ESR study of molecular dynamics and orientation of TEMPO included in organic 1-D nanochannel

A mixture of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical and 2,2,6,6-tetramethyl-1-piperidine (TEMP) was included into organic 1-D nanochannels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) crystal. Dilution of the paramagnetic TEMPO radical was achieved with excess TEMP, thereby isolating a TEMPO molecule in the nanochannel. For inclusion compounds of TPP with TEMPO and TEMP (TEMPO/all guest compounds = 0.017, and 0.15), temperature-dependent electron spin resonance (ESR) spectra were observed to investigate their molecular dynamics and orientation. In the temperature range from 112 K to room temperature, the spectra depended remarkably on the temperature. Temperature dependence was well interpreted by uniaxial rotation, suggesting that TEMPO molecules undergo uniaxial rotation about a channel axis with a molecular orientation in which the N-O bond in the nitroxide group is perpendicular to the channel axis. The activation energy of uniaxial rotation was evaluated as 4.5 +/- 0.3 kJ mol(-1).