- M. Wind, K. Saalwächter, U.-M. Wiesler, K. Müllen,
and H. W. Spiess.
Solid-State NMR Investigations of Molecular Dynamics in Polyphenylene
Dendrimers: Evidence of Dense-Shell Packing.
35, 10071-10086 (2002).
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-MAS recoupling techniques for the quantification of
dynamic averaging in the MHz range, we derive a clear picture of the complex
molecular dynamics in these systems. Fast processes in the MHz regime are shown
to be restricted to fast vibrations of terminal phenyl rings with amplitudes of
up to 40° at most, with a 5-30% fraction of rings performing
larger-amplitude motions. Slow processes on the timescale of ms to s 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
24° 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 to
the dependence on dendrimer generation, both the fast and the slow processes
follow a trend which 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.