growth, structural, thermal and optical studies on l-glutamic acid hydrobromide – a new...

Cryst. Res. Technol. 43, No. 7, 713 – 719 (2008) / DOI 10.1002/crat.200711089

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

Growth, structural, thermal and optical studies on L-glutamic

acid hydrobromide – A new semiorganic NLO material

S. Natarajan*, G. P. Chitra, S. A. Martin Britto Dhas, and S. Athimoolam

Department of Physics, Madurai Kamaraj University, Madurai - 625 021, India

Received 19 September 2007, revised 3 January 2008, accepted 11 January 2008 Published online 1 February 2008

Key words L-glutamic acid hydrobromide, crystal structure, TGA/DTA, FTIR, NLO.

PACS 61.05.cp, 61.10.-i, 78.30.-j, 65.40.-b, 62.20.-x, 42.70.mp

A new semiorganic crystal, L-glutamic acid hydrobromide, C5H10NO4Br (GHB) has been grown from aqueous solution. The single crystal X-ray analysis of the crystal showed that it belongs to the non-centrosymmetric P212121 space group with protonated glutamic acid as cation and bromine as anion. The back-bone conformations of the amino acid are in cis and trans form. The side-chain conformations are observed to be in gauche I / trans / cis / trans forms. The characteristic 'head-to-tail' hydrogen bonding interaction is observed through a chain C(5) motif. Further, the crystal structure is stabilized by an intricate three-dimensional hydrogen bonding network. TGA/DTA showed that the grown crystals are thermally stable upto 219°C without any phase transition. The functional groups responsible for the various modes of vibrations were identified by using FTIR spectroscopy. UV-Vis-NIR spectra showed that the crystals have excellent transparency in the visible and infrared regions. The second harmonic generation (SHG) conversion efficiency was investigated to explore the NLO characteristics of the material.

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

1 Introduction

In the recent past, efforts have been taken by many researchers to develop ultraviolet (UV) lasers for industrial and medical applications. The frequency conversion technique of solid state laser radiation in nonlinear optical (NLO) crystals is an effective method for obtaining UV radiation with high beam quality and stability. The challenge faced by the researchers in this field is the identification of new types of functional materials by rational construction of molecular assemblies exhibiting nonlinear optical effects. An added difficulty of this task is the fulfillment of secondary requirements such as thermal, mechanical and chemical stabilities in addition to the ease of growth in the case of single crystals [1].

Organic nonlinear optical materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultra fast response times and high optical thresholds for laser power as compared with inorganic materials. Organic molecules with significant nonlinear optical activity generally consist of a π-electron conjugated moiety substituted by an electron donor group on one end of the conjugated structure and an electron acceptor group on the other end, forming a “push-pull” conjugated structure. The conjugated π-electron moiety provides a pathway for the entire length of conjugation under the perturbation of an external electric field. The donor and acceptor groups provide the ground state charge asymmetry of the molecule, which is required for second order nonlinearity. In this context, amino acids are interesting materials for NLO applications as they contain a proton donor carboxyl acid (-COO) group and the proton acceptor amino (-NH2) group in them [2]. But, most organic NLO crystals have usually 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

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* Corresponding author: e-mail: [email protected]

714 S. Natarajan et al.: Thermal and optical studies on L-glutamic acid hydrobromide

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

materials have excellent mechanical and thermal properties, but possess relatively modest optical nonlinearity because of the lack of extended π - electron delocalization [3,4]. 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 [5], L-Histidine hydrochloride [6], L-Histidine tetrafluroborate [7], L-Hisidinium bromide [8], L-Histidine hydrofluoride dihydrate [9] and L-Glutamic acid hydrochloride (GHC) [10] are some examples of semiorganic amino acid-related NLO materials reported recently. Presently, the single crystals of a new semiorganic NLO material viz., L-Glutamic acid hydrobromide (GHB) have been grown by slow evaporation method. GHB is isomorphous to the chloride analogue (GHC), but is more stable. GHC crystals were reported to be deliquescent [11]. The structure elucidation of GHB was carried out by single crystal X-ray diffraction. Fourier transform infrared (FTIR) spectra, TGA/DTA and UV-Vis-NIR spectra were recorded and studied. Further, SHG efficiency of the powdered material was tested using Kurtz and Perry method. The crystal structure analysis of GHB showed that the carboxylic group is protonated. Thus the π – π* transition occurs in the carboxylate group. Possibly, the principal optical nonlinearities of GHB arise from the delocalized π electrons associated with the carboxylate group of the glutamic acid.

