Chapter 11

Fabrication of Organic Microcrystals and Their Optical Properties

H. Oikawa, H. Kasai, and H. Nakanishi

Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

Organic microcrystals occupy the mesoscopic phase between a single molecule and bulk crystal, and they are expected to exhibit peculiar optical- and electronic-properties, depending on crystal size. The reprecipitation method was available for fabrication of organic microcrystals. The crystal size was in the range of about ten nanometer to several hundred nanometer. The excitonic absorption peak positions were shifted to short-wavelength region wi th decreasing crystal size. This phenomenon is not explainable by quantum confinement effect, and it is now speculated to be due to a certain coupled interaction between exciton and lattice vibration in thermally softened microcrystal lattice.

Nano-particles and super-fine particles in inorganics and metals have been investigated extensively from the viewpoints of both fundamental science and applications (1-4). Microcrystais occupy the mesoscopic phase between a single molecule and bulk crystal (3-6). In particular, it was worth noting that several reports supporting the enhancement of nonlinear optical ( N L O ) properties on the basis of quantum confinement effect have recently been published in semi-conductor nano-particles with sizes below 10 nm (7-14). These nano-particles were fabricated either by the deposition methods in a molten glass-matrix or by the vacuum-evaporation processes (15). O n the other hand, organic compounds have essentially an abundance of physicochemical properties (16), in comparison with inorganic materials. However, little attention had been paid so far to fabrications of organic microcrystais, when our studies on organic microcrystais were started (17,18).

We have demonstrated that the "reprecipitation method" is useful and convenient to prepare some kinds of organic microcrystais (19): Polydiacetylene ( P D A ) derivatives (20-22), low-molecular weight aromatic compounds such as perylene and

158 © 2002 American Chemical Society

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C6Q(23-26), and organic functional chromophores of pseudo-isocyanine, merocyanine and phthalocyanine (27,28). In any case, the crystal size was commonly in the range of several tens of nanometers to sub-micrometer (20,23,28). Some interesting phenomena have been confirmed, e.g., the enlargement in conversion of solid-state polymerization in diacetylene monomer microcrystais (29), the shift of the excitonic absorption peak position to the short-wavelength region with decreasing crystal size (20-22), and the appearance of the emission peak from free-exciton energy level in perylene microcrystais with decreasing crystal size and the subsequent shift of the emission peak position to the high energy region (23-25).

In the present chapter, w e w i l l provide an interpretation of the reprecipitation method, and then discuss the fabrications of fibrous P D A microcrystais as well as ordinary P D A ones, microcrystallization processes to control the crystal size and shape, and linear optical properties dependence on crystal size.

Reprecipitation Method

Figure 1 shows the scheme of the reprecipitation method (20-27). A target compound is first dissolved in an alcohol or in acetone so that its concentration is about 10"3 M . Next , a few micro-liter of the diluted solution should be injected rapidly into a vigorously stirred poor solvent (10 m L ) , using a microsyringe. It follows that the target compound is reprecipitaled and microcrystallized in a poor solvent. Finally, one can obtain organic microcrystais dispersed in the dispersion medium- If the target compound would be solid-state polymerizable diacetylene monomer as shown in Figure 1, then the monomer microcrystais dispersed are further polymerized by U V -irradiation, and then the corresponding P D A microcrystais are formed (30-32). Water is commonly used as a poor solvent, whi le hydrocarbon such as n-hexane, cyclohexane and decalin are employed as a poor solvent in the case of water-soluble ionic chromophores (27). W e can control the crystal size and shape by changing some factors in the reprecipitation process: Concentration of an injected solution, temperature of the poor solvent, and an added surfactant (33).

ν hfi

5 m M D C H D acetone solution 200 μΐ (addition of" surfactant)

I I Retention time for I crystallization

Stirring bar Monomer microparticle Monomer microcrystal Poor solvent (Water ΙΟ ml)

Solid-state polymerization in monomer microcrystal UV-irradiation (254 nm) for 20 min

Polymer microcrystal

Figure 1 Reprecipitation method schematically exemplified for the case of a diacetylene such as DCHD [ly6-di(N-carhazolyl)-2,4-hexadiyne] and the corresponding solid-state polymerized DCHD, poly (DCHD).

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However, no suitable water-miscible good solvents are found in some cases such as slightly soluble titanyl-phthalocyanine. The supercritical fluid crystallization ( S C F C ) technique was attempted in this case (28,34). W e have succeeded in controlling the crystal size and the crystal forms by changing the temperature of supercritical acetone fluid and the composition of acetone-water mixture used as a cooling solvent as shown in Figure 2. In particular, γ-form of titanyl-phthalocyanine (micro)crystals is noted in the field of xerography (35).

