# Synthesis and Physicochemical Investigation of THz Material: 4-Ethoxy Benzaldehyde-4'-N'-Methyl Stilbazolium Hexafluorophosphate (EMBSHP)

A Karolin Martina, P Arul, J. Arul Martin Mani, N S Nirmala Jothi, P S Sagayaraj

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A Karolin Martina, P Arul, J. Arul Martin Mani, N S Nirmala Jothi, P S Sagayaraj. Synthesis and Physicochemical Investigation of THz Material: 4-Ethoxy Benzaldehyde-4'-N'-Methyl Stilbazolium Hexafluorophosphate (EMBSHP). Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.62.28.0. hal-01503668

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# Synthesis and Physicochemical Investigation of THz Material: 4-Ethoxy Benzaldehyde-4'-N'-Methyl Stilbazolium Hexafluorophosphate (EMBSHP)

A. Karolin Martina¹, J. Arul Martin Mani¹, N.S. Nirmala Jothi¹, P. Sagayaraj¹

1 – Department of Physics, Loyola College, Nungambakkam, Chennai – 600 034, India

a – jmjnirmala@yahoo.co.in

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.62.28.0 provided by [Seo4U.link](https://seo4U.link)

**Keywords:** organic compound, crystal growth, nonlinear optical, thermal behaviour.

**ABSTRACT.** Single crystals of organic optic material 4-Ethoxy Benzaldehyde-4'-N'-Methyl Stilbazolium Hexafluorophosphate (EMBSHP) were grown by slow evaporation technique at room temperature using acetone – water (3:1) mixed solvent. The grown crystal was subjected to single crystal X – ray diffraction analysis to identify the crystal structure and lattice parameter. The crystal belongs to the triclinic crystal system. The composition of the crystal was studied by CHN analysis. The presence of functional groups of the title crystal were confirmed from the FTIR spectral studies. The optical transmission range of the grown crystal was measured by UV – Vis – NIR spectral studies. The melting point of the grown crystal was found from DSC analysis.

**Introduction.** Organic NLO materials with excellent optical characteristics such as high sensitivity and short response time have attracted much attention due to their potential applications in optical switching, optical processing, optical computing, optical data storage and terahertz (THz) technology. Among the numerous organic materials, the well-known and most investigated organic NLO crystal is the stilbazolium salt *trans*-4-N-(dimethylamino)-N-Methyl-4-stilbazolium tosylate (DAST). It has very high electro optical (EO) and NLO figures of merit. Among all the other organic THz material, DAST has the largest NLO coefficient, $d_{11}$ of 540 ± 110 pm/V at $\lambda$~1540 nm, EO figure of merit $r_{11}$=47 ± 8 pm/V at $\lambda$~1535 nm and lower dielectric constant making it suitable for optoelectronic applications [1,2]. The advancement in the field of photonics have increased the demand for new nonlinear optical (NLO) materials. Especially, molecules exhibiting strong two photon absorption (TPA) are of practical importance in photons applications such as frequency up conversion lasing, three dimensional fluorescence imaging and multi photon microscopy, eye and sensor protection, optical signal reshaping and stabilizing fast fluctuations of laser power. To expand these utilizing, designing the molecule with large TPA cross section plays a vital role [3]. DAST crystal can generate short THz pulse by optical rectification with a sub-picosecond laser [4, 5], or widely tunable THz waves by different frequency generation (DFG) with a dual wavelength nanosecond laser [6]. In this present study, the tosylate anion of DAST is replaced by Hexafluorophosphate ($PF_6$) and two methyl group along with nitrogen in the cation is replaced by ethoxy group for achieving the stilbazolium crystal 4-Ethoxy benzaldehyde 4' - N' -methyl stilbazolium Hexafluorophosphate (EMBSHP). The grown crystal has been subjected to single crystal XRD, CHN, FTIR, Optical absorption and thermal analysis.

**Synthesis.** EMBSHP was synthesized by metathesization of the 4-ethoxybenzaldehyde-N-methyl-4-stilbazolium iodide (EMSI) salt with sodium hexafluorophosphate. EMSI was synthesized by the condensation of 1,4-dimethyl pyridinium iodide (2.35g, 10 mmol), methanol (30 ml) and 4-ethoxy benzaldehyde (1.36g, 10 mmol) in the presence of piperidine (0.2 ml). The total mixture was
taken in a round-bottom flask (1000 ml capacity) of a Dean-Stark apparatus and refluxed for 15 hours, then cooled to room temperature. The product was filtered and recrystallized from methanol four times for purification. During the next stage, the metathesization reaction was carried out with aqueous sodium hexafluorophosphate. A yellowish precipitate was formed as a result of exchange reaction between anion and cation. The remaining aqueous sodium iodide solution was removed from the precipitation. The purity of EMBSHP was further improved by successive recrystallization from acetone.

