- S. Rosenfeldt, N. Dingenouts, D. Pötschke, M. Ballauff,
A.J. Berresheim, K. Müllen, P. Lindner, K. Saalwächter.
Analysis of the spatial structure of rigid polyphenylene
dendrimers by small-angle neutron scattering.
J. Lumin. 111, 225-238 (2005).
The analysis of the spatial structure of a rigid polyphenylene dendrimer G4-M of
fourth generation by small-angle neutron scattering (SANS) is presented. This
dendrimer is composed of phenyl units and is therefore devoid of any flexible
unit. The scattering intensity of dilute solutions of the dendrimer was measured
by SANS at different contrast which was adjusted by mixtures of protonated and
deuterated toluene. Hence, the method of contrast variation could be applied and
the data yield the scattering function extrapolated to infinite contrast. The
comparison of this data with simulations demonstrates that the scaffold of the
dendrimer is rigid as expected from its chemical structure. The positions of the
various units setting up consecutive shells of the dendrimer are relatively well
localized and the entire structure cannot be modeled in terms of spherically
symmetric models. No backfolding of the terminal groups can occur and the model
calculations demonstrate that higher generations of this dendritic scaffold must
exhibit a dense shell and a congestion of the terminal groups. This finding is
directly corroborated by recent solid-state NMR data. All results show that the
rigid dendrimer investigated here presents the first example for a dendritic
structure whose segment density does not have its maximum at the center. Rigid
scaffolds are therefore the only way to achieve the goal of a "dense-shell"
dendrimer whereas flexible scaffolds leads invariably to the "dense-core" case.