(a) Crystal size (b) Crystal form

T S C F / K T S C T / K

Figure 2 Crystal size (a) and crystal forms (b) for titanyl-phthalocyanine microcrystais prepared by SCFC technique. T^p represents temperature of supercritical acetone, and RA is the volume ratio of acetone in acetone-water mixture used as a cooling solvent. Reproduced with permission from Ref. 34. Copyright 1999.

Various Types of Organic Microcrystais and Microcrystallization Processes

Figure 3 shows the typical S E M (scanning electron microscopy) photographs of p o l y ( D C H D ) [poly(l,6-di(^-carbazolyl)-2,4-hexadiyne)] microcrystais (20,21). The crystal size, which was also determined b y D L S (dynamic light scattering) technique, was evidently influenced by the water temperature. These obtained microcrystais are suggested to be a single crystal in principle from H R T E M (high resolution transmission electron microscopy) observation (36). In addition, p o l y ( D C H D ) microcrystais prepared in the presence of a surfactant such as SDS [sodium dodecylsulfate] at the elevated temperature of 6 0 ° C have grown as fibers with retention time after reprecipitation as shown in Figure 4 (37). The contour length of fibrous microcrystais is more than 1 μπι , and the diameter was about 50 nm. Hence, this diameter was not so different from those of ini t ial ly formed amorphous-like D C H D particles as described below (20,21,33).

A s already mentioned, the crystal size and shape of microcrystais are changeable by the reprecipitation conditions. In other words, it is important to investigate the microcrystallization processes for the purpose of controlling the crystal size and shape. Here, w e have focused on p o l y ( D C H D ) and perylene microcrystais mainly by the measuremenrs with S E M and S L S (static light scattering) measurements.

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Figure 3 SEM photographs of poly (DCHD) microcrystais fabricated by the reprecipitation method. The values ofd and WT mean the average crystal sizes and water temerature, respectively: (a), d = 150 nm,WT = 0 °C; (b) d = 100 nm, WT = 20 0°C; (c) d = 70nm,WT= 50°C.

Figure 4 SEM photographs of DCHD micropartides and fibrous microcrystais with various retention times of (a) 0 minute, (b) 7 minutes, and (c) 20 minutes at 333 Κ in the presence of SDS. Reproduced with permission from Ref 37. Copyright 2001 John Wiley & Sons.

Figure 5 depicts the S E M photographs of D C H D particles and microcrystais wi th retention time after injecting DCHD-acetone solution into water (20,21). In Figure 5, the shape was converted from sphere-like to cubic-like, but the average size seems to be almost same. During this period the excitonic absorbance based upon π-conjugated p o l y ( D C H D ) chains, measured at - 650 nm, increased gradually and saturated wi th retention time. The l ow absorbance at the init ial stage means that solid-state polymerization did not proceed enough. Thus, sphere-like D C H D particles at the initial stage are said not to be solid-state polymerizable microcrystal but to be amorphous-like particle (20,21,38). In fact, we could not observe any X-ray diffraction pattern peaks from D C H D particles formed at the ini t ial stage (21).

On the other hand, we have investigated the microcrystallization process of perylene by S L S measurements (33). A t constant temperature, the scattered light intensity I s increased gradually with retention time, and then saturated. The saturated

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Figure 5 SEM photographs of DCHD micropartides and microcrystais with various retention times of (a) 0 minute, (b) 5 minutes, and (c) 10 minutes. Reproduced with permission from Ref. 20. Copyright 1996.

\ were almost proportional to the amount of the injected perylene solutions, although the crystal sizes were almost the same and about 200 nm in any cases. It follows that the saturated I s correspond to the number of perylene microcrystais. Next , at the constant amount of the injected solution, the increments of \ were mult ipl ied exponentially wi th increasing temperature at above 4 0 ° C , and then also saturated with retention time at the given temperature. The saturated I s was almost the same at any temperature above 4 0 ° C , and the crystal sizes were also about 200 nm. On the contrary, the saturated \ became lower below 4 0 ° C , and the crystal size was reduced to about 120 nm in this case.

According to these data, the microcrystallization processes of D C H D and perylene were speculated to occur as illustrated in Figure 6 (20-22,33). In any cases, just after reprecipitation, fine droplets are first formed in an aqueous l iquid. In the case of D C H D , after removing solvent into the surrounding water, the amorphous and supersaturated D C H D particles are formed, and then nucleation and crystal growth may occur in the individual amorphous particles. On the other hand, the cluster-like fine particles are considered to be once produced in the case of perylene, and then nucleation and crystal growth proceeds through thermal collision between these clusters. Therefore, the initial size of the droplets formed should be minimized to control the crystal size of p o l y ( D C H D ) . W e have tried to reduce the size of the droplets by both decreasing concentration of the injected solution and adding S D S . A s a result, we could obtain the smallest p o l y ( D C H D ) microcrystais wi th a size of about 15 nm in our research (21).

