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

Abstract

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).

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

“…Thus, nitroxide groups of uniaxially rotating DTBN molecules confined in TPP nanochannels are almost perpendicular to the rotation axis, which can be assumed to be parallel to the channel axis as for the [(TPP) 2 -(TEMPO) 0.017 (TEMP) 0.983 ] IC. 11 These results are consistent with the expectation from the results of the [(TPP) 2 -(DTBN) 1.24 ] IC. 16 The experimental results in the range of 54−70 K and above 116 K are well reproduced using this model.…”

“…This is considered to be related to the order of E a for the uniaxial rotational motion of guest radicals in TPP nanochannels. In contrast, although τ R for DTBN and TEMPONE molecules in TPP nanochannels reached the order of 10 −9 s around room temperature, the τ R for TEMPO was, at most, in the order of 10 −8 s, even at 293 K. 11 The rotational motion of guest molecules confined in nanospaces is generally influenced by the size of the nanospace. For example, in the [(CLPOT) 2 -(TEMPO) 0.022 (TEMP) 0.98 ] IC, where the diameter of the CLPOT nanochannels is 1.1 nm, the τ R of TEMPO molecules in the nanochannels at room temperature is on the order of 10 −10 s, according to extrapolation, 23 although the combination and composition of guest radical and nonradical molecules are similar.…”

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

“…In fact, to prepare homogeneous ICs, and to investigate the molecular motion and orientation of TEMPOL in the TPP nanochannels more precisely, a new specimen should be prepared by the co-precipitation method, such as a [TPP=(TEMPOL) x -(TEMP-OH) y ] IC , analogous to our previous study in [TPP=(TEMPO) x -(TEMP) y ] IC [22,24]. Further works are now underway.…”

“…Uniaxial reorientation of TEMPOL will give rise to a triplet peak broadened by axial symmetric g and A tensors, similar to TEMPO in [TPP=TEMPO] IC [24]. Assuming that the TEMPOL molecules have similar orientation to that of TEMPO in [TPP=TEMPO] IC , and assuming that uniaxial reorientation of TEM-POL molecules takes place around the principle y axis approximately lying on the channel axis, the g and A tensors for the rigid component in Fig.…”

“…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]. 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 .…”