Synthesis, crystal growth and characterization of a D-π-A type novelorganic nonlinear optical single crystal

Anthoni Praveen Menezes a,n, Jayarama Ab, Seik Weng Ng c,d

a Department of Physics, Mangalore Institute of Technology & Engineering (MITE), Moodabidri 574225, Indiab Department of Physics, Sadguru Swami Nithyananda Institute of Technology (SSNIT), Kanhangad 671315, Indiac Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysiad Chemistry department, Faculty of Science King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia

a r t i c l e i n f o

Article history:Received 18 November 2013Received in revised form19 April 2014Accepted 22 April 2014Communicated by: M. FleckAvailable online 4 May 2014

Keywords:A1. CharacterizationA1. Crystal structureA2. Growth from solutionsB1. Organic compoundsB2. Nonlinear optic materialsB3. Nonlinear optical

a b s t r a c t

A new organic nonlinear optical material 3-(4-chlorophenyl)-1-(pyridin-3-yl) prop-2-en-1-one (CPP) havinghigh second harmonic generation efficiency has been synthesized by a Claisen–Schmidt condensation reactionmethod. Single crystals of CPP were grown by a slow evaporation solution growth technique. The details aboutthe various functional groups present are obtained by FTIR, FT-Raman and 1H NMR spectroscopic studies. Thesingle-crystal X-ray diffraction (XRD) shows that CPP crystallizes in a monoclinic crystal system withnoncentrosymmetric space group P21 and the cell parameters are a¼5.9339(2) Å, b¼4.8881(2) Å,c¼19.7804(8) Å and V¼568.21(4) Å3. The second harmonic generation (SHG) efficiency of the crystal is 3.67times that of Urea. The molecular dipoles aligned in zig-zag head to tail fashion along the crystallographicb-axis result in a net polarization and hence large SHG conversion efficiency. From UV–visible study it is foundthat the crystal is transparent in the entire visible region. The energy band gap of the material is found usingTauc's plot and by a direct method. The molecular origin of SHG efficiency is discussed in detail based on theexperimental results. The position of nitrogen atom in the pyridine ring has considerable effect on the NLOresponse of the chalcone derivatives. The crystal is thermally stable up to 133 1C and can be used for NLOdevice applications.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

At present, the modification of linear and nonlinear opticalproperties of organic materials by the appropriate design of thesesystems at molecular level is at the forefront of research. Materialswith large and fast nonlinear response are of current interestbecause of their increasing use in photonic device applications[1,2]. It has been understood that the second-order molecularnonlinearity can be enhanced by large delocalized π-electronsystems with strong donor and acceptor groups [3–5]. By optimiz-ing the charge transfer across the molecule, the microscopic firsthyperpolarizability (β) of NLO chromophores at molecular levelcan be enhanced. The extent of charge transfer across the moleculecan be increased by placing a strong electron donor and acceptorgroups at the ends of the highly polarizable conjugated bridge. Forefficient second harmonic generation, one requires a highlyhyperpolarizable molecule with zig-zag head to tail parallel

alignment connected through strong hydrogen bond interactions,crystallizing in noncentrosymmetric structures [6].

Chalcones are a class of cross conjugated NLO chromophores thatexhibit high SHG efficiency, good crystallizability and excellent bluelight transmittance [7,8]. The substitution of electron acceptors/donors on either of the phenyl rings of chalcones greatly influencesthe noncentrosymmetric crystal packing structures required forsecond harmonic generation (SHG) [8]. It was suggested by Zhaoet al. [9] that electron donors such as –OCH3, –CH3, –Cl and –Br arethe best donor groups for enhancing SHG in chalcones. It is alsoreported that replacement of phenyl group present near the carbonylgroup by pyridine results in significant NLO response due to thedonor–acceptor arrangement with π conjugated bond [10]. Moreover,a pyridine based chalcone derivative 3-(2, 3-dichlorophenyl)-1-(pyr-idin-2-yl) prop-2-en-1-one (DCPP) [11] was found to exhibit verygood SHG efficiency. DCPP contains two electron donating chlorogroups substituted at the meta and ortho positions of the phenylenegroup. However a single electron donor group substituted at the paraposition of the phenylene group is much more effective in enhancingthe nonlinear behavior [12]. This led us to design a pyridine basedchalcone derivative with single donor group attached at the paraposition of the phenyl group. The present work describes the

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.04.0260022-0248/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 9844910835.E-mail addresses: anthoni@mite.ac.in,

mapraveen.80@gmail.com (A.-n. Praveen Menezes).

