WO2008034272A1

WO2008034272A1 - Stilbazolium salt

Info

Publication number: WO2008034272A1
Application number: PCT/CH2007/000453
Authority: WO
Inventors: Peter Günter; Mojca Jazbinsek; Zhou Yang; Lukas Mutter; Blanca Ruiz
Original assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Current assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Priority date: 2006-09-18
Filing date: 2007-09-17
Publication date: 2008-03-27
Anticipated expiration: 2009-03-18

Abstract

The invention relates to a Stilbazolium salt with both high second-order nonlinear optical properties and very favorable crystal growth characteristics, which is especially suited for nonlinear optical applications such as frequency conversion, electro-optic modulation, optical parametric oscillation, and Terahertz (THz) generation and detection. The salt is represented by formula (I): wherein at least one of R1, R2, R3, R4, being identical or different, independently represents alkyl group, wherein hydroxyl group, carboxylic group, amino group may be included, preferably as ending group, and the remaining groups R1, R2, R3, R4 represent hydrogen atom or deuterium atom; X1 represents hydrogen atom, deuterium atom, halogen atom, alkyl, hydroxyl group, aldehyde group, amino group, nitro group, amide group or carboxylic group; X2 and X3, being identical or different, independently represent hydrogen atom, deuterium atom, alkyl group, aryl group, wherein hydroxyl group, amide group, carboxylic group, ether bond, double bond, or the like may be included in each group; and n = 1 or 2.

Description

STILBAZOLIUM SALT

FIELD OF THE INVENTION

The present invention relates to a stilbazolium salt as claimed in claim 1 and to a nonlinear optical material comprising such a salt as claimed in claim 11. The inventive salt has both high second-order nonlinear optical properties and very favorable crystal growth characteristics.

BACKGROUND OF THE INVENTION

During the last two decades, there have been intensive research efforts in exploring and developing second-order nonlinear optical (NLO) materials for applications such as frequency conversion, electro-optic modulation optical parametric oscillation and more recently, Terahertz (THz) generation and detection Compared with inorganic materials, molecular organic materials have been of considerable interest due to their large nonlinear optical properties, ultrafast response times and almost unlimited design possibilities. Among them, organic crystals based on the charged chromophores and strong Coulomb interactions have several advantages over non- ionic species, such as a large molecular nonlinearity or first-order hyperpolarizability (β), a better long-term stability and a higher tendency to override the dipole-dipole interactions and thus form non-centrosymmetric macroscopic packing For example, the organic salt crystal, 4-N₅N-dimethylamino-4'-JV^>-methyl- stilbazolium tosylate (DAST) is one of the best electro-optic materials with large nonlinear optical susceptibilities χ₁₁₁ ⁽²⁾=(2020+200) pmV^"1 at 1.3 μm and Xπi^(2)=:(420+l 10) pmV¹ at 1.9 μm due to the good alignment of the chromophores in the crystal [1, 2]. DAST consists of a positively charged nonlinear optical chromophore stilbazolium and a negatively charged tosylate anion. The counter- anion tosylate is used to override the preferred antiparallel crystallization of the chromophores. Up to date, DAST is also the only commercially available organic nonlinear optical crystal [3] However, its growth is still a challenge and there are high efforts to investigate the growth of bulk and thin films of DAST For example, one of the challenges is to speed up the several weeks long growth time of high optical quality DAST crystals with dimensions exceeding lcm³ [3].

Although DAST shows large electro-optic effects, the development of new organic crystals with larger electro-optic coefficients and (or) faster and easier crystal growth procedure is an important challenge for future applications. It has already been demonstrated that varying the counter-ion to optimize the crystal packing and orient the dipoles as parallel as possible is an effective molecular engineering strategy to develop ionic organic crystals with non-centrosymmetric structure [4, 5, 6]. Up to now, however, the chromophore packing and crystal growth cannot be reliably predicted.

Using this approach, the synthesis and crystal growth of DSΝS (A-N₁N- dimethylamino-4'-N'-methyl-stilbazolium 2-naphthalenesulfonate) has been reported. DSΝS is a promising DAST derivative with perfectly aligned chromophores, leading to a 50% higher nonlinearity in the crystalline powder than DAST [7]. However, only very small crystalline needles of DSΝS with a diameter of less than 100 μm could be grown. Beside the N-methyl pyridinium based chromophore as used in DAST, new chromophores with N-aryl pyridinium groups have also been reported, which could lead to salts with even higher nonlinear optical figures of merit [8].

