Inclusion of 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl in a calixarene nanocapsule in the solid state

We present an X-ray diffraction (XRD) and multi frequency electron spin resonance (ESR) study of the structure and dynamics of an inclusion complex of p-hexanoyl calix4]arene (C6OH) with 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT). The single crystal XRD experiments reveal that MT along with ethanol (solvent) molecules are entrapped in a capsular type crystalline lattice of the host C6OH material. ESR measurements were performed at 9.2 GHz/0.33 T (X-band) and at 360 GHz/14 T. In order to avoid ESR line broadening resulting from electron dipole-dipole interaction between nitroxides occupying neighbouring capsules in the crystal lattice, the capsules containing nitroxides were separated from each other by capsules containing diamagnetic dibenzylketone (DBK). Due to the extremely high g-tensor resolution of ESR at 360 GHz, we were able to distinguish, by shifts of their g(xx) component, between encapsulated nitroxide molecules forming a hydrogen bond between their O-(N) group and the OH group of an ethanol molecule occupying the same capsule and nitroxides missing this interaction. Temperature dependent ESR measurements revealed an orientational anisotropy in the motion of MT encapsulated in C6OH. Solid lipid nanoparticles (SLN) prepared from C6OH and loaded with the nitroxide retained the microcrystalline capsular structure of the pertinent inclusion complex. We found that encapsulated MT in SLNs becomes inaccessible to reducing agents such as sodium ascorbate.