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

Abstract

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 × 10⁻¹⁰ s and 1.2 × 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.