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a stilbazolium salt that gives crystals of large area suitable for bulk and thin film applications, especially for nonlinear optical applications. It is a further object of the invention to provide a stilbazolium salt that has high nonlinear optical properties.

These and other objects are achieved by a stilbazolium salt as claimed in claim 1, and by a nonlinear optical material comprising such a salt according to claim 11.

The Stilbazolium salt according to the invention is represented by formula (I). The anion that has substituent groups Ri to R₄ on the positions 1 to 4, at least one of these groups being different from hydrogen or deuterium:

In this compound, at least one of the substituents Ri to R₄ is different from hydrogen or deuterium and represents an alkyl group, preferably with 1-4 carbon atoms. A hydroxyl group, carboxylic group, and/or amino group may be included in said alkyl group, preferably as ending group. Preferred substituents are -CH₃, -CH₂OH, - CH₂CHO, -CH₂COOH, -CH₂NH₂, or -CH=CH₂.

The Rj groups different from H/D enhance the solubility of the compound which is beneficial for crystal growth.

Preferably, at least two of the substituents R₁ to R₄ are different from hydrogen or deuterium; these can be identical or different from one another. It is preferred that these substituents are Rj and R₄ , more preferably Ri = R₄ = methyl, at positions 1 and 4, while R₂ and R₃ are hydrogen or deuterium, as represented by the following formula:

Xi represents hydrogen atom, deuterium atom, halogen atom, alkyl, hydroxyl group, aldehyde group, amino group, nitro group, amide group or carboxylic group group. Preferably, X₁ represents an alkyl group with less than 4 carbon atoms, most preferably a methyl group. In other preferred embodiments, Xi represents -H, -F, -Cl, -Br, -I, -C₂H₅, -OH, -OCH₃, -CHO, -COOH, -NH₂, -NHCH₃, -N(CH₃)₂, -NO₂, or - CH=CH₂. X₂ and X₃, being identical or different, independently represent hydrogen atom, deuterium atom, alkyl group, aryl group, wherein hydroxyl group, amide group, carboxylic group, ether bond, double bond, or the like may be included in each group. Preferably, X₂ and/or X₃ represent an alkyl group with less than 4 carbon atoms, preferably methyl and/or -CH₂D, -CHD₂, or -CD₃. In other preferred embodiments, X₂ and/or X₃ represent phenyl, 2,4-dinitrophenyl, and/or 2-pyrimidyl.

The number n equals 1 or 2.

According to the invention, organic crystals with similar nonlinear optical properties as DAST are feasible with a considerably faster crystal growth rate.

The salts are prepared by metathesization of 4-JV,N-dimethylamino-4'-JV-methyl- stilbazolium iodide with water solution of sodium salt of 2-mesitylenesulfonic acid, for example.

The salts are suited for second-order nonlinear optical applications such as electro- optical modulation, deflection, switching, filtering, frequency conversion, optical parametric oscillation_s and for THz generation and detection. The salts retain high second-order nonlinearities, i.e. y^ of 2000 pm/V at 1.3 μm.

Solubilities of the salts are high in methanol, i.e. comparable to or even about two times higher than the one of DAST (4-JV, N-dimethylamino-4'-JV-methyl- stilbazolium tosylate) at the same temperature. Since the crystal structure cannot be predicted in general, some of the claimed compounds may have an enhanced solubility but crystallize in a centrosymmetric structure and are thus less suited for second-order nonlinear optical applications. Generally, those compounds crystallizing in a non-centrosymmetric structure are chosen for second-order nonlinear optical applications.

The stilbazolium chromophore (cation) has preferably one of the following structures:

(C)

(X₂=2-pyrimidyl, X₃= -CH₃)

The counter anion has preferably one of the following structures

Combination of cation (a) with anion (e) or (f) yields salts with solubilities comparable to or higher than DAST, and with non centrosymmetric crystal structure and thus second-order nonlinear optical properties (see description of preferred embodiments).

