Enhanced NLO activity of organic 2-methyl-5-nitroaniline crystal: Experimental and computational investigation with and without silver addition

Optics and Laser Technology 113 (2019) 416–427

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Enhanced NLO activity of organic 2-methyl-5-nitroaniline crystal: Experimental and computational investigation with and without silver addition

T

⁎

Jerin Susan Johna, D. Sajana, , P. Prabukanthanb, Reji Philipc, Nithin Joyc a

Centre for Advanced Functional Materials, Postgraduate and Research Department of Physics, Bishop Moore College, Mavelikara, Alappuzha, Kerala 690110, India Materials Chemistry Lab, Department of Chemistry, Muthurangam Government Arts College (Autonomous), Vellore 632002, India c Light and Matter Physics Group, Raman Research Institute, Bangalore 560080, India b

H I GH L IG H T S

manuscript reports the first time vibrational characterization of M5NA. • The limiting property of M5NA has been established from OA Z-scan method. • Optical • Enhancement of hyperpolarizability values is observed when Ag is added to M5NA.

A R T I C LE I N FO

A B S T R A C T

Keywords: DFT NLO Silver nanoparticles HOMO-LUMO Vibrational spectroscopy

The nonlinear optical (NLO) material, 2-methyl-5-nitroaniline (M5NA), is synthesized and the crystals are grown by slow evaporation technique. The theoretical vibrational spectral analyses are done for the first time for M5NA using B3LYP computational method with the basis set cc-pVTZ. Natural Bond Orbital (NBO) and Atoms In Molecules (AIM) analyses are carried out for obtaining the charge transfer interactions and the Hirshfeld surface analysis with the fingerprint plot is performed for finding out the intermolecular interaction sites of the molecule. Using the theoretical and experimental IR and Raman spectra, the vibrations of M5NA are estimated. Changes in the linear and nonlinear optical properties with the addition of silver nanoparticles are studied from the UV–vis absorption spectra and the Z-Scan curves. A comparison of the hyperpolarizability values is done with pure and silver-added M5NA.

1. Introduction Nonlinear optical (NLO) materials with second or third order nonlinearities have been used widely for various photonic applications and in opto-electronic devices, for ultrafast optical switching and modulations [1]. The density functional theory (DFT) methods form an effective tool for optimizing the nonlinear optical parameters and are used for designing materials with superior NLO behaviour. Depending upon the wavelength range of applications of NLO crystals, DFT studies have been carried out for developing beryllium-free materials [2–4] that work in the deep ultraviolet (DUV) region. For applications in the visible region of the electromagnetic spectrum, aromatic organic crystals make a better option due to the existence of π-conjugation and the presence of substituted donor and acceptor groups [5]. Amino group attachment to a benzene ring gives the structure of aniline where the

⁎

NH2 group acts as a donor of electrons. When the benzene ring is extended by appropriate acceptor group substitutions, the whole framework gives a donor-π-acceptor structure suitable for NLO applications. Benzylidene-aniline derivatives [6–8] are commonly studied double ring compounds for NLO purposes. Polarizability components of several 4-nitroaniline type compounds [9] have been investigated for finding the nonlinear optical properties. For some para-nitroaniline derivatives, films have been developed for structural and nonlinear optical studies [10]. The D-π-A structure of p-nitroaniline has been utilized in metalorganic framework [11] to generate nonlinear optical activity. Single crystals of N-substituted derivatives of 4-nitroaniline and 2-methyl-4 nitroaniline (MNA) have been explored for second harmonic generation activity [12]. Methyl nitroanilines belong to the group of organic NLO materials, the NLO activity of which are affected by the positional variations of the substituent groups. MNA has been reported to have

Corresponding author. E-mail address: [email protected] (D. Sajan).

https://doi.org/10.1016/j.optlastec.2019.01.014 Received 18 August 2018; Received in revised form 9 December 2018; Accepted 8 January 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Comparison of PXRD patterns obtained from Mercury software using SCXRD data of GuHM and from PXRD experiment.

