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A fifth-generation poly(propylene imine) dendrimer decorated with palmitoyl- and azobenzene-containing alkyl groups forms giant vesicles in aqueous solutions with diameters from 50 nm up to 20 mm and a multilaminar onion-like structure. Dense and ordered arrangement of the azobenzene chromophores in the bilayer structure leads to fluorescence with lmax= 600 nm. The fluorescence intensity can be increased by irradiation with blue light, and at low pH a distinctive blue shift of the spectrum is observed. With the aid of a single-beam optical tweezers it is possible to trap vesicles and direct them in a billiard-like fashion against each other using forces in the range of several pN. In collision experiments, the vesicles behave like hard spheres, and merging is not observed.

Higher generations of poly(propylene imine) dendrimers functionalized with aliphatic chains form large micrometer-sized spherical objects in aqueous solution below pH 8. These spheres are giant vesicles with a multilaminar onion-like structure. The size distribution and the structure of the vesicles depend on the pH of the solution and the endgroups at the periphery of the dendrimer. The vesicles containing azobenzene units (2 and 3) fluoresce with a maximum at ¿max = 600 nm. This emission can be attributed to the dense and ordered arrangement of the azobenzene chromophores in the bilayer structure. Laser irradiation of a small area of giant vesicles of 2 or 3 with 1064 and/or 420 nm light leads to changes in the morphology of the vesicles. Infrared light induces a rearrangement, whereas the azobenzene units isomerize under the influence of 420 nm light. Both irradiations lead to a change in refractive index in the illuminated area. Irradiation using 420 nm light is accompanied by an increase in the emission intensity. In aqueous solutions at pH 1, the increase in fluorescence intensity is concurrent with a blue shift of the emission maximum to 540 nm. This blue shift is not observed when the experiment is performed in Milli Q water (pH 5.5). The enhanced fluorescence can be attributed to reorganization of the chromophores within the giant vesicle. The increase in emission proves that the giant vesicle is a kinetically formed system that reaches a thermodynamically more relaxed state after light-induced isomerization.

Gas-phase fragmentation patterns of poly(propylene imine) dendrimers DAB-dendr-(NH2)n with n = 4, 8, 16, 32, and 64 are described as a result of MSn electrospray ionization mass spectrometry (ESI-MS) studies. The generally accepted on-the-average globular shape of dendrimers is supported here by the linear relationship found between the extent of protonation and molecular mass (M2/3). Unique fragmentation patterns are observed due to this globular shape. Most fragment ions are readily accounted for when, in addition to nucleophilic displacement reactions, proton shifts and rearrangements are taken into consideration. There is a striking difference in fragmentation behavior between singly charged dendrimers and multiply charged dendrimers. Therefore, it is proposed that the site of protonation within the dendritic framework has a large impact on the fragments observed. The position of the proton is largely dependent on both the relative proton affinities and the Coulomb repulsion between neighboring protonated sites. In case of singly charged dendrimers, electrostatic interactions are absent and the fragmentation is predominantly governed by the relative differences in proton affinity between the basic sites present in the dendrimer. In the gas phase the nitrogen atoms in the core of the dendrimer are the most basic ones present, leading to [M + H]+ ions carrying a protonated diaminobuthane core in all cases. This triggers a cascade of nucleophilic displacement reactions, starting with a bond cleavage of the central dendritic core and finishing with the formation of bis(1,1-propylamine)azetidinium (m/z 172) and fragments derived thereof. In the case of multiply charged dendrimers, Coulomb repulsions lead primarily to protonation of the peripheral tertiary amines, leading to the direct formation of bis(1,1-propylamine)azetidinium. These gas-phase reactions were subsequently utilized as a model for reactions in solution. The gas-phase reactions in ESI-MS thus provided a unique "fingerprint" area of dendrimer fragmentation adducts in the lower m/z range (m/z

The self-assembly of amphiphilic dendrimers based on poly(propylene imine) dendrimers of five different generations with up to 64 end groups modified with long hydrophobic chains has been studied. At the air-water interface stable monolayers form in which the dendritic surfactants presumably adopt a cylindrical shape; all the chains are aligned perpendicular to the water surface, and the dendritic poly(propylene imine) core faces the aq. phase. Electron microscopy and dynamic light-scattering measurements performed on aq. solns. of the amphiphiles at pH = 1 showed the formation of small spherical aggregates with diams. varying between 20 and 200 nm. X-ray diffraction of cast films of these aggregates revealed bilayer thicknesses of 48-54 .ANG.. A phase transition was detected by measuring fluorescence anisotropy. The theor. vols. of the dendritic amphiphiles were in good agreement with those calcd. from the monolayer expts. and X-ray diffraction data. Hence, the amphiphilic dendrimers within the aggregates in soln. have the same highly asym. conformation as that proposed at the air-water interface. Calcns. showed that the shape of the dendritic poly(propylene imine) core in the aggregates is distorted and that the axial ratio (rb:ra) ranges from 1:2.5 for the first generation to approx. 1:8 for the three highest generations. This behavior illustrates the high flexibility of the poly(propylene imine) dendrimers