Crystal structure and characterization of a novel organic optical crystal: 4-chloro-3-nitrobenzophenone

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Crystal structure and characterization of a novel organic optical crystal:2-Aminopyridinium trichloroacetate

P.V. Dhanaraj a, N.P. Rajesh a,*, G. Vinitha b, G. Bhagavannarayana c

a Centre for Crystal Growth, SSN College of Engineering, Kalavakkam-603 110, Indiab Department of Physics, Crescent Engineering College, Chennai-600 048, Indiac C.G.C. Section, National Physical Laboratory, New Delhi-110 012, India

1. Introduction

Extensive studies have been made on the synthesis and crystalgrowth of nonlinear optical (NLO) materials over the past decadebecause of their potential applications in the field of telecommu-nications, optical signal processing, and optical switching. Organicmolecules containing conjugate systems have some advantagesover inorganic materials because of the possibility of highlyenhanced electronic nonlinear optical polarization responses. Thebasic structure of organic NLO materials is based on the p bondsystem; due to the overlap of p orbitals, delocalization of electroniccharge distribution leads to a high mobility of the electron density.Functionalization of both ends of the p bond system withappropriate electron donor and acceptor groups can enhancethe asymmetric electronic distribution in either or both groundand excited states, leading to an increased optical nonlinearity.While the engineering for enhancing second order NLO efficiency is

relatively well understood, the need for efficient third ordermolecules and materials still exists. In particular, the strongdelocalization of p electrons in the organic backbone determines ahigh molecular polarizability and thus third order opticalnonlinearity. In general, large hyperpolarizabilities are the resultof an optimum combination of various factors such as pdelocalization length, donor–acceptor groups, dimensionalityand orientation for a given molecular structure [1–3].

Trichloroacetic acid forms crystalline complexes with aminesand amino acids [4,5]. It is interesting to study the association oftrichloroacetic acid with heterocyclic molecule 2-aminopyridinefrom the crystal engineering viewpoint. We have successfullysynthesized the crystalline salt 2-aminopyridinium trichloroace-tate (2APTC) and crystals were grown using slow evaporationsolution growth technique for the first time. In the complexformation, protonation of pyridine ring nitrogen facilitateshydrogen bonding interaction between trichloroacetate and 2-aminopyridine. In this paper, we report material synthesis,solubility, crystal growth, structural, optical, thermal, dielectricaland mechanical studies and third order nonlinear optical proper-ties of 2APTC.

Materials Research Bulletin 46 (2011) 726–731

A R T I C L E I N F O

Article history:

Received 8 June 2010

Received in revised form 5 January 2011

Accepted 20 January 2011

Available online 26 January 2011

PACS:

61.66.Hq

61.50.�f

81.10.Dn

81.70.Pg

Keywords:

A. Optical materials

A. Organic compounds

B. Crystal growth

C. X-ray diffraction

D. Crystal structure

A B S T R A C T

2-Aminopyridinium trichloroacetate, a novel organic optical material has been synthesized and crystals

were grown from aqueous solution employing the technique of controlled evaporation. 2-

Aminopyridinium trichloroacetate crystallizes in monoclinic system with space group P21/c and the

lattice parameters are a = 8.598(5) A, b = 11.336(2) A, c = 11.023(2) A, b = 102.83(1)8 and volu-

me = 1047.5(3) A3. High-resolution X-ray diffraction measurements were performed to analyze the

structural perfection of the grown crystals. Thermal analysis shows a sharp endothermic peak at 124 8Cdue to melting reaction of 2-aminopyridinium trichloroacetate. UV–vis–NIR studies reveal that 2-

aminopyridinium trichloroacetate has UV cutoff wavelength at 354 nm. Dielectric studies show that

dielectric constant and dielectric loss decreases with increasing frequency and finally it becomes almost

a constant at higher frequencies for all temperatures. The negative nonlinear optical parameters of 2-

aminopyridinium trichloroacetate were derived by the Z-scan technique.

� 2011 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +91 044 27474844; fax: +91 044 27474844.

E-mail addresses: [email protected], [email protected] (N.P. Rajesh).

