Studies on the growth and characterization of 2-aminopyridinium maleate—A novel nonlinear optical crystal

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1185–1189

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Studies on the growth and characterization of 2-aminopyridinium maleate—A novel nonlinear optical crystal G. Anandha babu, P. Ramasamy  Centre for Crystal Growth, SSN College of Engineering, SSN Nagar, Tamilnadu 603 110, India

a r t i c l e in fo

abstract

Article history: Received 17 November 2008 Received in revised form 20 November 2008 Accepted 26 November 2008 Communicated by M. Schieber Available online 16 December 2008

The growth of a novel organic nonlinear optical (NLO) crystal of 2-aminopyridinium maleate (2APM) in larger size is reported for the first time by the slow evaporation method. Single-crystal X-ray diffraction analysis reveals that 2APM crystallizes in monoclinic system with space group Pc. The crystal was characterized by infrared (IR) and UV–vis–NIR spectra. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study its thermal properties. Powder second-harmonic generation was investigated to explore its NLO properties. The dielectric constant and dielectric loss of the specimen were also studied. The transmittance of 2APM has been used to calculate the refractive index (n) and both the real er and imaginary ei components of the dielectric constant as functions of photon energy. & 2008 Elsevier B.V. All rights reserved.

PACS: 61.66.Hq 42.70.Nq 81.10.Dn 61.10.i Keywords: A1. X-ray diffraction A2. Growth from solutions B1. Organic compounds B2. Nonlinear optic materials

1. Introduction Crystal engineering of new nonlinear optical (NLO) materials, structures and devices with enhanced figures of merit has developed over the last three decades as a major force to help drive nonlinear optics from the laboratory to real applications. The research of large quadratic susceptibilities w(2) depending on the quasi-perfect packing of highly polarizable molecules in the crystal network has been the main challenge [1,2]. The search for efficient nonlinear optical crystals is, in fact, the search for the ‘‘polar crystals’’ in which the macroscopic properties reflect the internal asymmetric molecular relationships. The structural flexibility of organic chromophores easily modifiable through precise chemical syntheses in view to increase the molecular hyperpolarizability b(ijk) and the possible grafting of chirality centers are remarkable assets compared to the difficulties of the engineering route of inorganic materials in which the requirements of noncentrosymmetry and high susceptibilities w(2) have to be accounted at crystal [3,4]. An essential condition to realize even-order NLO processes in materials is a noncentrosymmetric structure; however, optimal molecular orientations are required if

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E-mail address: [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.11.101

appreciable effects are to be achieved in molecular materials [5,6]. Organic nonlinear chromophores of fast optical responses to the laser light have encouraged the research of highly efficient organic materials. The problematic chemical and thermal stabilities and the weak mechanical resistance of organic crystals have at the present time limited their application in nonlinear optical devices. Optical nonlinearity of the crystals with O–H bond has been extensively studied [7–11]. In particular, p-conjugated systems linking a donor (D) and an acceptor (A) show a large NLO response and hence have been well studied. Many of the dicarboxylic salts are reported to be active in second-harmonic generation (SHG) and it may be useful to study complexes with carboxylic acids and their properties. Maleic acid with relatively large p-conjugation has attracted our attention. The intramolecular hydrogen bond in maleic acid is very strong. Maleic acid forms crystalline maleate of various organic molecules through hydrogen bonding and p–p interactions. It is known that maleic acid acts not only as an acceptor to form various p stacking complexes with other aromatic molecules but also as an acidic ligand to form salts through specific electrostatic or hydrogen bond interactions. Acentric molecules consisting of highly delocalized p electron systems interacting with suitably substituted electron donor and acceptor groups exhibit high-value second-order polarizability (b) [12]. 2-Aminopyridinium maleate (2APM) is one such p donor– acceptor molecular compound in which maleic acid transfers one

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of its proton to the 2-aminopyridine, thus the asymmetric unit consists of 2-aminopyridine molecules in protonated form and a maleic acid in monoionized state. Chitra et al. [13] have reported the crystal structure of 2APM. However, to our best knowledge no systematic studies of 2APM have been made. Hence, in the present investigation we report for the first time the bulk growth, optical, thermal, dielectric studies and second-harmonic generation properties of 2APM.

