6
1 General Introduction
1.3 Conditions
for Crystal-to-Crystal/Single-Crystal-to-Single-Crystal
Transformation
There are several types of the solid-state phase transition when some solid sample
changed to different solid structure by controlling external conditions such as light,
temperature, pressure, chemical gradients [1]. The category can be described as
below.
(a) Crystal-to-Amorphous
(b) Crystal-to-Crystal
(c) Single Crystal-to-Single Crystal
(d) Amorphous-to-Crystal.
In the phase transitions involving amorphous phase (a and d), it is impossible to
have concise structural information in the solid samples, thus, hard to understand
underlying mechanism of the phase transition with physical properties changes. By
contrast, if the phase transition is undergoing (b) Crystal-to-Crystal or (c) Single
Crystal-to-Single Crystal transition, we can have precise information of the crystal
structure change in terms of powder and single crystal X-ray diffraction analyses.
Because of the huge benefit, design and control of Crystal-to-Crystal [6c–d, 10], or
Single Crystal-to-Single Crystal [7, 11] phase transition via external stimulus has
been intensely researched. Even the high development of the crystal engineering,
it is still difficult to predict or design these types of phase transitions for molecular crystalline phase. Especially, realization of the Single Crystal-to-Single Crystal
transformation would be major challenge because exposing single crystals to external or internal forces by applying some stimulations generally results in the loss of
single crystallinity [11m].
Several conditions to realize and control the Crystal-to-Crystal or the Single
Crystal-to-Single Crystal transformations have been discussed despite there is no
rational rule [11m–n]. First, the sample should be able to form multiple crystal
structures, in other word, to show polymorphism. Second, reasonable energy barrier between the polymorphs should be formed for interconversion. Third, thermodynamic stability of the crystalline phase should be characterized because many
cases of these phase transitions undergo from thermodynamically unstable phase to
stable phase. These three factors should be intensely considered when the molecular or material blue-print is designed for the Crystal-to-Crystal or the Single
Crystal-to-Single Crystal phase transitions.
1.4 Features of Biaryl Group
In this section, biaryl group possessing two aryl plans connected by C–C bonding
will be focused. In 2,2
-disubstituted biphenyl compounds (Fig. 1.5a), the rotation
around the single bond between the phenyl groups is restricted to some extent by the
1 General Introduction
1.3 Conditions
for Crystal-to-Crystal/Single-Crystal-to-Single-Crystal
Transformation
There are several types of the solid-state phase transition when some solid sample
changed to different solid structure by controlling external conditions such as light,
temperature, pressure, chemical gradients [1]. The category can be described as
below.
(a) Crystal-to-Amorphous
(b) Crystal-to-Crystal
(c) Single Crystal-to-Single Crystal
(d) Amorphous-to-Crystal.
In the phase transitions involving amorphous phase (a and d), it is impossible to
have concise structural information in the solid samples, thus, hard to understand
underlying mechanism of the phase transition with physical properties changes. By
contrast, if the phase transition is undergoing (b) Crystal-to-Crystal or (c) Single
Crystal-to-Single Crystal transition, we can have precise information of the crystal
structure change in terms of powder and single crystal X-ray diffraction analyses.
Because of the huge benefit, design and control of Crystal-to-Crystal [6c–d, 10], or
Single Crystal-to-Single Crystal [7, 11] phase transition via external stimulus has
been intensely researched. Even the high development of the crystal engineering,
it is still difficult to predict or design these types of phase transitions for molecular crystalline phase. Especially, realization of the Single Crystal-to-Single Crystal
transformation would be major challenge because exposing single crystals to external or internal forces by applying some stimulations generally results in the loss of
single crystallinity [11m].
Several conditions to realize and control the Crystal-to-Crystal or the Single
Crystal-to-Single Crystal transformations have been discussed despite there is no
rational rule [11m–n]. First, the sample should be able to form multiple crystal
structures, in other word, to show polymorphism. Second, reasonable energy barrier between the polymorphs should be formed for interconversion. Third, thermodynamic stability of the crystalline phase should be characterized because many
cases of these phase transitions undergo from thermodynamically unstable phase to
stable phase. These three factors should be intensely considered when the molecular or material blue-print is designed for the Crystal-to-Crystal or the Single
Crystal-to-Single Crystal phase transitions.
1.4 Features of Biaryl Group
In this section, biaryl group possessing two aryl plans connected by C–C bonding
will be focused. In 2,2
-disubstituted biphenyl compounds (Fig. 1.5a), the rotation
around the single bond between the phenyl groups is restricted to some extent by the