Second Harmonic, Ferroelectric and Dielectric Properties in N

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Organic Multi-functional Materials: Second Harmonic, Ferroelectric and Dielectric Properties in N-Benzylideneaniline Analogues Rekha Kumari, Raviteja Seera, Arnab De, Rajeev Ranjan, and Tayur N. Guru Row Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00985 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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Crystal Growth & Design

Organic Multi-functional Materials: Second Harmonic, Ferroelectric and Dielectric Properties in N-Benzylideneaniline Analogues Rekha Kumari,a Raviteja Seera,a Arnab De,b Rajeev Ranjan,b Tayur N. Guru Row*a aSolid

State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012,

Karnataka, India. bDepartment

of Materials Engineering, Indian Institute of Science, Bangalore 560012,

Karnataka, India. KEYWORDS: N-benzylideneanilines, Second harmonic generation (SHG) efficiency, Ferroelectric, Dielectric. ABSTRACT: Search for multifunctional organic compounds which display technologically important properties has been pursued in recent years. Here we report the synthesis and structure of

a

series

of

single

component

N-benzylideneaniline

nitrobenzylideneamino)benzyl)oxazolidin-2-one(NBOA), chlorobenzylideneamino)benzyl)oxa-zolidin-2-one(CBOA)

analogues

4-(4-(44-(4-(4-

and

4-(4-(4-hydroxybenzy-

lideneamino)benzyl)oxazolidin-2-one (HBOA). The dynamic disorder observed in the structure of NBOA is investigated using variable temperature single crystal X-ray diffraction. All three compounds display second harmonic generation, well defined PE loops with hysteresis at room temperature typical of ferroelectric materials and significant dielectric behavior with tolerance towards high electric fields.

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INTRODUCTION Ferroelectric effects are observed in a wide variety of materials such as solids,1 liquid crystals,2 polymers3 and composite ceramics.4 Ferroelectricity in inorganic compounds like KH2PO4, BaTiO3 are well known for decades. On the other hand, organic ferroelectrics in recent times have received considerable attention in the area of material science due to their potential applications as switchable nonlinear optical devices, ferroelectric random access memories (FeRAM), light modulators, piezoelectric transducers, capacitors, Ferroelectric tunnel junctions (FTJ) and other electro-optical devices.5-9 The high degree of freedom for molecular design, easy tunability of their molecular and electronic properties in multi component organic compounds too offer avenues for inducing ferroelectric properties.10-17 However, single component organic compounds which show multifunctional properties have hardly been investigated. N-benzylideneanilines are used as pigments, dyes, catalysts, intermediates in organic synthesis, polymer stabilizers, and they also exhibit a wide range of biological activities such as antifungal, antibacterial, antimalarial, antiproliferative, anti-inflammatory, antiviral and antipyretic properties.18-21 It is observed that organic compounds such as stilbenes, azobenzenes, Nbenzylideneanilines and its derivatives exhibit conformational changes due to the rotation along longest molecular axis and consequently the molecules result in dynamic disorder displayed in their crystal structures.22-24 The conformational interchange is recognized based on variable temperature single crystal X-ray diffraction and in the case of stilbenes and azobenzenes the dynamic disorder is explained as a “pedal motion” in literature.25-27 These observations have been supported by solid-state NMR spectroscopy 28 and theoretical calculations.29, 30 In this article, we report the crystal structures of three N-benzylideneaniline analogues (NBOA, CBOA and HBOA respectively) and analyze their hydrogen bonding interactions, second harmonic generation (SHG) efficiency, ferroelectric and dielectric properties. These studies confirm the utility of single component organic compounds as possible candidates for multifunctional behavior. The disorder associated with NBOA is analyzed with respect to conformational interconversion and the concomitant polymorphism in HBOA is investigated. Molecular structures of the synthesized compounds are shown in Figure 1.


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Figure 1. Molecular structures of NBOA, CBOA and HBOA.

