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 fluores cence decay in toluene indicates the presence of two emitting conforma tions. Molecular modeling suggests that a conformation where the dendr ons interact with the core is not improbable. The relative weight of t he two decay times indicates that the contribution of that conformatio n in toluene increases as the dendrimer generation increases. In aceto ne and acetonitrile however the fluorescence decay is, except for the fourth generation, monoexponential. The hydrodynamical volume of the d endrimer is determined in toluene, a good solvent, acetone, a medium q uality solvent, and acetonitrile, a poor solvent, with the time-resolv ed fluorescence depolarization technique. No change of the hydrodynami cal volume is found in toluene in a temperature range between 20 and 9 4 degrees C. This suggests that the dendrimers of all the generations are fully expanded in this solvent. In acetonitrile however the hydrod ynamical volume of the dendrimers is substantially smaller than in tol uene and acetone. This effect is most clear for the fourth-generation dendrimer. For this compound the hydrodynamical volume is only a few p ercentages larger than the excluded volume.