2. Experimental procedure 2.1. Material synthesis All the starting materials are of analytic reagent grade. Equimolar quantities of the parent compounds 2-aminopyridine (2-AP) and maleic acid were dissolved in millipore water. The solution was left in an oven for few days at 50 1C, thereby salt was obtained. The material was purified from aqueous solution by the recrystallization process. The single-crystal growth of this material has been performed from aqueous solution. The solubility of 2APM in water was assessed as a function of temperature in the range 25–40 1C. The different experimental solutions were prepared at desired saturation temperature by adding 2APM. Then the solution was heated 5 1C above the saturation temperature and kept constant there for 1 h. The solution was cooled at 4 1C/h until nucleation occurred. The difference between saturation and nucleation temperatures was taken as metastable zone width (MZW) at arbitrary conditions. The knowledge of MZW is very important in terms of designing crystallization processes and obtaining desired crystal sizes, shapes and purities. Fig. 1 shows the solubility and nucleation curve for 2APM. The 2APM exhibits good solubility and a positive solubility in water. Thus, single crystals of 2APM have been grown from saturated solution of the synthesized salt of 2APM by the slow evaporation technique at room temperature. Single crystal of size 36  5  7 mm3 has been obtained. Grown single crystals of 2APM are shown in Fig. 2. 2.2. Characterization of 2APM The grown crystals were subjected to X-ray diffraction studies using Nonius CAD4/MACH 3 single-crystal X-ray difffractometer, using MoKa (l=0.71073 A˚). Cell parameters were obtained from least-squares refinement of the setting angles of 25 reflections. The various functional groups of 2APM crystal were identified by

Fig. 2. Grown single crystals of 2APM.

the KBr pellet technique using a Perkin Elmer FTIR spectrometer in the range 4000–450 cm1. The transmission spectrum of the 2APM crystal was studied in the range 200–1100 nm by Perkin Elmer spectrometer. Transparent single crystal of 1.5 mm thickness was used for this study. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out on a NETZSCH STA 409 instrument with a heating rate of flux 10 1C/min from 30 to 560 1C. Samples were weighed in an Al2O3 crucible with a microprocessor-driven temperature control unit and a data station. Dielectric properties are correlated with electro-optic property of the crystals. Agilent 4284A LCR meter was used to measure the capacitance and dielectric loss of the 2APM crystal as a function of frequency (temperature range 40–100 1C). Quantitative measurement of relative efficiency of 2APM with respect to KDP was made by the Kurtz and Perry powder technique [14]. The finely powdered crystal of 2APM was packed in capillary tube. An Nd:YAG laser (DCR11) was used as a light source. A laser beam of fundamental wavelength of 1064 nm, 8 ns pulse width, with 10 Hz pulse rate was made to fall normally on the sample cell. The power of the incident beam was measured using a power meter. The transmitted fundamental wave was passed over a monochromator (Czemy Turner monochromator), which separates 532 nm (second-harmonic signal) from 1064 nm, and absorbed by a CuSO4 solution, which removes the 1064 nm light, and passed through BG-34 filter to remove the residual 1064 nm light and an interference filter with bandwidth of 4 nm and central wavelength of 532 nm.

3. Results and discussion Solubility curve Nucleation curve

Concentration (gm/100ml)

55

2APM belongs to the monoclinic crystal system with space group Pc. The lattice parameters of 2APM are a=9.225(5)A˚, b= 4.853(1)A˚, c=11.084(2)A˚, a=90.00(0)1, b =102.46(3)1, g=90.00(1)1 and volume=484.5(3)A˚3 in close agreement with reported values

50 45 40 35 30 25 20 18

20

22

24

26

28 30 32 34 Temperature (°C)

36

Fig. 1. Solubility and nucleation curve of 2APM.