Journal of Crystal Growth 402 (2014) 130–137

synthesis of a donor-π-acceptor (D-π-A) type pyridine substitutednew nonlinear optical material, 3-(4-chlorophenyl)-1-(pyridin-3-yl)prop-2-en-1-one (CPP) and its characterization by using variousexperimental methods. The effect of pyridine ring on the SHGefficiency of chalcone derivatives has been discussed on the basis ofexperimental results and the literature.

2. Experimental procedure

2.1. Synthesis

Chalcone derivatives are synthesized by the Claisen–Schmidtcondensation reaction [13]. Claisen–Schmidt condensation reactionis the reaction of a substituted acetyl pyridine with substitutedbenzaldehyde in the presence of an alkali. The compound CPP wassynthesized by stirring a mixture of Analytical Reagent (AR) gradechemicals 3-acetyl pyridine (0.01 mol) and 4-chlorobenzaldehyde(0.01 mol) in methanol (60 ml) without further purification. Sodiumhydroxide (5 ml, 20%) was then added drop wise to the solutionand the mixture is stirred for 2 h. The mixture is stirred bymaintaining the solution within the temperature range of25–30 1C throughout the stirring process. The contents of the flaskwere poured into a beaker containing ice-cold water. The crudeproduct was collected by filtration, washed with excess of waterand dried. The crude product was then purified by successive re-crystallizations from acetone. The synthetic scheme for the titlecompound is shown in Fig. 1.

2.2. Solubility study and crystal growth

The solubility study of the compound in various solvents wascarried out to select a suitable solvent for the growth of the singlecrystals. It is found that the compound is insoluble in water andsparingly soluble in ethanol and methanol. It is also moderatelysoluble in acetone, N, N-dimethyl formamide (DMF). To grow largesingle crystals of CPP, acetone is used due to its moderate solubilityin this solvent. The solubility in this solvent increases almostlinearly (Fig. 2a) with the temperature and hence slow cooling or

slow evaporation at constant temperature could be a bettermethod to grow high quality single crystals of CPP [14].A saturated solution of CPP was prepared in acetone in a 100 mlbeaker at 30 1C and the solution was filtered to remove suspendedparticles. The beaker was covered with a polythene sheet and keptin dust free atmosphere. Tiny single crystals of CPP appeared in aperiod of 5 days. A photograph of the grown crystals of approx-imate dimension 4�0.5�1 mm3 is shown in Fig. 2b. High qualitysingle crystals were selected for further study after examiningthem under microscope. An attempt to grow large size singlecrystals is in progress.

3. Results and discussion

3.1. Molecular design

In order to optimize the first hyperpolarizability ‘β’ in thechalcone chromophores, the title chalcone derivative was designedbased on the design criterion reported in the literature [9,12]. Fromthe past investigations, it is clear that the substitution of electronacceptor/donor groups on phenylene and benzoyl moieties signifi-cantly alters the β values of the chalcones. It is found that anelectron acceptor group at para position of benzoyl moiety and anelectron donor group at the para position of the phenylene moietyresult in molecules with high β values [9]. Ganapayya et al. [10,11]reported two pyridine based chalcones DMPP and DCPP withconsiderable NLO response, where two methoxy groups and twochlorine groups respectively attached at the ortho and meta posi-tions of the phenylene moiety act as electron donors. It is evidentfrom these studies that the substitution of chlorine groups is moreeffective than methoxy groups in enhancing the SHG efficiency. Toget better NLO response, the structure of these pyridine basedchalcones was slightly modified by changing the position of thenitrogen atom in the pyridine ring and attaching a chlorine group atthe para position of the phenylene moiety. The chlorine group actsas an electron donor while pyridine group along with the carbonylgroup acts as an electron acceptor. This gives rise to an extendedπ-conjugated system formed by the carbon–carbon double bondwith donor–acceptor substituent at the terminals. Thus CPP moleculeis designed to possess an effective D-π-A type push–pull structure.

3.2. CHN analysis

The percentage compositions of the elements present in CPPwere confirmed by performing carbon, hydrogen, nitrogen (CHN)analysis on the recrystallized sample using the Elementar Vario ELIII CHN analyzer. The result of the analysis is presented in Table 1.Fig. 1. Synthetic scheme of CPP.