2 Crystal growth

GHB was crystallized from an aqueous solution containing L-Glutamic acid and hydrobromic acid. Small, optically clear and well-shaped crystals suitable for usage as seed crystals were obtained in a period of few days. Bulk crystals were grown from the seeds using a saturated solution of GHB in a crystallizer using submerged seed solution growth method. Transparent crystals of size: 7.0 × 4.0 × 3.0 mm3 were obtained in a period of about four weeks (Fig. 1).

Fig. 1 Photograph of GHB crystal. The density of the crystal was determined as 1.78(3) gm/cm3 by the floatation method using a liquid-mixture of carbon tetrachloride and bromoform. The expected density (ρ) of the complex was calculated from the crystallographic data, using the well-known expression (1).

ρ = MZ / NV (1)

where M is molecular weight, Z, the number of molecules in the unit cell, N, the Avogadro number and V, the volume of the unit cell. This value of calculated density is 1.793 gm/cm3. The melting point was found out as 219°C.

3 X-ray structural studies

Structure determination The unit cell parameters and the crystal structure were determined from the single-crystal X-ray diffraction data obtained with a four-circle Nonius CAD4 MACH3 diffractometer (graphite-monochromated, MoKα = 0.71073Å) at room temperature (293 K). The data reduction was done using XCAD4 [12]; Absorption correction was done by the method of ψ-scan [13]. The structure solution and refinement were performed using SHELXTL 6.10 [14]. The structure was solved by direct methods, and full-matrix least-squares refinements were performed on F2 using all the unique reflections. All the non-hydrogen atoms were refined with anisotropic atomic displacement parameters, and hydrogen atoms were refined with isotropic displacement factors. The H atoms participating in the H-bonds were located from the difference fourier and all other H atoms (-CH) were positioned geometrically and refined using a riding model with C-H =

Cryst. Res. Technol. 43, No. 7 (2008) 715

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

0.97 (-CH) or 0.98(-CH2) Å with Uiso(H) = 1.2 Ueq (parent C atom). The absolute configuration of the amino acid (L-form) is assigned from the starting material taken for reaction. The structure of GHB with the atom-numbering scheme and 50% probability displacement ellipsoids is shown in figure 2. The crystallographic data and structure refinement parameters are presented in table 1. Selected bond distances, bond angles and torsion angles are listed in table 2. The packing diagram viewed down the a-axis is shown in figure 3 [16]. The hydrogen-bond geometry is given in table 3 and also shown in figure 3.

Table 1 The crystallographic data and structure refinement parameters for GHB.

Empirical formula C5H10NO4Br Formula weight 228.05 Temperature 293(2) K Wavelength 0.71073 Å Crystal system, space group Orthorhombic, P 212121 Unit cell dimensions a = 5.3672(6) Å, b = 11.7515(9) Å, c = 13.3924(11) Å Volume 844.69(13) Å3 Z 4 Calculated density 1.793 Mgm-3 Absorption coefficient 4.836 mm-1 F(000) 456 Crystal size 0.21 x 0.18 x 0.15 mm Theta range for data collection 2.31 to 24.92° Limiting indices 0 ≤ h ≤ 6, 0 ≤ k ≤ 13, -1 ≤ l ≤ 15 Reflections collected 971 Independent reflections 952 [R(int) = 0.0113] Absorption correction Psi-scan Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 952 / 5 / 121 Goodness-of-fit on F2 1.071 Final R indices [I>2σ(I)] R1 = 0.0209, wR2 = 0.0501 R indices (all data) R1 = 0.0292, wR2 = 0.0526 Absolute structure parameter -0.008(19) Extinction coefficient 0.052(2) Largest diff. peak and hole 0.289 and -0.365 e/Å 3 Programs used for molecular graphics ORTEP-3 for windows [15], PLATON [16] and Mercury 1.4.1 [17]

Conformational analysis The unsymmetrical C-O bond distances and O-C-C bond angles of the carboxyl

groups clearly indicate the protonation on the glutamic acid residue (Table 2). Generally, the conformations of the amino acids are classified into two categories: backbone and side-chain conformations. The backbone conformations are described as the deviation of the carboxyl group plane with respect to NCαC' defined with reference to the torsion angles, viz., ψ1 (O1a-C1-C2-N1) and ψ2 (O1b-C1-C2-N1). The side chain conformational features are described as χi and χij. In general, the torsion angles of the side chain are found to occur in all the three staggered configurations, with the values of 60°, 180° and -60°, corresponding to gauche

