Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 156–164

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Growth, crystalline perfection, spectral and optical characterization of a novel optical material: l-tryptophan p-nitrophenol trisolvate single crystal N. Sivakumar a, J. Srividya b, J. Mohana a, G. Anbalagan a,⇑ a b

Department of Physics, Presidency College, Chennai 600 005, India Department of Physics, Queen Marys College, Chennai 600 004, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Synthesized by slow solvent

evaporation method.  The SHG efficiency is 3.85 times that

of KDP.  Single and multiple (10 pulses/s) shot

pulse values are 19.7 GW/cm2 and 18.6 GW/cm2, respectively.  The crystal belongs to a soft material category.  Complete vibrational assignments have been made.

a r t i c l e

i n f o

Article history: Received 11 October 2014 Received in revised form 2 December 2014 Accepted 15 December 2014 Available online 24 December 2014 Keywords: Crystal growth HRXRD Vibrational analysis Laser damage threshold Photoluminescence Optical properties

a b s t r a c t l-tryptophan p-nitrophenol trisolvate (LTPN), an organic nonlinear optical material was synthesized using ethanol–water mixed solvent and the crystals were grown by a slow solvent evaporation method. The crystal structure and morphology were studied by single crystal X-ray diffraction analysis. The crystalline perfection of the LTPN crystal was analyzed by high-resolution X-ray diffraction study. The molecular structure of the crystal was confirmed by observing the various characteristic functional groups of the material using vibrational spectroscopy. The cut-off wavelength, optical transmission, refractive index and band gap energy were determined using UV–visible data. The variation of refractive index with wavelength shows the normal behavior. The second harmonic generation of the crystal was confirmed and the efficiency was measured using Kurtz Perry powder method. Single and multiple shot methods were employed to measure surface laser damage of the crystal. The photoluminescence spectral study revealed that the emission may be associated with the radiative recombination of trapped electrons and holes. Microhardness measurements revealed that LTPN belongs to a soft material category. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Organic nonlinear optical (NLO) materials attract more attention due to their potentially high nonlinearities and rapid response in the electro-optic effect compared with inorganic NLO ⇑ Corresponding author. Tel.: +91 94871 40051. E-mail address: [email protected] (G. Anbalagan). http://dx.doi.org/10.1016/j.saa.2014.12.044 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

compounds [1]. Nonlinear optical phenomena (NLO) form the basis for a wide range of devices, including frequency doublers, electrooptic modulators, optical limiters, high speed optical gates and parametric amplifiers. Nonlinear optical properties were associated with the ability of a material to undergo nonlinear polarization under the influence of electric fields [2]. Organic molecules containing p electron systems asymmetrized by the donor and acceptor groups are highly polarizable entities which may give rise

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Experimental Synthesis, solubility and growth of LTPN crystal The l-tryptophan p-nitrophenol trisolvate (LTPN) was synthesized by taking 20.42 g of l-tryptophan (Merck: 99%, AR grade) with 13.91 g of p-nitrophenol (Merck: 99%, AR grade) dissolved in 250 ml of ethanol–water (1:1) in equimolar ratio. The chemical reaction scheme is given in Fig. 1. The crystal structure of the title compound is resulting from cocrystallization of chiral molecules, ltryptophan and p-nitrophenol. The l-tryptophan is a zwitterion with overall neutral charge. From the molecular structure of LTPN, we infer that the main component of the crystal to provide enhanced NLO efficiency is due to p-nitrophenol. The delocalization of the electron cloud of the OH group is the cause for such enhanced NLO efficiency. The co-existence of l-tryptophan along with p-nitrophenol observes to produce significant contribution to enhanced NLO efficiency. The equilibrium solubility and its temperature dependence are essential for solution growth. The data from the solubility curve NO2

