Structural, spectral and nonlinear optical analysis of Bis(2-methyllactato)borate tetrahydrate: a new nonlinear optical crystal for laser applications

Journal Pre-proof Structural, spectral and nonlinear optical analysis of Bis(2-methyllactato)borate tetrahydrate: a new nonlinear optical crystal for laser applications Gokila G, Aarthi R, Ramachandra Raja C

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https://doi.org/10.1016/j.ijleo.2020.164384

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IJLEO 164384

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Optik

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Please cite this article as: G G, R A, Raja C R, Structural, spectral and nonlinear optical analysis of Bis(2-methyllactato)borate tetrahydrate: a new nonlinear optical crystal for laser applications, Optik (2020), doi: https://doi.org/10.1016/j.ijleo.2020.164384

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Structural, spectral and nonlinear optical analysis of Bis(2-methyllactato)borate tetrahydrate: a new nonlinear optical crystal for laser applications Gokila.G, Aarthi.R and Ramachandra Raja.C* Government Arts College (Autonomous), Kumbakonam 612002, India.

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Abstract A new nonlinear optical crystal, ‘Bis(2-methyllactato)borate tetrahydrate’ (BMBT) has been

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crystallized by slowly evaporating the solvent. It crystallizes in the space group P2 1212. Sharp peaks present in powder XRD profile reveals the good crystallinity. Its transmission bandwidth (220 nm – 1100 nm) suggest that it can be used to generate UV radiation upto 220 nm and also in

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optoelectronic applications. Vibrations of functional groups (B-O, CH3, CO) associated with the crystal structure have been identified through FTIR and FT-Raman spectral studies. OH stretching

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vibrations have been observed near 3000 cm-1 clearly endorses the existence of water molecule in the BMBT structure. The chemical shifts observed in 1H and 13C NMR spectral results establishes

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the molecular structure of BMBT crystal. Second harmonic generation (SHG) efficiency is obtained as 0.9 times of KDP. The third order nonlinear susceptibility (χ3), nonlinear refractive index (n2) and nonlinear absorption co-efficient (β) were found using Z-scan technique. χ3 of title

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crystal is found to be 4.16 X10-5 esu. The existence of SHG efficiency and the enhanced χ3 value are due to the hydrogen bonded intermolecular interactions present in the BMBT crystal structure. The observed results suggest that BMBT can be used in Q-switching, mode locking and optical sensors like night vision devices.

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Keywords: Crystal structure; crystal growth; NMR spectroscopy; second harmonic generation; Z-scan analysis

1. Introduction Nonlinear optical (NLO) crystals are enormously developed due to their applications in

photonics and optical communications [1-5]. Inorganic crystals such as BBO, LBO, KBBF, KBOC, BBOF, etc. possess greater physical properties and deep UV generation and possess low nonlinear efficiency [6-8]. Organic crystals are excellent NLO materials due to their tailoring

ability and larger nonlinear response. Organic NLO crystals such as DAST and MMONS provide good NLO efficiency and finds applications in logic devices, biological imaging, optical switching etc [9,10]. But these crystals have low mechanical and thermal stability. Semiorganic crystals combines the properties of these crystals. Functionalizing the organic moiety with appropriate inorganic counterpart generates new crystal structures. Attempts are still made by solid state researchers to develop an ideal NLO crystal. It is reported that hydrogen bonded intermolecular interactions gives increased NLO efficiency with good physical properties [11]. Among various

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organic materials, 2-methyllactic acid promotes enhanced charge transfer mechanism due to their ability to coordinate in aqueous media by changing their solubility [12]. The crystal structure of Mn(IV), Mn(II), Zn(II) complexes of 2-methyllctic acid [12,13], polymeric lithium bis(2-

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methyllactato)borate monohydrate [14] have already been reported. All these crystal structure consists of several hydrogen bonded intermolecular interactions. G. Cammas et al through

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vibrational analysis confirms that the lacticacid based complexes may form several hydrogen bonded inter and intramolecular interactions when reacted with suitable moieties [15]. In

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continuation of this, four new structures belonging to ML based derivatives were identified and their structures were reported by the authors [16-19]. In this work, the hydrogen bonded structure

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of BMBT crystal was confirmed through various characterization techniques. The hydrogen

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bonded intermolecular interactions promotes SHG efficiency and χ3 value.