A preferred embodiment of the invention is the combination of cation (a) with anion (e), the novel stilbazolium salt 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6- trimethylbenzenesulfonate (DSTMS). DSTMS can be prepared from metathesis reaction. With a similar molecular structure and crystal packing, DSTMS shows a much higher solubility in methanol, and therefore much better growth characteristics than the known material DAST. Large bulk crystals and thin films with good optical quality without using seed crystals can be obtained by carefully controlling the crystal growth conditions. Nonlinear optical measurements reveal that DSTMS possesses a high nonlinear optical susceptibility χ⁽²\π = 430 ± 40 pm/V at 1.9 μm, which is comparable to the one found in DAST. This material is therefore very promising for second-order nonlinear optical applications such as electro-optics, frequency conversion into the Mid IR and THz generation.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows solubility/temperature curves of DSTMS and DAST in methanol. The solid curves are according to equation (1) with ΔH= (81 ± 4) kJ moF¹ and ΔS = (170 ± 10) J mol^KT¹) for DSTMS and AH= (74 ± 3) kJ moF¹ and ΔS = (140 ± 10) J mol^"

Fig. 2 shows in the upper panel a) a DSTMS bulk crystal (size 33 x 33 x 2 mm³), the large surface is the (001) face. The lower panel b) shows a DSTMS thin film crystal (size 6 x 5 x 0.03 mm³) grown by the capillary method.

Fig. 3a-c show crystal packing of DSTMS projected along the crystallographic axes a (Fig. 3a), b (Fig. 3b), and c (Fig. 3c). The mirror symmetry plane is perpendicular to the b-axis. The chromophores make an angle of approximately 23% with respect to the polar a-axis (c). Fig. 4 shows a crystal packing diagram of DSTMS projected along the [110] crystallographic vector. The molecules are linked by Coulombic interactions between the ionic parts and by hydrogen bonds that are indicated by dotted lines.

Fig. 5 shows transmission spectra of 0.77 mm thick DSTMS crystal for light polarized along the polar a-axis (solid curve) and along the crystal b-axis (dashed curve).

Fig. 6 shows the THz amplitude generated through optical rectification of 160 fs pulses at 1.45 μm wavelength in a 0.34 mm thick DSTMS crystal, as detected by electro-optic sampling in a 0.5 mm thick ZnTe crystal using a frequency doubled probe beam at 0.725 μm (left) and its Fourier transform (right). The oscillations for t > 2 ps are due to ambient water vapor absorption.

Fig. 7 shows crystallographic and other data of DSDMS, DSTMS and DAST crystals. θi₂ is the angle between the long axis of the cation chromophores and the polar axis of the crystal, T_m the melting temperature and X₁₁₁ the element of the first order hyperpolarizability tensor for second harmonic generation at fundamental wavelength of 1.9 μm.

Fig. 8 shows two DSDMS bulk crystals (size 10 x 2 x 0.4 mm and 8 x 1 x 0.2 mm³).

Fig. 9a-c show crystal packing of DSDMS projected along the crystallographic axes a (Fig. 9a), b (Fig. 9b) and c (Fig. 9c). Cations and anions from four unit cells are shown. The chromophores are aligned perfectly parallel, which is an ideal configuration for achieving high diagonal electro-optic and nonlinear optical coefficients.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example Ir DSTMS

A novel promising DAST derivative (see Scheme 1) with high solubility - more than two times the one of DAST in methanol at the same temperature - has been synthesized by combination of cation (a) with anion (e) as described above.

Scheme 1 Molecular structure of DSTMS

4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6- trimethylbenzenesulfonate (DSTMS) was obtained by condensation reaction between 4-methyl-N-methyl pyridinium iodide, which was prepared from 4-picoline and methyl iodide, and 4-N, N-dimethylamino-benzaldehyde in the presence of piperidine. It was then metathesized to 2,4,6-trimethylbenzenesulfonate by precipitation from water solution of sodium salt of 2,4,6-trimethylbenzenesulfonic acid. The product was purified by recrystallizing three times from methanol before characterization by NMR and elemental analysis. Similar to DAST, it was easy to obtain orange color hydrate centrosymmetric crystals in the presence of water. The water could be removed by heating the material above 100°C for 1-2 hours.

