ESR study of the molecular orientation and dynamics of stable organic radicals included in the 1‐D organic nanochannels of 2,4,6‐tris‐4‐(chlorophenoxy)‐1,3,5‐triazine

Abstract

The molecular orientation and dynamics of the organic stable radicals such as 2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPO) or 4-hydroxy-TEMPO (TEMPOL) included in the one-dimensional (1-D) organic nanochannels of 2,4,6-tris-4-(chlorophenoxy)-1,3,5-triazine (CLPOT) were investigated by examining the inclusion compounds (ICs) diluted by the co-inclusion of non-radicals using ESR spectroscopy. Spectral simulation showed that the axial rotation of TEMPO or TEMPOL molecules is excited in the nanochannels with activation energies of 8 and 7 kJ mol(-1) , respectively. The rotation axis was estimated to be tilted towards the principal x direction in the axis system of the g-tensor of the respective radicals. This is quite different from that for similar ICs in the nanochannels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP), in which the radicals are axially rotating around the principal axis y of the g-tensor. The difference is attributed to the larger nanospace of the CLPOT nanochannels.

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

“…23 Neither of these components can reproduce the spectra of 1 by itself in the range of 85−106 K. Nor could the fine structure with the formation of radical clusters in the TPP nanochannels suggested by Barbon et al 26 be confirmed. This may be due to the coexistence of low-and high-temperature phases, in which the molecular motion of the guest molecules is frozen out and excited to start, respectively (see the next section).…”

“…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).…”

“…Twocomponent ESR spectra observed in other than 4 are independent of (1) the type of host material, i.e., even in the IC with 2,4,6-tris-(4-chlorophenoxy)-1,3,5-triazine (CLPOT), 23 which is another example of an organic 1D porous material where the crystal has larger pore diameters than TPP (TPP, 0.45−0.9 nm; CLPOT, 1.1−1.2 nm). In addition, they are independent of (2) the type of guest radical and (3) nonradical.…”

ESR study of molecular orientation and dynamics of TEMPO derivatives in CLPOT 1D nanochannels

“…The CLPOT crystal has cavities that are similar to TPP nanochannels, but with somewhat larger pore diameters on the order of 1.1–1.3 nm . CLPOT was employed for the inclusion of TEMPO and 4‐hydroxy‐TEMPO (TEMPOL; 1b in Scheme ), and the molecular orientations and dynamics of 1a and b in the CLPOT nanochannels were examined by variable temperature ESR. In particular, the difference of the molecular orientation of 4‐X‐TEMPO with regard to the channel axis of the CLPOT crystals was suggested, compared with the case of TPP nanochannels: approximately the rotation around the principal x or y axis of the g ‐tensor of 4‐X‐TEMPO, respectively (Fig.…”