Fig. 2. Optimized Geometry of (a) M5NA, (b) M5NA-Ag, (c) M5NA-Ag2 and (d) M5NA-Ag3.

material shows very large linear electro-optic effect [19]. Literature on Fourier-Transform Infrared, NLO and hyperpolarizability studies on 2methyl 4-nitroaniline are available [20–23]. The single crystal growth of N-methyl-4-nitroaniline has been reported [24] recently with vibrational and electronic spectroscopic, dielectric, thermal and secondorder nonlinear optical studies.

exceptionally large second order NLO co-efficient [13] and is a potential material for second harmonic generation applications. Due to the large second harmonic coefficient of the material, good quality single crystals of MNA were grown by vapour growth method [14] and Bridgman method [15]. Various applications of MNA crystal as a second harmonic waveguide [16–18] have been reported and the 417

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Table 1 Comparison of theoretical bond parameters of the nitro group with respect to the silver atom adsorption. Parameters

M5NA

M5NA-Ag

M5NA-Ag2

Table 3 Charge density and Laplacian of charge density at BCPs. BCP

Bonds

ρ (au)

∇2 ρ (au)

Ellipticity, ∊ (au)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

C1eC2 C2eH3 C2eC4 C4eH5 C4eC6 C6eC7 C1eC8 C7eC8 C8eH9 C6eC10 C10eH11 C10eH12 C10eH13 C1eN14 N14eO17 C7eN15 N15eH18 N15eH19 N14eO16

0.3265 0.2978 0.3237 0.2941 0.3222 0.3127 0.3264 0.3211 0.2954 0.2593 0.2874 0.2830 0.2804 0.2617 0.5099 0.3083 0.3519 0.3516 0.5084

−1.0906 −1.2061 −1.0733 −1.1745 −1.0553 −0.9969 −1.0855 −1.0521 −1.1853 −0.6918 −1.1153 −1.0818 −1.0628 −0.7122 −1.1594 −0.9434 −1.8542 −1.8495 −1.1488

0.2115 0.0186 0.1969 0.0161 0.2128 0.1992 0.2260 0.2149 0.0186 0.0357 0.0078 0.0102 0.0124 0.1421 0.1132 0.1025 0.0466 0.0452 0.1130

M5NA-Ag3

Distances (Å) N14eO16 N14eO17 C1eN14 O16eAg20 O17eAg20 O17eAg21 Ag20eAg21 Ag21eAg22 Ag20eAg22

1.223 1.222 1.475 – – – – – –

1.351 1.347 1.419 2.349 2.387 – – – –

1.279 1.301 1.458 – – 2.378 2.622 – –

1.357 1.357 1.419 2.254 – 2.245 2.737 2.748 2.748

Angles (°) C1eN14eO16 C1eN14eO17 O16eN14eO17

117.83 117.74 124.42

120.458 121.235 117.464

119.894 118.058 122.034

119.197 118.937 121.830

The studies of the title compound 2-methyl-5-nitroaniline (M5NA) or 2-methyl-5-benzenamine or 5-nitro-o-toluidine have been done by Ellena et al. [25,26] on experimental details regarding the structure and linear absorption, and on the hydrogen-bonded charge density of molecules organized in chains using Bader’s Atoms In Molecules theory [27]. The topological properties of electron density and the molecular dipole moment of M5NA have been determined [28,29] elsewhere also. The compound M5NA is reported to have less second harmonic effect when compared to extensively studied MNA due to the relative positions of the substituents, but still has the potential for NLO applications due to the conjugation possible between the amino and the nitro groups. The present work intends to obtain a detailed spectroscopic account of the title compound, M5NA, using the experimental and theoretical means, along with the third-order NLO characterization. Incorporation of metal atoms in the structure changes the overall symmetry and charge distribution of M5NA which will be effected as variations in optical properties. This effect of the addition of silver nanoparticles to M5NA also forms a part of the present work. Density functional theory (DFT) studies using Gaussian’09 program, Hirshfeld analysis and AIM studies are done on the title molecule to find out the further possibilities of the material to be used in spectroscopic applications.