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) analyses have shown that the melting point of DSTMS is (258 ± I)⁰C and that DSTMS begins to decompose at around 250⁰C, which is also quite similar to DAST.

DSTMS can be well dissolved in some strong polar solvents such as methanol, DMF and DMSO etc. However, DMF and DMSO are not well suited for crystal growth because of their high boiling points. Therefore methanol was chosen as the solvent for the growth of DSTMS crystals. Fig.l shows the solubility curve of DSTMS in methanol solution compared with DAST. The solubility at a temperature T was measured by saturating the solution at a higher temperature (about 5⁰C above T), slowly cooling it in the presence of a precipitated solid to maintain equilibrium, and then analyzing the solution at the temperature T.

The solubility of DSTMS is in the temperature range of interest around two times as large as the one found in DAST in methanol at the same temperature. The large difference in the solubility can mainly be attributed to the different structure of counter-anions - the solubility of 2,4,6-trimethyl-benzenesulfonate that possesses three methyl groups should be higher than that of tosylate that possesses only one methyl group. The temperature dependence of the solubility of a dilute solution, considering that the solute dissolves into two ions, can be described by

x_s = exp (ΔS/2R) exp (- ΔH/2RT) (1)

where x_s is the molar fraction of the solute at saturation, R=8.314 J/(mol-K) is the gas constant and T the temperature in K. The thermodynamic parameters ΔS and ΔH are the changes of the entropy and enthalpy when dissolving the solute. The measured solubility S (in g of solute per 100 g solvent) is related to the molar fraction of the solute x_s as S = 100 (M_soi_ute/M_Soivent) x_s/(ϊ - x_s), where M_soi_ute/soivent is the molecular mass of the solute/solvent. Analyzing the solubility data using Equation 1 shows that both ΔS and ΔH are larger for DSTMS compared to DAST and can be considered constant in the temperature region of interest (see Fig. 1).

An important parameter that affects the growth rate in solution- growth systems is the driving force for crystallization Δμ = μ_s - μ_c, defined by the difference between the chemical potential of the solute in the solution μ_s and in the crystalline phase μ_c. If we increase the concentration of the solute in the solution from x_s to x_s', the driving force can be expressed simply as Δμ = 2RT(In x_s' - In x_s) or with the level of supersaturation σ = (Xs'-x_s)/x_s as Δμ = 2RTIn(I + σ) « 2RT σ. Supersaturation in our system is achieved by lowering the temperature of the saturated solution from T' to T. Using Equation 1 and considering small ΔT= T' -T we obtain for the driving force Δμ = (ΔT/T)ΔH. Therefore, an increased driving force for crystallization of DSTMS compared to DAST is expected due to larger ΔH when lowering the temperature of the saturated solution by ΔT. The solubility results imply that much larger DSTMS crystals could be grown as compared to DAST under the same conditions.

It was found that DSTMS nucleated very easy and fast. Optical quality thin single- crystalline plates with dimensions of 5 x 5 x 0.3 mm³ could be obtained already by the recrystallization procedure that is used to purify the material.

Moreover, superior to DAST, bulky size DSTMS crystal can be easily grown without a seed crystal because of the improved growth characteristics of the latter. Slow cooling technique was adapted for the growth of bulk crystals. First, a saturated solution of DSTMS in methanol at 35°C-40°C was prepared. Spontaneous nucleation could be observed after cooling down the saturated solution. Then, the temperature was increased to dissolve most of the nuclei. The solution was carefully cooled down again to make sure that only one or two nucleated crystals remained undissolved. After that, large crystals with very good optical quality for optical measurements could be grown by slowly cooling the solution at a rate of 0.2°C-0.3°C /day. Typically, the crystals first appear as red thin plates and continue to grow preferentially along the edges in all directions. DAST, on the other hand, prefers to grow along the direction of the polar axis [-100].

Using this self-nucleating method, the defects introduced by seeds can be eliminated and the process of selecting, processing and fixing the seed crystals can be omitted. Large single crystal with dimensions of 33 x 33 * 2 mm³ have been grown by this method in a period of eight weeks (Fig. 2a). Experiments show that high quality single crystals of DSTMS with surface area of about 1 mm can be grown in 2 weeks, thus much faster than that of DAST (4-6 weeks). For integrated optics applications the possibility of growing single crystalline thin films is very attractive. In view of the good quality of the thin plates spontaneously grown from DSTMS solution, experiments on thin film growth by the capillarity method were performed. Two glass plates are placed together in solution and the capillary force pulls up the liquid. The slow evaporation of the solvent allows for growth. Single crystalline thin films with an area of up to 6 x 5 mm² and a thickness between 5-30 microns were obtained (Fig.2b).

