Crystal engineering, structural and optical properties

Journal of Crystal Growth 498 (2018) 115–123

Contents lists available at ScienceDirect

Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Crystal engineering, structural and optical properties of 2-aminopyridinium diphenylacetate diphenylacetic acid crystal

T

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RO.MU. Jauhara, , V. Viswanathanb, Paavai. Erac, P. Vivekd, G. Vinithaa, P. Murugakoothanc a

Division of Physics, School of Advanced Sciences, VIT University, Chennai 600 127, India Centre of Advanced Study in Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai 600 025, India c MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India d Sri Sankara College of Arts and Science (Autonomous), Enathur, Kancheepuram 631 561, India b

A R T I C LE I N FO

A B S T R A C T

Communicated by M. Roth

Organic crystal of 2-aminopyridinium diphenylacetate diphenylaetic acid with dimensions 22 × 13 × 11 mm3 was grown by slow cooling technique. The structural confirmation has been done using single crystal X-ray diffraction study. The title compound crystallizes in the monoclinic crystal system with noncentrosymmetric space group P21. The C-H-O, N-H-O type of packing for the title compound has been reported. Optical transmittance shows a wide transparency in the visible region with lower cutoff wavelength at 349 nm. The four independent tensor coefficients of dielectric permittivity were found to be ε11 = 12.16, ε22 = 10.68, ε33 = 11.90 and ε13 = −4.24 from the dielectric measurements. The thermal stability of the 2APD compound was found to be 128˚C assessed by TG-DTA analyses. The particle size SHG of the 2APD reveals that it is a phase-matchable NLO crystal.

Keywords: A1. Crystal structure Crystal morphology A2. Phase matching behavior

1. Introduction The search of new materials with enhanced nonlinear optical (NLO) properties, with the aid of crystal packing arrangements could possibly be a criterion for the crystals to be employed for greater technological applications [1–3]. The discovery of new materials exhibiting nonlinear optical properties in the combination with desirable physical properties such as optical transparency, thermal, optical and mechanical stability continues to be an important goal in nonlinear optics [4]. Relatively, a very efficient new approach that has been developed for obtaining new molecular materials with NLO properties is the acid–base hydrogen bond interactions and molecular recognition [5]. A molecular crystal is built from an inorganic or organic acid and an organic base which is the chromophore molecule. It can be said that the strong hydrogen bond interactions in the acid stabilizes the crystal lattice results in a more favourable mechanical and thermal properties [6]. Perhaps it would be interesting to notice that the hydrogen bond also plays an important role in the creation of non-centrosymmetric structures of crystals and contributes to the molecular quadratic hyperpolarizability of such systems. In this aspect, diphenylacetic acid along with imidazole was synthesized and reported recently by our group [7]. Another article reported by Haynes et al. also elaborates the features of diphenylacetic acid [8]. The molecule of diphenylacetic acid has the ability of

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Corresponding author. E-mail address: [email protected] (R.M. Jauhar).

https://doi.org/10.1016/j.jcrysgro.2018.06.009 Received 16 February 2018; Received in revised form 7 June 2018; Accepted 10 June 2018 Available online 15 June 2018 0022-0248/ © 2018 Published by Elsevier B.V.

promoting absolute asymmetric synthesis that is to assist the formation of enantiomerically enriched products from achiral precursors without the intervention of chiral chemical reagents or catalysts. It is the propeller like arrangement of the phenyl rings in the diphenylacetic acid molecule that causes a chiral conformation in an otherwise achiral molecule [9]. Moreover, the introduction of small ground-state dipole molecules could help the obtained crystals possess a much lower cut-off wavelength and a wide transparency range [10], which is considered as an alternative to overcome the contradiction between the NLO coefficients and the lower cut-off for organic NLO materials. Here in, we report the structure, growth, optical transmittance, thermal, nonlinear optical and phase matching properties of a new organic nonlinear optical material 2-aminopyridinium diphenylacetate diphenylacetic acid (2APD). 2. Material synthesis, growth, solubility and morphology control The 2APD compound was synthesized using analytical reagent grade 2-aminopyridine to the diphenylacetic acid solution in a 1:1 equimolar ratio using methanol as solvent. The reaction scheme is shown in Fig. 1. The reaction is a proton transfer reaction where a proton is transferred from the electron donor group of diphenylacetic acid to the electron acceptor group of 2-aminopyridine. Initially 300 mL


R.M. Jauhar et al.