EXPERIMENTAL SECTION All starting materials were commercially obtained (Sigma Aldrich and Alfa Aesar, Bengaluru, India) and were used without any further purification. Solvents were of analytical or chromatographic grade and purchased from local suppliers. All synthesized compounds were characterized by FTIR (Bruker Tensor 72, equipped with diamond cell ATR), DSC-TGA, PXRD and single crystal X-ray diffraction. Crystal structures were solved by direct method with SHELXS-9731 and refined by full matrix least squares on F2 using SHELXL-97.32 Synthesis of NBOA, CBOA and HBOA The Schiff bases were synthesized from 4-(4-aminobenzyl) oxazolidin-2-one (1 mmol) and aromatic aldehyde derivatives (NO2, Cl, OH) (1 mmol) mixed in 10 mL absolute ethyl alcohol and added 3-4 drops of glacial acetic acid (catalyst). The reaction mixture was refluxed for 4-5 hours at 60-70 ⁰C (Scheme S1). The crude product was easily separated by filtration and then washed with cold water. The resultant pure product crystals were obtained by recrystallization from ethanol solvent. Fourier Transform Infrared Analysis A careful analysis of FTIR data on NBOA, CBOA and HBOA are found to be in full agreement with the proposed structures and confirm the formation of azomethine bond (-C=N-) with the vibrational frequency of azomethine group (-C=N-) in NBOA, CBOA and HBOA appearing at


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1515 cm-1, 1504 cm-1 and 1579 cm-1 respectively (Figure S1). The disappearance of stretching frequency of NH2 of the starting material at 3367 cm-1 and the appearance of new band of -C=Nat 1504-1579 cm-1 confirms the formation of the azomethine linkage. Thermal Analysis Differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) of all compounds were carried out in Mettler Toledo DSC 822e module and Mettler Toledo TGA/SDTA 851e module respectively. Measurements were carried out over a temperature range of 50-360 ⁰C with a heating rate of 5 ⁰C/min. Samples were placed in crimped but vented aluminium pans for DSC and open aluminium pans for TGA and purged by a stream of dry nitrogen flowing at 50 mL min-1. DSC peaks were analyzed using STARe (version 8) software and evaluated the melting points of the samples. The representative DSC thermogram of respective samples is shown in Figure S2. The DSC scan of NBOA, CBOA and HBOA has shown a sharp endothermic peak for melting at 196.13 ⁰C, 184.87 ⁰C and 243.90 ⁰C respectively. The compounds have shown high thermal stability which could be due to the strong intermolecular hydrogen bonding interactions. Thermogravimetric analysis on powder samples of NBOA, CBOA and HBOA has shown in Figure S3, which indicates that there is no weight loss in the material before their respective melting points. Powder X-ray Diffraction PXRD data of NBOA, CBOA and HBOA were recorded on PANalytical X’Pert diffractometer using Cu−Kα X-radiation (λ = 1.54056 Å) operated at 40 kV and 30 mA. X’Pert High Score Plus (version 1.0d)33 was used to analyze and plot the diffraction patterns. Diffraction patterns were collected over 2θ range of 5−50⁰ using a step size of 0.013⁰ 2θ and time per step of 1s. All PXRD patterns have been simulated with the resulting coordinates of respective single crystals through mercury.34 Profile fitting refinements (Table S1) were carried out using Jana 2006. Along with the cell dimensions, profile parameters such as GU, GV, GW, LX and LY were refined using Pseudo-Voigt function. The PXRD patterns match (Rp = 2.12, 2.75, 2.75 and Rwp = 3.20, 3.65, 4.00 for NBOA, CBOA and HBOA respectively) with the simulated patterns from SCXRD data confirming the purity of the sample (Figure S4-6). The concomitant nature of HBOA was also firmly established (Figure S6).


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Single Crystal X-ray Diffraction X-ray diffraction data for good diffracting single crystals of NBOA, CBOA and HBOA were collected on an Oxford Xcalibur (Mova) diffractometer equipped with an EOS CCD detector and a microfocus sealed tube using MoKα X-radiation (λ = 0.71073Å). Low temperature data collection was performed using an Oxford Cobra open stream non-liquid nitrogen cooling device. Data collection and reduction was performed by CrysAlisPro (version 1.171.36.32) software.35 Crystal structures were solved and refined using WinGX software.36 All nonhydrogen atoms were refined anisotropically and hydrogen atoms isotropically. Hydrogen atoms on oxygen atom of hydroxyl group were fixed from difference electron density maps. Other hydrogen atoms were fixed geometrically using HFIX command. Crystallographic information files (CIFs) and parameter tables were generated using WinGX package. Nonlinear Optical Measurement Powder second harmonic generation (SHG) measurements were carried out on all compounds using Kurtz-Perry method.37 The powder material was filled in borosilicate capillary tubes (1.8 mm x 90 mm) and SHG efficiency was measured at ambient conditions in dark using fundamental wavelength 1064 nm of a Q-switched Nd:YAG laser with pulsed width 10 ns at 10 Hz. The idler beam was focused onto the sample with a 20 cm focal length of plano-convex lens on the spot size of 1mm. The SHG signal was collected off axis and focused onto a fiber optic bundle. The output of the fiber optic bundle coupled to the entrance slit of a photomultiplier tube and was detected using a digital storage oscilloscope. The time taken for data collection was 10 sec. The SHG efficiency for NBOA, CBOA and HBOA was found to be 7mv, 7mv and 9mv respectively with respect to KDP (standard). Ferroelectric and Dielectric Measurements Polarization-electric field loop measurements were performed using Precision Premier II tester (Radiant Technologies, Inc.). Dielectric measurements were carried out using an impedance analyzer (HP 4294A, Agilent Technologies Inc., Santa clara, CA). Ferroelectric and dielectric studies were done on the powder sample of NBOA, CBOA and HBOA. Pellets (8mm in diameter and 1-2 mm thickness) were made by applying 5-ton pressure. These pellets were used