The crystallographic structure of DSTMS was determined via X-ray analysis of single crystals. The data obtained are listed in the table shown in Fig. 7 and compared to those of DAST.

The crystal packing, which belongs to the monoclinic space group Cc (point group m, z= 4), is shown in Figure 3. Crystallographic data show that beside the Coulombic interactions between the cation and anion parts, hydrogen bonds between the sulfonic oxygen atoms and double-bond hydrogen atoms also play a role in crystal packing and chromophores orientation.

Figure 4 shows the hydrogen-bonded network formed by two kinds of C-H... O hydrogen bonds between the cation layers and the anion layers with H... O distances of about 2.49 A and 2.44 A, respectively.

Light transmission spectra of DSTMS were measured for light polarized along the polar a-axis, which is the most interesting for applications, and for light polarized along the crystallographic b-axis. For this measurement, an as-grown c-plate sample with a thickness of 0.77 mm was optically polished. The sample transmission was measured using a Perkin-Elmer Lambda 9 spectrophotometer along the two main axes and is shown in Figure 5. Three absorption bands observed in the infrared region at about 1.2, 1.4, and 1.7 μm correspond to overtones of the C-H stretching vibrations and have also been observed in DAST [2].

The refractive indices were measured at the telecommunication wavelength of 1.55 μm by an interferometric technique. DSTMS crystals are strongly anisotropic with refractive indices of n_a = 2.07 ± 0.05 along the polar a axis, and n_& = 1.64 ± 0.05 along the b axis. Using the refractive index data, we calculated the optical absorption from the transmission measurement of Figure 5 by considering Fresnel losses due to multiple reflections at the crystal surfaces. We determined the absorption constants at 1.55 μm as α_a= (0.6 ± 0.1) cnT¹ and cc_b = (0.5 ± 0.1) cm^"1 for light polarized along the a-axis and along the b-axis respectively. Low optical loss measured shows that the sample has good optical surfaces and is free from scattering centers within the crystal.

The nonlinear optical properties were measured by the standard Maker Fringe technique. The first Stokes line at 1907 nm generated in a high pressure Raman cell filled with H₂ and pumped with a Q switched Nd: YAG at 1064 nm (pulse length of 7 ns) was used as fundamental wavelength. The generated second harmonic light at 953.5 nm was detected by a photomultiplier and referenced to quartz with X₁₁₁= 0.554 pm/V at 1907 nm. For the measurement of the nonlinear optical susceptibility element χm of DSTMS, polished c plates were used with the dielectric X₁ axis oriented along the rotation axis of the Maker Fringe experiment. We obtained a value of χiπ = 430 ± 40 pm/V, which is comparable with the one of DAST χm = 420 ± 110 ρm/V [9].

The nonlinear optical properties of DAST and DSTMS can be also compared by relating microscopic first-order hyperpolarizability tensor β_x7Z with the macroscopic second order susceptibility tensor χijk- By applying the simple oriented-gas model and considering only the largest optical hyperpolarizability element P₂₂₂ for generating second-harmonic light at frequency 2ω we obtain

χni (-2ω,ω,ω) = Nfi^2ω(f₁ ^ω)²cos³(θ_i2)β_2zz(-2ω,ω,ω), (2)

where N is the number of chromophores per unit volume, fi^ω'^2ω are the local field corrections, and θi_z is the angle between the polar axis 1 in the crystal and the molecular axis z of the chromophore. This angle is for DSTMS about θi_z ^DSTMS = 23° (see Fig. 3), which is slightly larger than the reported θi_z ^DAST = 20° of DAST. Assuming similar hyperpolarizabilities β_zzZ in the solid state of DAST and DSTMS due to the same nonlinear optical active chromophore, and taking into account the change of the angle Θ-|_Z and the volume of the unit cell (see Fig. 7), we can estimate the ratio X₁₁₁(DSTMSy X₁₁₁(DAST) = 0.9. The measured nonlinear optical susceptibility is however higher, which can be attributed to a decreased influence of the intermolecular interactions that are decreasing the first hyperpolarizibility on the solid state of the ionic compounds, due to the larger distance between the chromophores in DSTMS compared to DAST.

