13C and 31P MAS NMR investigations of spirocyclotriphosphazene nanotubes

The inclusion compounds (ICs) of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) with several small molecules have been studied, for the first time, by C-13 magic angle spinning NMR, The channel-type structure of TPP ICs (about 5 Angstrom wide) provides an aromatic environment to trap some molecules such as benzene, tetrahydrofurane and p-xylene. C-13 solid state NMR shows that the high symmetry of the adducts is consistent with an hexagonal crystal cell. The dynamic behaviour of the guest and host molecules has been studied by the measurement of C-13 T-1 relaxation times and compared with structures containing 10 Angstrom diameter channels. The release of the guest molecules has been followed by differential scanning calorimetry and by solid state NMR. There exist two crystal structures of the guest-free TPP molecule: the former is monoclinic and the latter retains the channel-type structure and hexagonal packing. C-13 CP MAS spectra and relaxation times of guest-free TPP structures demonstrate the existence of empty nanotubes stable up to 150 degrees C. These structures are easily available for uptaking guest molecules by a phenomenon which is quite unusual for molecular architectures. The peak multiplicity of P-31 CP MAS spectra reflect the symmetry of the crystal cells together with residual dipolar coupling with N-14.