Contents lists available at ScienceDirect

Materials Research Bulletin

journal homepage: www.e lsev ier .com/ locate /mat resbu

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.01.013


2. Experimental

2.1. Material synthesis

Single crystals of 2APTC were grown from saturated aqueoussolution containing 2-aminopyridine (SRL, India) and trichloroa-cetic acid (Merck) in equimolar ratio. After preparing saturatedsolution of 2-aminopyridine, the proportionate amount oftrichloroacetic acid was added slowly with slightly heating thesolution for bringing a homogeneous mixture. Precipitate ofcrystalline substance was obtained with continuous stirring of thesolution of pH value 6.1. The purity of synthesized compound wasimproved by successive recrystallization process and filtration.

2.2. Solubility and metastable zone width

The solubility of 2APTC at 30 8C was determined by dissolvingthe recrystallized salt in 100 ml Millipore water taken in anairtight container. The solution was stirred continuously for 6 h toachieve stabilization using a magnetic stirrer. After attaining thesaturation the concentration of the solute was estimatedgravimetrically. The studies were carried out in a constanttemperature water bath of cryostat facility with an accuracy of�0.01 8C. For the determination of metastable zone width, saturatedsolution of recrystallized salt was prepared in accordance with thepresently determined solubility data at the same saturationtemperature. Polythermal method [6] was adopted for the determi-nation of metastable zone width. The solubility and nucleation weredetermined for different saturation temperatures 35, 40, 45 and50 8C and the variation in solubility along with the metastable zonewidth at these temperatures are shown in Fig. 1.

2.3. Crystal growth

The saturated solution of recrystallized salt of 2APTC at roomtemperature of 35 8C was filtered and transferred to crystal growthvessels. Crystallization was allowed to take place by slowevaporation at room temperature. The single crystals of 2APTCwere obtained after 15 days. The photograph of as-grown crystalsof 2APTC is shown in Fig. 2(a). The indexed morphology of 2APTCcrystal is shown in Fig. 2(b). The crystal has eight developed faces,out of which (1 0 0) and ð1 0 0Þ are prominent faces.

3. Results and discussion

3.1. X-ray crystal structure determination

The unit cell parameters and the crystal structure weredetermined from single crystal X-ray diffraction data obtainedwith a Bruker SMART APEXyy CCD detector diffractometer(graphite monochromated, MoKa = 0.71073 A). The unit cellparameters of the 2APTC crystal were measured at 293 K. Thedata were integrated using Bruker SAINT; corrections for absorp-tion and decay were applied using Bruker SAINT. The crystalstructure was solved by a direct method with the SHELXS-97program and refined by the SHELXL97 program [7]. The ORTEPdrawing was performed with the ORTEP3 program [8].

Single crystal X-ray diffraction analysis reveals that 2APTCbelongs to monoclinic crystal system with space group P21/c. Thedetermined lattice parameters are a = 8.598(5) A, b = 11.336(2) A,c = 11.023(2) A, b = 102.83(1)8 and volume = 1047.5(3) A3. The2APTC crystal data, experimental conditions and structuralrefinement parameters are presented in Table 1. The packingdiagram of 2APTC is shown in Fig. 3. The hydrogen bonds, values ofbond lengths and bond angles present in the crystal structure arelisted in Table 2. Pyridine nitrogen is more basic than the aminogroup at its ortho position. The lower basic property of NH2 is dueto resonance interaction between NH2 and the pyridine ring.Because of this difference in basicity, the pyridine ring nitrogen canbe immediately protonated in preference to the amino group whentrichloroacetic acid is added. This protonation facilitates hydrogenbonding interaction between trichloroacetate and protonated 2-aminopyridine. Though the pyridine ring nitrogen proton isbonded to trichloroacetate, the positive charge on nitrogen isimparted to bring closer an another trichloroacetate hydrogenbonded to a separate protonated pyridine as illustrated in thepacking diagram.

CCDC no. 774669 contains the supplementary crystallographicdata for this paper. These data can be obtained free of charge viawww.ccdc.cam.ac.uk/data-request/cif, by e-mailing [email protected] or by contacting the Cambridge CrystallographicData Centre, 12 Union Road, Cambridge CB21 EZ, U.K.; Fax: +441223 336033.