**Crystal growth.** After testing several common solvents and their combinations, it was found that the solubility of EMBSHP can be improved by using acetone. The size of the grown crystals were small when acetone was used as solvent. The size of the grown crystals improved a lot when the mixed solvent of water and acetone was used as solvent. We compared solvent mixtures of acetone and water with 5:1, 4:1, 3:1, 2:1 and 1:1 proportion and found that EMBSHP dissolved best and the size of the crystals were bigger for the 3:1 ratio. 100 ml of the above mentioned solvent was taken in a beaker and 1.7 g of EMBSHP was added slowly and stirred continuously for three days to prepare a saturated solution at room temperature. The saturated solution was filtered, sealed with perforations and was set aside to undergo slow evaporation. After two weeks, crystals of size up to 7 mm x 1 mm x 1 mm were obtained. The photograph of as-grown crystal is shown in Fig. 1.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Photograph of EMBSHP crystals.</figcaption></figure>

**Results and discussion**

**Single crystal XRD.** Single crystal X-ray diffraction was analyzed using ENRAF NONIUS CAD-4 diffractometer. It is found that the grown EMBSHP Single crystal belongs to the triclinic system, and the observed lattice parameters are a = 6.38 Å, b = 9.51 Å, c = 14.33 Å, α = 90.14 °, β = 93.89 °, γ = 90.32 ° and volume V = 867 Å³.

**CHN analysis.** The elemental composition of slow evaporation method grown EMBSHP crystal was carried out by CHN analysis. The calculated value of C₁₆H₁₉N₁O₁P₁F₆ are C = 49.76% H = 4.91% and N = 3.6% the corresponding experimentally found values are: C = 49.36% H = 5.05% N = 3.8%. Thus there is a close agreement between the calculated and experimental values of CHN.

**FT-IR analysis.** The sample was characterized by FT-IR spectroscopy in order to identify the functional groups and detect the vibrational modes of the molecules of the sample. The FTIR spectrum was recorded using BRUKER IFS 66V FT-IR spectrometer. The vibration frequencies observed between 500 cm⁻¹ and 700 cm⁻¹ are due to the out of plane bending of the ring C-H bonds. The frequencies observed between 1100 cm⁻¹ and 1200 cm⁻¹ are corresponding to the plane ring confirmation modes [1]. The measurement was done with KBr method for the wavelength range 400-
4000 cm⁻¹ and the spectrum is shown in fig.2 The characteristic frequencies observed between 3347 cm⁻¹ due to the N-H stretching vibration of primary amine of 4-aminopyridine. The peak at 2903 cm⁻¹ is assigned to the alkyl C-H stretch. The peak at 1588 and 1525cm⁻¹ Attributed to the aromatic vibrations observed. 1183 cm⁻¹ are pertaining to the S=O stretch modes of sulfonate group and further peak 1043 cm⁻¹ confirm the presence of vC-O-C linkage. The peak at 844 cm⁻¹ is assigned for 1, 4 distribution aromatic vibration [7].

<figure><img src="image_4.png" /><figcaption>Fig. 2. FTIR Spectrum of EMBSHP crystal.</figcaption></figure>

Optical transmittance studies

The optical transmittance range and the transparency cut off wavelength of single crystal are most
significant optical parameters for laser frequency applications. The optical absorption spectrum of
EMBSHP crystal in solution form was recorded in the wavelength range of 200 - 1100 nm using
Varian Cary 5E UV-VIS-NIR spectrometer using acetone and water as solvent. Absorbance is not
found in the wavelength region of 445 – 1100 nm and it shows an excellent optical behavior of the
crystal. It is also reported that absence of absorption in the region between 445 and 1100 nm showed
that EMBSHP crystal could be exploited for second and third order nonlinear optical applications
using Nd:YAG laser [8].

<figure><img src="image_5.png" /><figcaption>Fig. 3. Optical Absorption Spectrum of EMBSHP crystal.</figcaption></figure>
Thermal Analysis

The DSC thermal analysis was carried out in nitrogen atmosphere at a heating rate of 10°C/minute from room temperature up to 500 °C. The endothermic peak at 275.2 °C in the DSC trace (Fig.4) corresponds to melting point of the material and the exothermic peak corresponds to the major decomposition of the material starting from the removal of stilbazolium ion followed by the hexafluorophosphate anion.

<figure><img src="image_6.png" /><figcaption>Fig. 4. DSC trace of EMBSHP crystal.</figcaption></figure>

**Summary.** The growth of single crystal of EMBSHP was achieved by slow evaporation method. From Single crystal XRD analysis the crystal system is found to triclinic. The composition of the sample was verified by CHN and FT-IR studies. The UV – vis – NIR studies show that the crystal is transparent in the wave length range 445 – 1100 nm. From the DSC curve, the melting point of the crystal is found to be 275.2 °C which is higher than the melting point of the well known oraganic THz crystal DAST(256°C). The material is found to be thermaly stable and decomposes above the melting point.

References

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Cite the paper

A. Karolin Martina, J. Arul Martin Mani, N.S. Nirmala Jothi, P. Sagayaraj, (2017). *Synthesis and Physicochemical Investigation of THz Material: 4-Ethoxy Benzaldehyde-4'-N'-Methyl Stilbazolium Hexafluorophosphate (EMBSHP)*. Mechanics, Materials Science & Engineering, Vol 9. Doi 10.2412/mmse.62.28.0