Example 2: DSDMS

A further DAST derivative (see Scheme 2) with a solubility comparable to the one of DAST in methanol at the same temperature has been synthesized.

Scheme 2: Molecular structure of DSTMS

4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4- dimethylbenzenesulfonate (DSDMS) was obtained by condensation reaction between 4-methyl-N-methyl pyridinium iodide, which was prepared from 4-picoline and methyl iodide, and 4-N, N-dimethylamino-benzaldehyde in the presence of piperidine. It was then metathesized to 2,4-dimethylbenzenesulfonate by precipitation from water solution of sodium salt of 2,4-dimethylbenzenesulfonic acid. DSDMS can be well dissolved in alcohol, especially in methanol. Therefore methanol was chosen as the most suitable solvent for the growth of DSDMS crystals. For DSDMS the slow evaporation technique at a constant temperature was adapted because we its solubility is mainly sensitive to the amount of the solvent and almost independent of temperature. Bulk single crystals with dimensions of 10 x 2 x 0.2 mm³ have been obtained in 2-3 weeks (see Fig. 8).

DSDMS is an effective second-order nonlinear optical material, as confirmed by the preliminary powder second-harmonic generation test at 1.9 μm, which gave the efficiency of about 0.7 relative to the powder efficiency of DAST. The powder second-harmonic generation test is a well known test to determine whether a substance has nonlinear optical properties before growing single crystals. It comprises illuminating a powder sample with a basic wavelength and detecting the presence of the second harmonic of this wavelengths. Single crystal X-ray analysis was carried out for a DSDMS crystal. The crystallographic data are listed in Figure 7, where also DAST is added for comparison. The crystal packing diagram is presented in Figure 9. The crystal structure of DSDMS is triclinic, having space group symmetry Pl with one ion-pair per unit cell. The three-dimensional packing exhibits alternating acentric sheets of stilbazolium cations and counter anions. The observed parallel arrangement of all stilbazolium chromophores with the highest possible order parameter <cos³#> = 1 presents an ideal packing for efficient electro-optic or second-order nonlinear optical effects. Therefore, high diagonal second-order nonlinear optical susceptibilities χ and electro-optic coefficients r are expected for these crystals.

Example 3: DAPSD

A further example is 4-N, N-dimethylamino-4'-N'-phenyl-stilbazolium 2,4- dimethylbenzenesulfonate (DAPSD) salt, which is a derivative of trans-4'- (dimethylamino)-N-ρhenyl-4-stilbazolium hexafluorophosphate (DAPSH) [8,10].

DAPSD (scheme 3) has been synthesized by combination of cation (b) with anion (f) as described above.

Scheme 3: Molecular structure of DAPSD

DAPSD was obtained by condensation reaction between 4-methyl-N-phenyl pyridinium chloride, which was prepared from 4-picoline and phenyl chloroform, and 4-N, N-dimethylamino-benzaldehyde in the presence of piperidine. It was then metathesized to 2,4-dimethylbenzenesulfonate by precipitation from water solution of sodium 2,4-dimethylbenzenesulfonate.

The solubility of DAPSD in acetonitrile is considerably higher than the one of the related DAPSH (with cation (b) and PF₆ counter anion).

The powder second-harmonic generation test of DAPSD gave the efficiency of about 2.0 relative to the powder efficiency of DAST at 1.9 μm, which is also much higher than the one of the DAPSH (about 0.9 relative to the powder efficiency DAST at 1.9 μm).

References

[1] S. R. Marder, J. W. Perry, W. P. Schaefer, Science 1989, 245, 626. [2] F. Pan, M. S. Wong, C. Bosshard, P. Gϋnter, Adv. Mater. 1996, 8, 592.

[3] P. Laveant, C. Medrano, B. Ruiz, P. Gϋnter, Chimia 2003, 57, 1.

[4] S. R. Marder, J. W. Perry, C. P. Yakymyshyn, Chem. Mater. 1994, 6, 1137.

