12 Inter-spin Interactions of Organic Radical Chains in Organic 1D. . .
455
between the highest and lowest temperatures are close to 3.6 mT. Above 100 K, the
experimental ΔB pp values for 1a/2a are smaller than the estimated dipolar peak-topeak line width (ΔB pp
D ; see Sect. 12.2.4) based on Van Vleck’s formula for a rigid
spin lattice (Eq. (12.8) in Sect. 12.2.4), indicating exchange narrowing due to the
temperature dependency of the ESR line profile (see below and Fig. 12.15).
On the other hand, the effect of temperature on the ΔB pp values of 1b/2a–d
is evidently more gradual than that for 1a/2a, although there is a slight decrease
with increases in temperature. The magnitude of the ΔB pp values for 1b/2a–d
was at most 2–3 mT even at low temperature, and the variation with increasing
in temperature was less than 1.4 mT, even at 300 K. The ΔB pp value is also
somewhat larger when the 4-position substituent group on the TEMPO molecule is
bulkier. Because the crystal structure of the CLPOT nanochannels and the quantity
of guest radicals per unit cell in each [CLPOT-(4-X-TEMPO)] are similar to the
values for 1a/2a (see Sect. 12.3.2), the dipolar peak-to-peak line width (ΔB pp
D ,
see Sect. 12.2.4) for 1b/2a–d was estimated assuming a hexagonal rigid spin lattice
formed by positioning the electron spin in the CLPOT nanochannels at the center
of gravity of the TEMPO molecule [39, 53] (see Sect. 12.2.4). Using this model,
the ΔB pp
D value for 1b/2a–d was estimated to be 11.2 or 11.3 mT. A significantly
larger ΔB pp
D was evidently obtained from the shorter c (i.e., shorter intra-channel
inter-spin distance of 0.7 nm) [50] of 1b/2a–d as compared to 1a/2a. These results
indicate the narrowing of ESR line width of 1b/2a–d due to molecular motion or
exchange interaction.
The ESR line profiles were determined according to the theory described in Sect.
12.2.5. Figure 12.14 shows the effect of temperature on the line profiles of [(TPP) 2 -
(TEMPO) 1.0 ] (1a/2a; diagonal crosses) and of [CLPOT-(4-X-TEMPO)], such as
[(CLPOT) 2 -(TEMPO) 1.0 )] (1b/2a, yellow triangles), [(CLPOT) 2 -(TEMPOL) 1.0 )]
(1a/2c, empty circles), [(CLPOT) 2 -(TEMPONE) 1.1 )] (1a/2b, magenta diamonds),
and [(CLPOT) 2 -(MeO-TEMPOL) 0.91 )] (1a/2d, cyan squares) according to Dietz’s
method (see Sect. 12.2.5) at 4.2, 155 K, and 290 K [39, 53]. Note that the host to
guest ratio notation for each sample in Fig. 12.14 has been abbreviated for the sake
of simplifying the figure.
In the case of 1b/2a–d, the molecular motions of guest radicals in CLPOT
nanochannels are temperature-dependent, as shown in Sect. 12.4, with a rigidlimit under about 120 K. With increases in temperature (above 200–240 K), the
rotational diffusion motion originally in the slow-motion regime on the ESR time
scale changed such that τ R moved into the fast-motion region of the ESR time
scale. In addition, τ R of 1b/2a–d was shorter than the value for TEMPO in the
TPP nanochannels in the same temperature range (see Figs. 12.7 and 12.11) [41,
43, 52]. The 1b/2a–d line profiles were almost independent of temperature, as seen
in Fig. 12.14a–c, indicating different results compared to those obtained for 1a/2a
(see below and Fig. 12.15). These data suggest that the molecular motions and/or
substituent groups of guest radicals in the CLPOT nanochannels are not correlated
with the inter-spin interactions of 1b/2a–d. Therefore, the narrower ESR line width
for 1b/2a–d may be considered to originate not from motional narrowing but
rather than 3D exchange interactions. This finding demonstrates that 3D exchange
455
between the highest and lowest temperatures are close to 3.6 mT. Above 100 K, the
experimental ΔB pp values for 1a/2a are smaller than the estimated dipolar peak-topeak line width (ΔB pp
D ; see Sect. 12.2.4) based on Van Vleck’s formula for a rigid
spin lattice (Eq. (12.8) in Sect. 12.2.4), indicating exchange narrowing due to the
temperature dependency of the ESR line profile (see below and Fig. 12.15).
