546
F. Ito
the spectra were unable to be reproduced with these five peaks even considering
that the actual fluorescence bands were broader. In particular, a transient peak was
observed at around 500 nm during solvent evaporation. In a previous study, a liquidlike cluster that acted as both the crystal nucleus and molten state of the molecules
was observed as an intermediate [8]. Based on this finding, the fluorescence spectrum and image of 1 upon heating a sample to over 235 °C were measured, which
are depicted in Fig. 31.6b and its inset, respectively. Green emission with a peak at
501 nm was exhibited from the molten state. In the fluorescence spectrum of 1, the
peak at shorter wavelength shifted during solvent evaporation, as shown in Fig. 31.6a.
Re-analyzing the fluorescence spectra revealed that the observed spectra at all times
during solvent evaporation could be reproduced well by adding a sixth Gaussian peak
at 501 nm for the clusters. These findings suggest that the molten state exists as an
intermediate during evaporative crystallization.
The relative abundance changes of each component as a function of time are shown
in Fig. 31.6c. The monomer fraction steeply decreased after 75 s. The green fraction
first increased slightly and then decreased from 76 to 78 s. The orange fraction
monotonically increased from 75 s. The red fraction increased after 75 s, became
constant between 79 and 81 s, and then increased again. The cluster (molten state)
fraction increased from 76 to 80 s when the monomer and green fractions decreased
and then decreased with increasing fractions of the orange and red components. The
green and red species were assigned to 1y and 1r, respectively [32]. Formation of the
amorphous state after complete solvent evaporation was confirmed by polarization
optical microscopy. Because the fluorescence peaks of Am and 1v appeared at 610
and 604 nm, respectively, it was difficult to distinguish them based on only the
fluorescence spectra data. Therefore, the orange fraction was considered to consist
of both Am and 1v. The cluster fraction was identified by its similarity to the liquidlike cluster of 1 reported previously [8], which is also consistent with the two-step
nucleation model. It is considered that the liquid-like cluster and 1y are in equilibrium.
The generation of 1r is influenced by that of 1y because 1r and 1y have similar
crystal structures. In contrast, because the changes of these three fractions seem to
be independent of each other, the orange fraction is unlikely to be influenced by
1y and 1r, leading to the conclusion that the orange fraction is not produced by
transformation of 1y or 1r.
The number density of molecules in the solution increased over time because of
the solvent loss during solvent evaporation. A solute dense state was formed from the
monomer molecules generated by the fluctuation in the solution; namely, a liquid-like
cluster state. These liquid-like clusters formed ordered arrays and acted as crystalline
nuclei, from which crystals formed and grew. If the liquid-like clusters cannot form
crystal nuclei, they will form disordered aggregates such as an amorphous solid
through liquid–liquid separation. Figure 31.7 shows a proposed scheme based on
the superposition of fluctuations of the two order parameters density and structure
[33]. These two parameters are related to crystal growth and polymorph formation
[34]. Liquid-like cluster states can be formed by fluctuation of the monomers in the
droplet resulting from solvent evaporation. Then, crystals of the three polymorphs
are produced from the liquid-like clusters through the formation of crystal nuclei
F. Ito
the spectra were unable to be reproduced with these five peaks even considering
that the actual fluorescence bands were broader. In particular, a transient peak was
observed at around 500 nm during solvent evaporation. In a previous study, a liquidlike cluster that acted as both the crystal nucleus and molten state of the molecules
was observed as an intermediate [8]. Based on this finding, the fluorescence spectrum and image of 1 upon heating a sample to over 235 °C were measured, which
are depicted in Fig. 31.6b and its inset, respectively. Green emission with a peak at
501 nm was exhibited from the molten state. In the fluorescence spectrum of 1, the
peak at shorter wavelength shifted during solvent evaporation, as shown in Fig. 31.6a.
Re-analyzing the fluorescence spectra revealed that the observed spectra at all times
during solvent evaporation could be reproduced well by adding a sixth Gaussian peak
at 501 nm for the clusters. These findings suggest that the molten state exists as an
intermediate during evaporative crystallization.
The relative abundance changes of each component as a function of time are shown
in Fig. 31.6c. The monomer fraction steeply decreased after 75 s. The green fraction
first increased slightly and then decreased from 76 to 78 s. The orange fraction
monotonically increased from 75 s. The red fraction increased after 75 s, became
constant between 79 and 81 s, and then increased again. The cluster (molten state)
fraction increased from 76 to 80 s when the monomer and green fractions decreased
and then decreased with increasing fractions of the orange and red components. The
green and red species were assigned to 1y and 1r, respectively [32]. Formation of the
amorphous state after complete solvent evaporation was confirmed by polarization
optical microscopy. Because the fluorescence peaks of Am and 1v appeared at 610
and 604 nm, respectively, it was difficult to distinguish them based on only the
fluorescence spectra data. Therefore, the orange fraction was considered to consist
of both Am and 1v. The cluster fraction was identified by its similarity to the liquidlike cluster of 1 reported previously [8], which is also consistent with the two-step
nucleation model. It is considered that the liquid-like cluster and 1y are in equilibrium.
The generation of 1r is influenced by that of 1y because 1r and 1y have similar
crystal structures. In contrast, because the changes of these three fractions seem to
be independent of each other, the orange fraction is unlikely to be influenced by
1y and 1r, leading to the conclusion that the orange fraction is not produced by
transformation of 1y or 1r.
The number density of molecules in the solution increased over time because of
the solvent loss during solvent evaporation. A solute dense state was formed from the
monomer molecules generated by the fluctuation in the solution; namely, a liquid-like
cluster state. These liquid-like clusters formed ordered arrays and acted as crystalline
nuclei, from which crystals formed and grew. If the liquid-like clusters cannot form
crystal nuclei, they will form disordered aggregates such as an amorphous solid
through liquid–liquid separation. Figure 31.7 shows a proposed scheme based on
the superposition of fluctuations of the two order parameters density and structure
[33]. These two parameters are related to crystal growth and polymorph formation
[34]. Liquid-like cluster states can be formed by fluctuation of the monomers in the
droplet resulting from solvent evaporation. Then, crystals of the three polymorphs
are produced from the liquid-like clusters through the formation of crystal nuclei