while the silver nanoparticles for adsorption to M5NA are prepared by Lee-Meisel method [30]. Structure determination of M5NA is done from the single crystal XRD data obtained from Bruker-AXS X-ray diffractometer. The powder X-ray diffraction (PXRD) pattern of M5NA is collected using Rigaku SmartLab powder X-ray diffractometer. The FTIR measurement is taken using PerkinElmer spectrometer in the range 4000–400 cm−1 by KBr pellet method. FT-Raman spectrum of M5NA in the range 4000–50 cm−1 is measured using Bruker RFS 27: Stand alone FT-Raman spectrometer with Nd:YAG 1064 cm−1 laser source having a resolution of 2 cm−1. UV–visible absorption characterization is performed using JASCO UV–visible spectrophotometer and the photoluminescence (PL) emission spectrum is obtained from Horiba Jobin Yvon spectrometer. The open aperture Z-scan measurements are taken using Q-switched frequency doubled Nd: YAG laser at a wavelength of 532 nm having a pulse duration of 5 ns with repetition rate of 10 Hz. 3. Computational details The geometry optimization and the spectroscopic characterization of pure M5NA are done by employing density functional theory (DFT) using Becke’s three parameter Lee-Yang-Parr functional with cc-pVTZ and 6-311++G(d,p) basis sets with the aid of Gaussian’09 program

2. Experimental details Synthesis of the M5NA crystal is done as per the literature [24] Table 2 NBO analysis of pure M5NA and Ag-adsorbed M5NA with B3LYP functional. Donor Orbital (i) → Acceptor Orbital (j)

E(2) (kcal/mol) M5NA

π(C1eC8) → π (C2eC4) π(C1eC8) → π*(C6eC7) π(C1eC8) → π*(N14eO17) π(C1eC8) → π*(N14eO16) π(C2eC4) → π*(C1eC8) π(C2eC4) → π*(C6eC7) π(C6eC7) → π*(C1eC8) π(C6eC7) → π*(C2eC4) n1(N15) → π*(C6eC7) n2(O16) → σ*(C1eN14) n2(O16) → σ*(N14eO17) n2(O17) → σ*(C1eN14) n2(O17) → σ*(N14eO16) n1(O17) → σ*(Ag20eAg21) n2(O17) → σ*(Ag20eAg21) *

MNA

6-311 ++G(d,p)

cc-pVTZ

18.12 18.53 26.12 – 22.24 19.79 21.17 21.13 26.81 12.03 18.89 12.12 19.03 – –

18.16 18.81 25.59 – 22.54 19.95 21.03 21.48 26.59 15.05 19.36 15.20 19.46 – –

418

M5NA-Ag

M5NA-Ag2

M5NA-Ag3

10.52 – – 20.11 9.51 – – – – 5.48 4.23 5.56 4.61 – –

18.39 19.81 – 34.56 23.29 21.08 22.67 21.52 37.00 10.38 18.39 13.17 7.12 3.17 5.56

10.84 – – 19.64 9.06 – – – – 5.07 1.85 4.98 1.77 – –

cc-pVTZ

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Fig. 3. AIM analyses (a) Contour map of charge density, ρ (b) Contour map of Laplacian of charge density, ∇2 ρ , (c) Relief map of charge density, ρ and (d) Relief map of Laplacian of charge density, ∇2 ρ

cases of adsorption of silver atoms to the M5NA molecule with geometry optimization in the level B3LYP/LANL2DZ are shown in Fig. 2(b)–(d). The adsorption of silver atoms to the M5NA molecule takes place at the site of oxygen atoms of the nitro group. Single and three atom adsorption cases show that the two oxygen atoms of the nitro group take part in the interaction with the silver atoms almost equally while the two-atom adsorption case shows that the adsorption is head-on type with only one of the oxygen atoms of the nitro group. A comparison of the bond lengths and bond angles of the nitro group, and the distance between the sliver atoms and the oxygen atoms are given in Table 1 for Ag-adsorbed M5NA. Both NO bonds of the nitro group have almost similar bond length values in the cases of M5NA-Ag and M5NA-Ag3 while the M5NA-Ag2 case shows the dissimilarity in the NO bond lengths because the adsorption of silver atoms take place only to the O17 atom, so N14eO1l7 bond is slightly elongated due to the adsorption. It can be seen from Table 1 that all the NeO bonds are lengthened compared to the pure M5NA. Also, the C1eN14 bond length is decreased from that of pure M5NA, but the values are the same for Ag and Ag3 adsorption, whereas the Ag2 adsorption shows higher value of bond length for M5NA-Ag2. The distance of silver atom to the two oxygen atoms of the nitro group of M5NA-Ag shows that the silver atom is placed at equidistance from both the oxygen atoms. Ag2 adsorption takes place at only one oxygen atom, but the distance from the oxygen atom (O17) to the nearest silver atom (Ag21) is equal to that of single-Ag adsorption case. In the case of M5NA-Ag3, the oxygen-silver distance is reduced to a value of 2.2 Å, and the equilateral triangular Ag3 structure is symmetrical to the NO2 group. The adsorption energies [39] for M5NA-Ag, M5NA-Ag2 and M5NA-Ag3 are calculated to be −0.523 eV, −0.395 eV and −1.510 eV respectively.