O

OH

H2N N

diphenylacetic acid

2 aminopyridine

Fig. 3. As grown tiny seed crystals at 4.2 pH at 45 °C in methanol solvent. H2N O

NH+

at 45 °C. But tiny transparent seed crystals with definite morphology were harvested from the solution of pH 4.2 at 45 °C and are shown in Fig. 3. This reveals that the growth morphology depends on the pH of the solution and 4.2 pH is more suitable for growing 2APD crystal. The solubility study was carried out as a function of temperature ranging from 30 °C to 50 °C using a constant temperature water bath (CTB) with temperature accuracy of ± 0.01 °C by gravimetric method. The solubility of the 2APD compound increases with increase in temperature revealing the positive solubility gradient. This can be understood from the fact that the 2APD compound undergoes endothermic reaction [11]. The solubility of the 2APD compound was found to be 15.2 g/100 mL at 45 °C. Fig. 4 shows the solubility curve of 2APD revealing the positive solubility gradient of the titular material. Now the solution was prepared in accordance with the solubility data and housed in a CTB at 45 °C. A transparent seed crystal harvested from the solution with pH 4.2 was suspended carefully into the solution. After a span of three days the seed crystal started to grow and then the temperature was started lowered by 0.02 °C/day. The as grown crystal with growth initiated at 45 °C with dimensions 22 × 13 × 11 mm3 is shown in Fig. 5. Initially the solubility of the title compound was tried using different solvents such as water, acetone and ethanol. Since the solubility of 2APD is found to better in methanol solvent, it was chosen to be the suitable solvent for growing bulk crystals of 2APD. The morphology of the 2APD crystal as shown in Fig. 6 was found using WinXMorph software. As seen in the figure the 2APD crystal has a block like morphology where (1 0 0), (0 0 1) and (0 1 0) faces are prominent. The obtained morphology is found to be in consistent with the grown crystal.

O-

2 aminopyridinium diphenylacetate Fig. 1. Reaction scheme of 2APD.

of methanol was taken in a beaker and diphenylacetic acid was added to the solvent. On adding 2-aminopyridine to the diphenylacetic acid solution, milky white precipitate was obtained. The obtained precipitate was dissolved using the same solvent. The solution was vigorously stirred for 6 h to attain homogeneity. While stirring, the pH of the solution was found to be 3.4 using ELICO Li 120 pH meter. Then the solution was filtered using Whatman filter paper in a beaker covered with polyethene sheet to control abundant evaporation of solvent methanol. Later, the solution was covered tightly with perforated polyethene sheet and kept in an undisturbed condition allowing it for slow evaporation. The solution was under surveillance every day to observe the changes happening in the solution. After a span of 16 days crystalline mass with irregular morphology were harvested and the photograph of the as developed crystalline mass is shown in Fig. 2. Later all the crystalline mass were collected grounded and dissolved using the same solvent. In order to get definite morphology serious attempts were taken to change the pH of the solution by adding NaOH dropwise. Three pH values such as 3.6, 3.8 and 4.2 were fixed by trial and error method. No change was observed for the crystals grown at pH values 3.6 and 3.8

18 16

Methanol Ethanol Acetone Water

Concentration (g/100mL)

14 12 10 8 6 4 2 30

35

40

45

Temperature ( C) Fig. 2. As grown crystals at 3.4 pH at 45 °C in methanol solvent.

