New inclusion compounds (ICs) were prepared using the organic 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) as a nanosized template and nitronyl nitroxide (NN) radicals such as phenylnitronylnitroxide (PhNN) and p-nitrophenylnitronylnitroxide (p-NPNN). ESR measurements below 255 K for the CLPOT ICs diluted with spacer molecules gave rigid limit spectra similar to that for PhNN molecules in a glassy ethanol matrix at low temperature, which suggests isolation of the radical molecules. ESR measurements for them in the range of 290-400 K gave a modulated quintet ESR signal, which suggested uniaxial rotational diffusion of NN radicals in the nanochannels approximately around the principal y-axis of the g-tensors. In the ESR measurements to 430 K for the [(CLPOT)2-(p-NPNN)0.07] IC without spacers, the broader line width than the case in dilution was observed by inter-radical dipolar interaction. In every case, the rotational diffusion activation energies of NN radicals in the CLPOT nanochannels were several times larger than those of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical derivatives (4-X-TEMPO) in CLPOT nanochannels. This is expected due to the larger molecular size of NN radicals than 4-X-TEMPO or stronger interaction between NN radicals and the surrounding host or guest molecules.

Imino and nitronyl nitroxide (IN and NN, respectively) radicals such as phenyliminonitroxide (PhIN) and phenylnitronylnitroxide (PhNN), respectively, were dispersed in the organic 1D nanochannels of tris( o-phenylenedioxy)cyclotriphosphazene (TPP). Electron spin resonance (ESR) measurements were conducted on these inclusion compounds (ICs) in the temperature range 4.2-300 K. The modulated-septet ESR spectra of TPP ICs using PhIN observed in the range 165-258 K were reproduced with the EasySpin program package using a model in which some of the PhIN molecules underwent uniaxial rotational diffusion in the TPP nanochannels around the molecular long axis corresponding to the principal y-axis of the g tensor. However, for the TPP IC using PhNN, complicated ESR spectra were observed, which were not consistent with the modulated quintet observed in solution or in the fast-motion limit in solids. These spectra were reproduced by the superposition of a quintet originating from rotational diffusion of PhNN molecules and a septet based on rotational diffusion of PhIN molecules generated in the synthetic process. The rotational diffusion activation energies of PhIN and PhNN in the TPP nanochannels were estimated to be 19 and 45 kJ mol using an Arrhenius plot, respectively. These were consistent with that for NN radicals in the 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) (37-54 kJ mol) with a larger pore diameter than TPP reported in our previous study, or with that for 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-X-TEMPO) in TPP nanochannels (5-26 kJ mol) with regard to molecular size or host-guest or guest-guest interactions. These results indicate that not only the NN group but also IN may be used for the clarification of chemical or biological structures of nanomaterials such as nanosized cavities.

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.