Cryst. Res. Technol. 43, No. 5, 561 – 564 (2008) / DOI 10.1002/crat.200711048

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Growth and characterization of a new semi organic NLO

material: L-tyrosine hydrochloride

S. Natarajan*, G. Shanmugam, and S. A. Martin Britto Dhas

School of Physics, Madurai Kamaraj University, Madurai – 625 021, India

Received 17 September 2007, accepted 26 October 2007

Published online 16 November 2007

Key words L-tyrosine hydrochloride, XRD, NLO, FTIR, TGA/DTA, microhardness.

PACS 61.10.-i, 78.30.-j, 65.40.-b, 62.20.-x, 42.70.mp

Single crystals of L-Tyrosine hydrochloride were grown by using the submerged seed solution method. The

grown crystals were characterized by using single crystal X-ray diffraction. Functional groups and the modes

of vibrations were identified by FTIR spectroscopy. The TGA/DTA studies showed that the crystal is stable

up to 232°C. Microhardness study revealed that the crystal is a hard material. It is transparent in the entire

visible region. The SHG efficiency was determined by the Kurtz and Perry method.

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The search for new materials with high optical nonlinearities is an important area due to their practical applications such as optical communication, optical computing, optical information processing, optical disk data storage, laser fusion reactions, laser remote sensing, color display, medical diagnostics, etc. In semi-organic materials, the organic ligand is ionically bonded with inorganic host. Due to this, the new semi-organic crystals have higher mechanical strength and chemical stability [1]. Most of the organic NLO crystals usually have poor mechanical and thermal properties and are susceptible for damage during processing even though they have large NLO efficiency. Also, it is difficult to grow larger size optical-quality crystals of these materials for device applications. Purely inorganic NLO materials have excellent mechanical and thermal properties, but possess relatively modest optical nonlinearity because of the lack of extended π - electron delocalization [2, 3]. Hence, it may be useful to prepare semiorganic crystals which combine the positive aspects of organic and inorganic materials resulting in useful nonlinear optical properties.

L-Cysteine hydrochloride [4], L-Histidine hydrochloride [5], L-Histidine tetrafluroborate [6], L-Hisidinium bromide [7], L-Histidine hydrofluoride dihydrate [8] and L-Glutamic acid hydrochloride [9], Glycine nitrate [10] are some of the semiorganic NLO materials of the amino acid family, reported recently. Presently, another semiorganic NLO material from the amino acid family, viz., L-Tyrosine hydrochloride(LTH) is identified as a NLO material. The π electron cloud present in the aromatic ring and the carboxylic group may be the origin for the nonlinearity of this material. Bulk size single crystals of LTH were grown and found to have monoclinic system with the space group, P21. The molecular structure of LTH is given in figure 1 (black balls indicate carbon, blue – nitrogen, red - oxygen, white - hydrogen and green -chlorine). The grown single crystal was confirmed by using XRD. The functional groups and the mode of vibrations were identified by using FTIR analysis. The optical, thermal and mechanical properties were also investigated. The results are presented in detail in the following sections.

2 Results and discussion

Crystal growth L-Tyrosine and hydrochloric acid (E. Merk, India) were mixed in aqueous solution in eqimolar ratio and stirred continuously for an hour. The saturated solution was filtered and allowed to evaporate at room temperature. Small, optically clear and well-shaped crystals suitable for usage as seed ____________________

* Corresponding author: e-mail: s_natarajan50@yahoo.com

562 S. Natarajan et al.: A new semi organic NLO material: L-Tyrosine hydrochloride

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

crystals were obtained in the next few days. Bulk crystals were grown using the seeds in a saturated solution of LTH in a crystallizer using submerged seed solution growth method. Transparent crystals of size: 21.0 × 5.0 × 3.0 mm3 were obtained in a period of about three weeks (Fig. 2).

Fig. 1 Molecular structure of LTH.

(Online color at www.crt-journal.org)

Fig. 2 Photograph of LTH crystal.

(Online color at www.crt-journal.org)

Fig. 3 FTIR spectra of LTH.

Fig. 4 TGA/DTA of LTH.

Table 1 FTIR spectral data of LTH. (vs-very strong s-strong w-weak b-broad m-medium).

Wave number (cm-1) Tentative assignments

3200 vs, b NH3

+ symmetric stretching

2929 vs,b CH2 symmetric stretching

1726 vs C = O stretching

1609 vs NH3

+ asymmetric stretching

1594 vs C = C stretching

1559 s COO- asymmetric stretching

1515 vs NH3

+ symm deformation

1360 s O-H in-plane bending

1330 s CH2 wagging

1303 s Aromatic combination band vibration

1103 s C-H deformation

1041 m CH2 rocking

942 s C-H out of plane deformation

840 vs C-H out of plane bending

822 s C-H out of plane bending

776 m C-H out of plane deformation (ring)

740 m C-H out of plane deformation (ring)

714 w COO- bending

642 m C-H out of plane deformation

Single crystal X-ray diffraction The unit cell parameters were determined from the single-crystal X-ray diffraction data obtained with a four-circle Nonius CAD4 MACH3 diffractometer (MoKα, λ = 0.71073Å). The cell parameters are: a = 5.092(1) Å, b = 9.022(1) Å, c =11.070(2) Å and β = 91.80(2)°. The density of the single crystals of LTH was determined as 1.44(2) gm/cc using the floatation method. The melting points was found out as 230(2)°C.

