The molecular orientation and dynamics of di-t-butylnitroxide (DTBN) and 4-oxo-2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPONE) radicals in the organic 1D nanochannels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were investigated by examining inclusion compounds (ICs) diluted by the co-inclusion of nonradicals using electron spin resonance (ESR). Spectral simulation showed that the axial rotation of DTBN or TEMPONE molecules is excited in TPP nanochannels with activation energies of 3 and 10 kJ mol(-1), respectively. The rotation axes for both DTBN and TEMPONE were determined to be almost parallel to the principal y-axis of the g tensor, which agrees with the result reported for 2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPO) in TPP nanochannels, which has the activation energy of 5 kJ mol(-1). These results indicate that the molecular orientations of guest nitroxide radicals are almost independent of the molecular sizes of the guest radicals in TPP nanochannels, although the molecular dynamics are dependent on the molecular sizes.

ESR measurements of [CLPOT-(4-X-TEMPO)] inclusion compounds (CLPOT = 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine, and 4-X-TEMPO = 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl radical) with 1D organic-radical molecular chains constructed in organic 1D nanochannels used as templates were conducted in the range of temperatures from 4.2 to 300 K. The peak-to-peak line width (ΔBpp) of all isotropic ESR spectra of the [CLPOT-(4-X-TEMPO)] was much narrower in the whole temperature range than the estimation based on the Van Vleck’s formula for the second moment of the rigid lattice model. In addition, the line profiles of all ESR spectra of the [CLPOT-(4-X-TEMPO)] became Lorentzian in the whole temperature range, compared with our previous results using tris(o-phenylenedioxy)cyclotriphosphazene (TPP) as templates in which the ESR profiles were temperature-dependent. These results indicate that the narrower ESR line width of the [CLPOT-(4-X-TEMPO)] may be caused not by motional narrowing but by 3D exchange interaction even at room temperature although the magnitude was less than 1 K.

Variable-temperature electron spin resonance (ESR) was measured for one-dimensional (1D) molecular chains formed using different-sized organic radicals incorporated into the 1D nanochannels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP). The ESR spectra for the molecular chains of 4-oxo-2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPONE) incorporated in the TPP nanochannels ([TPP-TEMPONE]) exhibited anisotropic three-dimensional (3D) spin diffusion at temperatures close to room temperature. In contrast, 1D spin diffusion was observed even at low temperatures indicating a longer rotational diffusion correlation time or the termination of molecular motion of the guest radicals dispersed in the TPP nanochannels. The temperature range for 1D spin diffusion in [TPP-TEMPONE] was higher and wider than that of TPP inclusion compounds incorporating smaller nitroxide radicals, such as di-t-butyl nitroxide (DTBN), or 2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPO) radicals, as previously reported. Thus, inter-spin interactions of the organic-radical 1D molecular chains formed in size-adjustable nanochannels, such as TPP, are influenced by the molecular size and dynamics of guest radicals, and temperature.

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