On the other hand, the effect of temperature on the ΔB pp values of 1b/2a–d
is evidently more gradual than that for 1a/2a, although there is a slight decrease
with increases in temperature. The magnitude of the ΔB pp values for 1b/2a–d
was at most 2–3 mT even at low temperature, and the variation with increasing
in temperature was less than 1.4 mT, even at 300 K. The ΔB pp value is also
somewhat larger when the 4-position substituent group on the TEMPO molecule is
bulkier. Because the crystal structure of the CLPOT nanochannels and the quantity
of guest radicals per unit cell in each [CLPOT-(4-X-TEMPO)] are similar to the
values for 1a/2a (see Sect. 12.3.2), the dipolar peak-to-peak line width (ΔB pp
D ,
see Sect. 12.2.4) for 1b/2a–d was estimated assuming a hexagonal rigid spin lattice
formed by positioning the electron spin in the CLPOT nanochannels at the center
of gravity of the TEMPO molecule [39, 53] (see Sect. 12.2.4). Using this model,
the ΔB pp
D value for 1b/2a–d was estimated to be 11.2 or 11.3 mT. A significantly
larger ΔB pp
D was evidently obtained from the shorter c (i.e., shorter intra-channel
inter-spin distance of 0.7 nm) [50] of 1b/2a–d as compared to 1a/2a. These results
indicate the narrowing of ESR line width of 1b/2a–d due to molecular motion or
exchange interaction.
The ESR line profiles were determined according to the theory described in Sect.
12.2.5. Figure 12.14 shows the effect of temperature on the line profiles of [(TPP) 2 -
(TEMPO) 1.0 ] (1a/2a; diagonal crosses) and of [CLPOT-(4-X-TEMPO)], such as
[(CLPOT) 2 -(TEMPO) 1.0 )] (1b/2a, yellow triangles), [(CLPOT) 2 -(TEMPOL) 1.0 )]
(1a/2c, empty circles), [(CLPOT) 2 -(TEMPONE) 1.1 )] (1a/2b, magenta diamonds),
and [(CLPOT) 2 -(MeO-TEMPOL) 0.91 )] (1a/2d, cyan squares) according to Dietz’s
method (see Sect. 12.2.5) at 4.2, 155 K, and 290 K [39, 53]. Note that the host to
guest ratio notation for each sample in Fig. 12.14 has been abbreviated for the sake
of simplifying the figure.
In the case of 1b/2a–d, the molecular motions of guest radicals in CLPOT
nanochannels are temperature-dependent, as shown in Sect. 12.4, with a rigidlimit under about 120 K. With increases in temperature (above 200–240 K), the
rotational diffusion motion originally in the slow-motion regime on the ESR time
scale changed such that τ R moved into the fast-motion region of the ESR time
scale. In addition, τ R of 1b/2a–d was shorter than the value for TEMPO in the
TPP nanochannels in the same temperature range (see Figs. 12.7 and 12.11) [41,
43, 52]. The 1b/2a–d line profiles were almost independent of temperature, as seen
in Fig. 12.14a–c, indicating different results compared to those obtained for 1a/2a
(see below and Fig. 12.15). These data suggest that the molecular motions and/or
substituent groups of guest radicals in the CLPOT nanochannels are not correlated
with the inter-spin interactions of 1b/2a–d. Therefore, the narrower ESR line width
for 1b/2a–d may be considered to originate not from motional narrowing but
rather than 3D exchange interactions. This finding demonstrates that 3D exchange