Multifrequency EPR study of the mobility of nitroxides in solid-state calixarene nanocapsules.

Multifrequency continuous wave (cw) and echo-detected (ED) electron paramagnetic resonance (EPR) was employed to study the mobility of nitroxides confined in nanocapsules. The complexes p-hexanoyl calix[4]arene with 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT) and N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl (DEPN) were studied by X-, W-band and 360 GHz cw EPR at temperatures between 90 and 370 K. Thereby we were able to extract the canonical values of the hyperfine and g-tensors of the encapsulated radicals as well as information on restricted orientational dynamics of the caged spin probes. Comparing cw and ED-EPR data obtained on MT@C6OH we found that between 90 and 200 K the caged nitroxide undergoes isotropic small-angle fluctuations (librations), whereas at higher temperatures restricted rotations of the radical with correlation times of 0.75 x 10(-10) s and 1.2 x 10(-10) s dominate at 325 and 300 K, respectively. The activation energy of the rotational motion of encapsulated MT radicals was evaluated as E(a) = 1.0 kcal mol(-1), which is comparable to the magnitude of van der Waals interactions. Compared to MT, the reorientational motion of DEPN was found to be slower and more isotropic.

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 calix[4]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 gxx 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.

para-Acylcalix[n]arenes: from molecular to macroscopic assemblies.

The para-acylcalix[n]arenes possess a very rich capacity to self-assemble into a wide variety of structures and sizes ranging from molecular assemblies through dimeric capsules, molecular sheets to nanoparticles. All these assemblies are capable of taking guest molecules and in the process of this inclusion discrete nanoscopic reaction vessels may be formed for photochemistry. Interestingly this uptake of quite large organic molecules occurs in the bulk in non-porous crystals without loss of crystallinity. At the air-water interface either as Langmuir monolayers or as colloidal suspensions the para-acylcalix[n]arenes show interaction with ionic species. The extension from para-acylcalix[4]arenes to para-acylcalix[8]arenes is in its infancy but already there is much promise for novel assemblies to be found.

Crystalline inclusion complex of a calixarene with a nitroxide.

The first capsular crystalline inclusion complex of para-hexanoyl calix[4]arene with stable nitroxyl radical DEPN has been isolated and showed a low mobility of the radical in the capsule.

A molecular turnstile in para-octanoyl calix[4]arene nanocapsules.

The thermal treatment of different inclusion complexes of para-octanoyl calix[4]arene leads to the formation of a guest-free van der Waals nanocapsular structure possessing a remarkable stability caused by the high mobility of alkanoyl arms.

Supramolecular stabilization of hydroxylamine TEMPOH by complexation with an amphiphilic calixarene.

In an ethanol/water mixture, the nitroxyl radical TEMPO abstracts a hydrogen atom from a phenolic OH group of the amphiphilic para-hexanoylcalix[4]arene, and the hydroxylamine TEMPOH formed yields a stable inclusion complex with another molecule of the calixarene.

Phototransformation of stilbene in van der Waals nanocapsules.

We have utilized para-hexanoylcalix[4]arene nanocapsules as hosts to carry out phototransformations of cis- and trans-stilbene. Single-crystal X-ray diffraction studies were performed to define precisely the location of encapsulated stilbenes inside the capsule and to analyze possible pathways of phototransformation. cis-Stilbene stacks as a pi-pi dimer located at the center of the capsule, whereas trans-stilbene does not form such a dimer. Irradiation of the crystalline inclusion complexes of each isomer of stilbene in the solid state leads to the appearance of the second isomer, and after prolonged photolysis, photodimerization also occurs. syn-Tetraphenylcyclobutane is formed as the major product of dimerization and its yield depends on the time and intensity of irradiation. In most cases, the single crystals of the complexes remain intact during irradiation; hence, the nanocapsules have the potential to serve as robust nanoreactors in the solid state. The confinement in the nanocapsules is sufficient to keep the reacting molecules together, although this is less restrictive than for trans-stilbene crystals, in which the molecules cannot achieve a favorable orientation for dimerization.