Effect of sodium toluene sulfonate on the nucleation, growth and characterization of DAST single crystals

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Journal of Crystal Growth 292 (2006) 510–514

Effect of sodium toluene sulfonate on the nucleation, growth and

characterization of DAST single crystals

A.S. Haja Hameed

a

, W.C. Yu

b

, C.Y. Tai

a

, C.W. Lan

a,

a_{Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan}

b_{Department of Molecular Science and Engineering, National Taipei University of Technology, Taipei 106, Taiwan} Available online 9 June 2006

Abstract

The metastable zone width and nucleation parameters such as interfacial tension were estimated for pure and sodium p-toluene sulfonate (NaTS)-doped 4-N,N dimethylamino-40_{-N-methyl-stilbazolium tosylate (DAST) single crystals. Pure and additive-doped}

DAST single crystals were grown by the slow-cooling technique. The grown crystals were morphologically interpreted. The crystalline powders of the grown crystals were examined by X-ray diffraction to estimate the crystal lattice parameter values. From Vicker’s microhardness test, the mechanical strength on (0 0 1) face of the crystals was found.

PACS: 81.10.Dn; 81.70.Pg

Keywords: A1. Metastable zone width; A1. Nucleation; A1. Solubility; A2. Growth from solution; A2. Recrystallization; B1. Organic compounds; B2. Nonlinear optical materials

1. Introduction

Ionic organic materials such as DAST (4-N,N dimethy-lamino-40_{-N-methyl-stilbazolium tosylate) have been}

inves-tigated by many researchers for their use in optical applications. The materials have the advantages that the alignment of ionic chromophore can be controlled in a polar structure by changing the counter ion[1]. Thus, there is no need to modify the chemical structure of the chromophore itself after having determined the ionic species with large second-order hyperpolarizability. Re-cently, a series of stilbazolium derivatives has been studied due to their potential use for second-order harmonic generation (SHG) and electro-optic (EO) modulation

[2–6]. One of the stilbazolium derivatives is DAST, which is the most useful nonlinear optical material researched to date, and which has been demonstrated to have a very large nonlinear optic (NLO) susceptibility and the largest EO coefﬁcient [3,7–10]. It belongs to the monoclinic crystal

system with the non-centrosymmetric space group Cc and the point group m, and has the following lattice parameters: a ¼ 10.365 A˚, b ¼ 11.322 A˚, c ¼ 17.893 A˚, Z ¼ 4 and b ¼ 92.241 [3]. Duan et al. [11] found that sodium p-toluene sulfonate (NaTS) has a short cut-off wavelength of 280 nm as well as comparatively large hyperpolarizability. It is a potential chromophore for second-order NLO. The chemi-cal structures of DAST and NaTS are shown inFig. 1.

It is believed that NaTS is a possible dopant for DAST; both having NLO properties. NaTS is considered to dissociate in methanol generating p-toluene sulfonate anions and sodium cations. DAST has the anionic toluene sulfonate, which is responsible for the non-centrosym-metric nature in its own molecular structure. In the present study, we used NaTS as a dopant in the DAST solution. The nucleation behavior in the presence of NaTS in the DAST solution was investigated through induction time and metastable zone width (MZW) measurements. The pure and NaTS-doped DAST crystals were grown by the slow-cooling method. Moreover, the grown crystals were examined by X-ray diffraction analysis and Vicker’s microhardness tester.

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Corresponding author. Tel./fax: +886 2 2363 3917. E-mail address:[email protected] (C.W. Lan).

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2. Experimental procedure

DAST was synthesized by the condensation of 4-methyl-N-methyl pyridinium tosylate and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine [12–14]. The 4-methyl-N-methyl pyridinium tosylate used in the con-densation reaction was prepared from equimolar quantities of 4-picoline and methyl-p-toluene sulphonate. The entire synthesis process was conducted in dry nitrogen atmo-sphere to avoid the formation of the orange hydrated form of DAST. The resulting DAST material was further puriﬁed by recrystallization from methanol. The recrystal-lized DAST crystals were used for the nucleation and crystal growth experiments.