Fig. 2. [a] Solubility of CPP in acetone. [b] Photograph of CPP single crystals.

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137 131

Theoretical values of CHN were found by the molecular formulaC14H10ClNO. The experimental and calculated values of C, H and Nagree with each other, confirming the formation of the product.

3.3. Fourier transform infrared (FT-IR) and FT-Ramanspectral analysis

The FT-IR and FT-Raman spectral analyses of CPP were carriedout to investigate the presence of functional groups and theirvibrational modes. In order to obtain FTIR spectrum, the samplewas prepared by mixing the compound with KBr pellets. Thespectrum was recorded between 500 and 3500 cm�1 using aThermo Nicolet, Avatar 370 FT-IR spectrometer equipped with aKBr beam splitter and a DTGS detector, with a spectral resolutionof 4 cm�1. The FT-Raman spectrum was recorded using a BRUKERRFS 27, Stand-alone FT-Raman Spectrometer, having spectralresolution of 2 cm�1 with an exciting wavelength of 1064 nmfrom an Nd:YAG laser source. FTIR and FT-Raman spectra are

shown in Fig. 3 and Fig. 4 respectively. The assignments ofcharacteristic FT-Raman and FTIR bands are presented in Table 2.The vibrational bands observed in the FTIR and FT-Raman spectrasupport the structure of CPP.

3.4. 1H NMR study

CPP crystals were further analyzed by 1H NMR studies for theconfirmation of the molecular structure. The spectrum is recordedusing a Bruker Avance III FT-NMR spectrometer with TMS asinternal standard and CDCl3 as the solvent. The resulting spectrumis shown in Fig. 5. The observed characteristic peaks are assignedas follows. The NMR spectral study reconfirms the functionalgroups present in the molecule and hence the molecular structureof CPP shown in Fig. 6.

1H NMR spectrum (CDCl3, ppm), δ:9.217–9.221 (1H, d, C5–H),8.797–8.813 (1H, dd, C3–H), 8.272–8.302 (1H, m, C1–H), 7.774–7.813 (1H, d, C7–H, J¼15.6 Hz), 7.579–7.600 (2H, d, C11–H, C13–H,J¼8.4 Hz), 7.438–7.477 (1H, d, C8–H, J¼15.6 Hz), 7.425 (2H, s, C10–H,C14–H), 7.404 (1H, s, C2–H).

3.5. Single crystal X-ray diffraction study

CPP was subjected to single crystal X-ray diffraction (XRD)studies. The data was collected using a Supernova Dual diffract-ometer with an Atlas detector [Agilent (2012)] using mirrormonochromated Cu Kα radiation of wavelength 1.54184 Å. TheX-ray generator was operated at 50 kV and 1 mA using enhancedCu Kα radiation. Crystal data was collected with a scan width of 11.The data reduction and cell refinement were done using CrysAlisPRO [Agilent, 2012]. The crystal structures were solved by directmethods and refined by a full matrix least squares method usingSHELXL-97 [15] in the WinGx package suite [16] with isotropictemperature factors for all the atoms. Carbon-bound H-atomswere placed in calculated positions and included in the refinementin the riding model approximation. 154 parameters were refinedwith 2304 unique reflections which saturated the residuals toR1¼0.0526. For molecular graphics X-seed [Barbour 2001] andMercury [17] software was used. The details of the crystal data andrefinement are given in Table 3. CCDC 992964 contains thesupplementary crystallographic data for this paper. These datacan be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge CrystallographicData Centre, 12, Union Road, Cambridge CB2 1EZ, UK).

From single-crystal XRD studies, it has been found that thecompound crystallizes in the monoclinic crystal system with a spacegroup P21 indicating that the crystal is non-centrosymmetric in nature.This supports the second order nonlinear property of the crystal as this

Table 1CHN analysis data of CPP.

Element Experimental (%) Computed (%)

Carbon 67.44 69.00Hydrogen 4.10 4.14Nitrogen 5.80 5.75

Fig. 3. FTIR spectrum of CPP.

Fig. 4. FT-Raman spectrum of CPP.

Table 2Assignment of vibrational frequencies.

