Fabrication of Organic Microcrystals and Their ... - ACS Publications

Chapter 11

Fabrication of Organic Microcrystals and Their Optical Properties

Downloaded by OHIO STATE UNIV LIBRARIES on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch011

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. T h e crystal size was in the range o f about ten nanometer to several hundred nanometer. The excitonic absorption peak positions were shifted to short-wavelength region w i t h decreasing crystal size. T h i s phenomenon is not explainable by quantum confinement effect, and it is n o w 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 o f 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 o f nonlinear optical ( N L O ) properties on the basis o f quantum confinement effect have recently been published in semi-conductor nanoparticles w i t h sizes below 10 n m (7-14). These nano-particles were fabricated either by the deposition methods i n 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), i n comparison w i t h inorganic materials. H o w e v e r , little attention had been paid so far to fabrications o f organic microcrystais, w h e n our studies on organic microcrystais were started (17,18). W e have demonstrated that the "reprecipitation method" is useful and convenient to prepare some kinds o f organic microcrystais (19): Polydiacetylene ( P D A ) derivatives (20-22), low-molecular weight aromatic compounds such as perylene and

158

© 2002 A m e r i c a n Chemical Society

In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

159 C (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). S o m e 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 w i t h decreasing crystal size (20-22), and the appearance of the emission peak from free-exciton energy level i n perylene microcrystais w i t h decreasing crystal size and the subsequent shift of the emission peak position to the high energy region (23-25). Downloaded by OHIO STATE UNIV LIBRARIES on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch011

6Q

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 3

about 10" M . N e x t , 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 i n a poor solvent. Finally, one can obtain organic microcrystais dispersed in the dispersion medium- If the target compound w o u l d be solid-state polymerizable diacetylene monomer as shown i n 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

c o m m o n l y used

as a poor solvent, w h i l e hydrocarbon such

as

n-hexane,

cyclohexane and decalin are employed as a poor solvent i n the case of water-soluble i o n i c chromophores (27). W e can control the crystal size and shape by changing some factors

in the

reprecipitation

process:

Concentration

temperature of the poor solvent, and an added surfactant

ν

of

an

injected

(33).

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

I

hfi

Stirring bar Poor solvent (Water ΙΟ ml)

I I

Retention time for crystallization

Monomer microparticle

Monomer microcrystal

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 [l 6-di(N-carhazolyl)-2,4-hexadiyne] and the corresponding solid-state polymerized DCHD, poly (DCHD). y


solution,

160 H o w e v e r , no suitable water-miscible good solvents are found in some cases such as

slightly soluble titanyl-phthalocyanine.

( S C F C ) technique

The supercritical fluid

was attempted in this case (28,34).

crystallization

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 i n the field o f xerography (35).


(a) Crystal size

(b) Crystal form

TSCF/K

T

Figure 2 Crystal size (a) and crystal forms (b) for

S C T

/K

titanyl-phthalocyanine

microcrystais

prepared

supercritical

acetone, and R is the volume ratio of acetone in acetone-water

by SCFC technique. T^p represents temperature of A

used as a cooling solvent. Reproduced Copyright

with permission

mixture

from Ref. 34.

1999.

Various Types of Organic Microcrystais and Microcrystallization Processes Figure 3 shows the t y p i c a l 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, w h i c h 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

transmission

be

a

single crystal

electron

microcrystais dodecylsulfate]

prepared

microscopy) in the

in principle observation

presence of

a

from (36).

HRTEM In

surfactant

(high

addition, such

resolution

poly(DCHD)

as S D S

[sodium

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 o f fibrous microcrystais is more than 1 μ π ι , and the diameter was about 50 n m . Hence, this diameter was not so different from those of initially formed amorphous-like D C H D particles as described below

(20,21,33).

A s already mentioned, the crystal size and shape o f 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.



161

Figure 3 SEM photographs reprecipitation

of poly (DCHD)

microcrystais

fabricated

by the

method. The values ofd and WT mean the average crystal sizes and

water temerature,

respectively:

(a), d = 150 nm,WT

0°C; (c) d = 70nm,WT=

50°C.

Figure 4 SEM photographs

of DCHD

micropartides

= 0 °C; (b) d = 100 nm, WT = 20

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 w i t h 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. D u r i n g this period the excitonic absorbance based upon π-conjugated p o l y ( D C H D ) chains, measured at - 650 n m , increased gradually and saturated w i t h retention time. The l o w absorbance at the initial stage means that solid-state polymerization d i d 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). I n fact, we could not observe any X - r a y diffraction pattern peaks from D C H D particles formed at the initial stage (21). O n 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 increased gradually with retention time, and then saturated. The saturated s



162

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 permission from Ref. 20.

Copyright

with

1996.