O

NO2

COO-

Characterization The grown LTPN crystal was subjected to various characterization studies. A Bruker kappa APEX II single crystal X-ray Diffractometer with MoKa (k = 0.71073 Å) radiation was used to measure the cell parameters of LTPN crystal. The morphological planes of the grown crystal were identified with the help of ENRAF NONIUS CAD4 single crystal X-ray diffractometer. A Perkin Elmer Spectrum One Fourier transform infrared spectrometer was employed to determine the infrared spectrum at room temperature in the range of 4000–450 cm1. The sample was prepared by pressing the crystal powder with KBr to a pellet form. Powder Fourier Transform Raman (FT-Raman) spectrum was taken with an FRA-106 attachment to the Bruker IFS-88 spectrometer equipped with Ge detector cooled to liquid nitrogen temperature. Nd3+: YAG air-cooled diode pumped laser of power 200 mW was used as an excitation source. The incident laser excitation line was 1064 nm. The scattered light was collected at the angle of 180° in the region 4000–450 cm1, resolution 2 cm1, 256 scans. The UV–vis/NIR transmission spectrum of LTPN crystal was obtained at room temperature on a Perkin Elmer Lambda 35 Spectrophotometer with a wide wavelength range of 200–800 nm. LTPN crystal used in this test was polished to make it totally transparent and the thickness of the crystal was 1.59 mm. UV– vis radiation was allowed to incident normally on the (0 0 1) face of the crystal. Photoluminescence study on LTPN was recorded over the wavelength range from 400 to 550 nm using Jobin Yvon-Spex make spectrofluorometer (Fluorolog version-3: Model FL3-11). TGA/ 15.5 15.0

Solubility curve

14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0

OH NH3

NH2 N H

will suffice to start growing fair quality single crystals [11]. The solubility of LTPN in ethanol–water (1:1) was assessed as a function of temperature in the temperature range 30–50 °C. A thermostatically controlled vessel (100 ml) was filled with the solution of LTPN with some undissolved LTPN and stirred for 12 h. A small amount of solution was pippetted out and the concentration of the solute was determined gravimetrically. This procedure was repeated for the temperatures 35, 40, 45 and 50 °C. Fig. 2 shows the solubility curve of LTPN. The solubility curve dictates that the title compound exhibits good solubility and a positive solubility temperature gradient (direct solubility) in ethanol–water solvent. Single crystals of LTPN have been grown from saturated solutions (pH 3.26) of the synthesized salt of LTPN by the slow evaporation solution growth technique at 30 °C using a constant temperature bath having a control accuracy (±0. 01 °C). Brown crystals have been obtained over a time span of 20–25 days (Fig. 3).

Concentration (g/100 ml)

to organic polar crystals for NLO applications. The properties of individual molecules and their organization in the bulk crystalline structure are the key factors that determine the properties of the resulting molecular materials. An essential condition to obtain even-order NLO processes in materials is a noncentrosymmetric crystal structure. However, optimal molecular orientations are required if appreciable effects are to be achieved in molecular materials [3–5]. Materials of amino acids often possess specific features, such as weak Van der Waals and hydrogen bonds, wide transparency range in the visible region and zwitterionic natured molecules [6,7]. l-tryptophan is a naturally occurring amino acid, with a non-polar chain and no net charge at physiological pH. The crystal structure of the LTPN results from the cocrystallization of the chiral molecule, l-tryptophan, and p-nitrophenol, which has a high ground state dipole moment. The crystal structure results from the packing of sheets parallel to the (0 0 1) planes. These sheets are formed by a pair of screw axis related parallel networks bound by hydrogen bonding and p–p stacking interactions. The intermolecular cohesion of all organic residues in each of the latter two-dimensional networks is achieved via strong hydrogen bonding, nitro-p and p–p stacking interactions. Rodrigues et al. [8] synthesized l-tryptophan p-nitrophnol trisolvate using methanol as solvent and reported the crystal structure of LTPN. Certain optical properties of LTPN (UV–visible, FT IR and NLO) have been studied by Suresh et al. [9]. Omegala Priakumari et al. [10] studied the optical properties of LTPN crystal synthesized using deionised water as solvent. In the present work, we have synthesized LTPN using the mixed solvent ethanol–water and systematic characterizations like crystal morphology, crystalline perfection, complete vibrational analysis, optical transparency with optical band gap, refractive index from UV data and thermal analysis have been carried out. In order the find the suitability of the material for optical devices, the SHG efficiency, laser damage threshold and microhardness of the grown single crystal have also been carried out and the results are discussed in detail.

OH

+

.3

N H

Fig. 1. Reaction scheme of l-tryptophan p-nitrophenol trisolvate.

10.5 30 OH

35

40

45

Temperature (°C) Fig. 2. Solubility diagram of LTPN.

50

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Fig. 3. (a) Photograph, (b) morphology of as grown LTPN crystal.

