Benzimidazole Single Crystal

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Research Journal of Chemistry and Environment ___________________________________ Vol.17 (7)July (2013)Res. J. Chem. Environ.

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Thermal, Structural and Optical Analyses ofBenzimidazole Single Crystal Grown with Organic

Dopants for Nonlinear Optical ApplicationsVijayan N.,1*Madhurambal G.,2Bhagavannarayana G.1, Maurya K. K.1and Mojumdar S. C.3, 4

1. CSIR-National Physical Laboratory, New Delhi, INDIA2. Department of Chemistry,A.D.M. College for Women, Nagapattinam, INDIA3. Department of Chemistry, University of Guelph, Guelph, ON, CANADA

4. Department of Chemical Technologies and Environment, Trenn University of A, SLOVAKIA*[email protected]

Abstract

Nowadays nonlinear optical (NLO) materials are

gaining attention because of their numerous

applications in the area of fibre optic communication

and optical signal processing. In the present study,

the nonlinear optical material of benzimidazole

(BMZ) has been grown by slow evaporation solution

growth technique using two different organic dopants(Urea and N-methyl urea) at different molar

percentages. The effect of dopants on the growth andother physical properties has been analyzed by

different instrumentation methods. The addition of

urea and N-methyl urea (NMU) to the host material

of benzimidazole has not changed the existing

crystalline system, but there was a trivial variation in

the lattice dimensions which is confirmed by powder

X-ray diffraction analysis.

The crystalline perfection was analyzed by highresolution X-ray diffraction analysis. It was found

that urea dopant enhances the crystalline perfection

in comparison with the N-methyl urea, which is in

tune with the scanning electron microscopic analysis.

The thermal performance has been examined by TG-

DTA for all the doped specimens. Its relative second

harmonic generation efficiency was evaluated by

Kurtz powder technique and it was found that the

doped specimens showed enhanced efficiency in

comparison with the pure benzimidazole. Its optical

properties have been examined by UV-VIS spectralanalysis and found variations in the observed

transmittance values were found.

Keywords: Single crystal, dopants, characterizationmethods, nonlinear optical material, second harmonicgeneration, TG, DTA.

IntroductionIn the recent past, organic nonlinear optical materials aregaining attention due to their efficient second harmonicgeneration and tailor made flexibility. They are very much

useful in the filed of fibre optic communication, opticaldata storage and frequency doubling applications.1-3

The above said applications stimulated the materialsscientists to search new model NLO material to satisfy theday-to-day technological requirements. In the present study,we have attempted to grow single crystals ofbenzimidazole by using urea and N-methyl urea as thedopants. Benzimidazole (BMZ-C7H6N2) is a fused, tworing conjugated system with six carbon atoms in one ringand five atoms in the other. The smaller ring has nitrogenatoms in the first and third positions. BMZ crystallizes inorthorhombic crystal system with space group Pna21having lattice dimensions of a=13.504(3) , b=6.806(3) ,c=6.939(3) .

In our recent studies, the relative second harmonicgeneration (SHG) efficiency of BMZ was found to be 4.5times than that of KDP and it is having relatively largerlaser damage threshold4. The doped BMZ single crystalshave been grown by slow evaporation solution growthtechnique using methanol as the solvent. The addition ofdopants did not affect the existing crystal system, but led to

enhance thermal stability, crystalline perfection, SHGefficiency and optical properties. Thermal, XRD, opticaland spectral properties are very important properties ofmaterials. Therefore, many authors have studied theseproperties of various materials.5-32

Material and MethodsThe commercially purchased chemicals are of considerablyvariable quality, with purity lying in the range of 95-99 %.To improve the purity of the material, recrystallization isone of the easiest and inexpensive methods. In the presentstudy, the commercially available benzimidazole, urea and

NMU were dissolved in methanol and its purity wasimproved by repeated re-crystallization processes and therecrystallized salts were used as the charge material forpreparing the saturated solution.

The calculated amount of the host material ofbenzimidazole was prepared in four different beakers using100 mL of methanol solution. The solubility of the purebenzimidazole was determined in three different solvents.33Then the dopants of urea and NMU were added to theabove mentioned solution in 0.5M% and 1M%,respectively. Then the solution was stirred well by using

magnetic stirrer and the temperature was maintainedslightly above the room temperature in order to completethe reaction process.

* Author for Corr espondence


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Then, the prepared solution was filtered using a filter paperand then covered using a thick plastic sheet in order toavoid fast evaporation. Then the beakers were housed in aconstant temperature bath (CTB) with a settingtemperature of 34 C and the complete growth process wascarefully examined. After a span of 12 days, urea added

single crystals were grown in the bottom of the beaker withnatural facets. At the same time NMU added crystals wereharvested after a time span of 20-22 days and it was foundthat the dopant of urea enhanced the rate of growth as wellas the crystalline quality of the specimens which is laterconfirmed by high resolution X-ray diffraction analysis.The harvested single crystals are shown in fig. 1 (a-d). Theharvested single crystals have been subjected to differentcharacterization analyses.

