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S. Kobatake and D. Kitagawa
to the deformation of the material due to a large free volume around the molecule. It
is necessary to use a molecular assembly with a small free volume.
The phenomenon in which a chemical species reversibly isomerizes between
two isomers having different colors upon photoirradiation is called photochromism.
Various physicochemical properties such as conductivity, dielectric constant, refractive index, redox potential, and fluorescence are also reversibly changed by the
photochromic reaction. Azobenzene, spiropyran, furylfulgide, and diarylethene are
representative photochromic compounds. Among them, diarylethene has excellent
properties such as thermal stability of both isomers, high durability, high sensitivity,
and rapid response [1, 2]. When a diarylethene is irradiated with ultraviolet (UV) light
in a colorless state called an open-ring isomer, the molecular structure is changed to
a colored state called a closed-ring isomer. Moreover, when the colored closed-ring
isomer is irradiated with visible light, it returns to the original colorless open-ring
isomer. Many diarylethenes exhibit photochromism not only in solution but also in
the crystalline state. Crystals are considered as a molecular assembly with a small
free volume in which molecules are regularly arranged and packed, and the photomechanical behavior of diarylethene crystals associated with the photochromic reaction
is investigated. This chapter focuses on photomechanical organic crystals, especially
diarylethene crystals, and describes future developments of this research field.
25.2 Discovery of Photomechanical Crystals
The first example of a molecular crystal that bends reversibly in response to light
(photomechanical crystal) is the rhodium semiquinone complex crystal reported by
Abakumov and Nevodchikov [3]. When thin acicular crystals were irradiated with
light in the charge-transfer absorption band having an absorption maximum near
1600 nm, it was confirmed that the crystals bend 45°. It has been concluded that this
bending behavior is due to the formation of Rh–Rh bonds between the complexes
within the crystal. However, since oxygen is required for the cleavage of the Rh–Rh
bond in the reverse reaction, it does not return exactly to the original shape. Also, if
left in the presence of air all day long, the bending ability is lost.
Since then, little attention has been paid to research on photomechanical crystals,
and during that time, research was focused on photochemical reactions in solid and
crystalline states. In 2001, a photoreversible crystal surface morphology change in a
diarylethene single crystal was reported [4]. In the diarylethene crystal with a size of
several millimetres, a step of 1 nm or valley is formed with UV light irradiation, and
the original smooth surface is formed by visible light irradiation. This result reflects
the reversible molecular structure change of the diarylethene molecule due to UV
and visible light irradiation. In 2006, Bardeen et al. found that crystal nanorods of an
anthracene derivative were elongated by 15% due to [4 + 4] photodimerization [5].
This is different from an intramolecular reaction such as the cyclization and cycloreversion reaction of diarylethene, and utilizes the volume change in the crystal due to
the intermolecular reaction. In 2007, a reversible crystal shape change of diarylethene
S. Kobatake and D. Kitagawa
to the deformation of the material due to a large free volume around the molecule. It
is necessary to use a molecular assembly with a small free volume.
The phenomenon in which a chemical species reversibly isomerizes between
two isomers having different colors upon photoirradiation is called photochromism.
Various physicochemical properties such as conductivity, dielectric constant, refractive index, redox potential, and fluorescence are also reversibly changed by the
photochromic reaction. Azobenzene, spiropyran, furylfulgide, and diarylethene are
representative photochromic compounds. Among them, diarylethene has excellent
properties such as thermal stability of both isomers, high durability, high sensitivity,
and rapid response [1, 2]. When a diarylethene is irradiated with ultraviolet (UV) light
in a colorless state called an open-ring isomer, the molecular structure is changed to
a colored state called a closed-ring isomer. Moreover, when the colored closed-ring
isomer is irradiated with visible light, it returns to the original colorless open-ring
isomer. Many diarylethenes exhibit photochromism not only in solution but also in
the crystalline state. Crystals are considered as a molecular assembly with a small
free volume in which molecules are regularly arranged and packed, and the photomechanical behavior of diarylethene crystals associated with the photochromic reaction
is investigated. This chapter focuses on photomechanical organic crystals, especially
diarylethene crystals, and describes future developments of this research field.
25.2 Discovery of Photomechanical Crystals
The first example of a molecular crystal that bends reversibly in response to light
(photomechanical crystal) is the rhodium semiquinone complex crystal reported by
Abakumov and Nevodchikov [3]. When thin acicular crystals were irradiated with
light in the charge-transfer absorption band having an absorption maximum near
1600 nm, it was confirmed that the crystals bend 45°. It has been concluded that this
bending behavior is due to the formation of Rh–Rh bonds between the complexes
within the crystal. However, since oxygen is required for the cleavage of the Rh–Rh
bond in the reverse reaction, it does not return exactly to the original shape. Also, if
left in the presence of air all day long, the bending ability is lost.
Since then, little attention has been paid to research on photomechanical crystals,
and during that time, research was focused on photochemical reactions in solid and
crystalline states. In 2001, a photoreversible crystal surface morphology change in a
diarylethene single crystal was reported [4]. In the diarylethene crystal with a size of
several millimetres, a step of 1 nm or valley is formed with UV light irradiation, and
the original smooth surface is formed by visible light irradiation. This result reflects
the reversible molecular structure change of the diarylethene molecule due to UV
and visible light irradiation. In 2006, Bardeen et al. found that crystal nanorods of an
anthracene derivative were elongated by 15% due to [4 + 4] photodimerization [5].
This is different from an intramolecular reaction such as the cyclization and cycloreversion reaction of diarylethene, and utilizes the volume change in the crystal due to
the intermolecular reaction. In 2007, a reversible crystal shape change of diarylethene