Fig. 4. Solubility curve of 2APD in different solvents. 116

50


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Table 1 Crystal data and structure refinement for 2APD. Identification code Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions Volume (Å3) Z, Calculated density (Mg/m3) Absorption coefficient (mm−1) F(0 0 0) Crystal size (mm3) Theta range for data collection (deg.) Limiting indices

Fig. 5. As grown 2APD crystal by slow cooling method at 4.2 pH at 45 °C in methanol solvent.

Reflections collected/unique Completeness to theta Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole (e Å−3)

2APD C33 H30 N2 O4 518.59 293(2) K 0.71073 Å Monoclinic, P21 a = 13.744(12) Å, β = 111.302(3) ° b = 7.913(6) Å, c = 13.793(13) Å 1397.8(2) 2, 1.232 0.081 548 0.20 × 0.15 × 0.10 2.622–27.819 −17≤h≤17, −10≤k≤10, −18≤l≤17 19,741/6465[R(int) = 0.0354] 99.80% Semi-empirical from equivalents Full-matrix least-squares on F2 6465/1/354 1.013 R1 = 0.0434, wR2 = 0.0832 R1 = 0.0743, wR2 = 0.0955 0.043(3) 0.175 and −0.137

aminopyridinium cation, diphenylacetate anion and a neutral diphenylacetic acid molecule formed in a 1:1:1 ratio. The ORTEP of the compound is shown in Fig. 7. In the crystal structure, all the phenyl rings and pyridine ring have a planar conformation. The dihedral angle between the two phenyl rings in the diphenylacetate molecule (C1–C6) and (C8–C13) is 88.53(11)° whereas the dihedral angle of the neutral diphenylacetic acid molecule (C15–C20) and (C22–C27) is 73.32(11)°. The N1 atom deviates by 0.0633(2) Å from the mean plane of the pyridine ring. The N1 atom attached to the pyridine ring lies in a plane is evidenced from the torsion angle of C31–C32–C33–N1 = 176.3° and C29 - N2 - C33 - N1 = 177.3°. The torsion angle values of the anionic diphenylacetate molecule are O1 - C14 - C7 - C8 = −110.4° and O2 - C14 - C7 C1 = −164.4° respectively. The torsion angle values of the neutral diphenylacetic acid molecule are O4 - C28 - C21 - C22 = −93.4° and O3 - C28 - C21 - C15 = −145.3° respectively. The bond distance of the neutral diphenylacetic acid (C28 = O4) is 1.196 Å and (C28—O3) is 1.301 Å. The bond distance of the anionic diphenylacetate (C14 = O1) is 1.234 Å and (C14 = O2) is 1.253 Å. In the crystal structure the title molecule is linked through N1—H1A…O1 and N1—H1B…O1 hydrogen bonds running along (0 1 0) plane and it generates C(4) zigzag chain running along “b” axis as shown in Fig. 8. The N2—H2A…O2 and O3—H3A…O2 interactions constitute a pair of bifurcated acceptor bonds. The N2—H2A…O2, O3—H3A…O2 and C29—H29…O4 intermolecular interactions generating an R33 (9) ring motif [16] viewed down “c” axis shown in Fig. 9. The O1 atom involved in a well-defined trifurcated acceptor hydrogen bonds with N1eH1A…O1, N1eH1B…O1 and C32eH32…O1 bonds is shown in Fig. 10. As seen from Fig. 10, the N1eH1A…O1 and N2eH2A…O2 intermolecular interactions viewed down “b” axis generates a R22 (8) ring motif [16]. The N1eH1B…O1 and C32–H32…O1 intermolecular interactions constitute a pair of bifurcated acceptor bonds generating an R21 (6) ring motif [16] shown in Fig. 10. There are several other weak CeH…π interactions which contributes to the supramolecular aggregation which is given in Table 2. The C26–H26…Cg1 [where Cg1 is the centroid of phenyl ring (C15–C20) and the corresponding symmetry code is x, 1 + y, z] interactions link the molecules in a head-to-tail fashion, forming chains extending along (0 1 0) plane as shown in Fig. 11. The two molecules are also held together by C30–H30…Cg2 interaction with the centroid of the phenyl

Fig. 6. Morphology of 2APD crystal.