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as an alternative to crystalline samples since attempts to grow large single crystals were unsuccessful. Pellets were sintered at 100 ⁰C for 5h to get maximum density. Subsequently, samples were coated with silver paste and dried at room temperature. RESULTS AND DISCUSSION Single Crystal Structure Analysis Crystallographic data of NBOA, CBOA and HBOA is shown in Tables 1 and 2. Table 1. Summary of crystallographic data for NBOA.

Compound CCDC Number Chemical formula Formula weight Crystal system Space group a (A0) b (A0) c (A0) α (0) β (0) γ (0) V(A03) Z T (K) Dcalc(g cm-3) Abs.coefficient (mm-1) F (000) θ Range for data collection (ᵒ) Limiting indices

293K 1915496 C17H15N3O4 325.30 Monoclinic P21 11.6534(8) 7.4548(5) 18.4671(12) 90.000 104.534(7) 90.000 1552.97(29) 4 293(2) 1.39 0.101




Fl. _100K 1915499 C17H15N3O4 325.30 Monoclinic P21 11.7602(4) 7.2762(2) 18.1326(7) 90.00 105.314(4) 90.00 1496.51(17) 4 100(1) 1.44 0.105


678.0 3.3-27.5

680.0 3.3-27.5

680.0 3.3-27.5

680.0 3.3-27.5

680.0 3.3-27.5

680.0 3.3-27.5

-15 ≤ h ≤ 15, -9 ≤ k ≤ 9, -23 ≤ l ≤ 23 24043 6983(0.055)

-15 ≤ h ≤ 15, -9 ≤ k ≤ 9, -23 ≤ l ≤ 23 25694 6914(0.055)

-15 ≤ h ≤ 15, -9 ≤ k ≤ 9, -23 ≤ l ≤ 23 27823 6815(0.053)

-15 ≤ h ≤ 15, -9 ≤ k ≤ 9, -23 ≤ l ≤ 23 25919 6864(0.060)

-15 ≤ h ≤ 15, 9 ≤ k ≤ 9, 24≤ l ≤ 24

27.42 (99.79%)

27.42 (99.79%)

25.24 (99.89%)

27.42 (99.76%)

27.42 (99.77%)

-15 ≤ h ≤ 15, -9 ≤ k ≤ 9, -23 ≤ l ≤ 23 Reflections collected 24908 Unique reflections 7102(0.077) (Rint) Completeness of θ 27.42 (99.77%)

24639 7106(0.058)


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Data/restraints/param eters Goodness-of-fit on F2 Final R indices [I>2σ(I)] R indices (all data) Δρmax (e. A0 -3) Δρmin (e. A0 -3)

7102/1/441

6983/1/441

6914/1/441

6815/1/461

6864/1/449

7106/1/450

1.044 R1= 0 .086, wR2 = 0.143 R1= 0.164, wR2 = 0.184 0.208 -0.183

1.073 R1= 0.066, wR2 = 0.112 R1= 0.093, wR2 = 0.127 0.301 -0.224

1.063 R1= 0.063, wR2 = 0.109 R1=0 .080, wR2 = 0.119 0.318 -0.272

1.047 R1= 0.054, wR2 = 0.107 R1= 0.069, wR2 = 0.117 0.452 -0.295

1.032 R1= 0.058, wR2 = 0.122 R1= 0.078, wR2 = 0.137 0.398 -0.273

1.046 R1 = 0.066, wR2 = 0.119 R1 = 0.142, wR2 = 0.160 0.168 -0.174

Table 2. Summary of crystallographic data for CBOA and HBOA.

