(S)-4. Thus the chirality in the alkyl side chains was transferred as a helical
molecular arrangement in the self-assemblies, affording an emergence of supramolecular chirality. Owing to the supramolecular chirality, the self-assembly of (S)-(E,
E)-4 gave a positive CPL signal with the g lum value of 0.008. Although the photoreaction of azobenzene moiety could not be induced in the self-assembling state, the
dissociation of self-assembly with heating could enable the E-to-Z photoisomerization. The photo-irradiated solution was cooled below 300 K and the
inversion of CD signal sign was induced, thus reversing the supramolecular chirality.
Along with the supramolecular chirality inversion, the CPL signal became negative
with the g lum value of À0.002. After the photo-irradiation, the supramolecular
helicity in the fibrous assembly was also reversed. Photo-reactive spiropyran moiety
was also introduced in a self-assembling chiral glutamate organogelator 5 (Fig. 8.14)
[54]. 5-SP formed helical nanofibers to gelatinize organic solvents, while fluorescent
property was absent. Upon UV irradiation, the colorless organogel of 5-SP turned
blue with the formation of a merocyanine form (5-MC) with changing the microstructures. 5-MC exhibited red-color fluorescence, switching on the CPL activity.
Alternate UV and visible light irradiations reversibly switched the CPL activity of
organogel of 5 between ON and OFF states, respectively. A similar chiral glutamate
compound having a photo-reactive cinnamic acid moiety was also demonstrated to
reverse CPL sign in response to photo-dimerization reaction [55].
The photo-isomerization of azobenzene unit involves the rotational motion
around the N–N bond in the excited state, which may prohibit the trans-to-cis
isomerization in the self-assembling state of (E,E)-4. Therefore, dynamic in situ
photo-switching of CPL activity in a supramolecular system had still remained a
challenge. In comparison to the photo-reaction of azobenzene, the photo-cyclization
reaction of 6π-system requires a smaller structural change, affording very efficient
solid-state reactivity [17]. The CPL photo-switching molecules D- and L-2o were
employed as a building unit of self-assembly [56]. The emission turn-off behavior is
expected to be amplified in such aggregation systems because the energy transfer
based emission quenching operates not only in intramolecular but also in
intermolecular manners [57, 58]. Compound 2o was molecularly dispersed in
chloroform, while it gave dispersion of nanoparticle assemblies in a mixture of
chloroform/methylcyclohexane(MCH) (1:9 or 1:99) solvents. The transmission
electron microscopy (TEM) observation revealed the formation of nanoparticle
aggregates with an average diameter of ca. 250 nm in the chloroform/MCH (1:9)
mixture at a concentration of 1.0 Â 10
À4 M (Fig. 8.15a). CD spectra exhibited
mirror-image profiles in response to the stereochemistry in the amino-acid spacer
with the same sign of first Cotton effect regardless of the solvent composition
(Fig. 8.15b). The peak splitting patterns of MCH-rich solutions differ from those
in molecularly dispersed chloroform solutions, suggesting the difference in the
exciton coupling mode. The CD spectra for the MCH-rich solution should include
intermolecular exciton coupling effect in the aggregates. The molar CD (Δε), which
is the parameter of CD amplitude, exhibited a pronounced increase in the MCH-rich
solution compared to that in molecularly dispersed state.
8 Photo-Switching of Circularly Polarized Luminescence
189
molecular arrangement in the self-assemblies, affording an emergence of supramolecular chirality. Owing to the supramolecular chirality, the self-assembly of (S)-(E,
E)-4 gave a positive CPL signal with the g lum value of 0.008. Although the photoreaction of azobenzene moiety could not be induced in the self-assembling state, the
dissociation of self-assembly with heating could enable the E-to-Z photoisomerization. The photo-irradiated solution was cooled below 300 K and the
inversion of CD signal sign was induced, thus reversing the supramolecular chirality.
Along with the supramolecular chirality inversion, the CPL signal became negative
with the g lum value of À0.002. After the photo-irradiation, the supramolecular
helicity in the fibrous assembly was also reversed. Photo-reactive spiropyran moiety
was also introduced in a self-assembling chiral glutamate organogelator 5 (Fig. 8.14)
[54]. 5-SP formed helical nanofibers to gelatinize organic solvents, while fluorescent
property was absent. Upon UV irradiation, the colorless organogel of 5-SP turned
blue with the formation of a merocyanine form (5-MC) with changing the microstructures. 5-MC exhibited red-color fluorescence, switching on the CPL activity.
Alternate UV and visible light irradiations reversibly switched the CPL activity of
organogel of 5 between ON and OFF states, respectively. A similar chiral glutamate
compound having a photo-reactive cinnamic acid moiety was also demonstrated to
reverse CPL sign in response to photo-dimerization reaction [55].
The photo-isomerization of azobenzene unit involves the rotational motion
around the N–N bond in the excited state, which may prohibit the trans-to-cis
isomerization in the self-assembling state of (E,E)-4. Therefore, dynamic in situ
photo-switching of CPL activity in a supramolecular system had still remained a
challenge. In comparison to the photo-reaction of azobenzene, the photo-cyclization
reaction of 6π-system requires a smaller structural change, affording very efficient
solid-state reactivity [17]. The CPL photo-switching molecules D- and L-2o were
employed as a building unit of self-assembly [56]. The emission turn-off behavior is
expected to be amplified in such aggregation systems because the energy transfer
based emission quenching operates not only in intramolecular but also in
intermolecular manners [57, 58]. Compound 2o was molecularly dispersed in
chloroform, while it gave dispersion of nanoparticle assemblies in a mixture of
chloroform/methylcyclohexane(MCH) (1:9 or 1:99) solvents. The transmission
electron microscopy (TEM) observation revealed the formation of nanoparticle
aggregates with an average diameter of ca. 250 nm in the chloroform/MCH (1:9)
mixture at a concentration of 1.0 Â 10
À4 M (Fig. 8.15a). CD spectra exhibited
mirror-image profiles in response to the stereochemistry in the amino-acid spacer
with the same sign of first Cotton effect regardless of the solvent composition
(Fig. 8.15b). The peak splitting patterns of MCH-rich solutions differ from those
in molecularly dispersed chloroform solutions, suggesting the difference in the
exciton coupling mode. The CD spectra for the MCH-rich solution should include
intermolecular exciton coupling effect in the aggregates. The molar CD (Δε), which
is the parameter of CD amplitude, exhibited a pronounced increase in the MCH-rich
solution compared to that in molecularly dispersed state.
8 Photo-Switching of Circularly Polarized Luminescence
189