Fig. 1. Solubility and metastability curves.

Fig. 2. (a) As-grown single crystals of 2APTC. (b) Morphology diagram of 2APTC crystal.

P.V. Dhanaraj et al. / Materials Research Bulletin 46 (2011) 726–731 727


3.2. High-resolution X-ray diffraction analysis

The crystalline perfection of the grown 2APTC single crystalswas characterized by high-resolution X-ray diffraction (HRXRD) byemploying a multicrystal X-ray diffractometer with MoKa1

radiation. Fig. 4 shows the high-resolution diffraction curve (DC)recorded for a typical 2APTC single crystal using (1 0 2) diffractingplanes in symmetrical Bragg geometry. As seen in the figure, the DCcontains a single peak and indicates that the specimen is free fromstructural grain boundaries. The FWHM (full width at halfmaximum) of the curve is 20 arc s which is only slightly morethan that expected for an ideally perfect crystal from the planewave theory of dynamical X-ray diffraction [9], but close to thatexpected for a nearly perfect real life crystal.

It is interesting to see the asymmetry of the DC. For a particularangular deviation (Du) of glancing angle with respect to the peakposition, the scattered intensity is much more in the positivedirection in comparison to that of the negative direction. Thisfeature clearly indicates that the crystal contains predominantlyinterstitial type of defects than that of vacancy defects. This can bewell understood by the fact that due to interstitial defects (selfinterstitials or impurities at interstitial sites), which may be due to

fast growth and/or impurities present in the raw material, thelattice around these defects undergo compressive stress [10] andthe lattice parameter d (interplanar spacing) decreases and leads togive more scattered (also known as diffuse X-ray scattering)intensity at slightly higher Bragg angles (uB) as d and sin uB areinversely proportional to each other in the Bragg equation(2dsin uB = nl; n and l being the order of reflection andwavelength respectively which are fixed). However, the single

Table 1Crystal data and structure refinement for 2APTC.

Empirical formula C7H7Cl3N2O2

Formula weight 257.50

Temperature 293(2) K

Wavelength 0.71073 A

Crystal system, space group Monoclinic, P21/c

Unit cell dimensions a = 8.598(5) A, a= 908b = 11.336(2) A, b= 102.83(1)8c = 11.023(2) A, g= 908

Volume 1047.5(3) A3

Z 4

Calculated density 1.633 Mg/m3

Absorption coefficient 0.849/mm

F(0 0 0) 520

Crystal size 0.2 mm�0.19 mm�0.15 mm

Theta range for data collection 2.43–28.478Index ranges �11(h(11, �15(k(15, �14( l(14

Reflections collected/unique 8713/2552 [R(int) = 0.0648]

Completeness to u= 28.47 96.2%

Absorption correction None

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 2552/0/155

Goodness-of-fit on F2 1.104

Final R indices [I>2s(I)] R1 = 0.0854, wR2 = 0.2494

R indices (all data) R1 = 0.1180, wR2 = 0.2736

Largest diff. peak and hole 1.098 and �0.570 e A�3

Fig. 3. Molecular packing diagram of 2APTC crystal viewed down origin.

Table 2Hydrogen bonds in 2APTC molecule (A and deg.).

D–H� � �A d(D–H) d(H� � �A) d(D� � �A) <(DHA)

N(1)–H(6)� � �O(2) 0.8153 1.9925 2.7979 169.39

N(2)–H(7)� � �O(1) 0.6923 2.1222 2.8085 171.36

N(2)–H(8)� � �O(2) #1 0.9128 2.1315 2.9654 151.40

C(5)–H(5)� � �O(1) #2 0.9550 2.3983 3.3016 157.63

Bond lengths (A) in 2APTC

molecule

Bond angles (deg.) in 2APTC mole-

cule

C(1)–N(2) 1.314(8) N(2)–C(1)–N(1) 119.8(6)

C(1)–N(1) 1.350(7) N(2)–C(1)–C(2) 123.8(6)

C(1)–C(2) 1.422(8) N(1)–C(1)–C(2) 116.4(5)

