Use of a paramagnetic core to affect longitudinal nuclear relaxation in dendrimers - A tool for probing dendrimer conformation

The longitudinal relaxation time constants (T-1) of the protons in a series of dendrimers that alternatively had paramagnetic ([Fe4S4(SR)(4)](2-), R = dendron) and diamagnetic (tetraphenylmethane) cores were compared. The T-1 values of the phenyl and benzyl protons in the paramagnetic core dendrimers were attenuated compared to those of analogous protons in the diamagnetic core dendrimers. This observation indicated that protons in each set of topologically different repeat units (generation) of the dendrimer approach the core of the molecule closely in space. This conclusion is consistent with the computed radial density distributions of the different generations calculated from molecular dynamics simulations. In addition, by comparing T-1 values of protons at two slightly different temperatures, the terminal groups in both sets of dendrimers were concluded to be, on average, more mobile than the other generations within the dendrimers. This conclusion is consistent with the computed mean square displacement correlation functions for the different generations also calculated from molecular dynamics simulations.