(I), trans or gauche(II) conformations. In the present investigation, the back bone conformation angles of glutamic acid molecule are observed to be in cis (ψ1) and trans (ψ2) conformations. The side chain conformations χ1 [N(1)-C(2)-C(3)-C(4)] / χ2 [C(2)-C(3)-C(4)-C(5)] / χ31 [C(3)-C(4)-C(5)-O(2a)] / χ32 [C(3)-C(4)-C(5)-)O(2b)] are observed to be in gauche(I) / trans / cis / trans rotamaric conformations (Table 2). The value of the backbone conformation ψ1 is negative as is usually observed in L-amino acids [18]. The positive value of the side chain conformation χ31 leads to a decrease in the intramolecular C3...O2a distance, which is similar to the observation made by Sundaralingam & Putkey [19] for the backbone conformation angle ψ2. The dihedral angle between the planes of the carboxylic groups of the cation has a value of 66.2(5)°.

Hydrogen bonding interactions Although the bromine anions are mainly exploited for space filling in the crystal structure, they also play important role in the hydrogen bonding interaction as shown in figure 2 and table 3. There are three two centered hydrogen bonds from the amino group leading to a class I hydrogen-bonding scheme [20]. The three-dimensional hydrogen bonding network in the crystal structure can be explained with the aid of the graph-set notations followed by Etter et al. [21]. The carboxyl groups of the cations directly interact through O-H...O hydrogen bond leading to an open dimeric structure with the C(8)



motif and the chain propagating along the c-axis of the unit cell (Fig. 4). The characteristic 'head-to-tail' interaction is observed through N1-H1d...O1a [x+1/2,-y+3/2,-z] hydrogen bond forming a chain C(5) motif, the corresponding zig-zag chain running along the a-axis of the unit cell (Fig. 5).

Fig. 2 The structure of GHB with the atom-numbering scheme and 50% probability displacement ellipsoids. H-bond is shown as dashed lines.

Fig. 3 The packing diagram of GHB viewed down the a-axis. H-bonds are shown as dashed lines. (Online color at www.crt-journal.org).

Table 2 Selected bond distances(Å), bond angles(°) and torsion angles(°)

C1-O1a 1.212(5) C1-O1b 1.310(5) C2-N1 1.488(5) C5-O2a 1.198(5) C5-O2b 1.320(5) O1a-C1-C2 123.2(4) O1b-C1-C2 111.8(4) O2a-C5-C4 124.0(4) O2b-C5-C4 112.1(4) O1a-C1-C2-N1 -20.6(6) O1b-C1-C2-N1 161.6(4) N1-C2-C3-C4 -66.5(5) C2-C3-C4-C5 -169.4(3) C3-C4-C5-O2a 11.3(6) C3-C4-C5-O2b -169.5(4)

Table 3 Hydrogen-bond geometry (Å, °). (Symmetry transformations used to generate equivalent atoms: (i) -x-1/2,-y+1,z-1/2 (ii) x-1,y,z (iii) x+1/2,-y+3/2,-z (iv) x-1/2,-y+3/2,-z+1).

D-H...A d(D-H) d(H...A) d(D...A) <(DHA) O1b-H1b...O2ai 0.817(11) 1.845(14) 2.658(4) 173(6) N1-H1a...Br1 0.895(10) 2.411(13) 3.296(4) 170(4) N1-H1c...Br1ii 0.892(10) 2.448(14) 3.329(4) 170(4) N1-H1d...O1aiii 0.891(10) 2.040(20) 2.841(4) 149(4) O2b-H2b...Br1iv 0.821(10) 2.400(15) 3.206(3) 167(5)



Bromine anions act as acceptors and connect two adjacent cations along the a-axis of the unit cell forming a chain C2

1(4) motif (Fig. 6). Further, the combination of the carboxyl group open dimer and the N-H...O bond lead to the chain motifs: C2

2(11) / C22(13). The hydrogen bonds N1-H1a...Br1 and O2b-H2b...Br1 form a

closed ring R42(18) motif.

Fig. 4 A view of the carboxyl-carboxyl open dimer [C(8) motif] leading to a zig-zag chain running along the c-axis of the unit cell. H-bonds are shown as dashed lines. (online color at www.crt-journal.org).

Fig. 5 A view of the zig-zag 'head-to-tail' chain C(5) motif running along the a-axis of the unit cell. H-bonds are shown as dashed lines. (online color at www.crt-journal.org).

