Microcrystals of polydiacetylene derivatives and their linear and nonlinear optical properties

PII: S0968-5677 (98) 00021-2

Supramolecular Science 5 (1998) 289—295  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

Microcrystals of polydiacetylene derivatives and their linear and nonlinear optical properties Hachiro Nakanishi* and Hideyuki Katagi Center of Interdisciplinary Research, Tohoku University, Sendai, Miyagi 980-8577, Japan

Organic microcrystals which are expected to have interesting and fascinating physical properties were fabricated by a reprecipitation method as aqueous dispersions. Many kinds of organic compounds have been microcrystallized by this convenient method. The size control has been extensively investigated for a polydiacetylene and succeeded in the range from several tens of nanometers to several micrometers by adjusting the temperature and concentration. Linear optical properties of these well-defined polydiacetylene microcrystals have been investigated and interesting size and temperature dependences of excitonic absorption are demonstrated. Nonlinear optical (NLO) properties of polydiacetylene microcrystals have also been evaluated by means of a Z-scan technique, and an extremely high nonlinear refractive index at the resonant wavelength was shown.  1998 Elsevier Science Limited. All rights reserved. (Keywords: organic microcrystals; excitonic absorption; nonlinear optical property)

INTRODUCTION The fabrication and construction of nano-structures are topics of growing interest in the fundamental science and technology of organic materials owing to their characteristic chemical and physical properties. A remarkable number of investigations, such as self-assembling monoand multilayers (SAMS)1—3 and molecular beam epitaxy (MBE) and so on, have been performed and are still of great interest. These methods are surely a powerful way of constructing nano-structured materials but have the limitation of compounds for practical use. On the other hand, microcrystal fabrication is considered to be a new type of approach at the point where finely separated particles, i.e. the free standing state of particles, are able to show physical properties arising not from the aggregated state like samples of the above technique, but from only one isolated microcrystal. Fabrication of microcrystals has extensively been investigated for metals5—7 and semiconductors8—11, and many characteristic properties have already been shown. For example, a novel and unique optical properties called the quantum size effect was found for such inorganic microcrystals in having a size of a few nanometers. For organic microcrystals, however, microcrystal fabrication has so far rarely been investigated, owing to their thermal instability, except for the thermally stable phthalocyanine and a kind of low molecular weight aromatic compounds which were microcrystallized by an evaporation method in an inert gas or by a crystal growth technique in a porous polymer film. Recently,

* Also affiliated to the Institute for Chemical Reaction Science, Tohoku University, Sendai, Miyagi 980-8577, Japan

we have proposed a simple and convenient method for the fabricaiton of organic microcrystals, that is to say, reprecipitation technique. In brief, the hydrophilic solution of the compound is injected into water, which results in a reprecipitation owing to its insolubility in water to give a microcrystal dispersion. By this method, many kinds of organic compounds such as polydiacetylene18—20, phthalocyanine, perylene and so on can be microcrystallized successfully. These aqueous organic microcrystal dispersions may have great potential for use in fundamental and applied optics, e.g. as NLO materials having large s without optical loss and having easy processability to other material forms. In this study, we briefly review, how to control the microcrystal size of polydiacetylene microcrystals, and discuss their linear and nonlinear optical properties with particular reference to crystal size.

EXPERIMENTAL Organic compounds so far microcrystallized in our laboratory are summarized in Figure 1. In this paper, the diacetylene compound, i.e. 1,6-di(N-carbazolyl)-2,4hexadiyne abbreviated as DCHD whose solid-state polymerization has been known to proceed through single-crystal-to-single-crystal-phase transition like manner in its bulk with 100% conversion to the polymer structure was mainly used. In a typical experimental condition, 100—200 ll of DCHD acetone solution was injected into 10 ml of water using a microsyringe, then microcrystals were obtained as an aqueous dispersion. Solid-state polymerization of DCHD microcrystals was carried out by irradiating the aqueous dispersion with ultraviolet (UV) light from a low-pressure

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi

Figure 1 Organic compounds so far microcrystallized by the reprecipitation method

mercury lamp (254 nm) for 20 min. It is worth noting that the size and shape of the microcrystal did not change at all during the solid-state polymerization. The microcrystal size was verified by both the scanning electron microscope (SEM, Hitachi S-900) with filtered and dried-up samples, and a light scattering instrument (DLS, Otsuka electronics DLS-7000) for in situ measurements. Optical absorption spectra were recorded on a UV—Vis—NIR spectrophotometer (JASCO V-570DS).