software [31]. The assignments of the vibrational peaks are done using MOLVIB program package [32,33] while NBO version 3.1 [34] is used for Natural Bond Orbital (NBO) analysis. Quantum Theory of Atoms In Molecules (QTAIM) analysis is done by AIMAll program package [35] and the Hirshfeld surface analysis to find the intermolecular interaction sites is performed using the software CrystalExplorer [36] version 3.1. Time dependent DFT studies are performed for linear optical absorption studies with Polar Continuum Model (PCM) [37] applied for solution phase. For silver adsorption studies, B3LYP functional itself is used but with LANL2DZ basis set. 4. Results and discussions 4.1. Optimized geometry and XRD structure M5NA is a trisubstituted benzene ring with CH3, NH2 donor and NO2 acceptor groups. The single crystal XRD of M5NA reveals that it belongs to monoclinic crystal system with P21/m space group. The cell lengths obtained are a = 9.5554 Å, b = 5.6771 Å and c = 13.581 Å, cell angles are α = 90°, β = 92.713° and γ = 90°, and the cell volume is 735.886 Å3. The powder XRD pattern obtained for the M5NA crystal using Mercury 3.10 software [38] is compared with that obtained from the powder-XRD experiment and the resultant is shown in Fig. 1. It reveals the similarity between both the patterns and in turn the reliability of both the methods to obtain the crystal details. A comparison of optimized bond lengths, bond angles and dihedral angles of M5NA with experimental data is given in Table S1 (Supplementary Information) and the cc-pVTZ basis set with B3LYP functional is found to have closer values with the experimental data than the 6-311++G(d,p) basis set. So, the structure optimized with ccpVTZ basis set is used for further theoretical analyses. The optimized geometry of M5NA using B3LYP/cc-pVTZ is given in Fig. 2(a), while the 419

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Fig. 4. (a) dnorm surface of M5NA, (b) shape index curve of M5NA, (c), (d), (e) and (f) Fingerprint plots.

stabilization energy, E(2), is estimated from NBO Fock matrix using second-order perturbation theory. For an n → σ* interaction, the energy E(2) can be evaluated as [40]:

4.2. Natural Bond Orbital (NBO) analysis The comparison of stabilization energy obtained from the computational data using two basis sets for M5NA and single basis set for different cases of silver adsorption is given in Table 2, from which it can be concluded that the basis set difference does not vary the energy values considerably. Of the three cases of silver adsorption to M5NA, M5NA-Ag2 is found to have comparable energy values and similar interactions with that of pure M5NA. The charge transfer or the

(2) Enσ ∗ = 2

| σ ∗ n |F ∊σ ∗ − ∊n

(1)

where ∊i represents the diagonal element of the NBO matrix of the Fock . operator F The ring resonant interactions are observed between the alternate 420

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Fig. 5. Experimental UV–Visible spectrum of M5NA and M5NA-Ag. Inset shows the theoretical UV–Vis spectrum using TD-DFT.

chief parameters that are used in AIM analysis for finding out the types of interactions and the critical points where ∇ρ vanish [42]. The eigenvalues λ1, λ2 and λ3 of the Hessian of charge density are related to ellipticity ∊, a parameter which gives the extension of charge accumu-

Table 4 Computational results of UV–Vis absorption of M5NA using TD-DFT. excited state

% of contribution

Excitation energy (eV)

Excitation wavelength (nm)