Results and DiscussionPowder X-ray diffraction analysis:The urea and NMU

added BMZ single crystal was crushed into fine powderand the crushed material was subjected to powder X-raydiffraction analysis. The analysis was carried on a powderX-ray diffractometer at a scanning speed of 1 /minute. Theobtained two-theta values were used to calculate the latticedimensions of the respective compounds by using PROZKIsoftware package (APPLEMAN programme). Therecorded XRD patterns are shown in the fig. 2 (a-d). Fromthe measurement we found that the addition of dopants didnot affect the existing crystal system.33But there is a slightvariation in the lattice dimensions due to the inclusion ofUrea and NMU in the host material interstitials. Thecalculated lattice parameters are given in table 1.

Evaluation of crystalline perfection by HRXRD :In thepresent analysis, the crystalline perfection of the pure anddoped specimens of BMZ was characterized by HRXRDon multicrystal X-ray diffractometer developed at NPL.34The well-collimated and monochromated MoK1 beamobtained from the three monochromator Si crystals set indispersive (+,-,-) configuration has been used as theexploring X-ray beam. The specimen crystal is aligned inthe (+,-,-,+) configuration. Due to dispersive configuration,though the lattice constant of the monochromator crystal(s)and the specimen are different, the unwanted dispersion

broadening in the diffraction curve of the specimen crystalis insignificant. The schematic line representation is givenin our earlier reports.4

Fig. 3a shows that the diffraction curve (DC) contains oneadditional peak at an angular separation of 46 arc sec fromthe main peak. The additional peak shows that thespecimen contains one very low angle boundary. The halfwidths of the main peak and the peak due to very low angleboundary are 23 and 19 arc secs respectively. The halfwidths as well as the peak separation are within a minuteof arc which shows the crystal quality is reasonably good.

The very low angle boundary obtained for this solutiongrown specimen may be attributed to the segregation of

solvent molecules (methanol) which were entrapped withinthe crystal during growth.

Fig. 3b shows DC recorded for (002) diffracting planes.The FWHM of the main peak is 40 arc sec. The slightbroadening of the DC is due to the inclusion of dopants in

the crystalline matrix. As seen in the fig. 3c, the DC ishaving a single peak having the FWHM of 26 arc sec. Thepeak intensity of this curve is very high as compared tothat of SEST grown BMZ crystal which also shows bettercrystalline perfection similar to fig. 3b due to the inclusionof NMU dopants.

Fig. 3d shows the diffraction curve which containsmultiple peaks in an angular range of ~ 1000 arc sec. Thereare five main peaks whose FWHM values are in the rangeof several arc min. These multi-peaks indicate that thespecimen contain many structural internal low angle grainboundaries. It is worth to mention here that the addition of1M% of urea enhances the growth rate but reduces thecrystalline perfection.

The solid line in the figure is the convoluted curve of threepeaks shown with dotted lines. The additional peaks,which are 7 and 37 arc sec away from the main peak,indicate that the specimen contains two very low angleboundaries whose tilt angles are 7 and 37 arc sec. The halfwidths of the main peak and the peaks due to very lowangle boundaries are 95, 18 and 25 arc sec respectively.The formation of very low angle boundaries due to dopants(1M% of NMU) suggests that at higher dopant

concentrations, the crystal matrix could not accommodatethem.

Surface analysis by SEM: The surface features of thespecimen can be easily examined by scanning electronmicroscopy and the surface defects, growth spirals can beeasily identified. From the micrograph we have observedthat the specimen contains line defects on the surface.These types of behavior deteriorate the quality of thespecimen. No such line defects were observed in the SEMof the 0.5M% of urea added BMZ specimen. This may bedue to the addition of dopants in the host material.

Addition of dopants enhances the growth rate and reducesthe defects as compared to the pure BMZ crystal.

From the growth experiments we observed that NMUadded solutions take more time to form a single crystal andthat yielded a good quality specimen. The smoothness ofthe surface indicates betterment of the specimen ascompared to urea added specimen. The size of the crystalis also reasonably good. It can be seen that the surfaceroughness of the 1M% of urea added BMZ crystal is morecomparable to that 0.5M% urea added BMZ. Similarobservation was also found from the HRXRD. It is due tofaster growth rate and leads to internal boundaries in thespecimen. The surface of the specimen is quite smooth.These observations show that NMU doping enhances the


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surface smoothness as well as the bulk crystallineperfection as no boundaries were observed in thesespecimens but for urea doping, it is not the same.

Assessment of thermal behavior: In order to know thethermal stability of the grown specimen, the powder form

of the relevant specimen is subjected to TG-DTA analysis.Fig. 4a shows the thermal behavior of the 0.5% M of ureaadded BMZ specimen. The melting point of the specimenis shifted to the higher temperature of 176 C which is 4 Cmore than that of the pure specimen.

Fig. 4b shows the 1M% of urea added BMZ specimen. Theendothermic peak gives the melting point at 178 C whichis 6 C more than that of the pure specimen. Afteranalyzing the effect of urea dopants on the BMZ singlecrystal, it is interesting to conclude that the dopantsenhance the thermal stability of the compound. Theadjacent figs. 4d and 4e depict the thermal behavior of0.5M and 1 M% NMU added BMZ. Similar to that of ureaas seen above, NMU also enhances the thermal stability asthe melting point increases up to 8 C at a concentration of1M%.