Wave number (cm�1) Assignments

Infraredlines

Raman lines

3090 3064.5 Aromatic C–H stretching vibration1662 1658 CQO stretching vibration1599 1599.7 Aromatic ring vibrations1562, 1492,1404

1561.2, 1484.3,1405.8

CQC aromatic stretching vibrations

1348 1347.3 C–N stretching vibration1235, 1188,1086

1240.8, 1173.6,1085.5

Aromatic C–H in-plane bending vibrations

807 – Aryl C–Cl stretching vibrations725, 699,643

707.4, 639.4 Out of plane C–H aromatic bending orwagging vibrations

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137132

is one of the main criteria for any material to exhibit second harmonicgeneration [18]. Moreover the final R values of the refinement areR1¼0.0526 and wR2¼0.1441 for 2-theta maximum of 150.281. Sincesmall organic molecules commonly refine to better than R¼0.05 [19],there exists a good agreement between observed and calculatedintensities. In addition the low value of R shows the purity of thegrown single crystal [20]. An ORTEP plot of the molecule at 70%probability is shown in Fig. 6.

The molecule of the title heteroaryl chalcone derivative (Fig. 5)exists in E configuration with respect to the C7¼C8 double bond[1.334 Å]; the torsion angle of C6–C7–C8–C9 is 176.48 Å. Themolecule exhibits a planar structure as indicated by the dihedralangle between the pyridine and 4-chlorophenyl rings of 19.531.The carbonyl group is also planar with respect to the adjacentpyridine unit showing a torsional angle of O1–C6–C4–C3 as 5.711.The degree of planarity of molecules correlates with the degree ofπ-conjugation throughout the molecule. This, in turn, increasesthe donor–acceptor interactions across the molecule therebyincreasing the degree of molecular charge transfer and degree ofnonlinearity [21]. The observed high SHG efficiency (3.67 timesurea) of CPP can be attributed to its planar structure as the planarconformations would increase the electron mobility within theπ-conjugated molecular systems resulting in an enhancement inthe NLO response [22]. The chlorine atom is displaced from C9–C14 mean plane by 0.025 Å. Atoms of the propenone bridge (C6,C7, C8 and O1) lie almost on the same plane and the torsion angleis O1–C6–C7–C8 is �10.61. The mean plane through this bridgemakes dihedral angles of 7.621 and 19.721 with the planes ofpyridine and benzene rings respectively.

The hydrogen bond interactions play a vital role in increasingthe SHG in addition to stabilizing the crystal packing. It has beenobserved that, the hydrogen bond interactions (C–H…X whereX¼O, N or π) and aromatic π–π stacking balances the molecular

and supramolecular charge transfer processes, by creating anoncentrosymmetric structure [23]. The crystal structure of CPPis stabilized by weak C–H…O and C–H…N intermolecular inter-actions in the crystalline lattice. The partial packing diagram ofCPP crystal is shown in Fig. 7. This weak intermolecular interactionhelps molecules to pack in a parallel and zig-zag head to tailfashion along the crystallographic b-axis (Fig. 8). Also the relativelyshort distances Cl1…Cl1 (3.441 Å) with symmetry codes �x,�1/2þy,�z indicate the presence of intermolecular Cl…Cl interactionwhich contributes in stabilizing the crystal packing [24].

3.6. Thermal stability

The Differential Scanning Calorimeter (DSC) plot for the crystalCPP is shown in Fig. 9. The analysis is done using a Mettler ToledoDSC 822e thermal analyzer on a sample weight 1.448 mg in thetemperature range 50–350 1C, at a rate of 10 1C/min, in nitrogenatmosphere. The measurement resolution was 0.04 mW at roomtemperature. The sharp peak at 133 1C corresponds to the meltingpoint of the crystal. The sharpness of the peak at 133 1C in theresponse curve indicates the purity and good crystalline nature ofthe sample. The chalcone crystal CPP has significantly betterthermal stability than some of the NLO chalcones that are alreadyreported in the literature [25,26] and also slightly better than urea(130 1C) [12]. The high melting point of the crystal may be havingan advantage in device fabrication. The melting points of someorganic nonlinear optical materials by DSC plot are listed inTable 4.

The TG/DTA/DTG response curve for CPP is shown in Fig. 10.The analysis was done using a PerkinElmer simultaneous TG/DTAanalyzer. 4.067 mg of powder sample of the compound wasanalyzed under the nitrogen atmosphere at a heating rate of10 1C/min in the temperature range from 25 1C to 350 1C. As can

Fig. 5. 1H NMR spectrum of CPP.

Fig. 6. Molecular structure of CPP. Thermal ellipsoids were drawn at 70% probability level.