[5] S. Okada, K. Nogi, Anwar, K. Tsuji, X.-M. Duan, H. Oikawa, H. Matsuda, H. Nakanishi, Jpn. J. Appl. Phys. Part 1 2003, 42, 668.

[6] Z. Yang, S. Aravazhi, A. Schneider, P. Seiler, M. Jazbinsek, P. Giinter, Adv. Funct Mater. 2005, 15, 1072.

[7] B. Ruiz, Z. Yang, V. Gramlich, M. Jazbinsek, P. Gϋnter, J. Mater. Chem. 2006, 16, 2839.

[8] B. J. Coe, J. A. Harris, I. Asselberghs, K.Wostyn, K. Clays, A. Persoons, B. S. Brunschwig, S. J. Coles, T. Gelbrich, M. E. Light, M. B. Hursthouse, K. Nakatani, Adv. Funct. Mater. 2003, 13, 347.

[9] U. Meier, M. Bosch, C. Bosshard, F. Pan, P. Gϋnter, J. Appl. Phys. 1998, 83.

[10] B. Ruiz, B. J. Coe, R. Gianotti, V. Gramlich, M. Jazbinsek, P. Gϋnter, CrystEngComm 9, 772 (2007).

Claims

WHAT IS CLAIMED IS:

1. Stilbazolium salt represented by formula (I) :

(I)

wherein

at least one of Ri, R₂, R₃, R₄, being identical or different, independently represents alkyl group, wherein hydroxyl group, carboxylic group, amino group may be included, preferably as ending group, and the remaining groups R₁, R₂, R₃, R₄ represent hydrogen atom or deuterium atom;

Xi represents hydrogen atom, deuterium atom, halogen atom, alkyl, hydroxyl group, aldehyde group, amino group, nitro group, amide group or carboxylic group;

X₂ and X₃, being identical or different, independently represent hydrogen atom, deuterium atom, alkyl group, aryl group, wherein hydroxyl group, amide group, carboxylic group, ether bond, double bond, or the like may be included in each group; and

n = 1 or 2.

2. Stilbazolium salt according to claim 1, wherein at least one of Ri, R₂, R₃, R₄ represents an alkyl group with 1-4 carbon atoms, preferably -CH₃.

3. Stilbazolium salt according to claim 1 or 2, wherein at least one of Ri, R₂, R₃, R₄ represents -CH₃, -CH₂OH, -CH₂CHO, -CH₂COOH, -CH₂NH₂, or -CH=CH₂.

4. Stilbazolium salt according to one of the preceding claims, wherein Xi represents an alkyl group with less than 4 carbon atoms.

5. Stilbazolium salt according to one of the preceding claims, wherein X] represents -H, -F, -Cl, -Br, -I, -CH₃, -C₂H₅, -OH, -OCH₃, -CHO, -COOH₅ - NH₂, -NHCH₃, -N(CH₃)₂, -NO₂, or -CH=CH₂.

6. Stilbazolium salt according to one of the preceding claims, wherein X₂ and/or X₃, represent an alkyl group with less than 4 carbon atoms, phenyl, 2,4- dinirophenyl, or 2-pyrimidyl.

7. Stilbazolium salt according to one of the preceding claims, wherein at least one of Ri and R₄ are different from hydrogen or deuterium atom, and R₂ and R₃ represent a hydrogen or deuterium atom, as represented by one of the following formulas (II) or (III):

(II)

(III)

8. Stilbazolium salt according to claim 7, wherein Ri and/or R₄ represent -CH₃

9. Stilbazolium salt according to one of the preceding claims, wherein X₂ and X₃ independently represent -CH₃, -CH₂D, -CHD₂, or -CD₃.

10. Stilbazolium salt according to one of the preceding claims, as represented by formula (IV):

11. Nonlinear optical material, comprising stilbazolium salt according to one of the preceding claims.

12. Nonlinear optical material according to claim 11, having a nonlinear optical susceptibility χ⁽²⁾ of more than 300 pm/V at 1.9 μm wavelength.

13. Nonlinear optical material according to one of claims 11 or 12, having a non- centrosymmetric crystal structure.

14. Use of a stilbazolium salt according to one of claims 1-10 for the preparation of second-order nonlinear optical crystals.