ESR study of molecular orientation and dynamics of nitronyl nitroxide radicals in CLPOT 1D nanochannels

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.