C(2)–C(3) 1.35(1) C(3)–C(2)–C(1) 120.1(6)

C(2)–H(2) 0.93(7) C(3)–C(2)–H(2) 126(4)

C(3)–C(4) 1.40(1) C(1)–C(2)–H(2) 113(4)

C(3)–H(3) 1.05(8) C(2)–C(3)–C(4) 121.1(6)

C(4)–C(5) 1.353(9) C(2)–C(3)–H(3) 118(4)

C(4)–H(4) 0.93(7) C(4)–C(3)–H(3) 121(4)

C(5)–N(1) 1.355(8) C(5)–C(4)–C(3) 118.1(6)

C(5)–H(5) 0.96(7) C(5)–C(4)–H(4) 118(4)

C(6)–C(7) 1.571(7) C(3)–C(4)–H(4) 124(4)

C(6)–Cl(3) 1.754(5) C(4)–C(5)–N(1) 120.6(6)

C(6)–Cl(2) 1.768(6) C(4)–C(5)–H(5) 122(4)

C(6)–Cl(1) 1.773(6) N(1)–C(5)–H(5) 118(4)

C(7)–O(1) 1.221(7) C(7)–C(6)–Cl(3) 112.6(4)

C(7)–O(2) 1.240(7) C(7)–C(6)–Cl(2) 111.4(4)

N(1)–H(6) 0.82(7) Cl(3)–C(6)–Cl(2) 107.9(3)

N(2)–H(8) 0.91(8) C(7)–C(6)–Cl(1) 107.6(4)

N(2)–H(7) 0.69(9) Cl(3)–C(6)–Cl(1) 109.4(3)

Cl(2)–C(6)–Cl(1) 107.9(3)

O(1)–C(7)–O(2) 129.7(5)

O(1)–C(7)–C(6) 115.3(5)

O(2)–C(7)–C(6) 115.1(5)

C(1)–N(1)–C(5) 123.6(5)

C(1)–N(1)–H(6) 115(5)

C(5)–N(1)–H(6) 121(5)

C(1)–N(2)–H(8) 126(5)

C(1)–N(2)–H(7) 116(7)

H(8)–N(2)–H(7) 118(8)

Symmetry transformations used to generate equivalent atoms:#1 1� x,1/2 + y,1/

2� z; #2 1�x,�1/2 + y,1/2�z.

Fig. 4. High-resolution X-ray diffraction curve recorded for a typical 2APTC single

crystal specimen using (1 0 2) diffracting planes.

P.V. Dhanaraj et al. / Materials Research Bulletin 46 (2011) 726–731728


diffraction curve with reasonably low FWHM indicates that thecrystalline perfection is fairly good. The density of such interstialdefects is however very meager and in almost all real crystalsincluding nature gifted crystals, such defects are commonlyobserved and are many times unavoidable due to thermodynami-cal conditions and hardly affect the device performance. It is worthto mention here that the observed scattering due to interstitialdefects is of short order nature as the strain due to such minutedefects is limited to the very defect core and the long order couldnot be expected and hence the change in the lattice parameter ofthe crystal is not possible.

3.3. FTIR spectral analysis

The FTIR spectrum of 2APTC was recorded using Perkin-ElmerFTIR spectrum RXI spectrometer by KBr pellet technique in therange 400–4000 cm�1. The observed vibrational frequencies andtheir tentative assignments are given in Table 3. The strong band at824 cm�1 is due to antisymmetric stretching type vibrations ofCCl3 group. The similar band was observed at 839 cm�1 for thesolid complex betaine-trichloroacetic acid [5] and at 829 cm�1 formelaminium trichloroacetate crystals [11]. The strong peakobserved at 734 cm�1 is corresponds to the in-plane deformationmodes of COO� group derived from trichloroacetic ions. Theanalogous strong infrared band is present at 745 cm�1 in thespectrum of melaminium trichloroacetate [11].