38

40

42

[13]. The maleic acid transfers one of its proton to the aminopyridine. The maleate anion is not aromatic; it contains p electrons and polarized multiple bonds. The close overlap of p orbitals of the maleate ions are observed in melaminium maleate crystal [15]. The recorded infrared spectrum is shown in Fig. 3. The observed vibrational frequencies and the tentative frequency assignments of 2APM are given in Table 1. The peak observed at 1712 cm1 is due to the carboxyl carbonyl stretching. Optical window width is an important characteristic of an NLO material. Hence, the study of the transmission of UV–vis range through the NLO material is necessary. The UV–vis–NIR spectrum of 2APM is shown in Fig. 4. The low absorption at the fundamental wavelength (1064 nm) of the Nd:YAG laser, contributes to crystal’s resistance to laser-induced damage. Further there is

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1187

60

4000

3500

3000

2500

2000

1500

40 30 20 10 0

648

608533

1174

945 867 788

989

1206

1244 1679 1622 1520

0

50

1349

3135

20

1390

2577

40

1907

2056

60

3326

Transmittance

80

% Transmission (arb.unit)

100

200

1000

500

400

600 800 wavelength (nm)

1000

1200

Fig. 4. UV–vis–NIR spectrum of 2APM.

wave number(cm-1) 1.0

Fig. 3. FTIR spectrum of 2APM.

100

Exo

0.5

148°C

Wavenumber (cm1)

Tentative assignment

3135 s 2577 m 1907 m 1679 s 1622 s 1520 s 1390 m 1349 s 1244 m 1206 m 1174 m 989 m 945 s 867 s 788 m 648 s 553 m

C–H symmetry stretching C–H stretching C–C stretching C=N stretching COO asymmetric deformation NH2 symmetric deformation C–NH2 stretching O–H in-plane deformation COO symmetric deformation C–H bending C–OH stretching O–H out-of-plane deformation C–N bending in plane C–H out-of-plane deformation C–N–C in-plane bend and N–C–C in-plane bend ring NH2 out-of-plane bending C–N–C out-of-plane bend

very little absorption at the wavelength of 532 nm, which can improve the second-harmonic throughput [16]. The characteristic absorption band is observed at 340 nm leading to electronic excitation, and there is no absorption band between 340 and 1100 nm; hence the crystal is expected to be transparent between these two wavelengths. Fig. 5 shows the thermal properties of the 2APM crystal carried out by TGA and DTA. The DTA curve of 2APM shows an endothermic peak at 148 1C which can be attributed to the melting point of the sample. The compound starts to decompose at around 150 1C. Further heating above 150 1C, results in the formation of volatile substances, probably carbon dioxide, ammonia, CH4 and CO molecule. Prolonged heating up to 560 1C does not produce any significant endothermic or exothermic peaks in the DTA curve, because DTA becomes inactive due to improper contact with the molten substance, whereas TGA shows complete weight loss. Lowering the dielectric constant value of interlayer dielectric (ILD) decreases the RC delay, lowers power consumption and reduces ‘‘cross-talk’’ between nearby connects [17]. The dielectric constant and the dielectric loss (tan d), are inversely proportional to the frequency. The dielectric constant of dispersive medium decreases because the term contributing to dielectric constant from ion–dipole

μV/mg

0.0

DTA TGA

60

-0.5 40 -1.0

weight loss (%)

80

Table 1 FTIR spectral data of 2APM.

20 -1.5 0 -2.0 0

100

200 300 400 Temperature (°C)

500

600

Fig. 5. TGA–DTA curves of 2APM.