Oscillator strength f

State 1 HOMO-5 → LUMO HOMO-4 → LUMO HOMO-1 → LUMO + 1 HOMO → LUMO + 1 HOMO → LUMO + 2

13 3.1 13 10 58

5.8123

213.31

0.1403

State 2 HOMO-5 → LUMO HOMO-4 → LUMO HOMO → LUMO + 1 HOMO → LUMO + 2

73 6.1 2.2 16

5.6849

218.10

0.1418

State 3 HOMO-1 → LUMO HOMO-1 → LUMO + 2 HOMO → LUMO + 1 HOMO → LUMO + 2

2.5 4.8 79 11

5.1015

243.04

0.1635

95 3.3

4.2362

292.68

0.2102

98

2.9314

422.95

0.0444

State 4 HOMO-1 → LUMO HOMO → LUMO + 1 State 4 HOMO → LUMO

lation in a plane, by the equation ∊ =

( ) − 1. For the closed-shell λ1 λ2

interactions of the type as ionic bonds, hydrogen bonds and the van der Waal’s bonds, the value of ∇2 ρ should be positive and ρ should be low. From Table 3, it can be seen that all the ∇2 ρ values are negative and there are no bond critical points (BCPs) with ρ < 10−2. This conveys the fact that the interactions between the atoms in M5NA molecule are all of covalent nature and no hydrogen bonds are present in the molecule. Of the 19 BCPs, the largest charge density values are observed for NeO bonds of the nitro group and it can be seen that the methyl group attachment reduces the charge density of BCP of C6eC10 and C1eN14 than that of other BCPs present in M5NA. The charge density has a maximum value at the nucleus and it decreases as the distance from the nucleus increases. This can be expressed in two ways: one as a projection from a plane towards the third dimension, called as a relief map, whereas the other way is to represent in a plane itself as a contour map whose lines have greater values when nearing an atom. Fig. 3(a) and (b) show the contour maps of ∇2 ρ and ρ, and Fig. 3(c) and (d) represent the relief maps of ∇2 ρ and ρ. 4.4. Hirshfeld surface analysis Hirshfeld surface or dnorm surface is a representation of intermolecular interactive sites of a molecule in terms of distances to the nearest nuclei inside and outside a particular point. The normalized contact distance (dnorm) surface (Fig. 4(a)) for M5NA shows strong interaction sites at O16, O17, H19 and H18, while less intense red colour is marked at H9 and another position around O16. The intermolecular interactions take place through OH bonds between the oxygen atoms of the nitro group and the hydrogen atoms H9 and that of amine group. Thus, the nitro and amine groups form the reactive parts of M5NA while the methyl group doesn’t take part in any kind of interactions. Shape index (S) plot (Fig. 4(b)) of the dnorm surface is formed from the two local principal curvatures, κ1 and κ2, at a point of the dnorm surface according to the following equation [43]:

double bonds of the phenyl ring and also with the N]O double bonds of the nitro group. The lone pair of amine nitrogen interacts with the π* anti-bonding orbital of C6eC7, while the lone pairs of oxygen atoms of the nitro group take part in hyperconjugative interactions with C1eN14 and the respective other NeO bond of the nitro group. The M5NA-silver interaction occurs through the oxygen lone-pair to the σ* antibond of Ag20-Ag21 for M5NA-Ag2 case, as a weak interaction. 4.3. Atoms In Molecules (AIM) analysis The Quantum Theory of Atoms in Molecules (QTAIM), introduced by Bader [41], is an extension of quantum mechanics which defines atoms in a molecule and can be implemented to find the intramolecular interactions including the hydrogen bonds that exist with a molecule. The charge density ρ and the Laplacian of charge density ∇2 ρ are the

S= 421

2 κ + κ2 ⎞ arctan ⎛ 1 π ⎝ κ1 − κ2 ⎠ ⎜

⎟

(2)

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Fig. 6. HOMO-LUMO transitions of M5NA for different excitation wavelengths (a) 213 nm, (b) 243 nm and (c) 293 nm.

M5NA in the range 200–600 nm. When the excited state wavefunction is represented as a linear combination of the contributing transitions, the coefficients in the expansion give the extent of contribution of each configuration towards a particular excitation wavelength. The occupied and unoccupied orbitals which participate in the electronic transitions are shown in Fig. 6.