RESULTS AND DISCUSSION Preparation of organic microcrystals We have succeeded in preparing DCHD microcrystals in the size range of 30 nm to 1 lm. The upright dispersions of these microcrystals in water were stable even after one year. Figure 2 indicates scanning electron micrographs of DCHD microcrystals prepared under different conditions. It has become apparent that there are two important factors that control the size of the DCHD microcrystal. The first is the concentration of the DCHD acetone solution. Figures 2a and b correspond to 2.5 and 10 mM of acetone solution, respectively. The lower the concentration of the injected solution the smaller the size of the microcrystal obtained. To confirm this fact alternatively, the static light scattering was measured. Figure 3 shows the relationship between scattered light intensity (I /I ) for the aqueous dispersion and 1 concentration of the injected solution. According to the Rayleigh theory, I /I is expressed by 1 I /I "FNl 1 -

(1)

I /I &N at l"const. 1 -

(2)

I /I &v&[(c)]&c at N"const. 1 -

(3)

where l and N are the volume and the number of scattering bodies, respectively, and F is a numerical constant depending on the dielectric constant. In Figure 3, it is clearly seen that I /I is proportional to the injected 1 amount at any concentration. This means that the value of N depends on the injected amount without change of l [Eq. (2)]. On the other hand, the I /I profiles could be 1 fitted evidently to the square of the concentration, c, at the same injected amount. This result can be explained by Eq. (3) with that N as a constant. In fact, the decrease in the concentration leads to smaller DCHD particle sizes as shown in Figures 2a and b. The second critical factor is the temperature. Figure 2c indicates the microcrystals whose aqueous dispersion was kept at 60°C and the other conditions were the same as in Figure 2b. Fibrous DCHD microcrystals were formed by raising the temperature. To clarify how these fibrous crystals were formed, we investigated the crystallization process for DCHD during the reprecipitation. SEM micrographs shown in Figure 4 indicate the mirocrystallization process of DCHD at 60°C. It has already been presented that DCHD just after the injection of the solution at room temperature showed no solid-state polymerizability by UV-irradiation and were spherical amorphous particles. When these amorphous particles were allowed to stand for 20 min at 20°C of water temperature, quantitative polymerization became possible to proceed by UV-irradiation, and then cubic poly-DCHD microcrystals were obtained without a change in size and shape. Figure 4a shows the amorphous spherical particles, whose size is almost the same as that at 20°C. Standing over 20 min, rod-like particles appear as shown in Figure 4b, indicating that the binding these amorphous particles and subsequent crystallization occurred. However, many amorphous particles not yet crystallized remain at this stage. The crystallization, accompanied with the binding of amorphous particles, was completed in 90 min at 60°C, because saturation of excitonic absorbance was observed after solid-state polymerization. The

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi

Figure 2 Scanning electron micrographs of DCHD microcrystals with different sizes: (a) 50 nm, (b) 100 nm, and (c) more than 1 lm. These samples were prepared by injecting (a) 2.5 mM of DCHD aceton solution into 20°C water, (b) 10 mM of DCHD acetone solution into 20°C water, and (c) 10 mM of DCHD acetone solution into 60°C water and keeping it at this temperature for 90 min

temperature influences the rate of crystallization as well as the shape of the microcrystal. In the case of DCHD, the rate of crystallization becomes significantly slow at 60°C compared with that at 20°C, thus the crystal growth to fiber became possible. Figure 5 shows a schematic drawing of the crystallization process of DCHD in the reprecipitation method. Fine droplets including acetone

and DCHD molecules are formed first, and subsequently acetone is rapidly extracted into the surrounding water to give non-polymerizable amorphous particles having spherical morphology. If it stands at room temperature (20°C) crystallization takes place in 20 min and almost the same size of solid-state polymerizable microcrystals as that of amorphous particles are formed. As suggested

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi microcrystals. Thus, attempts were to make small amorphous particles using surfactants. As a result, an anionic surfactant, sodium dodecylsulfate (SDS), was found to be effective in reducing the microcrystal size significantly to give poly-DCHD microcrystals 15 nm in size (Figure 6). This result seems to be strongly correlated to the f-potential of DCHD microcrystals, which was evaluated to be about !40 mV. It is likely that fine micelles are formed with SDS, which leads to the increase of negative surface charge resulting in the generation of smaller amorphous particles.