3.4. Thermal analysis

Differential thermal analysis (DTA) and thermogravimetricanalysis (TGA) of 2APTC were carried out simultaneously byemploying TA instrument Model Q600 SDT thermal analyzer. Thesharp endothermic peak at 124 8C in DTA trace (Fig. 5) may be dueto melting reaction of 2APTC. The sharpness of the endothermicpeak shows the good degree of crystallinity and purity of thesample [12]. Since a weight loss is observed simultaneously withthe endothermic reaction, there is not only a melting reaction butalso the evaporation. This phenomenon should be the sublimationof the compound.

3.5. Dielectric studies

The dielectric constant (er) and dielectric loss (tan d) weremeasured using the conventional parallel plate capacitor methodwith the frequency range 100 Hz to 1 MHz using the Agilent 4284ALCR meter at various temperatures 313–353 K. A good qualitycrystal of 2 mm thickness was electroded on either side withgraphite coating to make it behave like a parallel plate capacitor.Fig. 6 shows the plot of dielectric constant versus appliedfrequency at different temperatures. The dielectric constantdecreases with increasing frequency and finally it becomes almosta constant at higher frequencies for all temperatures. It is also

indicates that the value of dielectric constant increases withincrease in temperature particularly at low frequencies. Themagnitude of dielectric constant depends on the degree ofpolarization of charge displacement in crystals. Also moderatelyhigher dielectric constant at lower frequencies is due to thecontribution of electronic, ionic, dipolar and space chargepolarizations, which might be active at low frequencies andhigher temperatures. And the decrease in dielectric constant athigh frequencies is attributed due to the absence of space chargepolarization near the grain boundary interfaces [13]. The sametrend is observed in the case of variation of dielectric loss withfrequency at different temperatures. Lowering the value ofdielectric constant of interlayer dielectric (ILD) decreases the RCdelay, lowers the power consumption and reduces the crosstalkbetween nearby interconnects [14]. Also the materials with quitelow dielectric constant lead to a small RC constant, thus permittinga higher bandwidth in the range of 1012 Hz for light modulation.The cycle time of the device or rate at which it can transfer imagesis limited by heat dissipation in the materials. Thus materials withlow dielectric constant have considerable advantages in thisregard.

3.6. Optical studies

Linear optical properties of 2APTC crystals were studied using aLambda 35 spectrophotometer in the region 190–1100 nm and therecorded spectrum is shown in Fig. 7(a). The characteristicabsorption band is observed at 354 nm leading to electronic

Table 3FTIR spectral data of 2APTC.

3353 N–H stretching

3073 C–H stretching

1669 Stretching mode of carboxylate anion

1637 C55C stretching

1484 C55N stretching

1434, 1254 C–H in-plane bending

1374, 1326 C–NH2 stretching

1160, 990, 620 C–H out-of-plane bending

1059 C–N stretching in the ring

940 CCC in-plane bending

680, 430 COO�

Fig. 5. TGA–DTA curves of 2APTC.

Fig. 6. Plot of dielectric constant versus log frequency.



excitation, and there is no absorption band between 354 and1100 nm; hence the crystal is expected to be transparent betweenthese two wavelengths.

3.7. Nonlinear optical studies

Z-scan technique [15,16], based on the spatial distortion of alaser beam, passed through a NLO material, is widely used inmaterial characterization because of their simplicity, highsensitivity and well-elaborated theory. A CW He–Ne laser ofwavelength 632.8 nm (Melles Griot 0.5-LHP-928) is used as theexcitation source for the Z-scan technique. The laser of Gaussianbeam profile was focused by a 3.5 cm focal length convex lens toproduce a beam thickness 25.1 mm and the Rayleigh length to be3.125 mm. An optical cell of 1 mm wide containing the sample istranslated across the focal region along the axial direction that isthe direction of the propagation laser beam. The transmission ofthe beam through an aperture placed in the far field is measuredusing photo detector fed to the digital power meter (Field masterGs-coherent). For an open aperture Z-scan, a lens to collect theentire laser beam transmitted through the sample replaced theaperture.