Cryst. Res. Technol. 43, No. 5 (2008) 563

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FTIR Studies The FTIR spectra of the grown crystal was recorded in the KBr phase in the frequency region of 400 – 4000 cm-1 using a Jasco Spectrometer (FTIR, model 410) at a resolution of 4 cm-1 and with a scanning speed of 2 mm/sec. The recorded FTIR spectra (Fig. 3) were compared with the standard spectra of the functional groups [11]. The tentative assignments are given in table 1.

Thermal studies Simultaneous thermogravimetric analysis(TGA) and differential thermal analysis (DTA) were carried out for the LTH crystals, using a SDT Q600 V8.2 Build 100 thermal analyzer. The characteristic curves are shown in figure 4. A powder sample was used for the analysis in the temperature range of 26°C to 900°C with a heating rate of 10 K/min. The crucible used was of alumina (Al2O3), which served as a reference for the sample. It is evident from the TGA/DTA curve that the crystal is stable up to 230°C without any phase transition. The endothermic peak around 232°C is due to the melting of the crystal. After this point, it started to decompose. Decomposition takes place in three steps. There is no loss of weight observed around 100° C showing the absence of any absorbed water molecules in the sample.

UV-Vis-NIR spectrum The UV-Vis-NIR transmission spectrum (Fig. 5) of the crystal was recorded in the range: 190 – 1100 nm using an AGILENT (8453), UV-Vis-NIR spectrophotometer. It is seen that a strong absorption band occurs at 290 nm and this absorption is due to the n → π

* transition. The lower cutoff wavelength occurs at 240 nm and is due to π → π

* transition [12]. After that, no absorption takes place in the entire visible region. This transmittance window (300-1100 nm) is sufficient for the generation of second harmonic light (λ=532 nm) as well as third harmonic generation (λ=354.6 nm) from the Nd:YAG laser (λ= 1064 nm).

Fig. 5 Transmittance spectrum of LTH.

Fig. 6 Load vs hardness number.

Second harmonic generation A preliminary study of the powder SHG conversion efficiency was also carried out with Nd: YAG laser beam of wavelength 1064 nm using the Kurtz and Perry method [13]. A Q-switched Nd:YAG laser beam of wavelength 1064 nm was used with an input power of 10.9 mJ/pulse, pulse width of 10 ns and the repetition rate being 10 Hz. The crystals of LTH were ground to a uniform particle size of about 125 – 150 µm and then packed in a capillary of uniform bore and exposed to laser radiation. A powder of KDP, with the same particle size, was used as the reference. The second harmonic generation was confirmed by the green emission of wavelength 532 nm, from the crystalline sample. It was found that the efficiency of second harmonic generation is 15% of that of the standard KDP.

Microhardness study The microhardness of the grown crystals was measured using a Shimadzu Microhardness Tester(Model No HMV2T) with a diamond indenter. The well polished crystals were mounted on the platform of the microhardness tester and loads of different magnitudes (25-100 gm) were applied over a fixed interval of time. The indentation time was fixed as 15 s. A plot between the load and the hardness number is given in figure 6, which shows that the microhardness number increases with increasing load. On further increase of the load beyond 100 g, crakes developed on the surface of the crystal due to the release of internal stress generated locally by indentation. A plot obtained between log (p) and log (d) gives a straight line. The relation connecting the applied load and diagonal length d of the indenter is given by the Meyer’s law:

P = adn

564 S. Natarajan et al.: A new semi organic NLO material: L-Tyrosine hydrochloride

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

where, n is the Meyer’s index or work hardening coefficient that could be calculated from the slope of the straight line. The value of n obtained for LTH is 1.08. Onitsch [14] has pointed out that n lies between 1.0 and 1.6 for hard materials, and it is more than 1.6 for soft materials. Based on the above criterion, LTH could be considered as a hard material.

3 Conclusions

Single crystals of L-Tyrosine hydrochloride were grown by using the submerged seed solution method. The grown crystals were characterized by using single crystal X-ray diffraction. Functional groups and the modes of vibrations were identified by FTIR spectroscopy. The TGA/DTA studies showed that the crystal is stable up to 232°C. Microhardness study revealed that the crystal is a hard material. It is transparent in the entire visible region. The SHG efficiency was determined as 15% of that of the standard KDP.

Acknowledgements The authors thank the UGC-SAP and DST-FIST Programmes and SAMB thanks the Madurai

Kamaraj University, Madurai for providing a fellowship

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