542
F. Ito
process in saturated and unsaturated glycine solutions by small-angle X-ray scattering measurements. In nuclei precursor clusters (liquid-like clusters) in glycine
solution, it was found that domains formed through the interaction between dimers
and solvent molecules. It was suggested that the liquid-like clusters are not perfectly
isotropic, but possess an internal packing structure similar to that of the crystal.
These experimental results strongly suggest that the liquid-like clusters proposed
in the two-step nucleation theory act as important intermediates during the crystallization process including polymorph formation. There are some reports on the
polymorph formation process from a theoretical perspective using both CNT and
two-step nucleation models [22, 23].
As described in Sect. 31.2, fluorescence visualization was achieved for the evaporation crystallization process of BF 2 DBMb in 1,2-dichloroethane [8]. Fluorescence
spectroscopy could be used to probe the molecular assembly processes on scales from
only a few molecules to the bulk material. The concentration-dependent fluorescence
spectral changes of organic molecules during solvent evaporation can provide information about molecular assembly and crystal nucleation and growth [24–26]. This
approach has the advantage of being able to detect changes in the surrounding environment in real-time using fluorescence spectral changes as a probe. Moreover, it is
expected that the formation of polymorphs will be affected by the cluster structure
before nucleation. Therefore, insight about the nuclei precursor clusters involved in
polymorph formation obtained by assessing fluorescence changes will enable the
prediction of polymorph appearance.
In this section, the 1,3-dipyrrol-2-yl-1,3-propanedione boron difluoride complex
1 (Fig. 31.3a) is reviewed as a model system with three polymorphs showing different
luminescence characteristics [27, 28]. It has been reported that the addition of
anions to a solution of 1 changes its conformation by inversion (flipping) of two
pyrrole rings [15, 29–31]. Aryl substitution at the α- and β-positions of pyrrole
can introduce a variety of substituents around the anionic receptor, facilitating the
anion-driven modulation of assembly structure, electronic and optical properties,
and charge carrier mobility in the bulk state. An innovative design that takes into
account the structural and electronic properties of pyrrole molecules has made it
possible to obtain attractive molecular systems. From this background, the detection
of molecular dynamics during the assembly process is considered to be important
for predicting the ordered arrangements of π-electron systems. Despite having the
simplest structure (without modification of the pyrrole rings), 1 has three polymorphs
abbreviated as 1r, 1y, and 1v [29, 32] that exhibit red, green, and vermilion fluorescence, respectively. The pyrrole rings of the molecules in 1r and 1y are oriented
toward the BF 2 unit. While 1r contains two stacked structures with anti-orientation
of the BF 2 unit, 1y forms a stacked structure with syn orientation. The crystal structure of 1v is different from those of 1r and 1y: one of the pyrrole rings in 1v is
inverted and the molecules adopt an anti-oriented stacking structure. In this section,
the relationship between the nuclei precursor and polymorph formed is examined by
fluorescence detection of the polymorphic expression process based on the spectral
changes detected during evaporative crystallization.
F. Ito
process in saturated and unsaturated glycine solutions by small-angle X-ray scattering measurements. In nuclei precursor clusters (liquid-like clusters) in glycine
solution, it was found that domains formed through the interaction between dimers
and solvent molecules. It was suggested that the liquid-like clusters are not perfectly
isotropic, but possess an internal packing structure similar to that of the crystal.
These experimental results strongly suggest that the liquid-like clusters proposed
in the two-step nucleation theory act as important intermediates during the crystallization process including polymorph formation. There are some reports on the
polymorph formation process from a theoretical perspective using both CNT and
two-step nucleation models [22, 23].
As described in Sect. 31.2, fluorescence visualization was achieved for the evaporation crystallization process of BF 2 DBMb in 1,2-dichloroethane [8]. Fluorescence
spectroscopy could be used to probe the molecular assembly processes on scales from
only a few molecules to the bulk material. The concentration-dependent fluorescence
spectral changes of organic molecules during solvent evaporation can provide information about molecular assembly and crystal nucleation and growth [24–26]. This
approach has the advantage of being able to detect changes in the surrounding environment in real-time using fluorescence spectral changes as a probe. Moreover, it is
expected that the formation of polymorphs will be affected by the cluster structure
before nucleation. Therefore, insight about the nuclei precursor clusters involved in
polymorph formation obtained by assessing fluorescence changes will enable the
prediction of polymorph appearance.
In this section, the 1,3-dipyrrol-2-yl-1,3-propanedione boron difluoride complex
1 (Fig. 31.3a) is reviewed as a model system with three polymorphs showing different
luminescence characteristics [27, 28]. It has been reported that the addition of
anions to a solution of 1 changes its conformation by inversion (flipping) of two
pyrrole rings [15, 29–31]. Aryl substitution at the α- and β-positions of pyrrole
can introduce a variety of substituents around the anionic receptor, facilitating the
anion-driven modulation of assembly structure, electronic and optical properties,
and charge carrier mobility in the bulk state. An innovative design that takes into
account the structural and electronic properties of pyrrole molecules has made it
possible to obtain attractive molecular systems. From this background, the detection
of molecular dynamics during the assembly process is considered to be important
for predicting the ordered arrangements of π-electron systems. Despite having the
simplest structure (without modification of the pyrrole rings), 1 has three polymorphs
abbreviated as 1r, 1y, and 1v [29, 32] that exhibit red, green, and vermilion fluorescence, respectively. The pyrrole rings of the molecules in 1r and 1y are oriented
toward the BF 2 unit. While 1r contains two stacked structures with anti-orientation
of the BF 2 unit, 1y forms a stacked structure with syn orientation. The crystal structure of 1v is different from those of 1r and 1y: one of the pyrrole rings in 1v is
inverted and the molecules adopt an anti-oriented stacking structure. In this section,
the relationship between the nuclei precursor and polymorph formed is examined by
fluorescence detection of the polymorphic expression process based on the spectral
changes detected during evaporative crystallization.