312
Y. Haketa et al.
and 16
C8− ·TBA
+ from 15
C8 and 16
C8 , respectively, failed because the high solubility
and low melting points of the resulting materials hindered their purification.
The thermal properties and assembled structures demonstrated the dependence
on the degrees of interactions at the carbonyl groups and the number and the length
of alkyl chains. Carboxylic acids 15
Cn (n = 8, 12, 16), possessing two aliphatic
chains, formed lamellar structures in the solid state, and the repeating distances were
slightly smaller than the summed length of two molecules, suggesting the formation
of a tilted or interdigitated bilayer structure through hydrogen bonding. On the other
hand, 16
Cn (n = 8, 12, 16), possessing fan-shaped three aliphatic chains, formed
discotic columnar structures. Ion pairs 15
Cn− ·TBA
+ and 16
Cn− ·TBA
+ (n = 12, 16)
provided lamellar structures through ionic and van der Waals interactions, with the
repeating distances corresponding to the summed lengths of azobenzene carboxylates
and TBA
+ . Interestingly, the ion pair 16
C16− ·TBA
+ provided a stable lamellar phase
with repeating distances of 3.8 nm at 22–55 °C only in the heating process, as the
metastable crystalline lamellar phase with repeating distances of 4.5 nm was formed
by rapid cooling from the isotropic state (>55 °C).
18.2.3 Photo-Responsive Properties of Ion-Pairing
Assemblies
The photo-induced phase transitions were examined by POM and XRD analyses
under photoirradiation. Although film samples are suitable for these analyses because
of the difficulty experienced by light in penetrating thick samples, it is challenging
to prepare films of low-molecular-weight species due to the dewetting behavior. This
problem was resolved by sandwiching the samples into glass plate cells with 8 μm
spacers. The diffraction profiles were obtained by using an X-ray diffractometer
equipped with a two-dimensional detector. The X-ray beam (Cu-Kα) was made
parallel to the in-plane direction of the cells using pulse controllers. UV light (365 nm)
and visible light (436 nm) were irradiated into the cells from an out-of-plane direction
(Fig. 18.11a).
In contrast to the ion pairs 15
C12− ·TBA
+ , 15
C16− ·TBA
+ , and 16
C12− ·TBA
+ ,
which showed the photo-induced phase transitions from ionic crystals to liquids
(Fig. 18.11b), the birefringence of 16
C16− ·TBA
+ at 40 °C was completely transformed
under the photoirradiation at 365 nm (100 mW/cm
2 ), suggesting a phase transition.
UV (365 nm, 100 mW/cm
2 ) irradiation onto the sample induced a gradual disappearance of the lamellar phase (Phase A) with repeating distances of 3.8 nm, as well as the
appearance of a new crystalline lamellar phase with a repeating distance of 3.4 nm
(Phase B) (Fig. 18.12a(i–iii)). The calculated lengths of 16
C16− including the countercation are approximately 3.9 and 3.3 nm for the trans and cis forms, respectively,
(Fig. 18.12b). Thus, the difference between Phase A and Phase B can be rationalized
by the trans-to-cis photo-isomerization. In addition, XRD peaks assignable to Phase
A and Phase B were independently observed, indicating that the phase transition from
Y. Haketa et al.
and 16
C8− ·TBA
+ from 15
C8 and 16
C8 , respectively, failed because the high solubility
and low melting points of the resulting materials hindered their purification.
The thermal properties and assembled structures demonstrated the dependence
on the degrees of interactions at the carbonyl groups and the number and the length
of alkyl chains. Carboxylic acids 15
Cn (n = 8, 12, 16), possessing two aliphatic
chains, formed lamellar structures in the solid state, and the repeating distances were
slightly smaller than the summed length of two molecules, suggesting the formation
of a tilted or interdigitated bilayer structure through hydrogen bonding. On the other
hand, 16
Cn (n = 8, 12, 16), possessing fan-shaped three aliphatic chains, formed
discotic columnar structures. Ion pairs 15
Cn− ·TBA
+ and 16
Cn− ·TBA
+ (n = 12, 16)
provided lamellar structures through ionic and van der Waals interactions, with the
repeating distances corresponding to the summed lengths of azobenzene carboxylates
and TBA
+ . Interestingly, the ion pair 16
C16− ·TBA
+ provided a stable lamellar phase
with repeating distances of 3.8 nm at 22–55 °C only in the heating process, as the
metastable crystalline lamellar phase with repeating distances of 4.5 nm was formed
by rapid cooling from the isotropic state (>55 °C).
18.2.3 Photo-Responsive Properties of Ion-Pairing
Assemblies
The photo-induced phase transitions were examined by POM and XRD analyses
under photoirradiation. Although film samples are suitable for these analyses because
of the difficulty experienced by light in penetrating thick samples, it is challenging
to prepare films of low-molecular-weight species due to the dewetting behavior. This
problem was resolved by sandwiching the samples into glass plate cells with 8 μm
spacers. The diffraction profiles were obtained by using an X-ray diffractometer
equipped with a two-dimensional detector. The X-ray beam (Cu-Kα) was made
parallel to the in-plane direction of the cells using pulse controllers. UV light (365 nm)
and visible light (436 nm) were irradiated into the cells from an out-of-plane direction
(Fig. 18.11a).
In contrast to the ion pairs 15
C12− ·TBA
+ , 15
C16− ·TBA
+ , and 16
C12− ·TBA
+ ,
which showed the photo-induced phase transitions from ionic crystals to liquids
(Fig. 18.11b), the birefringence of 16
C16− ·TBA
+ at 40 °C was completely transformed
under the photoirradiation at 365 nm (100 mW/cm
2 ), suggesting a phase transition.
UV (365 nm, 100 mW/cm
2 ) irradiation onto the sample induced a gradual disappearance of the lamellar phase (Phase A) with repeating distances of 3.8 nm, as well as the
appearance of a new crystalline lamellar phase with a repeating distance of 3.4 nm
(Phase B) (Fig. 18.12a(i–iii)). The calculated lengths of 16
C16− including the countercation are approximately 3.9 and 3.3 nm for the trans and cis forms, respectively,
(Fig. 18.12b). Thus, the difference between Phase A and Phase B can be rationalized
by the trans-to-cis photo-isomerization. In addition, XRD peaks assignable to Phase
A and Phase B were independently observed, indicating that the phase transition from