DTA experiments were carried out using a SDT Q 6000 V 8.2 Built 100 thermal analyzer with a heating rate of 15 °C/min starting from RT to 1000 °C. Second harmonic generation property of the LTPN crystal was studied using Nd: YAG laser (continuum, series) with IR radiation having a wavelength of 1064 nm. The LDT value of LTPN crystal was measured using Q-switched Nd: YAG laser system with laser input pulse width of 10 ns at 1064 nm in the rate of 10 Hz. The microhardness of the LTPN crystal was measured at room temperature on the (0 0 1) plane with the Leitz Wetzler hardness tester fitted with the Vickers pyramidal indenter. Flat, parallel and polished surfaces cut perpendicular to a-axis (0 0 1) are used in the measurements. Results and discussion Single crystal X-ray diffraction analysis The single crystal X-ray diffraction studies reveal that LTPN belongs to the monoclinic system with space group P21, and the lattice parameters are a = 13.0311 Å, b = 6.7417 Å, c = 17.3157 Å, and volume V = 1473.12 Å3, Z = 2. The calculated lattice parameters are in close agreement with the reported values [8].

that the (0 0 1) plane is the dominant surface plane and the major growth is directed along the b-axis of the crystal.

High resolution X-ray diffraction analysis (HRXRD) The crystalline perfection of the grown single crystal was characterized by HRXRD studies using an Analytical X’pert PRO MRD high resolution XRD system with CuKa1 radiation. The experimental details are well established in the in literature [12]. The specimen was first lapped, then chemically etched with a nonpreferential etchant of water and acetone mixture in 1:2 volume ratio in order to remove the non-crystallized solute atoms retained on the surface of the crystal along with the possible layers which may occur on the surface of crystals grown by solution methods [12] and also to ensure the surface planarity. The high resolution X-ray diffraction curve recorded for (0 0 1) diffraction planes of LTPN crystal specimen is shown in Fig. 4. It can be seen from the figure that deconvolution of the diffraction curve shows the presence of two peaks. The experimental points represented by the open circles fits well with the theoretical one, shown as solid line which is 324.700 arc s away from the main peak on the left. This additional peak corresponds to one internal structural low angle boundary (tilt angle > 1 arc min. but 2, there is a reverse ISE behavior. This is in good agreement with the experimental data and thus confirms the reverse ISE. According to Onitsch [41], ‘n’ should lie between 1 and 1.6 for harder materials and above 1.6 for softer materials. Hence, LTPN crystal belongs to a soft material category. Conclusion

50 45 40 35 30 25 log P

Hv (kg / mm2)

ranging from 1 to 50 g measured on the (0 0 1) plane of the LTPN crystal. It is observed from the figure that the hardness number (Hv) increases with the increase of the load which can be considered as the reverse indentation size effect [RISE] [37–39]. Hydrogen bonded two-dimensional networks, parallel to the (0 0 1) planes, involving all four independent molecules (l-tryptophan and 3 pnitrophenol) and strong hydrogen bonding, nitro-p and p–p stacking interactions is responsible for the high value of hardness. The Mayer’s work hardening coefficient was calculated from the Meyer’s law [40], which relates the load and the size of indentation as:

20 15 10

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.2

n=2.99

1.3

5 0

10

20

30

1.4

1.5 1.6 log d

40

1.7

1.8

50

Load P (g) Fig. 14. Variation of Vickers hardness number with load and load versus distance (inset) for LTPN crystal measured on (0 0 1) plane.

The organic l-tryptophan p-nitrophnol trisolvate was synthesized and LTPN single crystal with 10  4  4 mm3 was grown from ethanol–water mixed solvent using slow evaporation technique. Single crystal X-ray diffraction study confirmed that the grown crystal belongs to the monoclinic system to space group of P21. The crystal morphology was predicted using goniometry and found that (0 0 1) plane is dominant. The thermal analysis revealed the stability of the as grown LTPN crystal up to 139 °C. The HRXRD study confirmed the absence of major defects like structural grain boundaries and inclusions in them. The as grown LTPN crystal is transparent in the region 450–800 nm. The band gap energy (Eg) of the as grown LTPN crystal was found to be 2.81 eV. Photoluminescence spectral study revealed the electron excitation in the grown crystal. The laser damage threshold and SHG efficiency were found to greater than that of KDP. The microhardness study indicates that the LTPN crystal belongs to a soft material category.

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