The nucleation experiments were carried out in a constant temperature bath, with a control accuracy of

70.01 1C, provided with a cryostat for cooling below room

temperature. Hundred milliliter of the saturated DAST solution was used in all the experiments. NaTS (1 mol%) was used in the DAST solutions. Several trials were carried out to arrive at the optimal conditions. To measure the MZW, the conventional polythermal method was em-ployed [15,16]. In this method, the equilibrium-saturated solution was cooled from overheated temperature under a stirred condition until the first visible crystal was observed. Since the time required for the formation of a visible crystal after the attainment of a critical nucleus is minimal, the first crystal observed may be taken as the critical nucleus. In order to learn the induction period for the DAST solutions in different supersaturations, the conventional isothermal method was used. Here, the saturated solution was cooled to the desired temperature and maintained at that temperature, and the time taken for formation of the first crystal was measured, which gave the induction period for a selected supersaturation.

The solubility of DAST was estimated for the solvent methanol by the gravimetric method. For the growth of DAST crystals, a known amount of DAST salt was saturated in 100 ml of methanol at 40 1C. For the NaTS-doped DAST crystals, 1 mol% of NaTS was used in the saturated solution. Spontaneously grown good-quality seed crystals from pure and doped DAST solutions were

used to initiate the growth. The solution temperature (40 1C) was then reduced initially by 0.5 1C/day in constant temperature bath for two weeks.

By means of a diffractometer (MAC Science Diffract-ometer MXP-3), the X-ray diffraction pattern was recorded for the powdered pure and doped DAST samples in the range 101–351, where the monochromatic wavelength 1.5418 A˚ (CuKa) was used. Vickers hardness measurements

were done on (0 0 1) face of the grown crystals using a microhardness tester (Model HMV-2, Shimadzu, Japan). 3. Results and discussion

To calculate the interfacial tension, the following theoretical consideration can be used [17]. The kinetics of nucleation depends on the thermodynamic driving force, which in turn depends on the supersaturation, temperature and impurities present in a system. The induction period, a measure of the nucleation rate is determined experimen-tally for the pure and NaTS-doped DAST solutions at different supersaturations. According to the classical theory of homogeneous formation of a spherical nucleus, ln t ¼ ln B þ 16pg3V2NA=3R3T3ðln SÞ2, (1)

where V is the molar crystal volume, NA is Avogadro’s

number, R is the gas constant and S is the relative supersaturation (S ¼ C/C*) with C as the actual concen-tration and C* as the equilibrium concenconcen-tration. The function ln B weakly depends on temperature and hence, there is a linear dependence between ln t and (ln S)2at a constant temperature. The above equation suggests a straight line for ln t and (ln S)2with the slope given by m ¼ 16pg3V2NA=3R3T3. (2)

The interfacial tension of the solid relative to this solution has been calculated from the slope of the line as

g ¼ RT ½3m=16pV2NA1=3. (3)

The crucial parameter involved in between a growing crystal and the surrounding mother liquor is the interfacial tension g. In the present study, the interfacial tension was calculated using the experimentally determined induction period values. This complex parameter was determined by conducting the nucleation experiments on saturated solu-tions of pure and NaTS-doped DAST. From the induction period measurements, it was found that the induction period increases with the addition of NaTS.

From the induction period measurements, the interfacial tensions of pure and doped DAST were estimated and are shown in Fig. 2. The interfacial tension decreases with the addition of NaTS. The interfacial tensions of pure and doped DAST are estimated in the ranges 2.4–3.8 and 1.8–2.8 mJ/m2, respectively.

The MZW of pure and NaTS-doped DAST solutions as a function of temperature is shown in Fig. 3. The results obtained from the MZW experiments show that the addition of NaTs enhances the zone width. Pure DAST

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solution has the MZW of 5 1C. The MZW of NaTS-added DAST solutions were about 11 1C. The MZW is increased by the addition of NaTS. It is believed that the polarity of the solutes in the solvent would affect the relative motion of nuclei and hence, the metastable zone. The polar solute molecules have a positive and a negative end to the molecule. If the solvent molecule is also polar, then positive ends of solvent molecules attract negative ends of solute molecules due to dipole–dipole interaction.