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2. Materials and methods

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Figure 1: Ortep diagram of molecular structure of BMBT crystal [19]

BMBT is crystallized by reacting 2-methyllactic acid and boric acid in 2:1 molar ratio by

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using deionized water as solvent. The mixture is stirred well using magnetic stirrer and kept covered using perforated paper at room temperature. Good quality crystals were harvested over a period of 4 months.

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3. Results and discussion 3.1 Structural analysis

The structure of BMBT is already reported [19] and the molecular structure is given in fig.1. XPERT-PRO powder X-ray diffractometer with Cukα wavelength 1.5418 Å is employed to record the diffractogram and is presented in figure.2. Presence of sharp peaks ensures fine crystalline quality of BMBT. Rietveld indx software is used to index the PXRD pattern. The lattice constants

were calculated using Rietveld unit cell package. The calculated values matches well with the reported values and are given in table I.

033

1600

260

042

012

30

40

50

60

ro 122

70

80

re

20

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0

2100

033 252 302 351

200

290

210

400

170

140

141

600

241 161

112 122

002

800

062 331

202

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312

1000

370

Intensity (a.u)

1200

231

150 051

121

1400

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two theta (degree)

Figure:2 Powder X-ray diffraction pattern of BMBT crystal

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Table 1: Lattice parameter of BMBT crystal

Single crystal XRD [19]

Powder XRD

a(Å)

7.0809

7.0928

b(Å)

16.7912

16.7731

c(Å)

6.5001

6.4948

V(Å3)

772.84

772.69

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Lattice parameters

3.2 Linear transmission analysis

PERKIN ELMER Lamda 35 UV-Vis-NIR Spectrophotometer is employed to investigate the linear optical transmittance window of BMBT crystal. The spectrum is presented in figure.3. The lower cut off wavelength is observed at 220 nm and the transmittance bandwidth is extended upto to 1100 nm. This made the crystal a potential candidate to generate wavelength upto 220 nm by

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frequency mixing process and it can be used in optoelectronic applications [20, 21].

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80

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60

40

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Transmittance (%)

100

20

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0

400

600 800 Wavelength (nm)

1000

1200

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200

Figure:3 UV-Vis-NIR spectrum of BMBT crystal

3.3 Vibrational Analysis

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Perkin-Elmer FTIR spectrometer and Bruker RFS 27 FT-Raman spectrometer were

employed to record the vibrational spectra of BMBT crystal and are depicted in figure 4 and 5 respectively. Water molecules present in the crystal is established through the observed vibrations above 3000 cm-1 in IR and Raman spectrum. Vibrations observed at 1706 cm-1 in IR and 1713 cm1

in Raman spectrum corresponds to the bending mode of OH group. The stretching frequencies of

OH are shifted towards lower frequency side and bending frequency shifted towards higher

frequency side when compared with the OH vibrations of 2-methyllactic acid [22]. This shifts in frequencies is due to the O-H---O intermolecular interaction present in BMBT crystal. CH3 stretching vibrations were observed from 2942 cm-1 to 2987 cm-1 in IR and 2987 cm-1 to 2722 cm-1 in Raman spectrum. Bending vibrations of methyl group is observed at 1542 cm-1, 1464 cm-1, 1349 cm-1, 1389 cm-1 in IR and 1449 cm-1, 1362 cm-1, 1390 cm-1 in Raman spectrum. This clearly confirms the presence of methyl group in BMBT. Observed vibrations such as 1106 cm-1 and 1167 cm-1 in IR and Raman spectrum corresponds to the BO4 asymmetric stretching vibrations and the symmetric stretching vibrations were observed at 614 cm-1, 549 cm-1 in IR and