Fig. 6 A view of a straight chain C2

1(4) motif running along the a-axis of the unit cell through two N-H....Br hydrogen bonds (dashed lines). (online color at www.crt-journal.org).

Fig. 7 TGA/DTA curves for GHB.

4 Thermal studies

Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out for the crystals, using a SDT Q600 V8.2 Build 100 thermal analyzer. The characteristic curves are shown in figure 7. A powder sample was used for the analysis in the temperature range of 26°C to 900°C with a heating rate of 20 K/min. The crucible used was of alumina (Al2O3), which also served as a reference for the sample. It is seen that the sample has very good thermal stability upto 219°C. Another important observation is that, there is no phase transition and no decomposition till the material melts. The endothermic peak at 219°C is due to the melting of the crystal and immediately afterwards it starts to decompose. The decomposition occurs in three steps. A weight loss (54% ) occurs due to the liberation of bromine anion and CO2 molecules in the temperature region of 220°C to 250°C. In the next stage, the loss of weight of 19.3%, in the temperature region of 440°C to 580°C may be assigned to the liberation of one more carbon dioxide from the glutamic acid, gradually. On further heating, the remaining molecules got decomposed simultaneously in the temperature



region of 450°C to 575 °C. The absence of any peak in the DTA in the region around 100°C shows the absence of water molecules in the crystal.

5 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. 8) were compared with the standard spectra of the functional groups [22]. The characteristic peak at 3121 cm-1 is due to the asymmetric stretching vibration of the NH3

+ group and the corresponding peak at 1725 cm-1 is due to the symmetric stretching of the C=O group. The strong peak at 1680 cm-1 is due to the COO- asymmetric stretching vibration and the peak at 1419 cm-1 due to the symmetric stretching of the COO- group. The other peak observed at 1502 cm-1 is assigned to the C-N asymmetric stretching and the one at 994 cm-1 to the C-O stretching. Strong absorption at 1369 cm-1 is assigned to the bending and the peak at 2902 cm-1 to the symmetric stretching of the CH2 group. The stretching vibration of the C-C group is observed at 1119 cm-1 and the deformation of the C-C group is observed at 488 cm-1. The C-C-N symmetric stretching is observed at 858 cm-1.

Fig. 8 FTIR spectra of GHB.

Fig. 9 Transmission spectra of GHB.

6 UV-Vis-NIR spectrum

The UV-Vis-NIR transmission spectrum (Fig. 9) of the crystal was recorded in the range: 190 – 1100 nm using a AGILENT (8453), UV-Vis-NIR spectrophotometer. It is seen that the UV transparency cutoff occurs at 220 nm and there is no remarkable absorption in the entire region of the spectra. The absence of absorption in the region between 220 and 1100 nm shows that the crystal of GHB is useful for the second harmonic generation of Nd: YAG laser of wavelength λ = 1064 nm.

7 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 [23]. A Q-switched Nd:YAG laser beam of wavelength 1064 nm was used with an input power of 3.5 mJ/pulse, pulse width of 10 ns and the repetition rate being 10 Hz. The crystals were ground to a uniform particle size of about 125 – 150 µm and then packed in a capillary of uniform bore and exposed to the 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 25% of that of KDP.



8 Conclusions

A new semiorganic NLO material, viz., L-Glutamic acid hydrobromide (GHB) was successfully grown using slow evaporation method at room temperature and its structure elucidated by single crystal X-ray diffraction methods. The back bone and side chain conformations were analyzed. The three-dimensional hydrogen bond patterns were assigned to different hydrogen bonding motifs. TGA/DTA studies showed that the crystals are thermally stable without any phase transition upto the melting point (219°C). The functional groups were identified using FTIR spectroscopy. UV-Vis-NIR study showed that the crystal is transparent for the fundamental and second harmonic of Nd: YAG (λ = 1064) laser. The SHG efficiency was measured using the Kurtz and Perry method and is found to be about 25% of that of the standard KDP crystal. These results suggest that GHB shows promise as a new NLO material for frequency conversion in the UV and near IR. Acknowledgements The authors thank the DST, Government of India, for establishing the single crystal diffractometer facility at the School of Physics, Madurai Kamaraj University, through the FIST Programme. The authors also thank the UGC for the DRS programme. SAMB thanks the Madurai Kamaraj University for providing a Research fellowship.

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growth, structural, thermal and optical studies on l-glutamic acid hydrobromide – a new...

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