40
F. Ito
was no transparency in the image without UV irradiation, indicating that the droplet
of BF 2 DBMb solution has no crystalline region. With time, the fluorescence color
changed to orange from the peripheral of the droplet; this change was associated with
the solvent evaporation from 20 to 40 s after the dropping, which can be assigned to
the emission of the amorphous phase according to the previous report. After 60 s,
birefringence was observed via the polarized optical image, and the texture in both
the polarized optical images with and without UV irradiation gradually propagated
throughout the droplet. The texture in the images implies the phase transformation
of the amorphous state, followed by the formation of a crystalline state. Compared
with both images at 85 s, the region with orange emission shows no birefringence,
suggesting the optically isotropic phase. We have postulated that the amorphous
phase as an intermediate can be identical to the liquid-like cluster state proposed in
the two-step nucleation model of the crystallization process. These findings indicate
that the photophysical and optical properties of the liquid-like cluster state have an
isotropic aggregated state similar to an amorphous phase.
Next, we attempted QCM measurements during the evaporative crystallization of
the droplet. Prior to discussing the results of the QCM measurements, to confirm
that the Au electrode does not affect the fluorescence behavior in the evaporative
crystallization of the droplet, we measured the fluorescence images and spectra during
the solvent evaporation on Au-coated AT-cut quartz substrate, simultaneously. The
behavior of the fluorescence spectral changes of BF 2 DBMb droplet onto Au electrode
is almost the same as that in the previous report, indicating that Au electrode does not
affect the spectral changes that occur during solvent evaporation, such as the surface
plasmon resonance of Au thin film [34, 35].
We attempted the QCM measurements adapted for the evaporative crystallization
of the BF 2 DBMb droplet, to assess the changes in the dynamic viscoelastic properties.
First, to confirm the effects of just solvent evaporation, we measured f and R
changes by the evaporation of 1,2-DCE as a function of time. Both values indicate
the amount of change from before the dropping. Just after dropping, f was −2 kHz
and began to return to the initial value from approximately 50–156 s. Meanwhile, R
exhibited behavior similar to that of f just after dropping and then monotonically
recovered from 93 to 156 s. Changes in both values correspond to the solvent mass
change m based on the Sauerbrey equation described below; therefore, recovery
to the initial values suggested that the solvent on the Au electrode fully evaporated
with time.
Next, we performed the QCM measurements of BF 2 DBMb in the 1,2-DCE solution droplet on the Au-coated At-cut quartz electrode. Figure 2.9a shows the changes
in f and R as a function of time after the dropping of the solution on the Au electrode. Just after dropping, f showed a value of −2 kHz until 70 s, which is comparable to that in the 1,2-DCE solvent. From 70 to 86 s, f temporarily decreased to
−4.5 kHz and then reached −6.9 kHz at 95 s. This value was maintained constant
between 95 and 107 s. Afterward, f decreased again, ultimately exhibiting a value
of −13.8 kHz. Just after dropping, R exhibited a value of 0.37 k until 85 s, which
is also comparable to that in the 1,2-DCE solvent. From 85 to 93 s, R temporarily
increased to 1.2 k and then steeply increased to 1.6 k until 115 s. Afterward,
F. Ito
was no transparency in the image without UV irradiation, indicating that the droplet
of BF 2 DBMb solution has no crystalline region. With time, the fluorescence color
changed to orange from the peripheral of the droplet; this change was associated with
the solvent evaporation from 20 to 40 s after the dropping, which can be assigned to
the emission of the amorphous phase according to the previous report. After 60 s,
birefringence was observed via the polarized optical image, and the texture in both
the polarized optical images with and without UV irradiation gradually propagated
throughout the droplet. The texture in the images implies the phase transformation
of the amorphous state, followed by the formation of a crystalline state. Compared
with both images at 85 s, the region with orange emission shows no birefringence,
suggesting the optically isotropic phase. We have postulated that the amorphous
phase as an intermediate can be identical to the liquid-like cluster state proposed in
the two-step nucleation model of the crystallization process. These findings indicate
that the photophysical and optical properties of the liquid-like cluster state have an
isotropic aggregated state similar to an amorphous phase.
Next, we attempted QCM measurements during the evaporative crystallization of
the droplet. Prior to discussing the results of the QCM measurements, to confirm
that the Au electrode does not affect the fluorescence behavior in the evaporative
crystallization of the droplet, we measured the fluorescence images and spectra during
the solvent evaporation on Au-coated AT-cut quartz substrate, simultaneously. The
behavior of the fluorescence spectral changes of BF 2 DBMb droplet onto Au electrode
is almost the same as that in the previous report, indicating that Au electrode does not
affect the spectral changes that occur during solvent evaporation, such as the surface
plasmon resonance of Au thin film [34, 35].
We attempted the QCM measurements adapted for the evaporative crystallization
of the BF 2 DBMb droplet, to assess the changes in the dynamic viscoelastic properties.
First, to confirm the effects of just solvent evaporation, we measured f and R
changes by the evaporation of 1,2-DCE as a function of time. Both values indicate
the amount of change from before the dropping. Just after dropping, f was −2 kHz
and began to return to the initial value from approximately 50–156 s. Meanwhile, R
exhibited behavior similar to that of f just after dropping and then monotonically
recovered from 93 to 156 s. Changes in both values correspond to the solvent mass
change m based on the Sauerbrey equation described below; therefore, recovery
to the initial values suggested that the solvent on the Au electrode fully evaporated
with time.
Next, we performed the QCM measurements of BF 2 DBMb in the 1,2-DCE solution droplet on the Au-coated At-cut quartz electrode. Figure 2.9a shows the changes
in f and R as a function of time after the dropping of the solution on the Au electrode. Just after dropping, f showed a value of −2 kHz until 70 s, which is comparable to that in the 1,2-DCE solvent. From 70 to 86 s, f temporarily decreased to
−4.5 kHz and then reached −6.9 kHz at 95 s. This value was maintained constant
between 95 and 107 s. Afterward, f decreased again, ultimately exhibiting a value
of −13.8 kHz. Just after dropping, R exhibited a value of 0.37 k until 85 s, which
is also comparable to that in the 1,2-DCE solvent. From 85 to 93 s, R temporarily
increased to 1.2 k and then steeply increased to 1.6 k until 115 s. Afterward,