Solvent dependence of the hydrodynamical volume of dendrimers with a rubicene core

Four generations of a dendrimer with a fluorescent core consisting of a rubicene moiety are synthesized. The biexponential nature of fluorescence decay in toluene indicates the presence of two emitting conformations. Molecular modeling suggests that a conformation where the dendrons interact with the core is not improbable. The relative weight of the two decay times indicates that the contribution of that conformation in toluene increases as the dendrimer generation increases. In acetone and acetonitrile however the fluorescence decay is, except for the fourth generation, monoexponential. The hydrodynamical volume of the dendrimer is determined in toluene, a good solvent, acetone, a medium quality solvent, and acetonitrile, a poor solvent, with the time-resolved fluorescence depolarization technique. No change of the hydrodynamical volume is found in toluene in a temperature range between 20 and 94 degrees C. This suggests that the dendrimers of all the generations are fully expanded in this solvent. In acetonitrile however the hydrodynamical volume of the dendrimers is substantially smaller than in toluene and acetone. This effect is most clear for the fourth-generation dendrimer. For this compound the hydrodynamical volume is only a few percentages larger than the excluded volume.