3. Results and discussions 3.1. Single crystal X-ray diffraction study X-ray diffraction intensity data were collected at room temperature (293 K) on a Brukeraxs SMART APEXII single crystal X-ray diffractometer equipped with graphite monochromatic MoKα (λ = 0.71073 Å) radiation and CCD detector. A crystal of dimensions 0.230 × 0.170 × 0.110 mm3 was mounted on a glass fiber using cyanoacrylate adhesive. An empirical absorption correction (multi-scan) was performed using the SADABS program [12]. The crystal structure was solved by direct methods using SHELXS-97 and refined by fullmatrix least-squares using SHELXL-2014 [13]. Molecular geometry was calculated using PARST [14]. All non-hydrogen atoms were refined using anisotropic thermal parameters. The hydrogen atoms were included in the structure factor calculation at idealized positions by using a riding model, but not refined. Images were created with the ORTEPPLATON program [15,16]. The hydrogen atoms were placed in calculated positions with CeH = 0.93 Å–0.98 Å, NeH = 0.86 Å and OeH = 0.82 Å refined in the riding model with fixed isotropic displacement parameters: Uiso(H) = 1.2Ueq(C) for C aromatic. Table1 shows the crystal data and structure refinement details for the compound. The title compound crystallizes in the monoclinic P21 space group with cell parameters a = 13.7449(12) Å, b = 7.9133(6) Å, c = 13.7936(13) Å, β=111.302(3)° and Z = 2. The three dimensional molecular structure of this compound was determined by X-ray crystallography using SHELXS-97 and later refined by SHELXL-2014 to a final R-value 4.3%. The title compound 2APD comprises of 2117


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Fig. 7. ORTEP of the 2APD, showing the atom labelling.

ring (C22–C27) and the symmetry code is 1 + x, −1 + y, z as shown in Fig. 12. The N1–H1B…Cg3 interaction with the centroid of the phenyl ring (C1–C6) and the symmetry code is 1 − x, ½ + y, 1 − z viewed down “b” axis shown in Fig. 13. The crystal parameters are given in Table 1 and hydrogen bonds are given in Table 2. Cambridge crystallographic

data centre (CCDC No: 1444071) contains the crystallographic data for the 2APD compound. 3.2. Optical transmittance study The UV–vis-NIR transmission spectrum of 2APD crystal with

Fig. 8. NeH…O hydrogen bonds generate zigzag chain running along “b” axis. 118


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Fig. 9. NeH…O, OeH…O and CeH…O interactions generating R33 (9) ring motif.

Fig. 10. NeH…O interactions generating R22 (8) ring motif and NeH…O and CeH…O bifurcated acceptor bonds generating R21 (6) ring motif. 119


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3.3. Thermal analysis

Table 2 Hydrogen bonds for 2APD. DeH…A

DeH(Å)

H…A(Å)

D…A(Å)

DeH…A[°]

N1eH1A…O1 (ii) N1eH1B…O1 (iii) N2eH2A…O2 (ii) O3eH3A…O2 (iv) C29eH29…O4 (v) C32eH32…O1 (iii) C26eH26…Cg1 (ii) C30eH30…Cg2 (vi) N1eH1B…Cg3 (iii) C2eH2…Cg4 (i)

0.86 0.86 0.86 0.82 0.93 0.93 0.93 0.93 0.86 0.93

2.08 2.15 1.88 1.74 2.46 2.56 2.79 2.78 2.42 2.69

2.910(3) 2.937(3) 2.728(3) 2.550(3) 3.195(4) 3.266(4) 3.750(4) 3.712(5) 3.280(3) 3.622(3)

