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

• Published in: Physical chemistry chemical physics : PCCP 2009-01, Vol.11 (31), p.6700-6707
• Main Authors: Bagryanskaya, Elena G, Polovyanenko, Dmitriy N, Fedin, Matvey V, Kulik, Leonid, Schnegg, Alexander, Savitsky, Anton, Möbius, Klaus, Coleman, Anthony W, Ananchenko, Gennady S, Ripmeester, John A
• Format: Article
• Language: English
• Subjects: Calixarenes - chemistry; Cyclic N-Oxides - chemistry; Electron Spin Resonance Spectroscopy; Nanocapsules - chemistry; Nitrogen Oxides - chemistry; Spin Labels; Temperature
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/b906827a

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.