5.5 Control Experiment and Proposed Mechanism of the Reversible SCSC
137
surface. Contrary to 3O scsc , 3O obtained by recrystallization exhibited smooth surface (Fig. 5.24). These results indicate that the crystal habit and surface conditions
should be important for the reversion.
In the mechanical-stimulation-triggered SCSC transitions of 1 and 2, the key
factors are the phase transitions from metastable to stable phases. In this work, the
key feature for the reversible phase changes in 3 is the release/inclusion of MeOH.
In the presence of low concentrations of MeOH, the MeOH molecules remain in
the 3G/MeOH crystal through an adsorption/desorption equilibrium of MeOH on
the crystal surface. By cutting the 3G/MeOH crystal, this equilibrium is altered and
the molecular arrangement changed, which results in a release of MeOH from the
crystal, which finally affords 3O scsc under such a low concentration of MeOH vapor.
The cut edge corresponds to the (100) or (−100) surface, which is oriented orthogonal to the channel of the MeOH molecules and the domain where phase changes
would occur. We propose that the release of MeOH molecules from the cut crystal
edge should affect the equilibrium on the very other side of the MeOH channels.
This should be the reason why the emission-color changes start not only at the cut
edge, but also at the other end of the crystalline needle (Fig. 5.2). For the reversion, increasing the MeOH vapor concentration should shift the equilibrium toward
the inclusion of MeOH and the SCSC phase changes from 3O scsc to 3G/MeOH.
When 3O scsc was contacted to a droplet of liquid MeOH, only the domain in which
MeOH liquid directly contacted exhibited emission change from orange to green
un-der ambient air. This result indicates that the contact of the crystal and MeOH is
pre-requisite for the phase transition from 3O scsc to 3G re /MeOH. It is reasonable to
assume that 3G/MeOH, which accommodates polar MeOH molecules in the crystal
structure, exhibits a “polar” molecular arrangement (Fig. 5.3b) and that, after releasing the MeOH molecules, its crystal structure changes to a non-polar phase, where
the molecular polarity is canceled in the centrosymmetric structure via the inversion
center (Fig. 5.27) [10].
5.6 Summary
We have discovered the first example for reversible mechanical-stimulation-triggered
SCSC transitions under concomitant changes of the luminescence properties. Interestingly, these SCSC transitions occur between a polar crystal containing a polar
solvent (MeOH) and a centrosymmetric crystal that does not contain solvent
molecules. This solvent-related phase reversion represents a new design strategy
for mechano-responsive SCSC materials.
137
surface. Contrary to 3O scsc , 3O obtained by recrystallization exhibited smooth surface (Fig. 5.24). These results indicate that the crystal habit and surface conditions
should be important for the reversion.
In the mechanical-stimulation-triggered SCSC transitions of 1 and 2, the key
factors are the phase transitions from metastable to stable phases. In this work, the
key feature for the reversible phase changes in 3 is the release/inclusion of MeOH.
In the presence of low concentrations of MeOH, the MeOH molecules remain in
the 3G/MeOH crystal through an adsorption/desorption equilibrium of MeOH on
the crystal surface. By cutting the 3G/MeOH crystal, this equilibrium is altered and
the molecular arrangement changed, which results in a release of MeOH from the
crystal, which finally affords 3O scsc under such a low concentration of MeOH vapor.
The cut edge corresponds to the (100) or (−100) surface, which is oriented orthogonal to the channel of the MeOH molecules and the domain where phase changes
would occur. We propose that the release of MeOH molecules from the cut crystal
edge should affect the equilibrium on the very other side of the MeOH channels.
This should be the reason why the emission-color changes start not only at the cut
edge, but also at the other end of the crystalline needle (Fig. 5.2). For the reversion, increasing the MeOH vapor concentration should shift the equilibrium toward
the inclusion of MeOH and the SCSC phase changes from 3O scsc to 3G/MeOH.
When 3O scsc was contacted to a droplet of liquid MeOH, only the domain in which
MeOH liquid directly contacted exhibited emission change from orange to green
un-der ambient air. This result indicates that the contact of the crystal and MeOH is
pre-requisite for the phase transition from 3O scsc to 3G re /MeOH. It is reasonable to
assume that 3G/MeOH, which accommodates polar MeOH molecules in the crystal
structure, exhibits a “polar” molecular arrangement (Fig. 5.3b) and that, after releasing the MeOH molecules, its crystal structure changes to a non-polar phase, where
the molecular polarity is canceled in the centrosymmetric structure via the inversion
center (Fig. 5.27) [10].
5.6 Summary
We have discovered the first example for reversible mechanical-stimulation-triggered
SCSC transitions under concomitant changes of the luminescence properties. Interestingly, these SCSC transitions occur between a polar crystal containing a polar
solvent (MeOH) and a centrosymmetric crystal that does not contain solvent
molecules. This solvent-related phase reversion represents a new design strategy
for mechano-responsive SCSC materials.