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590 cm-1 in Raman spectrum. Bending vibrations of BO4 group were observed at 463 cm-1 in IR and in Raman it is observed at 427 cm-1, 222 cm-1, 210 cm-1 respectively. All other functional

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groups associated with the BMBT crystal are also observed and assigned using literature [13, 15, 23-31] and are given in table 2. Presence of OH, CH3 and BO4 vibrations ensures the formation of

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BMBT molecule.

FT-Raman cm-1 3012

OH stretching

2987 - 2722

CH3 stretching

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C=O stretching

1706

1713

OH bending

1542, 1464

1449

CH3 asymmetric bending

2942 - 2987

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1772

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FTIR cm-1 3235 - 3485

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Table 2: Vibrational spectral assignments of BMBT crystal

1349, 1389

1362, 1390

Assignments

CH3 symmetric bending

1191

CO stretching vibration

1155

1116

BO4 asymmetric stretching

773

-

C=O bending

614, 549

590

BO4 symmetric stretching

463

427, 222, 210

BO4 bending

-

316

CH3 wagging

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1085, 1178

-

123, 81

Lattice vibrations

110

90

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80 70 60 50

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Transmittance (%)

100

40

20 4000

3000

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30

2000

1000

0

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Wavenumber (cm-1)

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0.07

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Figure:4 FTIR Spectrum of BMBT crystal

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Raman intensity

0.06 0.05 0.04 0.03 0.02 0.01 0.00

500

1000

1500 2000 2500 3000 -1 Wavenumber (cm )

3500

4000

Figure:5 Raman Spectrum of BMBT crystal

3.4 NMR spectral analysis 1

H and

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C NMR spectral analysis of BMBT crystal was carried out using Bruker 300MHZ

(ultrashield)TM instrument and DMSO is the solvent used. Figure 6 and 7 shows the 1H NMR and C NMR spectra of BMBT crystal. In 1H NMR, the peak noticed at 2.512 ppm is due to DMSO

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13

solvent. Chemical shift appears at 1.148 ppm and 1.262 ppm are assigned to CH3 group. In pure ML the shift for the same is present at 1.497 ppm [32]. Chemical shift noticed at 4.848 ppm is due

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to the water molecule present in the title crystal. 13

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C NMR profile shows the signal at 27.11 ppm and 27.77 ppm which are assigned to the carbon

atoms of methyl group (C3, C3i, C4, C4i). Chemical shift of (C1, C2i) carbon atoms are appeared at 71.19 ppm and 75.65 ppm respectively. Chemical shift observed at 178.34 ppm and 181.89 ppm

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are attributed to the C2 and C1i carbon atoms of BMBT crystal. DMSO solvent signal appears at

181.48 ppm (-COOH) [34].

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39.31 ppm [33]. For pure ML, these shifts are observed at 27.0 ppm (-CH3), 72.33 ppm (-C-) and

The changes in the values of chemical shifts observed between 2-methyllactic acid and the title

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crystal may be attributed to the bonding between electronegative boron atom and 2-methyllactic

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acid moiety and also due to the several hydrogen bonds present in BMBT crystal [19].

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Figure 6: 1 H NMR spectrum of BMBT crystal

Figure:7 13C NMR spectrum of BMBT crystal 3.5 Second harmonic generation analysis

SHG property of the BMBT is ensured through Kurtz and Perry powder method. Nd:YAG laser (1064 nm) with a input energy of 1.2 mJ is used as a radiation source to irradiate the BMBT crystal.

It results in the emission of green light and it is directed towards a photomulitplier tube which produces corresponding electricl pulses. Emission of green light from BMBT confirms the SHG. The output measured for BMBT crystal is 20 mV and 22 mV for KDP. SHG efficiency of BMBT is 0.9 times that of KDP. As reported [19] BMBT crystal consists of several hydrogen and B-O bonds and these interactions promotes charge transfer which results in SHG efficiency [35, 36].

