12 Inter-spin Interactions of Organic Radical Chains in Organic 1D. . .
445
Figure 12.6 shows continuous changes in the ESR spectra with increases in
temperature. In the vicinity of 351 K, the 1b/2g/3e spectra exhibit two narrower
peaks between lines 2 and 3 in Fig. 12.2b (see Sect. 12.2.3.4) [27, 66]. In addition,
a modulated quintet associated with the two 14 N atoms for which I = 1 in the
NN group appears above 369 K for p-NPNN. The temperature-dependent ESR
spectra of 1b/2g/3e were reproducible based on a model for assumption anisotropic
rotational diffusion of isolated p-NPNN molecules in CLPOT nanochannels within
the slow-motion regime with respect to the ESR time scale (10 −9 s < τ R < 10 −6 s)
over the entire temperature range in Fig. 12.6. The 1b/2g/3e spectra at 398 K
were obtained by assuming rotational diffusion around the axis tilted by (θ ,
φ) = (74 ◦ , 90 ◦ ) relative to the principal system of the g tensor, with Gaussian and
Lorentzian line width components and τ R of 0.11 and 0.30 mT and 1.0 × 10 −8 s,
respectively. The ESR spectra were also well-reproduced at other temperatures
using the anisotropic rotational diffusion model around the same rotation axis.
The rotational axes in 1b/2g/3e are approximately parallel to the principal y-axis
of the g tensor (θ = φ = 90 ◦ ). For this reason, p-NPNN molecules are expected to
be included in the CLPOT nanochannels with the longest molecular axis parallel to
the channel axis, assuming that the rotational axis is parallel to the channel axis. The
(θ , φ) parameters for the rotational axis relative to the principal axis for p-NPNN
are larger than those for PhNN ((θ , φ) = (68 ◦ , 78 ◦ )). This difference is likely the
result of the ratio of the molecular lengths of the NN radicals relative to the pore
diameter for the CLPOT nanochannels (0.9 nm for PhNN, 1.1 nm for p-NPNN, and
1.1–1.3 nm for CLPOT).
The effects of temperature on τ R for various [(CLPOT)-(4-X-TEMPO)] or
[(CLPOT)-(4-XPNN)] are shown in Fig. 12.7 [27, 51, 52]. The rotational activation energy, E a , for 4-X-TEMPO rotation in each [(CLPOT)-(4-X-TEMPO)] was
estimated to be in the range of 6–8 kJ mol −1 , using the τ R values plotted in this
figure. Thus, all [(CLPOT)-(4-X-TEMPO)] have similar E a values, and this value
is independent of the size or type of the substituent group or the total inclusion
amount. This independence may result from the relatively large pore diameter of
the CLPOT nanochannels relative to the sizes of the radical molecules. However, the
τ R values for [(CLPOT)-(4-X-TEMPO)] are an order of magnitude greater than the
value for 1a/2a/3a. This result suggests that the pore diameter of the nanochannels
is an important factor determining τ R .
