Page 624 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 42 FABRICATION TECHNIQUE OF ORGANIC NANOCRYSTALS
References [3] C. Mao, W. Sun and N.C. Seeman: J. Am. Chem. Soc.,
121, 5437–5443 (1999).
[1] M. Burst, M. Walker, D. Bethel, D.J. Stiffing and R. Why
[4] Y. Hatakeyama, M. Minami, S. Ohara, M. Umetsu,
man: J. Chem. Soc. Chem. Common., 801–802 (1994).
S. Takami and T. Adschiri: Kobunshi Ronbunshu, 61,
[2] A.C. Templeton, W.P. Wuelfing and R.W. Murray: Acc.
617–622 (2004).
Chem. Res., 33, 27–36 (2000).
APPLICATION 42
42 FABRICATION TECHNIQUE OF ORGANIC NANOCRYSTALS AND THEIR OPTICAL

PROPERTIES AND MATERIALIZATION

Organic nanocrystals are, for the definition, crystals direction in a DAST crystals, it is well known that the
of nanometer order of an organic compound. Organic crystals have large dipole moments and show very
nanocrystals are the nanomaterials that have been large second-order nonlinear optical properties. TPB
attracting attention in recent years, because they show and perylene are organic dye compounds having
some interesting characteristics for basic science, and strong fluorescence even in crystalline state, and
many applications using organic nanocrystals are attract attention as fluorescence and electrolumines-
expected as well. cence (EL) materials. Titanylphthalocyanine (TiOPc),
Regarding the fabrication techniques of organic quinaridone, C and so on were used as hardly soluble
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nanocrystals until around 1990, an evaporation tech- compounds. TiOPc is known to have a superior optical
nique in inert gas was reported as the bottom-up photoconductive property and has been really used in
method by Toyotama or Yase et al. [1, 2]. Evaporation usual photocopiers. Quinaridone has been used very
techniques, however, were not applicable for preparing well as a high-quality red pigment. All of the com-
nanocrystals of different organic compounds, because pounds shown in Fig. 42.1 are of interest for their opti-
ordinary organics are unstable at high temperatures. cal and electronic properties in the crystalline state.
On the other hand, the fabrication of organic pigments
nanocrystals has been performed using milling tech-
nique mainly by company researchers. The milling 2. Fabrication techniques of organic nanocrystals
method is effective to obtain the nanocrystals with par-
ticle size around 50–100 nm. However it is not easy for (1) Reprecipitation method
the milling method to reduce the particle size to 50 nm The scheme of reprecipitation method [3], which we
below because of structural rearrangement in crystals found for a fabrication technique of organic nanocrys-
induced by a mechano–thermal effect. In this review, tals is shown in Fig. 42.2. The reprecipitation method
the authors introduce “the reprecipitation method” [3] is a pure chemical technique to prepare organic
and the related technique to fabricate organic and nanocrystals simply by injecting solution of the target
polymer -conjugated nanocrystals. And their size- compound into a poor solvent. As distilled water is
dependent optical properties and application are usually used as poor solvent, water-soluble solvent
described here. such as alcohol, acetone or THF was selected as the
injected solution. The characteristics of reprecipitation
1. The organic compounds used for nanocrystallization method are that it is simple, quick and inexpensive. In
addition, nanocrystallizations using this method are
The -conjugated compounds attempted so far for the applicable for many kinds of organic compounds. In
preparation of nanocrystals are summarized in case of water-soluble organic dyes, the above-
Fig. 42.1. described reprecipitation method cannot be applied. In
Polydiacetylenes are -conjugated polymer materi- such a case, the reprecipitation method was improved
als that can be obtained by irradiation of UV light or like the following two ways: the reprecipitation of dis-
-ray or by thermal treatment through solid-state solved salts into saturated aqueous solution instead of
polymerization of diacetylenes [4]. The diacetylene pure water, i.e., salting-out method [5], and the repre-
derivative used in this paper was 1,6-di(N-carbazolyl)- cipitation into a hydrocarbon solvent such as hexane,
2,4-hexadiyne (DCHD), which has the carbazolyl decalin, and so on, i.e., inverse-reprecipitation method
group at both ends. 4-Dimethylamino-N-Methyl-4- [6]. In the reprecipitation method, the size and mor-
Stilbazolium Tosylate (DAST) is an ionic compound phology of nanocrystals could be controlled by chang-
with stilbazolium structure. As the electric dipole ing experimental condition such as combination
moment of each molecule almost falls in the same between poor and good solvents, concentration of

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