Table 5 Global reactivity descriptors for M5NA. Global reactivity descriptors

Relation with HOMO & LUMO energies

Values for M5NA (eV)

Ionization Potential, IP Electron Affinity, EA Electronegativity, χ Chemical Potential, μ Global Hardness, η Global Softness, σ Electrophilicity Index, ω

IP = -εHOMO EA = -εLUMO χ=-(ɛLUMO + ɛHOMO)/2 µ = (ɛLUMO + ɛHOMO)/2 η= (ɛLUMO - ɛHOMO)/2 σ = 1/η ω =µ2/2η

6.1509 2.6278 4.3893 −4.3893 1.7615 0.5677 5.4686

4.5.3. Global reactivity descriptors The HOMO energy (εHOMO) and LUMO energy (εLUMO) of M5NA are −6.1509 and −2.6278 eV respectively from which ionization potential (IP), electron affinity (EA), electronegativity (χ), chemical potential (μ), global hardness (η), global softness (σ) and electrophilicity index (ω) [44–47] can be calculated using Koopman’s theorem [48,49]. Table 5 gives the relations connecting these parameters, termed as global reactivity descriptors, and the values calculated for M5NA (see Table 6).

If the two shapes differ only by a change of sign, they form complementary pairs represented as hollows (red) and bumps (blue) and they help to identify the places where two molecular surfaces meet. Fig. 4(c) shows the fingerprint plot for M5NA which is a 2D depiction of dnorm surface and the contribution of each kind of bonds to the total surface gets revealed from this plot. Major and almost equal contributions are given by HH and OH interactions with contributions of 34.9% and 34.4% respectively.

4.5.4. Photoluminescence emission The emission properties of M5NA are investigated by getting the photoluminescence (PL) spectral data by exciting the sample at 350 nm wavelength. The emission spectrum in the range 365–600 nm shows an emission peak at 456 nm which corresponds to blue light emission. Generally, materials with good emissive property in the visible region are used for organic light emitting devices. The CIE colour space created by International Commission on Illumination (CIE) is used for linking the electromagnetic visible spectral wavelengths to perceivable colours. Using the PL data, the CIE co-ordinates calculated for M5NA comes to be (0.165,0.169,0.666) which corresponds to the blue colour as seen in Fig. 7(b).

4.5. Absorption and emission studies 4.5.1. UV–Visible absorption The computational and experimental optical absorption spectra of M5NA (Fig. 5) in the UV–visible range gives an insight into the linear electronic absorption properties of M5NA. The experimental UV–Vis spectrum of M5NA in the range 200–550 nm consists of three peaks with variable intensity. The high intensity peak at 207 nm and the peak at 242 nm are interpreted as benzene π → π* transition modified by the amine and nitro substituents, whereas the weakly intense peak observed at 292 nm arise from the electronic transition involving nitro and methyl groups [25]. The theoretical UV–Vis spectral analysis of M5NA gives the corresponding peaks at 216 (average of two theoretical peaks at 213.3 and 218.1 nm), 243 and 292 nm respectively.

4.6. Vibrational spectral analysis The vibrations of M5NA compound are analysed experimentally using FTIR (Fig. 8) and FT-Raman spectra (Fig. 9) at 1064 cm−1 and theoretically using B3LYP/cc-pVTZ method, for getting the vibrations of the benzene ring along with that of the substituent groups: nitro, amine and methyl groups. The stretching and deformation modes of vibration of each group appear as absorption peaks in the region which are specific to those vibrations. These spectral analyses form a mode of

4.5.2. HOMO-LUMO analysis Table 4 shows the oscillator strengths for the theoretical peaks of 422

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Table 6 Vibrational PED assignments of MNBA peaks computed using B3LYP/cc-pVTZ method. Theoretical wavenumbers

Experimental wavenumbers

Unscaled (cm−1)

Scaled (cm−1)

52 108 182 208 210 290 310 354 360 417 451 540 555 561 590 652 729 765 768 835 840 906 957 972 1018 1063 1079 1118 1167 1220 1297 1317 1369 1373 1418 1456 1487 1505 1532 1585 1631 1651 1668 3006 3049 3112 3174 3210 3235 3572 3671

50 104 176 201 203 280 299 342 347 402 435 521 536 541 570 629 704 738 741 806 811 874 923 938 982 1026 1041 1079 1126 1177 1252 1271 1321 1325 1368 1405 1435 1452 1479 1529 1574 1593 1610 2901 2943 3004 3063 3098 3122 3447 3543

