2 Direct Visualization of Crystal Formation and Growth …
37
approximately 32 s. The solvent evaporated, which resulted in a doughnut-shaped
orange emission formed at approximately 34 s. The region of purple emission shrank
from both inside and outside after 34 s. Finally, the entire droplet region turned to
blue emission with small parts of the orange emission remaining. The evaporation
of solvent from the inner region most probably originates from the analogous mechanism to gas bubbles, which has been reported for the molecular assembling during
the evaporation of low vapor pressure solvent [21]. The orange emission is exhibited
only in solution with non-equilibrium state. Actually, the concentration dependence
of fluorescence spectra of BF 2 DBMb in solution does not exhibit orange emission,
even under concentrated conditions. It is strongly suggested that the molecular form
or assembly with orange emission can be like a supersaturated solution. To obtain
the spectroscopic information for solvent evaporation, we observed the fluorescence
spectra of BF 2 DBMb in 1,2-dichroloethane during evaporation as a function of time,
which are shown in Fig. 2.6b. The fluorescence spectrum acquired immediately after
applying the droplet exhibits peaks at 433 nm with shoulders at 415, 460, 550 nm,
corresponding to the emission spectra of the monomer state. The peak near 550 nm
corresponding to the amorphous state monotonically increased with time up to 91 s.
The fluorescence peak at 433 nm decreased from 91 to 95 s. After 95 s, the peaks near
445 and 470 nm appeared concomitant with decreasing band intensity near 550 nm.
The series of fluorescence spectral changes correspond to the fluorescence image
change. Based on the information of the fluorescence properties of BF 2 DBMb as
described above, we can explain the molecular assembling by solvent evaporative
crystallization. The crystal of BF 2 DBMb formed from solution via the amorphous
state. The dynamic fluorescence change is identical to observations when increasing
the concentration in PMMA films during static trials. The Raman spectra of each
species were identical to the fluorescence spectral change, which strongly supports
the changes in molecular species in the ground state.
The fluorescence spectra were analyzed by nonlinear least squares fitting by six
Gaussians. All of the observed spectra matched these values well. Thus, we plotted
the relative abundances of monomer, crystal, and amorphous state as a function of
time, which is shown in Fig. 2.7a. Afterward, the dropping the fraction of monomer
species fell to approximately 0.9. The monomer fraction monotonically decreased,
whereas the amorphous fraction increased up to 95 s. The amorphous fraction reached
approximately 0.6 at 95 s; then, the fraction decreased considerably. The crystal
fraction was not observed before 95 s. The fraction suddenly increased after 95 s,
concomitant with the decrease of the amorphous state. These findings indicate that
the crystal can be formed from monomer species via the amorphous state, which is
presumed to show hierarchical change like a consecutive reaction, as schematically
shown in Fig. 2.7b.
Based on the observed phenomena by fluorescence change both depending on the
dispersion concentration in PMMA films and during the solvent evaporative crystallization of BF 2 DBMb, we can conclude that the direct visualization of proposed a
mechanism of the two-step nucleation model. The fluorescence color change from
purple to blue via orange corresponds to the molecular formation change from
monomer to crystal via amorphous state. The amorphous state is transiently formed
37
approximately 32 s. The solvent evaporated, which resulted in a doughnut-shaped
orange emission formed at approximately 34 s. The region of purple emission shrank
from both inside and outside after 34 s. Finally, the entire droplet region turned to
blue emission with small parts of the orange emission remaining. The evaporation
of solvent from the inner region most probably originates from the analogous mechanism to gas bubbles, which has been reported for the molecular assembling during
the evaporation of low vapor pressure solvent [21]. The orange emission is exhibited
only in solution with non-equilibrium state. Actually, the concentration dependence
of fluorescence spectra of BF 2 DBMb in solution does not exhibit orange emission,
even under concentrated conditions. It is strongly suggested that the molecular form
or assembly with orange emission can be like a supersaturated solution. To obtain
the spectroscopic information for solvent evaporation, we observed the fluorescence
spectra of BF 2 DBMb in 1,2-dichroloethane during evaporation as a function of time,
which are shown in Fig. 2.6b. The fluorescence spectrum acquired immediately after
applying the droplet exhibits peaks at 433 nm with shoulders at 415, 460, 550 nm,
corresponding to the emission spectra of the monomer state. The peak near 550 nm
corresponding to the amorphous state monotonically increased with time up to 91 s.
The fluorescence peak at 433 nm decreased from 91 to 95 s. After 95 s, the peaks near
445 and 470 nm appeared concomitant with decreasing band intensity near 550 nm.
The series of fluorescence spectral changes correspond to the fluorescence image
change. Based on the information of the fluorescence properties of BF 2 DBMb as
described above, we can explain the molecular assembling by solvent evaporative
crystallization. The crystal of BF 2 DBMb formed from solution via the amorphous
state. The dynamic fluorescence change is identical to observations when increasing
the concentration in PMMA films during static trials. The Raman spectra of each
species were identical to the fluorescence spectral change, which strongly supports
the changes in molecular species in the ground state.
The fluorescence spectra were analyzed by nonlinear least squares fitting by six
Gaussians. All of the observed spectra matched these values well. Thus, we plotted
the relative abundances of monomer, crystal, and amorphous state as a function of
time, which is shown in Fig. 2.7a. Afterward, the dropping the fraction of monomer
species fell to approximately 0.9. The monomer fraction monotonically decreased,
whereas the amorphous fraction increased up to 95 s. The amorphous fraction reached
approximately 0.6 at 95 s; then, the fraction decreased considerably. The crystal
fraction was not observed before 95 s. The fraction suddenly increased after 95 s,
concomitant with the decrease of the amorphous state. These findings indicate that
the crystal can be formed from monomer species via the amorphous state, which is
presumed to show hierarchical change like a consecutive reaction, as schematically
shown in Fig. 2.7b.
Based on the observed phenomena by fluorescence change both depending on the
dispersion concentration in PMMA films and during the solvent evaporative crystallization of BF 2 DBMb, we can conclude that the direct visualization of proposed a
mechanism of the two-step nucleation model. The fluorescence color change from
purple to blue via orange corresponds to the molecular formation change from
monomer to crystal via amorphous state. The amorphous state is transiently formed