The high-temperature side in Fig. 12.7 demonstrates the temperature dependence
of τ R for [(CLPOT)-(PhNN)/(N-PhMI)] (diagonal crosses) and [(CLPOT)-(pNPNN)/(N-PhMI)] (yellow circles) [27]. The τ R values for PhNN or p-NPNN
rotations in [(CLPOT)-(PhNN)/(N-PhMI)] and [(CLPOT)-(p-NPNN)/(N-PhMI)]
are much longer than those for 4-X-TEMPO radicals in these nanochannels at the
same temperature. As an example, τ R for TEMPO or TEMPOL is on the order of
10 −9 s at room temperature [51], whereas τ R for the NN radical was approximately
10 −7 s [27]. The E a values for PhNN and p-NPNN rotations in [(CLPOT)(PhNN)/(N-PhMI)] and [(CLPOT)-(p-NPNN)/(N-PhMI)] were estimated to be 37
and 54 kJ mol −1 , respectively, using the τ R values in the range of 293–400 K in
Fig. 12.7 [27]. These values are several times larger than those for 4-X-TEMPO [51,
445
Figure 12.6 shows continuous changes in the ESR spectra with increases in
temperature. In the vicinity of 351 K, the 1b/2g/3e spectra exhibit two narrower
peaks between lines 2 and 3 in Fig. 12.2b (see Sect. 12.2.3.4) [27, 66]. In addition,
a modulated quintet associated with the two 14 N atoms for which I = 1 in the
NN group appears above 369 K for p-NPNN. The temperature-dependent ESR
spectra of 1b/2g/3e were reproducible based on a model for assumption anisotropic
rotational diffusion of isolated p-NPNN molecules in CLPOT nanochannels within
the slow-motion regime with respect to the ESR time scale (10 −9 s < τ R < 10 −6 s)
over the entire temperature range in Fig. 12.6. The 1b/2g/3e spectra at 398 K
were obtained by assuming rotational diffusion around the axis tilted by (θ ,
φ) = (74 ◦ , 90 ◦ ) relative to the principal system of the g tensor, with Gaussian and
Lorentzian line width components and τ R of 0.11 and 0.30 mT and 1.0 × 10 −8 s,
respectively. The ESR spectra were also well-reproduced at other temperatures
using the anisotropic rotational diffusion model around the same rotation axis.
The rotational axes in 1b/2g/3e are approximately parallel to the principal y-axis
of the g tensor (θ = φ = 90 ◦ ). For this reason, p-NPNN molecules are expected to
be included in the CLPOT nanochannels with the longest molecular axis parallel to
the channel axis, assuming that the rotational axis is parallel to the channel axis. The
(θ , φ) parameters for the rotational axis relative to the principal axis for p-NPNN
are larger than those for PhNN ((θ , φ) = (68 ◦ , 78 ◦ )). This difference is likely the
result of the ratio of the molecular lengths of the NN radicals relative to the pore
diameter for the CLPOT nanochannels (0.9 nm for PhNN, 1.1 nm for p-NPNN, and
1.1–1.3 nm for CLPOT).
The effects of temperature on τ R for various [(CLPOT)-(4-X-TEMPO)] or
[(CLPOT)-(4-XPNN)] are shown in Fig. 12.7 [27, 51, 52]. The rotational activation energy, E a , for 4-X-TEMPO rotation in each [(CLPOT)-(4-X-TEMPO)] was
estimated to be in the range of 6–8 kJ mol −1 , using the τ R values plotted in this
figure. Thus, all [(CLPOT)-(4-X-TEMPO)] have similar E a values, and this value
is independent of the size or type of the substituent group or the total inclusion
amount. This independence may result from the relatively large pore diameter of
the CLPOT nanochannels relative to the sizes of the radical molecules. However, the
τ R values for [(CLPOT)-(4-X-TEMPO)] are an order of magnitude greater than the
value for 1a/2a/3a. This result suggests that the pore diameter of the nanochannels
is an important factor determining τ R .
The high-temperature side in Fig. 12.7 demonstrates the temperature dependence
of τ R for [(CLPOT)-(PhNN)/(N-PhMI)] (diagonal crosses) and [(CLPOT)-(pNPNN)/(N-PhMI)] (yellow circles) [27]. The τ R values for PhNN or p-NPNN
rotations in [(CLPOT)-(PhNN)/(N-PhMI)] and [(CLPOT)-(p-NPNN)/(N-PhMI)]
are much longer than those for 4-X-TEMPO radicals in these nanochannels at the
same temperature. As an example, τ R for TEMPO or TEMPOL is on the order of
10 −9 s at room temperature [51], whereas τ R for the NN radical was approximately
10 −7 s [27]. The E a values for PhNN and p-NPNN rotations in [(CLPOT)(PhNN)/(N-PhMI)] and [(CLPOT)-(p-NPNN)/(N-PhMI)] were estimated to be 37
and 54 kJ mol −1 , respectively, using the τ R values in the range of 293–400 K in
Fig. 12.7 [27]. These values are several times larger than those for 4-X-TEMPO [51,