Compound CCDC number Chemical formula Formula weight Crystal system Space group a (A0) b (A0) c (A0) α (0) β (0) γ (0) V(A03) Z T (K) Dcalc (g cm-3) Abs. coefficient (mm-1) F (000) θ Range for data collection (ᵒ) Limiting indices

CBOA 2 (100K) 1915491 C17H15ClN2O2 314.80 Monoclinic

CBOA 2 (293K) 1915490 C17H15ClN2O2 314.80 Monoclinic

P21 5.8094(1) 8.4033(2) 15.2776(3) 90.000 94.127(2) 90.000 743.89(1) 2 100(1) 1.41 0.265

P21 5.8783(7) 8.4832(9) 15.4442(16) 90.000 94.418(10) 90.000 767.86(6) 2 293(2) 1.36 0.257

HBOA (Orthorhombic) 1 (100K) 1 (293K) 1915494 1915493 C17H16N2O3 C17H16N2O3 296.30 296.30 Orthorhomb Orthorhomb ic ic P21 P21 P212121 P212121 6.2303(5) 6.6762(20) 5.6755(2) 5.7826(6) 7.3263(7) 7.3516(20) 7.8474(2) 7.9057(7) 15.7674(13) 15.4867(30) 31.4474(9) 31.4187(23) 90.000 90.000(30) 90.000 90.000 98.412(8) 98.382(30) 90.000 90.000 90.000 90.000(30) 90.000 90.000 711.96(9) 751.98(34) 1400.60(1) 1436.32(2) 2 2 4 4 100(1) 293(2) 100(1) 293(2) 1.38 1.31 1.41 1.37 0.096 0.091 0.098 0.095

328 3.5-27.5

328.0 2.6-27.5

312.0 3.4-25.0

312.0 3.5-27.5

-7 ≤ h ≤ 7, -10 ≤ k ≤ 10, -19 ≤ l ≤ 19 17466

-7 ≤ h ≤ 7, -6 ≤ k ≤ 11, -17 ≤ l ≤ 20 3877

-7 ≤ h ≤ 7, 8 ≤ k ≤ 8, 17 ≤ l ≤ 18 5172

-8 ≤ h ≤ 8, - -7 ≤ h ≤ 7, 9 ≤ k ≤ 9, - 10 ≤ k ≤ 10, 20 ≤ l ≤ 20 -40 ≤ l ≤ 40 11793 21954

2633 (0.062)

2505(0.054) 3434(0.058) 3221(0.044) 2806(0.086)

Reflections collected Unique reflections 3420 (0.095) (Rint)

HBOA (Monoclinic) 1 (100K) 1 (293K) 1915492 1915495 C17H16N2O3 C17H16N2O3 296.30 296.30 Monoclinic Monoclinic

624.0 3.6-27.5

624.0 3.6-26.0 -7 ≤ h ≤ 7, 9 ≤ k ≤ 9, 38 ≤ l ≤ 28 10258


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Completeness of θ Data/restraints/par ameters GOF Final R indices [I>2σ(I)] R indices (all data) Δρmax (e. A0 -3) Δρmin (e. A0 -3)

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27.42 (99.78%) 3420/1/203

27.42 (99.84%) 2633/1/199

27.42 (99.72%) 2505/1/200

27.42 (99.74%) 3434/1/211

27.42 (99.54%) 3221/1/200

27.42 (99.77%) 2806/1/200

1.035 R1= 0.040, wR2 = 0.094 R1= 0.043, wR2 = 0.098 0.181 -0.231

0.978 R1= 0.067, wR2 = 0.132 R1= 0.135, wR2 = 0.181 0.201 -0.235

1.010 R1= 0.063, wR2 = 0.091 R1= 0.086, wR2 = 0.100 0.247 -0.225

1.071 R1= 0.072, wR2 = 0.125 R1= 0.117, wR2 = 0.150 0.136 -0.192

1.123 R1= 0.042, wR2 = 0.091 R1= 0.044, wR2 = 0.093 0.248 -0.233

1.066 R1= 0.083, wR2 = 0.122 R1= 0.150, wR2 = 0.148 0.218 -0.157

Crystal Structure of NBOA

Figure 2. (a) ORTEP diagram of crystal NBOA at 100K drawn with 50% ellipsoidal probability (b) Chain formation through a strong N-H···O hydrogen bond (c) C-H···O and π-π auxiliary interactions along a-axis (d) Packing diagram showing the formation of a “herringbone” pattern through the strong hydrogen bond and other weak auxiliary interactions.


