No abstract

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 aqueous phase. Electron microscopy and dynamic light-scattering measurements performed on aqueous solutions of the amphiphiles at pH = 1 showed the formation of small spherical aggregates with diameters varying between 20 and 200 nm. X-ray diffraction of cast films of these aggregates revealed bilayer thicknesses of 48−54 Å. A phase transition was detected by measuring fluorescence anisotropy. The theoretical volumes of the dendritic amphiphiles were in good agreement with those calculated from the monolayer experiments and X-ray diffraction data. Hence, the amphiphilic dendrimers within the aggregates in solution have the same highly asymmetric conformation as that proposed at the air−water interface. Calculations showed that the shape of the dendritic poly(propylene imine) core in the aggregates is distorted and that the axial ratio (r b:r a) ranges from 1:2.5 for the first generation to approximately 1:8 for the three highest generation of dendrimer. This behavior illustrates the high flexibility of the poly(propylene imine) dendrimers.

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 λ 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.