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K. Higashiguchi and K. Matsuda
in the case of axially symmetric amphiphiles consisting of an ionic head and alkyl tails
[1], and more complex amphiphiles [2]. It is interesting to note that the energy difference between the different kinds of molecular packing is small, so that the packing of
the supramolecular architecture in water may change with various external stimuli.
Photoreaction is a well-known technique for changing the packing and
morphology of supramolecular architectures in water. Azobenzene, a representative photochromic compound, undergoes reversible photoisomerization between the
rod-shaped trans-isomer and the bent cis-isomer. Such a large change in molecular geometry may result in sizable transformations of the microstructure [3],
but the microstructure is often irreversibly destructed [4]. Even if the molecular packing changes reversibly before and after isomerization, the fragmented
microstructures do not return to the original position due to diffusion. However,
some examples of reversible morphological changes have been reported. Hamada
and co-workers showed that a sheet-shaped supramolecular architecture composed
of an azobenzene derivative and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)
exhibited photoreversible morphological change between sheet and capsule shapes
[5]. Kageyama et al. reported that a plate-like assembly composed of another azobenzene derivative and oleic acid show self-oscillations upon irradiation with continuous
blue light [6].
Diarylethene, a typical photochromic compound, is also used to form photoresponsive supramolecular architectures [7]. It undergoes reversible photoisomerization between a colorless open-ring isomer and a colored closed-ring isomer. The
characteristic photochromic features, e.g., color, quantum yield, and fatigue resistance, can be controlled within a wide range by means of molecular design [8]. We
recently studied amphiphilic diarylethenes showing photo- and thermo-responsive
morphological changes. Oligo(ethylene glycol) monomethyl ether chain (hereinafter,
referred to as “oligo(ethylene glycol) ether”), used here as a hydrophilic part of
the amphiphilic diarylethenes, is known to exhibit lower critical solution temperature (LCST) behavior, by which miscibility switches depending on temperature.
Below the LCST, the oligo(ethylene glycol) ether is in a hydrated phase by means
of hydrogen bonding with the surrounding water molecules and forms an extended
chain, and above this temperature, hydrogen bonding ceases to exist because the
contribution from the entropy becomes more significant than from enthalpy, estimated from the Gibbs energy of hydrogen bonding. The oligo(ethylene glycol) ether
becomes randomly coiled and the chains are tangled with each other. Since miscibility changes drastically near the LCST, this characteristic is often used to bring
about reversible morphological changes in the micrometer-sized supramolecular
architectures [9].
K. Higashiguchi and K. Matsuda
in the case of axially symmetric amphiphiles consisting of an ionic head and alkyl tails
[1], and more complex amphiphiles [2]. It is interesting to note that the energy difference between the different kinds of molecular packing is small, so that the packing of
the supramolecular architecture in water may change with various external stimuli.
Photoreaction is a well-known technique for changing the packing and
morphology of supramolecular architectures in water. Azobenzene, a representative photochromic compound, undergoes reversible photoisomerization between the
rod-shaped trans-isomer and the bent cis-isomer. Such a large change in molecular geometry may result in sizable transformations of the microstructure [3],
but the microstructure is often irreversibly destructed [4]. Even if the molecular packing changes reversibly before and after isomerization, the fragmented
microstructures do not return to the original position due to diffusion. However,
some examples of reversible morphological changes have been reported. Hamada
and co-workers showed that a sheet-shaped supramolecular architecture composed
of an azobenzene derivative and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)
exhibited photoreversible morphological change between sheet and capsule shapes
[5]. Kageyama et al. reported that a plate-like assembly composed of another azobenzene derivative and oleic acid show self-oscillations upon irradiation with continuous
blue light [6].
Diarylethene, a typical photochromic compound, is also used to form photoresponsive supramolecular architectures [7]. It undergoes reversible photoisomerization between a colorless open-ring isomer and a colored closed-ring isomer. The
characteristic photochromic features, e.g., color, quantum yield, and fatigue resistance, can be controlled within a wide range by means of molecular design [8]. We
recently studied amphiphilic diarylethenes showing photo- and thermo-responsive
morphological changes. Oligo(ethylene glycol) monomethyl ether chain (hereinafter,
referred to as “oligo(ethylene glycol) ether”), used here as a hydrophilic part of
the amphiphilic diarylethenes, is known to exhibit lower critical solution temperature (LCST) behavior, by which miscibility switches depending on temperature.
Below the LCST, the oligo(ethylene glycol) ether is in a hydrated phase by means
of hydrogen bonding with the surrounding water molecules and forms an extended
chain, and above this temperature, hydrogen bonding ceases to exist because the
contribution from the entropy becomes more significant than from enthalpy, estimated from the Gibbs energy of hydrogen bonding. The oligo(ethylene glycol) ether
becomes randomly coiled and the chains are tangled with each other. Since miscibility changes drastically near the LCST, this characteristic is often used to bring
about reversible morphological changes in the micrometer-sized supramolecular
architectures [9].