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The compound NBOA crystallizes in a monoclinic, P21 space group with Z = 4. Strong NH···O hydrogen bonds (2.808(4) Å, 172.92°; 100K) between the two oxazole units constitute the main packing feature. In 2-D, the molecules propagate through weak C-H···O (3.214(5) Å, 132.36°; 100K) and π···π (3.398(6) Å; 100K) interactions and a herring bone pattern in 3-D along the unique axis (Figure 2). There are two independent molecules in the asymmetric unit and one of these shows a disorder at the azomethine (C=N) bond (Figure 2). It was intended to quantitatively study the extent of torsional vibration by using variable temperature single crystal X-ray diffraction akin to the work described by Harada and Ogawa (2009). This effect was identified due to so called “pedal motion”, a consequence of conformational interconversion between different conformers. Diffraction data at variable temperatures (293K, 200K, 150K, 100K and 300K) were recorded on the same crystal to analyze the nature of motion associated with the disorder. In order to check the conformational ratios of major and minor components, a separate crystal was subjected to “flash cool” from RT to 100K. ORTEP diagrams at 300K, 200K, 150K and flash cool are shown in Figure S7-10. In general, molecules in crystals are not stationary and are associated with thermal vibrations and crystal structure description includes these displacements. However, several instances of molecules in crystals which are associated with extensive amplitude motions have been observed and are described in terms of either dynamic or static disorder. It is important to note that spectroscopic measurement in solution do not contain such disorder and only techniques like solid state NMR, solid state Raman are used to identify such motion in crystals. A quantitative evaluation of disorder can be performed using variable temperature X-ray diffraction. This is highlighted in series of studies performed on stilbenes,25 azobenzenes26 and N-benzylideneaniline analogues38 (Harada et al 2009 and references there in).39,40 The unusual shortening of the ethylene bond in several E-stilbenes was identified only in single crystal structures due to the torsional vibration across the C-Ph bonds and represent different possible conformers thus enabling the identification of percentage of major and minor conformers. The resultant provides the model for the description of dynamic disorder and quantification of the conformers.27 In the present study, we observed a significant shortening of the central C=N bond (1.256 Å at RT) and embarked upon variable temperature Xray diffraction studies in order to analyze the disorder and to evaluate the conformational aspects associated with a “pedal motion”.


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The refinement of disordered structure of NBOA was done carefully by refining four atoms (N5A, N5B, C24A, C24B) anisotropically. All H atoms were refined isotropically. The ratio of populations of the two orientations were determined and found to be changing with the variation of temperature suggesting “dynamic disorder”. At room temperature the ratio of the populations is 72:28. The population of major conformation increases with the decrease of temperature according to the Boltzmann distribution (Figure 7b). Therefore, the temperature dependence of the populations of the conformers is a decisive proof to establish the so called “pedal motion” in the crystal.

Crystal Structure of CBOA CBOA crystallizes in monoclinic system, space group P21 with Z=2. There is no disorder around the azomethine (-C=N-) bond and strong N-H···O hydrogen bonds (2.871(3) Å, 165.12°; 100K) between the two oxazole units constitute the main packing feature. In 2-D, the molecules propagate through weak C-H···O (3.303(3) Å, 156.27°; 100K), C-H···N (3.392(4) Å, 127.20°; 100K), C-Cl···O (3.196(2) Å, 147.21°; 100K) and π···π (3.456(4) Å; 100K) interactions and stair like formation in 3-D along the unique axis (Figure 3).


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Figure 3. ORTEP diagram of crystal CBOA at (a) 100K drawn with 50% ellipsoidal probability (b) Chain formation through a strong N-H···O bond along a-axis (c) Formation of weak CH···Cl, C-H···N, C-Cl···O and π···π interactions. (d) “Stair” like formation in 3-D along unique axis.

Crystal Structure of HBOA (I) The compound HBOA crystallizes in two polymorphs (concomitantly) from a mixture of methanol and acetonitrile solution with similar plate like morphology (HBOA (I) and HBOA (II) respectively).

Figure 4. ORTEP diagram of crystal HBOA (I) at (a) 100K drawn with 50% ellipsoidal probability (b) Chain formation through strong O-H···O and N-H···O hydrogen bonds along crystallographic b-axis (c) π···π and C-H···π auxiliary interactions (d) Packing diagram viewed down the bc plane showing the formation of “tape” motifs.