163 152 169 172 136 133 147 140 111 140

The thermal stability of 2APD was determined by thermogravimetric (TG) and differential thermal analyses (DTA). The 2APD sample weighing 6.246 g was analysed using STA 409 PL thermal analyzer in the range 30–600 °C under nitrogen atmosphere and is depicted in Fig. 16. From the TG curve, it is evident that the 2APD material is stable up to 128 °C and moisture free. The TG curve shows two stage weight loss patterns. A major weight loss starting from 128 °C to 248 °C with a mass change of 42.78% is due to the elimination of the neutral diphenylacetic acid. The neutral diphenylacetic acid comprises a mass of 212.24 g/mol (observed 42.78%, calculated 41%). The second stage weight loss in the TG curve starting from 248 °C to 297 °C with a mass change of 54.13% is due to the elimination of 2-aminopyridinium diphenylacetate molecule. The 2-aminopyridinium diphenylacetate molecule comprises a mass of 293.34 g/mol (observed 54.13%, calculated 56%). A broad peak found at 300 °C is found associated with second stage weight loss of the sample. This broad endotherm, in the DTA curve, found around 300 °C in association with the weight loss confirms the absorption of energy for breaking of bonds during decomposition. The decomposition process was carried up to 600 °C with the removal of material into gaseous products (mixture of CO, CO2, NO and hydrocarbons). The residual mass which remains after all the decomposition process is 0.44%. Thus, the 2APD crystal could be exploited for any applications below 128 °C.

Symmetry codes: (i) x, y, z; (ii) x, 1 + y, z; (iii) 1 − x, ½ + y, 1 − z; (iv) −1 + x, 1 + y, z; (v) 1 + x, y, z; (vi) 1 + x, −1 + y, z.

thickness 1.5 mm was recorded in the wavelength range from 190 to 900 nm using Labindia 3032 UV–vis-NIR spectrophotometer and is shown in Fig. 14. From the recorded spectrum, it is observed that 2APD crystal has a high transmittance of nearly 67% up to 900 nm with lower cut-off wavelength at 349 nm. The optical transmittance of the 2APD crystals may be further improved by reducing the amount of residual inclusions near their centre part. The optical band gap was estimated from the transmission spectrum and the optical absorption coefficient (α) near the absorption edge was calculated using

(α hν )2 = A(Eg−hν )

3.4. Dielectric study

where Eg is the optical band gap of the crystal and A is a constant. The variation of (αhν)2 with hν in the fundamental absorption region was plotted [17] and is shown in Fig. 15. The band gap of the crystal estimated by extrapolation of the linear part to the onset of the energy axis of the graph is 3.55 eV.

The HIOKI 3532–50 LCR HITESTER instrument was used to determine the relative dielectric permittivity of 2APD crystal as a function of frequency ranging from 50 Hz to 5 MHz at 40 °C. As the dielectric permittivity of the crystal is a second rank tensor, the monoclinic system has four independent components of permittivity ε11, ε22, ε33 and ε13 corresponding to a, b and c directions [18]. The magnitude and

Fig. 11. CeH…π interaction link the molecules in a head-to-tail fashion extending along (0 1 0) plane. 120


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Fig. 12. CeH…π interaction viewed down “b” axis.

permittivity values are in the ranges 23.89–12.16, 20.74–10.68, 23.01–11.90, 26.94–7.79. It is observed that the elimination of space charge polarization of molecular dipoles is responsible for higher relative permittivity at low frequency region and a dip in the relative permittivity in the higher frequency region. The dielectric permittivity ε13 is found to be −4.24. The negative value of this coefficient indicates that the polarization occurs in the negative direction of the c-axis when an electric field is applied along the positive direction of the a-axis. This can also be assumed from the fact that the structural interactions of the 2APD molecule is easily deviated along the c-axis than the ‘a’ and ‘b’ axes under an electric field [19]. This could be the reason for a relatively larger value along the c-axis than the ‘a’ and ‘b’ axes.