interactions is compensated by the thermal energy leading to the relaxation of polarization. At higher frequencies, the decreased dielectric constant could be due to the reduction in the space charge polarization. The results of dielectric measurements are shown in Figs. 6 and 7. The dielectric constant of 2APM crystal at 373 K is 3.48, and this value decreases to 2.25 at 313 K for 100 Hz. The low value of dielectric loss (tan d) indicates that the grown crystals of 2APM are of reasonably good quality [18]. The Kurtz–Perry powder technique remains an extremely valuable tool for initial screening of materials for secondharmonic generation. The SHG signal energy outputs are 55 and 195 mV for KDP and 2APM, respectively. The output power of 2APM was 3.5 times that of standard KDP. Table 2 shows melting point, relative SHG efficiency compared to KDP, cut-off wavelength of some NLO crystals. Table 2 strongly suggests the title compound as a potential candidate for SHG applications compared to other maleate crystals. The measured transmittance (T) was used to calculate the absorption coefficient (a) using the formula

a¼

2:3026 logð1=TÞ t

(1)

where t is the thickness of the sample. The optical band gap (Eg) was evaluated from the transmission spectra, and the optical absorption coefficient (a) near the absorption edge is given by [25] hva ¼ Aðhv  Eg Þ1=2

(2)

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The refractive index (n) can be determined from reflectance data using,

4.0

Dielectric constant

3.5 313K 333K 353K 373K

3.0

n¼

(4)

Fig. 9 shows the energy dependence of n in the range 1–3.5 eV for 2APM crystal. The refractive index increases with increasing energy. The refractive index (n) is 2.16 at 3.5 eV for 2APM crystal. From the optical constants, the electric susceptibility wc can be calculated according to the relation [26]

2.5 2.0 1.5 2

3

4 log frequency

5

r ¼ o þ 4pvc ¼ n2  K 2

6

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

vc ¼ ðn2  K 2  o Þ=4p

0.20

313K

0.18

333K

(5)

where e0 is the dielectric constant in the absence of any contribution from free carriers. The value of electric susceptibility wc is 0.045371 at l=1100 nm.

0.22

353K

0.16

350

373K

0.14 0.12

300

0.10

250

0.08

(αhν)1/2

Dielectric loss

pffiffiffi ðR þ 1Þ  2 R ðR  1Þ

0.06 0.04

200 150

0.02

100

0.00 3

2

4 log Frequency

5

6

50 0

Fig. 7. Plot of log-frequency versus dielectric loss.

1

Table 2 Melting point, relative SHG efficiency compared to KDP and cut-off wave length of some NLO crystals.

L-arginine maleate Dihydrate [19,20] L-alaninium maleate [21] L-arginine formomaleate [22] Ammonium malate [23] Potassium hydrogen malate Monohydrate [24] 2APM (present work)

Melting point (1C)

SHG (KDP=1)

Cut-off wavelength (nm)

99 150 90 75

3 1.5 1.2 2

300 310 315 210

78.5 148

1.2 3.5

238 350

R¼

expðatÞ þ expð2atÞT

4 Energy (eV)

5

6

7

2.2

2.0

where A is a constant, Eg the optical band gap, h Plank’s constant and n the frequency of the incident photons. The band gap of 2APM crystal was estimated by plotting (ahn)1/2 versus hn as shown in Fig. 8 and extrapolating the linear portion near the onset of absorption edge to the energy axis. From Fig. 8 value of band gap is obtained as 3.4 eV. The reflectance (R) in terms of the absorption coefficient can be obtained from the above equation. Hence, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi expðatÞ  expðatÞT  expð3atÞT þ expð2atÞT 2

3

Fig. 8. Plot of (ahn)1/2 versus photon energy of 2APM.

Refractive index, n

Crystals name

2

1.8

1.6

1.4

1.2 1.0

1.5

2.0 2.5 Energy (eV)

3.0

3.5

(3) Fig. 9. Plot of photon energy versus refractive index for 2APM crystal.