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Figure 5. (a) ORTEP diagram of crystal HBOA (II) at 100K drawn with 50% ellipsoidal probability (b) Chain formation through strong O-H···O and N-H···O hydrogen bonds. (c) Formation of layer motifs due to π···π and C-H···π auxiliary interactions (d) Packing diagram viewed down the bc plane showing the formation of “herringbone” motif. HBOA (I) crystallizes in monoclinic system, space group P21 with Z=2. Strong hydrogen bonds N-H···O (2.946(5) Å, 155.77°; 100K) between the hydrogen atom of amide group of one molecule and the oxygen atom of the hydroxyl group on the other and O-H···O (2.671(4) Å, 163.89°; 100K) hydrogen bond between the oxygen atom of the carbonyl group of the oxazole unit on one with the oxygen atom of hydroxyl group on the other molecule and in 3-D, the structure forms a “tape” motif (Figure 4). The polymorph II of HBOA crystallizes in an orthorhombic system, space group P212121, Z=4. Strong hydrogen bonds N- H···O (2.978(3) Å, 159.84°; 100K) and O-H···O (2.715(2) Å, 172.65°; 100K) in the bc plane generate the packing to form a “herringbone” pattern with auxiliary π···π (3.470(3) Å; 100K) and C-H··· π interactions (3.562(3) Å; 100K) (Figure 5). Analysis of the Disorder: Difference Fourier Maps of NBOA

Figure 6. Difference Fourier maps of NBOA at various temperatures. The contour lines are drawn at 0.1 e Å-3 intervals.


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Temp (K)

Population Ratio

300

72.0:28.0

200

74.8:25.2

150

79.2:20.8

100

83.0:17.0

Flash Cool to 100

79.1:20.9

Figure 7. (a) Van't Hoff plots ln (K2/K1) vs 1/T for the two orientations at the disordered site in NBOA (b) Populations of the two orientations at the disordered site in the crystal of NBOA. Difference Fourier maps of CBOA and HBOA do not show any residual peaks around the azomethine (-C=N-) bond which indicates that the molecules are not disordered and have a single stable conformation. It is due to the inhibition of conformational interconversion owing to the high barrier energy to the pedal motion and low stability of the minor conformation.38 Difference Fourier maps of NBOA around the azomethine (-C=N-) bond at different temperatures are shown in Figure 6. The contour maps are drawn by considering the plane (C24A, C24B, N5A, N5B) around the –C=N- bond. Two residual peaks (C24B, N5B) appear around the azomethine bond (C24A, N5A) representing the libration of -C=N- bond. The analysis is done by following the methodology suggested earlier in the case of Nbenzylideneaniline and derivatives which showed a static disorder (Harada et al, 2004)23 The electron density at the residual peak positions (C24B, N5B) is significantly higher for the RT data as compared to the data at lower temperatures, indicating the extent of disorder in the molecule. However, the electron density maps clearly bring out the pedal motion with the appearance of residual densities symmetrically disposed on either side of the central –C=Nbond.


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Figure 8. Representation of geometrical parameters of NBOA, CBOA and HBOA. Figure 7 describes the Van’t Hoff plot with the population ratios displayed along the y-axis as natural logarithm against 1/T. It can be inferred from the Van’t Hoff plot that the ratio of major to minor conformer at all temperatures is linear with the exception at the flash cool point which is due to non-equilibrium stabilization of major and minor conformers upon rapid cooling. The conformational interconversion around the –C=N- bond has been analyzed in terms of the torsion angles and the corresponding distances in –C-C-N-C- moiety which holds the phenyl rings and substituents as shown in Figure 8. Table S2 lists the values of distances, bond angles and torsion angles associated with this moiety and an examination of the table suggests that the torsion angle between azomethine bond and phenyl ring increases with the electron donating capability of the substituent. We extended our studies to evaluate the properties of these compounds despite the presence of disorder in NBOA since all these compounds crystallize in a non-centrosymmetric space group and exhibit electronic polarization from one end of the molecule to the other. Indeed, this feature might assist in the design of a material which displays a significant ferroelectric effect. Theoretical Nonlinear Optical Properties. All compounds NBOA, CBOA and HBOA exhibited NLO property with SHG conversion efficiency of 0.84, 0.84 and 1.08 times that of KDP respectively. The ground state geometries of the molecules were optimized by Gaussian09 program (Becke AD (1993)) using DFT/B3LYP/6-31G basis sets. The dipole moments, polarizability and first static hyperpolarizability of all aimed molecules are obtained by “polar” keyword after optimization and are listed in Table S3. The dipole moment and polarizability values of NBOA were found to be less (2.93 D and 7.29 a.u./1.080 x 10-24esu) as compared to