direction of four principal dielectric permittivities influences the dielectric properties of a crystal [18]. The crystal was cut along a, b, caxis and the fourth plane was cut along 45° between ‘a’ and ‘c’ directions with the thickness of 2.07 mm. To make the prepared samples behave like parallel plate capacitors the sample was coated with silver paste on either side. The fourth component, ε13 can be obtained from the following relation [18]: ε’33 = ε11 sin2θ + 2 ε13 sin θ cos θ + ε33 cos2 θ The variation of dielectric permittivity with frequency along the four crystallographic planes is shown in Fig. 17. The calculated relative

Fig. 13. NeH…π interaction viewed down “b” axis. 121


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70 60

25

Dielectric permittivity

50

% Transmittance

cut along 45 a-plane b-plane c-plane

40 30 20

349 nm

10

20

15

10

0 200

300

400

500

600

700

800

5

900

1

Wavelength (nm)

2

3

4

5

6

log f

Fig. 14. Optical transmittance of 2APD crystal. Fig. 17. Dielectric tensor analysis of 2APD crystal. 8

4.0x10

55 8

3.5x10

50

8

3.0x10

45

SHG output (mV)

eV m-1

8

2.0x10

8

h

KDP 2APD

40

8

2.5x10

1.5x10

8

1.0x10

3.5 eV

35 30 25 20 15

7

5.0x10

10

0.0 2.0

2.5

3.0

3.5

5

h eV)

0

50

Fig. 15. Tauc’s plot of 2APD crystal.

150

200

250

300

350

400

Particle size ( m) Fig. 18. Particle size dependent SHG of 2APD.

120 1.0

TG DTA

128 C

0.0

80

-0.5 60

-1.0

40

-1.5

54.13%

Endo

play a vital role in the SHG output. The resultant SHG is low when the particle size is minimal i.e., it is less than coherence length of radiation. When the average particle size increases, the output SHG also increases as the particle size equals the coherence length. Above a particular particle size the SHG output remains constant, which may be due to the phase-matchable nature of the grown NLO crystal [20]. Sieved KDP powder of 40 μm particle size was used as reference. KDP yielded a signal voltage of 6.2 mV, while the 2APD material gave a signal voltage of 12.22 mV. The SH output of the 2APD material was thus found to be 1.97 times that of the standard KDP.

0.5

42.78%

-2.0

20

DTA (mW/mg)

100

Weight %

100

-2.5 -3.0

0 0

50

100

150

200

250

300

350

400

450

500

550

-3.5 600

4. Conclusions

Temperature ( C)

To summarize, 2-aminopyridinium diphenylacetate diphenylacetic acid (2APD) was synthesized and good quality crystals of 2APD were grown by the slow evaporation and slow cooling methods. Single crystal X-ray diffraction study confirmed the cell parameters and the crystallographic planes of the 2APD crystal. Optical transparency of the grown crystal was around 70% and is transparent in the entire visible region ranging from 350 nm to 900 nm. The thermal behavior of 2APD observed from TG and DTA analyses reveals that the material is stable up to 128 °C. Anisotropic behavior of the grown crystal has been confirmed by dielectric measurements. Particle size SHG of the sample is 1.97 times that of the standard KDP.

Fig. 16. TG-DTA curves of 2APD.

3.5. Nonlinear optical study and phase matching property The second harmonic generation (SHG) efficiency of the 2APD material was found by grinding the 2APD crystal into different particle sizes viz., < 40 μm, 70 μm, 106 μm, 212 μm and 355 μm. The output of the 2APD material was found to increase as the particle size increases and found saturated at a particular range i.e., 212–355 μm as shown in Fig. 18. It is observed that the particle size and the coherence length 122


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