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Abstract:
We present solid-state NMR investigations of a series of shape- persistent polyphenylene dendrimers of generation 1-4 with different surface functionalization. Using a combination of traditional static and more advanced magic-angle spinning (MAS) exchange techniques for the elucidation of slow dynamics as well as fast-AMS recoupling techniques for the quantification of dynamic averaging in the megahertz range, we derive a clear picture of the complex molecular dynamics in these systems. Fast processes in the megahertz regime are shown to be restricted to fast vibrations of terminal phenyl rings with amplitudes of up to 40degrees at most, with a 5-30% fraction of rings performing larger-amplitude motions. Slow processes on the time scale of milliseconds to seconds are also restricted to terminal and doubly para-substituted phenyl rings. This type of motion is characterized by a two-site jump with a mean reorientation angle of 24degrees and a mean apparent activation energy of 34 kJ/mol and is presumably a concerted process involving several adjacent phenyl rings. The comparison of dendrimers with different surface functionalization allows us to conclude that the molecular dynamics are dominated by intramolecular steric constraints. As for the dependence on dendrimer generation, both the fast and the slow processes follow a trend that is expected from the evolution of the segment free volume at the periphery of the molecules, where most terminal rings are located. We therefore believe that our results represent the first experimental evidence of a class of dendrimers in which the radial segment density distribution is caused by truly extended arms and for which the dense-shell packing limit is reached for generation 4.
