+), the fluorescence of all the 24 dansyl units of the dendrimer is quenched with unitary efficiency. Quantitative measurements were performed in a variety of experimental conditions, including protonation of the dansyl units and measurements in rigid matrix at 77 K where a sensitized Eu(3+) emission could also be observed. The results obtained have been interpreted on the basis of the energy levels and redox potentials of dendrimer and metal ions.

Journal Article↗
PubMed2002

Monitoring apple flavor by use of quartz microbalances.

"Electronic noses", i.e. arrays of differently coated quartz microbalances (QMB), have been used for selective detection of, and discrimination between, volatile organic compounds (VOC) formed during the post-harvest ripening of apples. The flavor components to be differentiated are chemically rather similar carbonyl compounds, chiefly aldehydes and esters. Because their relative ratios change during the post-harvest ripening period, appropriately selected sensor-active layers lead to characteristic patterns of the sensor responses which can be analyzed via pattern-recognition methods. This enables qualitative and quantitative identification of individual components whereby the post-harvest ripening of apples and other fruits can be monitored. Different kinds of apple differ in type and concentration of individual carbonyl compounds.

Aldehydes↗
PubMed2002

Photochemical and photophysical properties of poly(propylene amine) dendrimers with peripheral naphthalene and azobenzene groups.

We report the preparation, the absorption spectra, and the photophysical and photochemical properties in dichloromethane solution of four dendrimers of the poly(propylene amine) family (indicated by POPAM or PPI) functionalised with naphthalene and trans-azobenzene units. Each dendrimer Gn, where n = 1 to 4 is the generation number, comprises 2(n+1)-2 (i.e., 30 for G4) tertiary amine units in the interior and 2(n1) (i.e., 32 for G4) naphthalene and trans-azobenzene units in the periphery. For comparison purposes, the photophysical and photochemical properties of model compounds of the peripheral units have also been investigated. We have found that the fluorescence of the naphthalene units is quenched by the tertiary amines (via electron transfer) as well as by the trans-azobenzene units (via energy transfer). The quantum yields of the trans-->cis and cis-->trans photoisomerisation of the azobenzene units have been measured at various excitation wavelengths. Quenching of the fluorescence of the excited naphthalene unit by the trans- and cis-azobenzene units is accompanied by the sensitisation of the cis-->trans (but not of the trans-->cis) isomerisation. The rate constant of the thermal cis-->trans isomerisation of the azobenzene units has also been measured. Comparison of the results obtained for model compounds and for the G4 dendrimer shows that the dendritic structure favours the trans configuration of the azobenzene units ("dendritic effect"), presumably because it is less demanding in terms of space and causes less crowding on the dendrimer surface.

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