Z-scan data for the closed aperture set up for 2APTC at atransmission of about 60% shows a peak followed by a valley-normalized transmittance which is the signature for negativenonlinearity [17], i.e. self-defocusing. Self-defocusing effect is dueto local variation of refractive index with temperature. Z-scan datafor the open aperture (S = 1) set up of the same specimen showsenhanced transmission near the focus, which indicates thesaturation of absorption at high intensity. Absorption saturationin the sample enhances the peak and decreases the valley in theclosed aperture Z-scan thus distorting the symmetry of the Z-scancurve about Z = 0. The defocusing effect shown in spectrum ofclosed aperture is attributed to a thermal nonlinearity resultingfrom absorption of radiation at 632.8 nm. Localized absorption of atightly focused beam propagating through an absorbing complexmedium produces a spatial distribution of temperature in the2APTC and consequently, a spatial variation of the refractive index,that acts as a thermal lens resulting in phase distortion of thepropagating beam.

In general, for the cases of closed and open aperture, themeasurements of the normalized transmittance versus sampleposition allows the determination of both nonlinear refractiveindex (n2) and the saturation absorption coefficient (b). Here, sincethe closed aperture transmittance is affected by both nonlinearrefraction and absorption, the determination of n2 is lessstraightforward from the closed aperture scans. So a method[16] was employed to obtain purely effective n2 by the division ofclosed aperture transmittance by the corresponding open aperture

scans. The ratio of closed aperture to open aperture scans is shownin Fig. 7(b). The data obtained in this way reflects purely the effectsof nonlinear refraction. The experimental measurements of n2 andb allow one to determine the third order nonlinear opticalsusceptibility x(3). The determined values of nonlinear parametersn2, b and x(3) of 2APTC are �4.41 � 10�8 cm2/W,�11.63 � 10�4 cm/W and �2.028 � 10�6 esu respectively. It indi-cates that 2APTC exhibits negative nonlinear optical properties.Both b and n2 contribute to the third order nonlinearity of thesample. It is shown that the nonlinear absorption can be attributedto saturation absorption process, while the nonlinear refractionleads to self-defocussing in the compound.

3.8. Microhardness studies

Hardness studies on (1 0 0) plane of 2APTC crystals wereconducted using Leitz Wetzlar microhardness tester fitted withVickers diamond pyramidal indenter for the loads in the range 10–100 g. From Fig. 8, it is observed that Vickers hardness (Hv) valueincreases initially with load up to 75 g and beyond which reaches asteady value of 33–34 kg/mm2. This type of load variation ofhardness is termed as reverse indentation size effect (reverse ISE).At low loads, the indenter penetrates only the top surface layersgenerating dislocations, which results in the increase of hardnessin this region. The load independence of hardness at higher loadscan be attributed to the mutual interaction or rearrangement ofdislocations [18]. By Mayer’s law, the value of Mayer’s index (n)estimated to be 2.7, which indicates that 2APTC crystal belongs tosoft material category.

Fig. 7. (a) Transmittance spectrum of 2APTC crystal. (b) Z-scan spectrum of 2APTC for the ratio of closed to open aperture.

Fig. 8. Plot of Vickers microhardness number versus load.

P.V. Dhanaraj et al. / Materials Research Bulletin 46 (2011) 726–731730


4. Conclusions

Single crystals of 2APTC were grown by solution growthtechnique for first time and its solubility and metastable zonewidth were determined. X-ray diffraction analysis reveals themolecular arrangements and the formation of hydrogen bonds inthe crystal. HRXRD analysis indicates that crystalline perfection of2APTC crystal is fairly good. Fourier transform infrared spectralanalysis was carried out to identify the functional groups in2APTC. The low values of dielectric constant and dielectric loss of2APTC at higher frequencies revealed from dielectric measure-ments. The optical studies show that 2APTC crystal hastransmission of about 60%. From the Z-scan technique, thedetermined nonlinear refractive index (n2) and nonlinearabsorption coefficient (b) of 2APTC are �4.41 � 10�8 cm2/Wand �11.63 � 10�4 cm/W.

Acknowledgements

This work, supported by Department of Science andTechnology, Government of India under the grant of projectref-SR/FTP/PS-20/2005, is hereby gratefully acknowledged. P.V.Dhanaraj is grateful to Council of Scientific and IndustrialResearch, Government of India, for the award of Senior ResearchFellowship.

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Documents

Crystal structure and characterization of a novel organic optical crystal: 4-chloro-3-nitrobenzophenone