3.6 Z-Scan Analysis

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Z-scan technique is employed to evaluate the third order NLO parameters of the BMBT crystal. Closed and open aperture traces of BMBT crystal is recorded using Nd:YAG laser of intensity 5

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mW and wavelength 532 nm. are depicted in figure 8 and 9 respectively. Closed aperture trace shows the pre-focal peak followed by post-focal valley which declares BMBT pronounces self-

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defocussing nature and gives negative n2 which is -4.6032X10-10 cm2/W. This makes BMBT a suitable candidate for laser Q-switching and mode locking applications [33]. The open aperture trace displays the reverse saturable absorption nature of grown crystal due to the minimum

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transmittance at Z=0 and β is found to be 1.6579X10-3 cm/W. This property is used in night vision devices [37]. BMBT crystal consists of several O-H---O intermolecular interactions. This

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hydrogen bonded interactions promotes the charge transfer mechanism and gives rise to molecular hyperpolarizability which results in enhanced χ3 value. It is found to be 4.1600X10-5 esu. χ3 value

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of BMBT crystal and some reported NLO materials is given in table 3.

0.8 0.6 0.4 0.2

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Normalized Transmittance

1.0

-20

-10

0 10 Distance(mm)

20

30

-p

-30

ro

0.0

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Figure: 8 Closed aperture trace of the BMBT crystal

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0.8 0.6

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Normalized Transmittance

1.0

0.4 0.2

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0.0

-30

-20

-10

0 10 Distance (mm)

20

30

Figure:9 Open aperture trace of the BMBT crystal

Table:3 Comparison of χ3 value of BMBT with some reported NLO crystals Name of the crystal Third order optical Reference susceptibility (χ3)

4.16 X10-5 esu

Present work

4MLBANO3

3.57 X 10-6 esu

[38]

4MLBDCB

5.04 X 10-6 esu

[39]

4MLBACH

3.57 × 10-6 esu

[40]

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BMBT

4.Conclusion

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Solution grown BMBT crystallizes in P21212 space group and the whole crystal structure is linked together through O-H---O intermolecular interactions. Fine intensified peaks observed through PXRD analysis reveals that BMBT is in perfect crystalline nature. BMBT is transparent

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from 220 nm to 1100 nm and this made it a suitable candidate in various optoelectronic and photonic applications. Existence of water molecule in the BMBT is confirmed through observed OH vibrations. Vibrations of B-O, CH3, CO were observed and this ascertains the formation of title crystal. Chemical shift observed at 4.848 ppm in 1H NMR spectral profile reveals the presence of water molecule in BMBT and supplements the results of vibrational spectrum. Carbon-hydrogen

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frame work of MBT is well acknowledged by the 1H and 13C NMR spectral profile. The shift in the signals of MBT compared with pure 2-methyllactic acid are be attributed to the presence of hydrogen bonds. SHG efficiency of the BMBT is found to be 0.9 times of KDP. Charge transfer mechanism due to intermolecular interactions promotes higher molecular polarizability and results in enhanced χ3 value. Declaration of interest We wish to confirm that there are no known conflicts of interest associated with this publication and there

has been no significant financial support for this work that could have influenced its outcome. No funding was received for this work.

Acknowledgments The authors thank the authorities of Sophisticated Analytical Instrument Facility, IIT, Chennai for single crystal XRD and FT-Raman. The authors are thankful to SASTRA University, Thanjavur, for having provided NMR facilities. Gratefully acknowledge the Instrumentation

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centre of St. Joseph’s College, Trichy for recording UV–Vis-NIR and FTIR spectrum. The authors are grateful to Professor Dr.P.K.Das, IISc Bangalore for providing SHG test and Dr. D. Sasti

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