In addition, the microcrystallization process of fibrous p o l y ( D C H D ) microcrystais is speculated in the fol lowing (37). In the presence of added S D S , amorphous-like D C H D particles seem to be stable thermodynamically even at the elevated temperature. Meanwhile, a part of these particles is microcrystallized, and then amorphous-like particles and microcrystais would co-exist. Next, already-formed microcrystais may act as a k ind of substrate, and amorphous particles may be bound epitaxially through thermal collision on the particular crystal plane. The amorphous-

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DCHD m ^ -

Monomer microcrystais

Cluster Microcrystais

Figure 6 Proposed scheme of microcrystallization processes for DCHD and perylene in the reprecipitation method.

l ike D C H D particles likely is not completely random but ordered in pseudo-crystalline state. In fact, the excitonic absorbance from solid-state polymerization was observed to be low but finite even at the init ial retention time. The added S D S and high temperature may contribute to stabilize the co-existence between amorphous-like D C H D particles and microcrystais, and to promote epitaxial microcrystal growth.

Crystal Sizes Dependence of Linear Optical Properties in Organic Microcrystais

Figure 7(a) shows VIS spectra of p o l y ( D C H D ) microcrystais dispersed in an aqueous l iqu id . The excitonic absorption peak positions, λ ^ , of π-conjugated polymer chains were shifted evidently to the short-wavelength region with decreasing crystal size (20-22). The relationship between and crystal size is plotted in Figure 7(b). On the other hand, the value of (= 670 nm) in fibrous p o l y ( D C H D ) microcrystais was almost similar to that of the corresponding bulk p o l y ( D C H D ) crystals (37). This fact suggests that π-conjugated polymer chains are extended along the long axis of the fibrous microcrystal (38). The size effects on linear optical properties were also observed in perylene microcrystais (23-25,39).

These blue-shift phenomena of \ a K with decreasing crystal size are apparently similar to the behaviors reported in semi-conductor nano-particles with sizes below 10 nm. However, the crystal sizes in the present organic microcrystais are about ten times greater than those of the semi-conductor nano-particles. We believe these experimental results are a peculiar size effect in organic microcrystais, and the mechanism cannot be explained by the so-called quantum confinement effect (7-14). To further promote our discussion on these phenomena, the temperature dependence of for p o l y ( D C H D ) microcrystais with three crystal sizes was measured as shown in Figure 8 (40). In every case, was red-shifted with lowering temperature, and these three plot lines are almost parallel within experimental errors. W e can regard the intercepts as the intrinsic at each crystal size. In addition, the half-width of the

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Temperature / Κ Figure 8 Excitonic absorption peak positions (cm'1) dependence on temperature for poly (DCHD) microcrystais with three different crystal sizes: #, 50 nm; β, 100 nm; A, 1 μm above. Reproduced with permission fromRef 40. Copyright 1998 Gordon and Breach.

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excitonic absorption peak was changed from ca. 770 cm"1 to ca. 600 cm" 1 with decreasing temperature in the case of p o l y ( D C H D ) microcrystais with 100 nm in crystal size, whereas the half-width changed from ca. 1050 cm"1 to ca. 750 cm" 1 as wel l in the case of 50 nm crystals.

Table 1 summarized the size effects on linear optical properties in p o l y ( D C H D ) microcrystais. Let us consider some factors to clarify the present size effects. In conclusion, the quantum size effect, light scattering effect from microcrystais dispersion, and some surface effects between microcrystais and the surrounding dispersion medium should be rejected or, at least, minor factors. A s a reasonable discussion, a certain coupled interaction between exciton and lattice vibration in thermally "softened" crystal lattice in microcrystais is now speculated to bring about instabilization in the lowest exciton level and/or high occupation in exciton band (40,41). This is the possible qualitative explanation at the present time. Theoretical analyses are now in progress as well .

Table I Crystal Sizes Dependence of Linear Optical Properties for Poly(DCHD) Microcrystais

Crystal Size Small Large

High energy shift ο L o w energy shift Δ ν ^ Broadening ο Narrowing Temperature High <=> L o w Crystal Lattice High frequency vibration <=> L o w frequency vibration

Concluding Remarks

We have established the reprecipitation method, including supercritical f luid crystallization technique, to fabricate well-defined organic microcrystais, i.e., to control crystal size, shape, and crystal forms. Next, linear optical properties of organic microcrystais were found to be dependent on crystal size, wh ich was qualitatively explained by a certain coupled interaction between exciton and lattice vibration in soften microcrystal lattice, rather than the so-called quantum confinement effect.

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