Measurement of SHG efficiency: The nonlinear opticalconversion efficiency has been carried out using the Kurtzand Perry35 technique at the Indian Institute of Science,Bangalore. It is an important and popular tool to evaluatethe conversion efficiency of a NLO material. A Q-switchedNd:YAG laser operating at the fundamental wavelength1064 nm, generating about 1.3 mJ and pulse of width 8 ns

was used for the present experimental study. The inputlaser beam was passed through an IR reflector and thendirected on the microcrystalline powdered sample of dopedBMZ packed in a capillary tube. The light emitted by thesample was detected by a photodiode detector integratedwith oscilloscope assembly. When the laser was passedthrough these specimens, second harmonic signal of532 nm is generated and it was confirmed by the emissionof green light. To estimate the relative SHG efficiency withrespect to KDP, microcrystalline powder (with sameparticle size as that of the specimen i.e. 120-125 m) ofKDP was used.The observed values are given in table 2.

UV-VIS spectral analysis: The optically polished singlecrystal of pure and doped benzimidazole having thethickness of 1.5 mm was subjected to Shimadzu UV-1061UV-VIS spectrophotometer in the wavelength range of 200800 nm. The recorded spectrum is shown in fig. 5. Fromthe figure, we can understand that the title compound istransmitting the entire incident light and there is noabsorption in the whole visible region of the spectrum. At280 nm a sharp fall of transmittance to zero was observedindicating a single transition in the near UV region ofBenzimidazole. The nearly sharp fall in transmittance at

280 nm suggests nearly similar distribution of energiesamong all molecules of Benzimidazole single crystal33.There are no noticeable changes in the pure and doped

benzimidazole single crystals.

ConclusionDoped benzimidazole single crystals (with urea and NMU)have been grown by conventional solution growth methodand it was found that growth period was varied depending

on the percentage of dopants. Even though NMU addedspecimens were taking more time for the growth but thecrystalline perfection is good in comparison with the ureaadded dopants. Its crystal system, lattice dimensions andfunctional groups were determined by powder-XRD andFTIR analyses. Its crystalline perfection has been evaluatedby HRXRD. Surface micro morphological analysis hasbeen carried out using SEM. Relative SHG efficiencieshave been measured and urea doping in BMZ crystalsleads to enhance the SHG efficiency as compared to that ofNMU.

AcknowledgementThe authors are very much thankful to Director, NPL forhis constant encouragement and support in carrying outthese studies. The authors are also thankful to Dr. S. K.Halder and Dr. S. K. Dhawan forproviding the powderXRD thermal characterization facilities.

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http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1http://prb.aps.org/abstract/PRB/v59/i5/p3276_1


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Fig. 1: Images of the grown single crystal of BMZ (a) (b) (c) (d)

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(b) (c)


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Fig. 2: Powder X-ray diffraction patterns of (a) (b) (c) (d)

Fig. 3: HRXRD patterns for the (a) (b) (c) (d)

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Fig. 4: TG-DTA curves of (a) (b) (c) (d)

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(d)

(b)

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Fig. 5: Absorbance spectra of ??

Table 1Calculated lattice dimensions of pure and doped BMZ crystal

Name of thespecimen

a/ b/ c/ Volume/3

Pure BMZ 6.9393 6.8063 13.5043 637.82100.5M% urea 6.8655 6.9765 13.4647 644.92091M% urea 6.9662 6.8561 13.5303 646.2201

0.5M% NMU 6.9692 6.8477 13.5502 646.65601M% NMU 6.9703 6.8503 13.5603 647.4859

Table 2Determined SHG efficiency of pure and doped BMZ single crystal

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Sample name Relative SHGefficiency/mV

KDP 7

Pure BMZ 32

0.5M% urea 168

1M% urea 188

0.5M% NMU 69

1M% NMU 22


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22. Rejitha K.S. and Mathew S., Investigations on the thermalbehavior of hexaamminenickel(II) sulphate using TG-MS andTR-XRD, Glob J Anal Chem., 1(1), 100-108 (2010)

23. Pajtov M., Ondruov D., Jna E., Mojumdar S.C.,alkov S., Bazylkov T. andGregor M., Spectral and thermalcharacteristics of copper(II) carboxylates with fatty acid chainsand their benzothiazole adducts, J Therm Anal Calorim., 100,769-77 (2010)

24. Mojumdar S.C., Thermoanalytical and IR spectroscopyinvestigation of Mg(II) complexes with heterocyclic ligands, JTherm Anal Calorim, 64, 629-36 (2001)

25. Gonsalves L.R., Mojumdar S.C. and Verenkar V.M.S.,Synthesis and Characterisation of Co0.8Zn0.2Fe2O4Nanoparticles,J Therm Anal Calorim.,104, 869-73 (2011)

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(Received 28thMarch 2013, accepted 10thMay 2013)
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