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 iminophosphorane model compound bearing two labels dissolved in acetonitrile, diglyme, 1,4-dioxane, triethylene glycol, and cyclohexanol revealed that the interior of dendrimers contained many solvent molecules, and movements of internally located pyrene labels were not reduced by interactions with the dendrimer core. This conclusion was based on the following findings. A ratio of the intensities of pyrene−pyrene excimer and pyrene monomer emissions (I E/I M) decreased with increasing solvent viscosity but, with exception of acetonitrile solutions, was very close for the dendrimers and for the model. Monomer and excimer emission intensity decays were fitted with two-exponential functions with time constants τ1 and τ2 characterizing formation of pyrene−pyrene excimers and recovery of pyrene moieties in their ground state, respectively. For solvent viscosities varying from 3.44 × 10-3 P (acetonitrile) to 4.78 × 10-1 P (triethylene glycol), τ1 increased from 7.1 to 37.7 ns and from 6.2 to 34.9 ns for the model and for the dendrimer. In acetonitrile 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 pyrene label mobility has been noticed for both the model and labeled dendrimer.