Spin–Spin Interaction of TEMPO Molecular Chains Formed in an Organic One-Dimensional Nanochannel as Studied by Electron Spin Resonance (ESR)

Abstract

The magnitude and dimensionality of spin–spin interactions among 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (TEMPO) molecules in a 1D nanochannel consisting of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were examined using electron spin resonance (ESR) spectroscopy. Dilution of TEMPO with 2,2,6,6-tetramethyl-1-piperidine (TEMP), a diamagnetic molecule, reduced the dipolar interaction in the TPP/(TEMPO/TEMP) inclusion compound (IC), leading to axially symmetric hyperfine coupling and g-tensors with a molar fraction of 0–0.45: A⊥ = 1.93 ± 0.03 mT and A|| = 0.7 ± 0.1 mT, and g⊥ = 2.0063 ± 0.0003 and g|| = 2.0068 ± 0.0005. The resonance line in TPP/(TEMPO/TEMP) IC narrowed when the molar fraction was more than 0.7, implying the existence of exchange narrowing. The ESR lineshape of TPP/TEMPO IC showed pure 1D spin diffusion between 139 and 166 K, while interchain exchange interactions contributed to the line shape in T > 166 K. The magnitude of intra-chain exchange interaction, |Jintra|/kB, in TPP/TEMPO IC depended on the temperature from ≈0.05 K in 139–166 K to 0.4 K at 383 K. This aspect implies that molecular motion strongly affects the spin–spin interaction between TEMPO molecules in the TPP crystal. The magnetic susceptibility of TPP/TEMPO IC was also measured, and it was found that the exchange interaction was antiferromagnetic.

Molecular Orientation and Dynamics of Different Sized Organic Radicals Included in Organic 1D Nanochannels

“…When TEMP is used as a spacer, the contribution of a small amount of TEMPO, generated by the oxidation of TEMP by heating during sample preparation, to the ESR signal should be considered. 10,23 However, such an ESR signal for 4 may be negligible because the ESR signal of TEMPO derived from TEMP is expected to be weaker than that of the target radical (e.g., TEMPONE in this case) by a few orders of magnitude. 10,23 The 17 Coincidentally, the pore diameter is comparable to the molecular cross-section of TEMPONE, and it is slightly large to accommodate TEMPO (see Table 1).…”

“…10,23 However, such an ESR signal for 4 may be negligible because the ESR signal of TEMPO derived from TEMP is expected to be weaker than that of the target radical (e.g., TEMPONE in this case) by a few orders of magnitude. 10,23 The 17 Coincidentally, the pore diameter is comparable to the molecular cross-section of TEMPONE, and it is slightly large to accommodate TEMPO (see Table 1). Therefore, the difference in the dynamics of TEMPO and TEMPONE included in TPP nanochannels and that in the intermolecular interaction between radical and spacer molecules are probably significant (see section 3.3).…”

Preparation and Characterization of Inclusion Compounds Using TEMPOL and an Organic 1-D Nanochannel as a Template

“…In addition, the temperature-dependent ESR spectra were characterized by the molecular motion of TEMPO depending on the temperature regions: the rigid-limit for T < 128 K; superposition of 60% uniaxial molecular rotation and 40% rigid components for 128 K < T < 173 K; and the uniaxial rotation of all TEMPO molecules for T > 173 K. It is reasonable to infer that the molecular orientation and dynamic behavior of TEMPO in [TPP=TEMPO] IC are approximated by those in [TPP=(TEMPO) 0.017 -(TEMP) 0.983 ] IC . These results can be related to temperature-dependent magnetism based on the following temperature ranges: paramagnetic interaction for T < 139 K; pure 1-D spin diffusion at 139 K < T < 166 K; and anisotropic 3-D antiferromagnetic interaction for T > 166 K [22]. For the design of a new organic magnet using TPP as a template, it is important to clarify the relationship between the inter-spin interaction and the molecular dynamics of guest molecules in the 1-D spin chain.…”

“…In an earlier study, we estimated a minimum diameter of the TPP nanochannels of [TPP=TEMPO] IC at 100 K to be 0.58 nm [22]. The TEMPO molecules are accommodated in the portion of the maximum diameter (0.85 nm) of the nanochannel in a molecular orientation in which the N-O bond is perpendicular to the channel axis.…”

“…The structure of TEMPO radicals in the TPP nanochannels could not be confirmed by single-crystal X-ray diffraction (XRD), even at 100 K. Instead, we obtained information about the molecular orientation of TEMPO in the TPP nanochannels by the spin probe technique using electron spin resonance (ESR) [18][19][20][21]. For the co-inclusion compound of TEMPO and 2,2,6,6-tetramethylpiperidine [TEMP, Scheme 1(c)], ([TPP=(TEMPO) 0.017 À (TEMP) 0.983 ] IC ), it was shown that the TEMPO molecule undergoes uniaxial reorientation around the channel axis of TPP [22]. Detailed analyses of the powder patterns [23] revealed the orientation of the principle axes of g and hyperflne (A) tensors in TEMPO with respect to the channel axis, showing that the average direction of the N-O group of TEMPO is nearly perpendicular to the channel axis [24].…”