Segmental mobility in phosphorus-containing dendrimers. Studies by fluorescent spectroscopy

Steady-state fluorescence spectra and decays of excitations of a phosphorus -containing dendrimer labeled with 12 internal labels and of an iminophosph orane model compound bearing two labels dissolved in acetonitrile, diglyme, 1,4-dioxane, triethylene glycol, and cyclohexanol revealed that the interi or of dendrimers contained many solvent molecules, and movements of interna lly located pyrene labels were not reduced by interactions with the dendrim er core. This conclusion was based on the following findings. A ratio of th e intensities of pyrene-pyrene excimer and pyrene monomer emissions (I-E/I- M) decreased with increasing solvent viscosity but, with exception of aceto nitrile solutions, was very close for the dendrimers and for the model. Mon omer and excimer emission intensity decays were fitted with two-exponential functions with time constants Ti and Tv characterizing formation of pyrene -pyrene excimers and recovery of pyrene moieties in their ground state, res pectively, For solvent viscosities varying from 3.44 x 10(-3) P (acetonitri le) to 4.78 x 10(-1) P (triethylene glycol), tau (1) increased from 7.1 to 37.7 ns and from 6.2 to 34.9 ns for the model and for the dendrimer. In ace tonitrile the rates of the pyrene-pyrene excimer formation were almost the same for the labels in solution and bound covalently in the interior of the dendrimer core. For more viscous solvents the extensive reduction of pyren e label mobility has been noticed for both the model and labeled dendrimer.