538
F. Ito
31.1 Introduction
Photochemistry of organic molecular aggregates plays an important role in various
systems such as organic light-emitting diodes and organic solar cells. The fluorescence properties of organic molecules in aggregates are different from those of
molecules dissolved in solution. Various emission colors have been observed from
samples depending on the assembled state, i.e., these samples are cooperative and
hierarchal photoresponsive molecular assemblies. To control these emissive properties, it is important to obtain detailed information about the origin and evolution
of the emission in the aggregated state, where specific interactions can lead to the
appearance of new emission colors.
Crystallization of organic crystals from solution is important for not only separation and purification, but also the formation of single crystals for pharmaceutical
production and structure determination and the development of organic solid materials. Because many organic crystals are formed by relatively weak interactions such
as van der Waals interactions, the properties of organic molecular solids depend on
both their molecular structure and molecular assembly. In other words, molecular
crystals are formed under the influence of weak interactions, and thus have different
integrated structures depending on slight differences in crystallization conditions
such as temperature, solution concentration, and the speed of solvent evaporation.
The crystallization process is roughly divided into nucleation and crystal growth.
Although crystal formation from solution is a relatively familiar phenomenon, the
formation and growth of crystal nuclei in a homogeneous solution is still not well
understood. Nucleation in the saturated state is the initial step of crystal formation from the solution [1]. The size, number, structure, and quality of crystals are
determined in the initial nucleation process. Therefore, understanding nucleation is
important to control crystal properties including polymorph formation. The classical
nucleation theory (CNT) is based on the Gibbs energies of surfaces and volume in the
growth process without considering molecular identities. The CNT has been widely
utilized to simply explain crystallization processes. In this theory, it is assumed that
the density and order fluctuations in a supersaturated solution lead to the formation
of clusters within the molecular packing, which reflects all possible polymorphs of
the solute and crystal nuclei with the same structure as a mature crystal. In some
cases, predictions based on the CNT differ from experimental results, suggesting
that nucleation from solution must involve a more complex mechanism. Recently,
the two-step nucleation model, which includes the formation of metastable precursor
clusters with a size of hundreds of nanometers in a dense liquid, was proposed by
several groups based on the induction time of crystal formation [2], nuclear magnetic
resonance spectroscopy [3, 4], electron microscopy [5], dynamic light scattering [6],
and nonphotochemical laser-induced crystallization data [7]. This model assumes the
separation of a dense, disordered liquid phase, and fluctuations in its density prior to
the formation of crystalline order. In other words, the initial clusters are liquid-like
and crystalline order appears in the clusters over time. This model is known to be
F. Ito
31.1 Introduction
Photochemistry of organic molecular aggregates plays an important role in various
systems such as organic light-emitting diodes and organic solar cells. The fluorescence properties of organic molecules in aggregates are different from those of
molecules dissolved in solution. Various emission colors have been observed from
samples depending on the assembled state, i.e., these samples are cooperative and
hierarchal photoresponsive molecular assemblies. To control these emissive properties, it is important to obtain detailed information about the origin and evolution
of the emission in the aggregated state, where specific interactions can lead to the
appearance of new emission colors.
Crystallization of organic crystals from solution is important for not only separation and purification, but also the formation of single crystals for pharmaceutical
production and structure determination and the development of organic solid materials. Because many organic crystals are formed by relatively weak interactions such
as van der Waals interactions, the properties of organic molecular solids depend on
both their molecular structure and molecular assembly. In other words, molecular
crystals are formed under the influence of weak interactions, and thus have different
integrated structures depending on slight differences in crystallization conditions
such as temperature, solution concentration, and the speed of solvent evaporation.
The crystallization process is roughly divided into nucleation and crystal growth.
Although crystal formation from solution is a relatively familiar phenomenon, the
formation and growth of crystal nuclei in a homogeneous solution is still not well
understood. Nucleation in the saturated state is the initial step of crystal formation from the solution [1]. The size, number, structure, and quality of crystals are
determined in the initial nucleation process. Therefore, understanding nucleation is
important to control crystal properties including polymorph formation. The classical
nucleation theory (CNT) is based on the Gibbs energies of surfaces and volume in the
growth process without considering molecular identities. The CNT has been widely
utilized to simply explain crystallization processes. In this theory, it is assumed that
the density and order fluctuations in a supersaturated solution lead to the formation
of clusters within the molecular packing, which reflects all possible polymorphs of
the solute and crystal nuclei with the same structure as a mature crystal. In some
cases, predictions based on the CNT differ from experimental results, suggesting
that nucleation from solution must involve a more complex mechanism. Recently,
the two-step nucleation model, which includes the formation of metastable precursor
clusters with a size of hundreds of nanometers in a dense liquid, was proposed by
several groups based on the induction time of crystal formation [2], nuclear magnetic
resonance spectroscopy [3, 4], electron microscopy [5], dynamic light scattering [6],
and nonphotochemical laser-induced crystallization data [7]. This model assumes the
separation of a dense, disordered liquid phase, and fluctuations in its density prior to
the formation of crystalline order. In other words, the initial clusters are liquid-like
and crystalline order appears in the clusters over time. This model is known to be