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be seen from the DTA curve, the material undergoes an irreversibleendothermic transition at 128 1C at which material starts melting.The sharp endothermic peak at 133 1C at which melting termi-nates corresponds to the melting point of the crystal. This value isin good agreement with the endothermic peak observed in the

DSC curve. No weight loss was observed in the TG curve at themelting temperature. Weight loss begins at around 169 1C. A majorweight loss of about 92% is observed in the temperature rangefrom 225 C to 288 1C which may be due to the decomposition ofthe compound into gaseous products. The DTG curve coincideswell with the TGA curve. It is important to note that the crystal hasno phase transitions till it reaches melting point and this enhancesthe temperature range of the crystal for applications in optoelec-tronics. The thermal analysis of CPP indicates that the compoundis thermally stable up to a temperature of 133 1C.

3.7. SHG efficiency

The SHG conversion efficiency of the compound was measuredby the method reported in the literature [32]. In this method theconversion efficiency of a given sample taken in powder form iscompared with that of a standard material such as urea orpotassium dihydrogen phosphate (KDP). The CPP and urea werepulverized to have identical range of grain sizes (90–125 μm) andwere used for relative SHG efficiency measurement. A Q-switched8-ns Nd:YAG laser beam of wavelength 1064 nm and 8 mmdiameter with 10 Hz repetition rate and the beam energy of6.3 mJ/pulse is used as source of light in this experiment. TheCPP and urea samples were tightly packed in separate micro-capillary of uniform bore size and exposed to fundamental laserradiation. The second harmonic (SH) output from the sample wasmonochromated to collect the 532 nm component while thefundamental wavelength was eliminated. The SH radiation gener-ated by the micro crystals was focused by a lens which is detectedby a photo multiplier tube. The SH signal originated from CPP andreference samples was averaged over 100 laser pulses. The relativeSHG efficiency of CPP was found to be 3.67 times greater than thatof urea.

Table 3Crystal data and structure refinement of CPP crystal.

Molecular formula C14H10ClNO

Crystal size (mm3) 0.10�0.20�0.30Formula weight (g mol�1) 243.68Crystal shape and color Prism, light yellowCrystal system MonoclinicSpace group P 21Unit cell parameters a¼5.9339(2) Å

b¼4.8881(2) Åc¼19.7804(8) Å

Cell angles (deg) α¼90.00, β¼97.962(3), γ¼90.00Z¼2

Volume 568.21(4) Å3

Radiation Cu Kα radiationRadiation wavelength 1.54184 ÅTemperature T¼100.0(2) KDensity 1.424Absorption coefficient (μ/mm�1) 2.810F (000) 252θ range for data collection (deg) 4.51–74.97hmin,max, kmin,max, lmin,max (�7 5), (�5 6), (�23 24)Refinement method Full-matrix least-squares on F2

No. of measured reflections 4513Measured no. of unique reflections 2306No. of reflections with I42s (I) 2295No. of parameters 154Goodness-of-fit on F2 1.054No. of restraints 1R_all, R_obs. 0.0531, 0.0526wR2_all, wR2_obs. 0.1445, 0.1441Δρ min, max/e Å�3 �0.409, 0.764

Fig. 7. Packing diagram of CPP veiwed down the b-axis.

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137134

3.8. UV–visible spectral studies

A good optical transmittance is desirable in a NLO crystalbecause the absorptions, if any, near the fundamental or theharmonics will lead to the loss of conversion efficiency in thosewavelengths. UV–vis–NIR absorption spectrum of the crystal wasrecorded using a Varian, Cary 5000 UV–vis spectrophotometer inthe wavelength range of 200–1100 nm. A solution of CPP inacetone was placed in a 1 cm thick cuvette for measurement.The recorded spectrum is shown in Fig. 11. As can be seen from thefigure, the crystal has less absorption in the visible region of thespectrum. The peak absorption observed in the curve can beassigned to the n-πn transition and may be attributed to theexcitation of the aromatic ring and CQO group of the molecule.From Fig. 11 it is clear that maximum absorption is centered at417 nm and this wavelength corresponds to the cut-off wave-length of the crystal [33] beyond which there are no significantabsorption peaks till the IR region. The transparency in the blue

region of CPP is better than 3, 4-dimethoxy-40-bromo-chalcone(DMBC)[25], 3-methyl-4-methoxy-40-nitrostilbene (MMONS)[29],and m-nitro aniline (m-NA)[31] which have lower optical cutoffwavelengths of 470, 520 and 515 nm respectively. CPP has a widertransparency range extending into most of the visible region andhence it can be used for device applications in its region oftransmittance at room temperature [34].

