We
examine the fluorescence anisotropy of rod-shaped guests held
inside the channels of tris(o-phenylenedioxy)cyclotriphosphazene
(TPP) host nanocrystals, characterized by powder X-ray diffraction
and solid state NMR spectroscopy. We address two issues: (i) are light
polarization measurements on an aqueous colloidal solution of TPP
nanocrystals meaningful, or is depolarization by scattering excessive?
(ii) Can measurements of the rotational mobility of the included guests
be performed at low enough loading levels to suppress depolarization
by intercrystallite energy transfer? We find that meaningful measurements
are possible and demonstrate that the long axis of molecular rods
included in TPP channels performs negligible vibrational motion.