FTIR (cm−1)

Assignments FT-Raman (cm−1) 77

419 441

564

737 756 819

815 872 943 996 1031 1102

944

1056 1099

1137 1278 1294

1290

1344

1339

1383 1437

1505

1505

1585 1629

1586 2900 2941 2979

2979 3082 3228 3395 3488

3081 3394

τ(N14eC1) (87), τR (11) τR (51), ωNCCC (21), ωHCCC (16) τ(C6eC10) (77) τR (26), ωNCCC (17), puckR (15), βNCC (12) βNCC (55) ωCCCC (27), τ(N15eC7) (25), ωNCCC (14), τR (12) βCCC (47), βNCC (24) τ(N15eC7) (69) δR (42), νCN (33), νCC (14) βNCC (32), ρNO2 (27), βCCC (14) τR (58), ωNCCC (20), ωCCCC (17) ωNH2 (48), νCN (16) ρNO2 (39), δR (16), βNCC (14) βNCC (19), ωNCCC (14), βCCC (10), δR (10) ωNCCC (26), ωNH2 (21), puckR (18), τR (10) δR (45), νCC (15), νCN (12), δsNO2 (10) puckR (54), ωNCCC (25), ωCCCC (12) ωNO2 (56), ωHCCC (25), ωNCCC (13) νCC (65), δR (13) δsNO2 (58), trigR (15) ωHCCC (74) ωHCCC (63), puckR (15), ωNCCC (12) νCN (27), νCC (17), trigR(14), ρCH3 (12), δR (10) ωHCCC (85) ρCH3 (54), trigR (17), νCC (15) ρCH3 (65), ωCCCC (10) νCC (36), ρNH2 (34), νCN (11) βHCC (34), νCC (32), ρNH2 (16), νCN (12) βHCC (59), νCC (22), ρNH2 (10) νCC (53), trigR (26) βHCC (34), νCN (34), νCC (20) βHCC (57), νCC (16) νCC (82) νNO (64), δsNO2 (18), νCN (16) δsCH3 (87), νCC (10) νCC (37), βHCC (15), δasCH3 (14), νCN (12) δasCH3 (92) δasNH2 (61), νCC (11) βHCC (45), νCC (43) νNO (73), νCC (14) νCC (60) δNH2 (38), νCC (36) δNH2 (46), νCC (29) νCH (100) νCH (100) νCH (100) νCH (99) νCH (99) νCH (99) νNH (100) νNH (100)

ν-Stretching; β-Bending; δ-Deformation; ρ-Rocking; ω-Wagging; τ-Torsion; puck-Puckering; R-Ring.

observed theoretically at 3543 and 3447 cm−1 with 100% PED contribution towards NH stretching. The absorption region of primary aromatic amines for NH2 scissoring is 1615–1580 cm−1 [51]. In the theoretical spectra, the peak at 1610 cm−1 is assigned to NH2 scissoring, while the corresponding peak is observed at 1629 cm−1 in the experimental IR spectrum. Out-ofplane bending vibrations for the amine group are observed at 570 and 521 cm−1 in the theoretical vibrational spectrum. The CN stretching vibration of aromatic amine occurs in the range 1340–1250 cm−1 and are observed at 1252 and 1278 cm−1 in the theoretical and experimental IR spectra respectively.

confirmation for the presence of the functional groups in the molecule. The assignments of the peaks are done by MOLVIB software from which the Potential Energy Distribution (PED) of each vibration is obtained. A uniform scaling by a factor of 0.9651, which corresponds to the scaling factor for B3LYP/cc-pVTZ level, is applied to the wavenumbers to bring the theoretical frequencies down to match the experimental values. 4.6.1. NH2 vibrations Generally, there are two modes of NH2 stretching vibrations [50]: one near 3500 cm−1 is the asymmetrical mode and the other near 3400 cm−1 is the symmetrical mode. For M5NA, the asymmetric stretching vibration is observed at 3488 cm−1 while the symmetric mode is detected at 3395 cm−1 in the experimental FT-IR spectrum. In FT-Raman spectrum, only one mode which is the symmetric stretching vibration is present and is found at 3394 cm−1. These peaks are

4.6.2. NO2 vibrations The strong NO2 stretching vibrations of aromatic nitro compounds occur at 1570–1485 cm−1 and 1370–1320 cm−1 for asymmetric and 423

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Fig. 7. (a) Emission spectrum of M5NA and (b) CIE co-ordinates of M5NA.