14

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CBOA and HBOA. The strong electron withdrawing group in NBOA increases the charge transfer as well as the first hyperpolarizability value. Indeed, the first hyperpolarizability β value of NBOA was found to be 70 times more than that of urea (0.77x10-30). The high stability and large first hyperpolarizability value of NBOA indicates that it can serve as a nonlinear optical material. However, it must be noted that the experimental measurements of SHG efficiencies are far from these theoretical estimates. It is obvious that the herringbone crystal packing literally cancels out the dipolar contributions. Ferroelectric Measurements. The polarization versus electric field loops for NBOA, CBOA and HBOA were measured over a range of frequencies at room temperature (Figure 9). The remanent polarization (Pr) and coercive (Ec) value of CBOA at a frequency of 0.5 Hz were found to be 0.49 µCcm-2 and 52.3 kV/cm respectively whereas for HBOA at the same frequency were 0.5 µCcm-2 and 48.8 kV/cm respectively. The hysteresis loop of CBOA and HBOA were measured by varying frequency (0.2 Hz – 1 Hz) at 105 kV and 90 kV (Figure 9a, c) respectively. At higher frequency (1Hz), the hysteresis loop is very slim with negligible remanent polarization whereas at lower frequency (0.2Hz) hysteresis loop is increasingly open with observable remanent polarization. The PE loops of CBOA and HBOA at 1 Hz with increasing electric field are shown in Figure S11. The hysteresis loop of NBOA was measured at 90 kV/cm with increasing frequency from 10 Hz to 100 Hz (Figure 9b) and found to be similar to that of CBOA and HBOA.


15


(b)

(a)

(d)

(e)

Page 16 of 24

(c)

(f)

Figure 9. Ferroelectric hysteresis loop of pellets obtained from powdered sample of CBOA, at room temperature at different frequencies and electric fields (a) PE loops of CBOA at 105 kV/cm (b) PE loops of NBOA at 90 kV/cm (c) PE loops of HBOA at 90 kV/cm (d) PE loops of NBOA at 10 Hz (e) Relation between Pr and PME of NBOA at 10 Hz (f) PE loops of NBOA at 100 Hz. The hysteresis loop was further measured up to 165 kV/cm at 100 Hz (Figure 9f). This frequency dependence suggests a competition of underlying rate processes that determine whether the hysteresis will be that of a remanent or non remanently polarized ferroelectric. Similar frequency dependent ferroelectric study has also been observed in PVDF films.41 At low frequencies there is adequate time for space charge migration to trap at the interfaces of polarized domains which stabilize the dipole orientation of some of these domains and cause an opening of the PE loops. At higher frequencies, there is not enough time for space charge to migrate and trap at the interfaces of rapidly switching polar domains. An interesting result was observed in case of NBOA that the molecule was able to withstand very high electric field upto 165 kV/cm (the maximum possible with our instrument) compared with other ferroelectric organic molecules (Bauer and Dormann (1992) studied on substituted diacetylene l,6-bis(2,4-dinitrophenoxy)-2, 4hexadiyne (DNP) at 33 kV/cm; Horiuchi et al. (2010) worked on croconic acid at 37 kV/cm; Venkataramudu et al. studied on 4-(4-(methylthio)phenyl)-2,6-di(1H-pyrazol-1-yl)pyridine (UOH1) at 140 kV/cm; Kamishina et al. (1991) investigated on trichloroacetamide at 7.5


