Nitroxide-labeled Ru(II) polypyridyl complexes as EPR probes of organized systems .3. Characterization of starburst dendrimers and comparison to photophysical measurements

Ruthenium(II) phenanthroline complexes labeled with a nitroxide radical through -NHCOO(CH2)- and -O(CH2)(8)O- links, RuPT and RuPC8T, respectively, have been utilized as EPR probes to monitor the binding and dynamics of the complexes on the surface of starburst dendrimers (SBDs) of varying size (generations). Analysis of the EPR signals by spectral computation provided structural and dynamic parameters. The cationic probes bind to the negatively charged surface of the half-generation dendrimers (n.5-SBDs) through electrostatic interactions, which is consistent with decreased binding upon protonation of the NR(2)R' internal groups of the SBDs. The results indicate that there is a fast exchange of RuPT between the n.5-SBD surface and the bulk solution and that the probe does not interact with the nonionic full-generation dendrimers (n.0-SBDs). The activation energies for the rotational motion of the probes are consistent with their localization in a microheterogeneous environment. RuPC8T exhibited both hydrophilic and hydrophobic interactions with the anionic n.5-SBDs, whereas only hydrophobic interactions were observed with the neutral n-SBDs. The N-O-. group of RuPC8T was deduced to be distributed throughout the various hydration layers at the SBD/water interface. A good correlation was found between the rotational mobility of RuPT evaluated by EPR analysis and the translational mobility of Ru(phen)(3)(2+) derived from photophysical measurements, especially for G = 4.5 and G = 5.5 SBDs.