The grown pure and NaTS-doped DAST crystals are shown inFig. 4. It was observed from the growth of DAST crystals that the growth rate along the a and b axes was the same, whereas the growth rate along the c-axis was very slow. The {0 0 1} habit faces grew predominantly; also, the growth was faster in both positive and negative a and b axes. In the grown crystal, the large (0 0 1) face was found and it was bounded by {1 1 0}, ð1 ¯1 0Þ and small {0 2 0} faces; the {0 2 0} faces disappearing in the prolonged

growth. In the case of NaTS-doped DAST crystals, the growth rate along the a-axis was much larger than that in the case of the pure crystals. The morphology of the NaTS-doped DAST crystal is shown inFig. 5(a), which resembles a rectangular shape rather than a square (as shown for the morphology of DAST crystal (Fig. 5(b)).

From the X-ray powder diffraction pattern of pure and NaTS-doped DAST samples (Fig. 6), the lattice parameter values were calculated using 2y values of high-intensity peaks corresponding to the h k l planes using the mono-clinic crystallographic equation. The lattice parameter values of the grown crystals are shown in Table 1. The calculated lattice parameter values of pure DAST crystals were consistent with those of the pure one and with the values reported by Marder et al.[3]. Little variation in the values of the lattice parameter was observed in the NaTS-doped DAST crystals. These values vary slightly with the values of pure DAST and this may be due to the presence of impurity in the grown crystals.

The microhardness was measured on the (0 0 1) face for pure and NaTS-doped DAST crystals under different

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 20 25 30 35 40 45 50 Temperature (°C) Concentration (g /100 ml)

Solubility Curve - DAST Nucleation Curve - DAST Nucleation Curve - DAST + NaTS

Fig. 3. Metastable zone width of pure and NaTS-doped DAST.

Fig. 4. Grown crystals, (a) NaTS-doped DAST (11 8 3 mm3), and (b) pure DAST (8 8 3 mm3). 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Interfacial Tension (mJ/m 2 ) Supersaturation DAST DAST + NaTS

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loads. The average diagonal length of the indented impression was calculated, and the Vickers microhardness number (Hv) was found from the relation Hv¼2P sin(y/2)/

d2. If the P is the applied load (kg) and d is the average diagonal length of the indentation impressions (mm), and the angle between the opposite faces of the diamond pyramid is y ¼ 1361, then Hvin kg/mm is given by

Hv¼1:8544P=d2. (4)

The variation of microhardness (Hv) with the applied loads

for pure and NaTS-doped DAST crystals is shown in

Fig. 7. The microhardness number decreased with increase in the applied load. From the microhardness studies, it was revealed that the hardness of pure and doped DAST crystals was almost the same. The microhardness values of pure DAST crystals were approximately 30.5, 27.5 and 20.3 kg/mm2 for the loads of 5, 10 and 25 g, respectively. The microhardness number was found in the range 19–31 kg/mm2for the doped DAST crystals.

4. Conclusion

In this study, the MZW of pure and NaTS-doped DAST solutions for different temperatures was determined. According to the classical theory of homogeneous forma-tion of a spherical nucleus, the nucleaforma-tion parameters were calculated. The interfacial tensions of pure and doped DAST were estimated as 2.4 and 1.8 mJ/m2, respectively, at

lower supersaturation. Pure and NaTS-doped DAST crystals were grown by the slow-cooling method. The growth rate along the a-axis was found to be faster in the NaTS-doped DAST crystals as compared to pure DAST. From the X-ray powder diffraction study, the lattice parameters were determined using the simulated h k l and experimentally observed d values. From microhardness studies, the hardness values of the pure and doped DAST crystals were determined on the (0 0 1) face. It was found that the addition of NaTS to DAST did not change the

10 15 20 25 30 35 0 250 500 750 1000 1250 1500 1750 2000 Intensity (counts) 0 250 500 750 1000 1250 1500 1750 2000 Intensity (counts)

Diffraction angle, 2 theta (deg.)

10 15 20 25 30 35

Diffraction angle, 2 theta (deg.) (b)

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Fig. 6. X-ray diffraction pattern of (a) pure DAST and (b) NaTS-doped DAST.

Table 1

Lattice parameter values of pure and NaTS doped DAST crystals Samples Lattice parameter values (A˚)

a b c

Pure DAST 10.366 11.323 17.891

DAST:NaTS 10.370 11.319 17.894

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hardness values as compared to the pure ones. Other characterization studies will be reported in due course. References

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Fig. 7. Variation of microhardness with the different loads for pure and NaTS-doped DAST crystals.