30
F. Ito
density fluctuations and grows into a stable crystal. Some computational and experimental results, however, cannot be explained based only on classical nucleation
theory [1]. Recently though, a two-step nucleation model involving a liquid-like
cluster intermediate prior to nucleation has been developed to explain protein crystallization and has been shown to be of more general validity [2]. It is postulated
that liquid-like clusters originate from disordered liquid or amorphous metastable
clusters in homogeneous solutions [3]. There have been many reports supporting this
two-step nucleation model, in which the intermediated phases play an important role
in crystallization.
Nucleation is the initial step of crystallization. The nuclei could not be directly
observed because the nuclei exist in the transition state. Understanding and controlling nuclei formation will provide a suitable process for crystallization because the
organic molecular crystals generally are formed by weak intermolecular interaction
such as van der Waals, π –π interaction, or hydrogen bonding [4]. However, direct
observation of such processes under realistic conditions in the real time remains
a challenge because of the lack of advanced techniques to discriminate the phase
boundaries and capture the intermediate states.
In this chapter, studies of the direct visualization of crystal formation and growth,
probed by organic fluorescent molecules by using fluorescence microscopy and spectroscopy, are introduced, with a particular focus on the fluorescence spectral change
of a dibenzoylmethane boron difluoride complex exhibiting mechanofluorochromism
and a cyanostilbene derivative displaying aggregation-induced emission (AIE).
2.2 Fluorescence Detection of Molecular Assembling
The fluorescence spectra of materials are sensitive to molecular environment and
aggregation. In principle, fluorescence spectroscopy can be used to probe the progress
of molecular assembly on the scale of just a few molecules or that of a bulk process.
In this section, the studies of molecular assembling probed by fluorescence detection
are described.
Yu et al. [5] monitored an amorphous-to-crystalline transformation through fluorescence color changes by the in situ microscopic observation of the crystallization of molecular microparticles. As a molecule, tetra-substituted ethene with novel
morphology-dependent fluorescence was applied, which can distinguish the interface between the crystalline and amorphous phase by fluorescence color, providing a
simple and practical method to probe the inner processes of a molecular microparticle.
The fluorescence images of the crystallization due to contact between microparticles were categorized into three cases by monitoring the crystallization evolution
of these defective microspheres. This method can clearly record the inhomogeneous crystallization of amorphous microparticles, whereby the perfect microparticles and those with defects demonstrate diverse destinies. The study presents a
realistic picture of the microscopic kinetics of not only solid–solid transitions but
also crystallizations that occur spontaneously in atmosphere or under external stimuli,
F. Ito
density fluctuations and grows into a stable crystal. Some computational and experimental results, however, cannot be explained based only on classical nucleation
theory [1]. Recently though, a two-step nucleation model involving a liquid-like
cluster intermediate prior to nucleation has been developed to explain protein crystallization and has been shown to be of more general validity [2]. It is postulated
that liquid-like clusters originate from disordered liquid or amorphous metastable
clusters in homogeneous solutions [3]. There have been many reports supporting this
two-step nucleation model, in which the intermediated phases play an important role
in crystallization.
Nucleation is the initial step of crystallization. The nuclei could not be directly
observed because the nuclei exist in the transition state. Understanding and controlling nuclei formation will provide a suitable process for crystallization because the
organic molecular crystals generally are formed by weak intermolecular interaction
such as van der Waals, π –π interaction, or hydrogen bonding [4]. However, direct
observation of such processes under realistic conditions in the real time remains
a challenge because of the lack of advanced techniques to discriminate the phase
boundaries and capture the intermediate states.
In this chapter, studies of the direct visualization of crystal formation and growth,
probed by organic fluorescent molecules by using fluorescence microscopy and spectroscopy, are introduced, with a particular focus on the fluorescence spectral change
of a dibenzoylmethane boron difluoride complex exhibiting mechanofluorochromism
and a cyanostilbene derivative displaying aggregation-induced emission (AIE).
2.2 Fluorescence Detection of Molecular Assembling
The fluorescence spectra of materials are sensitive to molecular environment and
aggregation. In principle, fluorescence spectroscopy can be used to probe the progress
of molecular assembly on the scale of just a few molecules or that of a bulk process.
In this section, the studies of molecular assembling probed by fluorescence detection
are described.
Yu et al. [5] monitored an amorphous-to-crystalline transformation through fluorescence color changes by the in situ microscopic observation of the crystallization of molecular microparticles. As a molecule, tetra-substituted ethene with novel
morphology-dependent fluorescence was applied, which can distinguish the interface between the crystalline and amorphous phase by fluorescence color, providing a
simple and practical method to probe the inner processes of a molecular microparticle.
The fluorescence images of the crystallization due to contact between microparticles were categorized into three cases by monitoring the crystallization evolution
of these defective microspheres. This method can clearly record the inhomogeneous crystallization of amorphous microparticles, whereby the perfect microparticles and those with defects demonstrate diverse destinies. The study presents a
realistic picture of the microscopic kinetics of not only solid–solid transitions but
also crystallizations that occur spontaneously in atmosphere or under external stimuli,