ARTICLE IN PRESS G. Anandha babu, P. Ramasamy / Journal of Crystal Growth 311 (2009) 1185–1189

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The real part dielectric constant er and imaginary part dielectric constant ei can be calculated following relations [27]

References

r ¼ n2  K 2 and i ¼ 2nK

[1] J. Zyss, J.F. Nicoud, M. Coquillay, J. Chem. Phys. 81 (1984) 4160. [2] G.R. Meredith, Nonlinear Optical Properties of Organic and Polymeric Materials, ACS Symposium Series 233 (2) (1983) 27. [3] S.R. Marder, J.W. Perry, W.P. Schaefer, Science 245 (1989) 626. [4] S.R. Marder, J.W. Perry, W.P. Schaefer, J. Chem. Phys. 2 (1992) 985. [5] W. Nie, Adv. Mater. 5 (1993) 520. [6] T.J. Marks, M.A. Ratner, Angew. Chem. Int. Ed. Engl. 34 (1995) 155. [7] D. Xu, D. Xue, J. Crystal Growth 310 (2008) 1385. [8] X. Ren, D. Xu, D. Xue, J. Crystal Growth 310 (2008) 2005. [9] D. Yu, D. Xue, H. Ratajczak, J. Mol. Struct. 792–793 (2006) 280. [10] D. Yu, D. Xue, H. Ratajczak, Physica. B: Condensed. Matter 371 (1) (2006) 170. [11] D. Xu, D. Xue, J. Crystal Growth 286 (2006) 108. [12] J. Oudar, R. Hierle, J. Appl. Phys. 48 (1977) 2699. [13] R. Chitra, P. Roussel, F. Capet, C. Murli, R.R. Choudhury, J. Mole. Struct. 891 (2008) 103. [14] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [15] J. Janczak, G.J. Perpetuo, Acta Crystallogr. C 60 (2004) 6211. [16] P. Srinivasan, T. Kanagasekaran, R. Gopalakrishnan, G. Bhagavannarayana, P. Ramasamy, Cryst. Growth Des. 6 (7) (2006) 1663. [17] B.D. Hatton, K. Landskron, W.J. Hunks, M.R. Bennett, D. Shukaris, D.D. Pervoic, G.A. Ozin, Materials Today 9 (2006) 22. [18] C.J. Benet, F.D. Gnanam, Cryst. Res. Technol. 29 (1994) 707. [19] Z.H. Sun, W.T. Yu, X.F. Cheng, X.Q. Wang, G.H. Zhang, G. Yu, H.L. Fan, D. Xu, Optical Mater. 30 (2008) 1001. [20] T. Mallik, T. Kar, Cryst. Res. Technol. 40 (2005) 778. [21] S. Natarajan, S.A. Britto, E. Ramachandran, Cryst. Growth Des. 6 (1) (2006) 137. [22] T. Mallik, T. Kar, Materials Lett. 61 (2007) 3826. [23] G. Anandha babu, G. Bhagavannarayana, P. Ramasamy, J. Crystal Growth 310 (2008) 1228. [24] G. Anandha babu, G. Bhagavannarayana, P. Ramasamy, J. Crystal Growth 310 (2008) 2820. [25] A. Ashour, N. El-Kadry, S.A. Mahmoud, Thin Solid Films 269 (1995) 117. [26] V. Gupta, A. Mansingh, J. Appl. Phys. 80 (1996) 1063. [27] M.A. Gaffar, A. Abu El-Fadl, S. Bin Anooz, Physica B 327 (2003) 43.

(6)

The value of real er and imaginary ei dielectric constants, at

l=1100 nm are 1.76 and 2.14  104, respectively. 4. Conclusion Optical-quality single crystals of 2APM were grown using solution growth technique. The unit-cell parameters of 2APM were confirmed by single-crystal X-ray diffraction analysis. The functional group was confirmed by FTIR. In the transmittance spectra, it is evident that the 2APM crystal has a wide transparency range in the entire visible range. The thermal behavior of the grown crystals was studied by using TGA–DTA. The dielectric constant and dielectric loss studies of 2APM establish the normal behavior. The SHG relative efficiency of 2APM is 3.5 times that of KDP. Thus, 2APM seems to be a promising material for NLO application. The optical band gap (Eg), absorption coefficient (a), refractive index (n), electric susceptibility (wc) and dielectric constants were also calculated as a function of energy. Acknowledgment The authors thank Prof. P.K. Das, Indian Institute of Science, Bangalore, for support in SHG measurement.