In crystalline materials, the intrinsic loss mechanisms and theore-tical transmittance achievable are defined by the region of transpar-ency to the electromagnetic radiation. The transparent spectral regionis defined by the electronic transitions across the band gap and bylattice vibrations. It is desirable to have low absorption coefficient forNLO materials. The larger the absorption coefficient, the more theenergy dissipation in materials which causes undesirable thermaleffects. The dependence of optical absorption coefficient (α) with thephoton energy helps to study the type of transition of electrons and

Fig. 8. Crystal packing diagram of CPP veiwed down the a-axis (for clarity hydrogen atoms are ignored). Arrow marks indicate the orientations of molecular dipoles.

Fig. 9. DSC thermograph of CPP.

Table 4DSC data of some organic NLO materials.

Crystal Melting point by DSC plot (1C)

MPP [27] 81DMMC [28] 94MMONS [29] 110CTDMP [30] 111m-NA [31] 112DMBC [25] 121PDBA [26] 131CPPa 133

MPP-3-(4-Methoxyphenyl)-1-(pyridin-2-yl) prop-2-en-1-one.DMMC-3-(3, 4-dimethoxyphenyl)-1-(4-methoxyphenyl) prop-2-en-1-one.m-NA-m-nitroaniline; MMONS-3-methyl-4-methoxy-4ʹ-nitrostilbene.DMBC-3, 4-dimethoxy-4ʹ-bromo-chalcone.CTDMP-1-(5-chlorothiophen-2-yl)-3-(2, 3-dimethoxyphenyl) prop-2-en-1-one.PDBA-p-methoxy dibenzylideneacetone.

a Present work.

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137 135

the band structure. The direct/indirect band gap transition, scatteringand background absorption make contributions to the absorptioncoefficient. The optical absorption coefficient (α) was calculated fromthe transmittance (T) using the relation

α¼ 2:303d

log 10ð1=TÞ

where ‘d’ is the thickness of the crystal and log 10ð1=TÞ is defined asthe absorbance (A) of the material. The dependence of absorptioncoefficient in terms of direct and indirect transitions is given by therelation (Tauc's expression) [35] αhν¼ Aðhν�EgÞPwhere ‘A’ is theband edge constant that depends on the transition probability, Eg isthe optical band gap, and ‘p’ is the index that characterizes the opticalabsorption process; theoretically it is equal to ½ and 2 for alloweddirect and indirect transitions respectively. The indirect band gap ofthe crystal can be obtained by plotting (αhυ)1/2 versus energy anddirect band gap can be obtained by plotting (αhυ)2 versus energy. Theplots are found to be a straight line in the high absorption region(Fig. 12a and b). The direct optical band gap is obtained as the interceptof linear fitting line of (αhυ)2 and the zero absorption coefficient lineand the value obtained is 3.045 eV (Fig. 12a inset). The indirect bandgap obtained from Fig. 12b is 2.88 eV. These values are very close tothe energy band gap value obtained using the relation, Eg¼hc/λC(2.974 eV) where λC is the cutoff wavelength of the crystal.

3.9. Structure-NLO property relationship

The title compound CPP has a D-π-A type push–pull structureand the structure is shown in Fig. 6. In CPP, the pyridine moietyalong with carbonyl group (–CQO–) acts as an electron acceptorand chloro phenyl group acts as an electron donor. This donor–acceptor groups are connected by carbon–carbon double bondwhich forms the π-conjugation path for the effective intermole-cular charge transfer.

The β value of CPP is compared with some of the reportedchalcone derivatives and is tabulated in Table 5. It is clear fromthese results that the substitution of two chlorine groups at theortho and meta position of the phenylene is better compared tothe two methoxy groups. This may be due to the higher electrondonating ability of the chlorine group than that of methoxy group.It is reported in our earlier communication [36] that a change inthe position of the nitrogen in the pyridine ring and an electrondonating –N(CH3)2 group substituted at the phenylene ringresulted in a centrosymmetric crystal structure and hence noSHG efficiency. In the present case a single electron donatingchlorine group is attached at para position of the phenylenemoiety and the observed SHG efficiency is 3.67 times that of urea.The substitution of a strong electron donating chlorine group atthe para position of the phenylene group leads to a push–pulleffect that results in an easy flow of charges towards the electronaccepting carbonyl group [12]. However, a pyridine based chalcone3-(4-Methoxyphenyl)-1-(pyridin-2-yl) prop-2-en-1-one, MPP [27]showed a SHG efficiency of 4.7 times that of urea. In MPP, an

Fig. 10. TGA/DTA/DTG thermographs of CPP crystal.