16

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kV/cm).42-45 At 165 kV/cm the loop has the remanent polarization (Pr) = 0.4 µCcm-2 and coercive field (Ec) = 110 kV/cm. It is noteworthy that all three molecules show good remanent polarization and coercive values at room temperature compared to the other compound (UOH1: Pr = 0.166 and Ec = 18 kV/cm).44 It is of interest to note that all three molecules withstand reasonably high electric fields and do not exhibit dielectric loss. DC electric fields of 105, 165 and 90 kV/cm were applied on all compounds, but no leakage current was observed. It confirms that the hysteresis loop is due to the ferroelectric nature of the material and not because the compounds are lossy dielectrics. In addition, Pr/PME plot (Figure 9e) shows that remanent polarization and polarization at maximum electric field do not increase in a similar fashion. Pr increases at a higher rate with increasing electric field in comparison to PME (which should not be the case for lossy dielectric) further confirming the ferroelectric nature of the compounds. In particular, NBOA is able to withstand very high electric field up to 165 kV/cm without any break down. It is noteworthy that the conformational interconversion could have a significant effect for this unusually large tolerance to higher electric fields. It has been shown that in polymeric materials the ability to withstand high electric fields without an electrical breakdown is due to highly dense packing of molecular species (Pure HDPF: 0.957 g/cc).46 All three compounds pack closely in the crystal structure with extensive hydrogen bonding and π-stacking resulting in high density (1.45 g/cc for NBOA, 1.41 g/cc for CBOA, 1.38 g/cc for HBOA (I) and 1.41 g/cc for HBOA (II)), which could result in materials withstanding higher electric fields. Dielectric Measurements. Dielectric constant and loss curve of NBOA, CBOA and HBOA versus frequency are shown in Figure 10. Real, Imaginary and loss values for all three materials are measured over a range of frequency (1Hz-1MHz) at room temperature. The dielectric constant and loss values in Table 3 indicate high dielectric constant as compared to other nonionic organic molecules to date.47,48 All three compounds show a decrease in dielectric constant with increase in frequency as expected due to contributions from electronic, ionic, dipolar and space charge distribution.49 A broad hump around 10 KHz suggests that the dielectric relaxes at higher frequency region due to non-cooperative motion of dipoles with the electric fields which enhance space charge and dipolar polarizations. The presence of disorder in NBOA reflects on the variation in the plot of dielectric loss observed at low frequency.50


17


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Figure 10. Dielectric constant and dissipation factor plots of NBOA, CBOA and HBOA. Table 3 Dielectric constant and loss values of NBOA, CBOA and HBOA. Compound

ε' (100 Hz)

ε' (1 kHz)

ε' (10 kHz)

Tan δ (Loss)

NBOA

193.2135

110.6273

56.9983

0.1282

CBOA

411.7056

263.9910

125.1950

0.2049

HBOA

899.1970

494.9066

194.3972

0.2925

CONCLUSIONS We have designed and synthesized three N-benzylideneaniline derivatives and studied their ferroelectric, dielectric properties. We carried out variable temperature X-ray diffraction analysis to evaluate the disorder in the crystal structure of NBOA in terms of conformational change through the “pedal motion”. The detailed analysis of temperature dependence of the populations revealed that the pedal motion was frozen at low temperature. SHG signal using Kurtz-Perry approach confirms the noncentrosymmetric nature of the structures. The high stability and large first hyperpolarizability value of NBOA indicates that it can be a good candidate as a nonlinear optical material. Ferroelectric and dielectric properties of NBOA, CBOA and HBOA were studied at room temperature. PE hysteresis loops of all molecules confirm that they exhibit ferroelectric properties. Remarkably, NBOA withstood up to 165 kV/cm. Low values of dielectric loss indicate that molecules are suitable as dielectric materials and can be good candidates for futuristic material applications.


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ASSOCIATED CONTENT Supporting Information Supporting information is available and where, listing the contents of the material supplied as supporting information. Synthetic route, FTIR, DSC, TGA, PXRD, Crystallographic parameters, ORTEP Diagrams, Selected geometrical parameters, Theoretical SHG measurements, Hysteresis loops. AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Rekha Kumari: 0000-0002-4992-6141 Raviteja Seera: 0000-0003-2157-3900 Tayur N. Guru Row: 0000-0001-7830-9532 Rajeev Ranjan: 0000-0002-3027-6396 Author Contributions Rekha Kumari, Raviteja Seera and Arnab De contributed equally to the work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Prof. P. K. Das from Department of Inorganic and Physical chemistry, IISc, Bangalore for SHG measurements. T. N. G. thanks D. S. T. for J. C. Bose fellowship. Rekha thanks UGC for DS. Kothari fellowship, Ravi Teja thanks DST Inspire for fellowship.

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"For Table of Contents Use Only"

Organic Multi-functional Materials: Second Harmonic, Ferroelectric and Dielectric Properties in N-Benzylideneaniline Analogues Rekha Kumari,a Raviteja Seera,a Arnab De,b Rajeev Ranjan,b Tayur N. Guru Row*a

Organic functional materials 900 Eps'

NBOA

800

Tan

700

2.0



500 1.0

400 300

Tan  (Loss)

1.5

600

' (Real)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

0.5

200 100

0.0 0

1

10

100

1000

10000

100000 1000000

Frequency [Hz]

Synopsis: A new class of organic compounds N-Benzylideneaniline analogues, have been designed to explore Second Harmonic Generation Efficiency, Ferroelectric and Dielectric properties via structure-property correlation.


24

Second Harmonic, Ferroelectric and Dielectric Properties in N

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