\ were almost proportional to the amount of the injected perylene solutions, although the crystal sizes were almost the same and about 200 nm i n any cases. It follows that the saturated I correspond to the number of perylene microcrystais. N e x t , at the constant amount of the injected solution, the increments of \ were multiplied exponentially w i t h increasing temperature at above 4 0 ° C , and then also saturated with retention time at the given temperature. The saturated I was almost the same at any temperature above 4 0 ° C , and the crystal sizes were also about 200 n m . O n the contrary, the saturated \ became lower below 4 0 ° C , and the crystal size was reduced to about 120 n m i n this case. s

s

A c c o r d i n g to these data, the microcrystallization processes of D C H D and perylene were speculated to occur as illustrated i n Figure 6 (20-22,33). In any cases, just after reprecipitation, fine droplets are first formed i n an aqueous liquid. In the case o f 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 i n the individual amorphous particles. O n the other hand, the cluster-like fine particles are considered to be once produced in the case o f perylene, and then nucleation and crystal growth proceeds through thermal collision between these clusters. Therefore, the initial size of the droplets formed should be m i n i m i z e d to control the crystal size o f 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 c o u l d obtain the smallest p o l y ( D C H D ) microcrystais w i t h a size o f about 15 n m i n our research (21). In addition, the microcrystallization process of fibrous poly(DCHD) microcrystais is speculated i n the following (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. M e a n w h i l e , a part of these particles is microcrystallized, and then amorphous-like particles and microcrystais w o u l d co-exist. Next, already-formed microcrystais m a y act as a k i n d of substrate, and amorphous particles may be bound epitaxially through thermal collision on the particular crystal plane. T h e amorphous-


163

m

DCHD

^-


Monomer microcrystais

Cluster

Microcrystais

Figure 6 Proposed scheme of microcrystallization in the reprecipitation method.

processes for DCHD

l i k e D C H D particles likely is not completely random but

and

perylene

ordered in pseudo-

crystalline state. I n fact, the excitonic absorbance from solid-state polymerization was observed to be l o w but finite even at the initial 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 V I S spectra of p o l y ( D C H D ) microcrystais dispersed i n an aqueous l i q u i d . 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 7(b). O n the other hand, the value of

and crystal size is plotted i n Figure (= 670 nm) i n 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). T h i s fact suggests that π-conjugated polymer chains are extended along the long axis o f the fibrous microcrystal (38).

The size effects on linear optical

properties were also observed i n perylene microcrystais

(23-25,39).

These blue-shift phenomena of \ with decreasing crystal size are apparently similar to the behaviors reported in semi-conductor nano-particles with sizes below 10 n m . However, the crystal sizes in the present organic microcrystais are about ten times greater than those of the semi-conductor nano-particles. W e believe these experimental results are a peculiar size effect i n organic microcrystais, and the mechanism cannot be explained by the so-called quantum confinement effect (7-14). T o 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 w i t h lowering temperature, and these three plot lines are almost parallel w i t h i n experimental errors. W e can regard the intercepts as the intrinsic at each crystal size. In addition, the half-width of the a

K


In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001. microcrystais

Reproduced

(b), Breach.

on crystal sizes. 1998 Gordon and

^^(nm)

with permission from Ref 40. Copyright

dependence of excitonic absorption peak positions

spectra;

with three different

of poly (DCHD)

Figure 7 Linear optical properties

crystal sizes dispersed in an aqueous liquid: (a), visible absorption

W

(·)

λ/am


165

4


1.57 Ι Ο È-

1:56 ΙΟ

S ο

ι

4

4

1.55 ΙΟ

1.54 Ι Ο

4

1.53 Ι Ο

4

1.52 Ι Ο

4

1.51 Ι Ο

4

1.5 Ι Ο

4

1.49 Ι Ο

É-

I Γ

I

4

Ο

50

100

150

200

250

300

Temperature / Κ Figure 8 Excitonic

absorption peak positions

for poly (DCHD) microcrystais

1

(cm' ) dependence on temperature

with three different crystal sizes: #, 50 nm; β, 100

nm; A, 1 μm above. Reproduced with permission fromRef Copyright

40.

1998 Gordon and Breach.


166 1

1

excitonic absorption peak was changed from ca. 770 cm" to ca. 600 cm" with decreasing temperature i n 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" to ca. 750 cm" as w e l l in the case of 50 n m crystals.


1

1

Table 1 summarized the size effects on linear optical properties i n 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 i n thermally "softened" crystal lattice in microcrystais is now speculated to bring about instabilization i n the lowest exciton level and/or high occupation in exciton band (40,41). T h i s 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

Broadening

ο

Narrowing

Temperature

High

L o w

Crystal Lattice

High frequency vibration L o w frequency vibration

Δν^

Concluding Remarks W e have established the reprecipitation method, including supercritical fluid 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, w h i c h 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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