Fig. 11. Optical absorption spectrum of CPP.

Fig. 12. Tauc's plots of CPP.

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137136

electron donating methoxy group is attached at the para positionof the phenelene moiety. Though electron donating chlorine groupis better in enhancing the NLO response than methoxy group, theobserved high SHG efficiency of MPP compared to CPP can beattributed to the position of nitrogen atom in the pyridine ring. InMPP the nitrogen is attached to the ortho position whereas in CPPit is attached to the meta position. The ortho substituted nitrogenwill increase the charge transfer across the molecule and hencethe optical nonlinearity.

Moreover, in some of the organic crystals such as PDBA [26],MMONS [29], MPP [27] and a chalcone co-crystal [12] a zig-zaghead to tail and parallel alignment of the molecular dipoles wasobserved which leads to the summation of the individual mole-cular dipole contribution resulting in a net polarization in thecrystal. This kind of zig-zag head to tail parallel dipole alignmentresulted in very high SHG conversion efficiency in these crystals.The packing diagram of CPP viewed along the a-axis is shown inFig. 8. It is clear from Fig. 8 that the molecular dipoles of CPP arestacked in a zig-zag head-to-tail fashion along b-axis in whichindividual dipoles add up to establish a net polarization in thecrystal resulting in high SHG efficiency of 3.67 times that of urea.Further, the C–H…O and C–H…N intermolecular interactionsobserved in crystal lattice of CPP are expected to play a significantrole in the excited state by extending the molecular chargetransfer into supramolecular realm. The CPP crystal has goodthermal stability, optical transparency and high SHG efficiencywhich make this material attractive for SHG applications.

4. Conclusions

Single crystals of the chalcone CPP were grown by slowevaporation of an acetone solution at ambient temperature.FT-IR, FT-Raman and 1H NMR spectral analysis confirms thefunctional groups of the grown crystal. The UV–vis–NIR spectrumreveals that the absorption takes place in the UV region and thecrystal is transparent in the visible region with lower optical cutoffwavelength being 430 nm. The optical band gap has been deter-mined by Tauc's plot and the value is in good agreement with thevalue obtained by the direct method. Single-crystal XRD studieswere carried out to determine the space group and the cellparameters. The SHG conversion efficiency of the crystal is foundto be 3.67 times that of urea. The high SHG conversion efficiency ofCPP is attributed to the zigzag head to tail alignment of moleculardipoles stacked along the b-axis. The influence of the functionalgroups on the second order nonlinearity has been discussed. Thechlorine group attached at the para position enhances the NLOactivity of the crystal. In addition, the position of nitrogen atom inthe pyridine ring plays significant role on the NLO response of thechalcone derivatives. The thermal analysis reveals that the crystalretains its texture up to 133 1C. The good transparency, good

thermal stability and high SHG efficiency of CPP indicate that itis a promising candidate for NLO applications such as frequencydoubling.

Acknowledgments

The authors express their grateful thanks to University ofMalaya for providing the single crystal X-ray diffraction facilityand Dr. P.K. Das, Department of Inorganic and Physical Chemistry,IISC Bangalore for SHG efficiency measurement. The authors alsoacknowledge Sophisticated Analytical Instrument Facility (SAIF)-STIC, Cochin University, SAIF, IIT Bombay and SAIF, IIT Madras forproviding experimental facilities.

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Table 5Comparison of SHG efficiencies of various NLO crystals.

Crystal SHG (�Urea)

DMPP [10] WeakDCPP [11] 1DMAPP [36] 0CPPa 3.67

DMPP, 3-(2, 3-dimethoxyphenyl)-1-(pyridin-2-yl) prop-2-en-1-one.DCPP, 3-(2, 3-dichlorophenyl)-1-(pyridin-2-yl) prop-2-en-1-one.DMAPP, 3-[4-(Dimethylamino) phenyl]-1-(3-pyridyl) prop-2-en-1-one.

a Present work.

A. Praveen Menezes et al. / Journal of Crystal Growth 402 (2014) 130–137 137