LINEAR AND NONLINEAR OPTICAL PROPERTIES

Figure 3 Plots of scattered light intensity I /I vs solution concentra1 tion just after injection of solution. The injected amounts are 100 ll (䊉) and 200 ll (䊏), respectively

by the concentration dependence of the DCHD microcrystal size, the average microcrystal size is determined by the density of molecules in a droplet. At high temperatures, however, crystallization does not occur rapidly, and rather the binding of amorphous particle happens easily. Repeat of this process results in a growth of fibrous DCHD crystals. A challenge to further reduce the DCHD microcrystal size has taken. The mechanism in Figure 5 suggests that the smaller size of the droplet or the amorphous particle led to the formation of smaller DCHD or poly-DCHD

Figure 7 shows excitonic absorption spectra of one-dimensionally n-conjugated poly-DCHD microcrystals, dispersed in water at room temperature. It is clearly seen that the excitonic absorption peak position (j ) de  pends on crystal size. The relation between j and

 crystal size including those of much wider size of crystals is plotted in Figure 8. The j is blue-shifted with de  creasing crystal size, while with increasing crystal size j approaches 663 nm, i.e. the limiting value of bulk

 poly-DCHD crystals. A similar size dependence has been found for perylene and phthalocyanine microcrystals in their absorption spectra. In addition, the luminescence of perylene microcrystals has also been confirmed to have a similar size dependence. An important point to note is that the present size-dependent blue-shift appears at a size an order of magnitude larger than the of semiconductors and metals.

Figure 4 Scanning electron micrographs of DCHD. The samples were kept for (a) 0 min, (b) 20 min, (c) 40 min, and (d) 90 min at 60°C

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi

Figure 4 continued

Figure 5 The proposed schematic model for the crystallization process of DCHD in the reprecipitation method

To investigate the reason for this characteristic size effect in organic microcrystals, we have tried to measure absorption spectra at low temperatures. For this purpose, poly-DCHD microcrystals, whose sizes were 50 nm, 100 nm, and 1 lm, were embedded in a gelatin thin film by the spin-coating method. Figure 9 shows the result of poly-DCHD microcrystals 100 nm in size and Figure 10 shows the change of the excitonic absorption peak position (l /cm\) with temperature for three

 different sizes of poly-DCHD microcrystals. By lowering the temperature from 298 to 4 K, about 300 cm\ of redshift was observed in any crystal size. The value of l at

 4 K is supposed to be an intrinsic excitonic feature without thermal effect. It is also observed that the slopes are roughly the same and independent of crystal size. These thermal effects are clearly related to phonons in the

Figure 6 Scanning electron micrograph of 15 nm size of poly-DCHD microcrystals prepared in the presence of sodium dodecylsulfate

microcrystal lattice. The lattice vibration must become highly frequent with increasing temperature, and also with decreasing crystal size. These speculations could be supported from the dependence of the half-width of the excitonic absorption peak on temperature and crystal

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi

Figure 7 Dependence of visible absorption spectra for poly-DCHD microcrystals on their crystal size Figure 9 Temperture dependence of visible absorption spectra of poly-DCHD microcrystals dispersed in a gelatin thin film. The average crystal size is 100 nm

Figure 8 Plots of excitonic absorption peak position vs poly-DCHD microcrystals size

size as shown in Figure 11, where the highly frequent lattice vibration, i.e., phonon, can be considered to bring instability of the exciton band, and its higher energy shift. Some interesting features in the nonlinear optical properties of organic microcrystals have also been found. From the measurement of the degenerate four-wave mixing method using an incoherent OPO light source, vanadyl oxophthalocyanine microcrystals showed an ultra-fast response time of 150 fs without any slower response than 1 ps due to the so-called molecular reorientation. The Z-scan measurement for polyDCHD microcrystals has shown a remarkably high

Figure 10 Temperature dependence of excitonic absorption peak position (l /cm\) of poly-DCHD: crystal sizes are 50 nm (䊉);

 100 nm (䊏); more than 1 lm (䉱)

nonlinear refractive index (n ) as listed in Table 1.  These facts indicate a great possibility of microcrystals being the applied to purely optical switching devices.

Figure 11 Plots of half-width of excitonic absorption vs. reciprocal of temperature. The average size of poly-DCHD microcrystals was (a) 50 nm and (b) 100 nm

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Optical properties of polydiacetylene microcrystals: H. Nakanishi and H. Katagi Table 1 n and Re s of poly-DCHD microcrystals  j (nm)

n (10\ cm/GW) 

Re s (10\ esu.)

620 630 640 650 660 670

#1.0 #1.7 #2.4 — !2.2 !1.4

#0.61 #1.0 #1.4 !1.3 !0.88

SUMMARY The fabrication of polydiacetylene microcystals in a variety of size was demonstrated using the reprecipitation method. The size effect on the absorption spectra, which seems to be a characteristic of organic microcrystals, has been experimentally verified. The evaluation of nonlinear optical properties suggest that organic microcrystal dispersions could be promising NLO materials.

ACKNOWLEDGEMENT The authors gratefully acknowledge the receipt of a grant from the Center of Interdisciplinary Research in Tohoku University in support for this reserach project.

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