Shape-Persistent Polyphenylene Dendrimers—Restricted Molecular Dynamics from Advanced Solid-State Nuclear Magnetic Resonance Techniques

Abstract

The molecular dynamics of polyphenylene dendrimers(e.g., the second generation dendrimer shown in the Figure) is investigated here using advanced solid‐state NMR techniques. It is found that well‐localized, restricted reorientation of single terminal phenyl substituents occurs around fixed axes only, clearly demonstrating the shape persistence of these dendrimers. The synthetic approach to these polyaromatic dendrimers is also described.

Analysis of the spatial structure of rigid polyphenylene dendrimers by small-angle neutron scattering

“…In the following we shall focus on the dynamics of this dendrimer that is intimately related to its peculiar structure. Solid state NMR was used to elucidate the highly restricted motions which are allowed within the scaffolds of polyphenylene dendrimers, [25,31], and these data provide sound proof of the feasibility of the assumptions underlying the modeling of our scattering data. Since the pioneering study of Meltzer et al on rapid arm fluctuations in poly(amido amine) dendrimers by solution-state NMR relaxation [32,33], which yielded conclusive evidence of an absence of dense-shell packing, solid-state NMR has emerged as another important method to characterize dendron structure [34] and mobility [35].…”

“…Direct evidence for shape persistence could also be derived from the solid-state NMR studies [25,31]. The sole existence of small-angle terminal ring librations on the 100-kHz scale and above could essentially be confirmed; only a rather small fraction of para-substituted rings was found to undergo fast large-angle fluctuations around the para axis.…”

Single-Crystal Structures of Polyphenylene Dendrimers

“…The interlocking of twisted phenyl rings has exciting structural and dynamic consequences and can explain the stiffness of the molecular framework. This has been experimentally proven recently by means of solid‐state NMR techniques 14. The measurements clearly support the shape‐persistence of highly substituted polyphenylene dendrimers 2, 3.…”

Shape-Persistent, Fluorescent Polyphenylene Dyads and a Triad for Efficient Vectorial Transduction of Excitation Energy This research was supported by the TMR European Research Program through the SISITOMAS project, the Volkswagen-Stiftung, and the Bundesministerium für Bildung and Forschung. We also thank S. Spang and C. Beer for support in the synthetic work.

“…Other drawbacks are often the very limited photostability9 and low fluorescence quantum yields of the chosen chromophores. The use of a rigid polyphenylene dendrimer10a, 10b should overcome most of the above‐mentioned difficulties11, 12 since no back‐folding of the dendritic branches can occur13 and self‐quenching of the chromophores is not observed 12. Furthermore, energy‐transfer processes, including those at the single‐molecule level, were investigated by using polyphenylene dendrimers decorated with very photostable perylene dicarboxmonoimide chromophores; collective effects, quite well known in biological systems, have been recognized 14a, 14b.…”