Fig. 8. (a) Experimental FTIR Spectrum and (b) Theoretical Scaled IR Spectrum of M5NA computed using B3LYP/cc-pVTZ method.

symmetric vibrations respectively. The peaks at 1529 and 1325 cm−1 in the theoretical spectra belong to the asymmetric and symmetric NeO stretching vibrations of the nitro group. The strong resonance and the presence of electron donating amino group in M5NA shift the NO2 stretching vibration to lower frequency and the intensity is increased. Therefore, the strong peaks at 1505 and 1344 cm−1 in the FTIR spectrum and at 1505 and 1339 cm−1 in the FT-Raman spectrum are assigned to the asymmetric and symmetric NO2 stretching vibrations of M5NA. The assignments are confirmed from the appearance of bands at 1475 and 1310 cm−1 for p-nitroaniline for the nitro group stretching vibrations [50].

The deformation vibrations of NO2 in M5NA are observed at 806 and 738 cm−1 in the theoretical spectrum while the same vibrations are assigned to FTIR peaks at 815 and 737 cm−1 and the Raman peak at 819 cm−1.

4.6.3. CH3 vibrations The CH stretching vibrations appear as medium-to-strong intensity peaks in the region 3000–2800 cm−1, while the methyl group deformation vibrations occur in the region 1470–1400 cm−1 of the FTIR spectrum whereas the Raman spectrum has the corresponding peaks in weak-to-medium intensity. The methyl group attached to the aromatic 424

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Fig. 9. (a) Experimental FT-Raman Spectrum and (b) Theoretical Scaled Raman Spectrum of M5NA computed using B3LYP/cc-pVTZ method.

Fig. 10. Open aperture Z-scan curves measured for (a) M5NA and (b) M5NA + Ag.

group stretches asymmetrically in the region 3010–2905 cm−1 while the symmetric stretch bands occur in the region 2945–2845 cm−1. Therefore, the peaks at 2979 cm−1 in the FTIR spectrum and at 3004 cm−1 in the theoretical vibrational spectrum belong to asymmetrical stretching, and the peaks at 2941 and 2900 cm−1 in the FTRaman and at 2943 and 2901 cm−1 in the theoretical spectra belong to symmetrical CH stretching vibrations of the CH3 group. The asymmetric methyl group deformation occurs in the region 1485–1400 cm−1 and the symmetric methyl vibrations occur at 1470–1260 cm−1. For M5NA, the theoretical asymmetric methyl group bending vibrations appear as a weak peak at 1435 cm−1 and the symmetric vibration at 1368 cm−1, while experimentally these peaks are observed at 1437 and 1383 cm−1 in the FTIR spectrum.

and CH in-plane bending vibrations, while the radial vibrations include radial skeletal vibrations (Vibrations 1, 12, 6a and 6b, and CeX stretching vibrations) and CH stretching vibrations [52]. Of the CC stretching vibrations, the normal mode 8a of M5NA is observed at 1585 cm−1 in the FTIR spectrum and at 1593 cm−1 in the theoretical spectrum. For asymmetric tri-substitution, the mode 19a has a frequency range 1370–1450 cm−1 and the range fixed for the vibration mode 19b is 1460–1530 cm−1. A peak is observed at 1321 cm−1 in the theoretical spectra due to the mode 19a and the peak at 1405 cm−1 is assigned to vibration 19b. The vibration 14, that also belonging to CC stretching vibration, has the frequency interval 1240–1290 cm−1. The CeX in-plane bending vibrations appear below 500 cm−1 and the frequency intervals of vibrations 9a, 9b and 18a overlap. The vibrations 3, 15 and 18b that are considered as CH in-plane bending vibrations have the frequency intervals 1260–1305 cm−1, −1 −1 1140–1170 cm and 1070–1120 cm respectively. Generally, the CH in-plane and out-of-plane deformations occur in the range

4.6.4. Ring vibrations The tangential vibrations of benzene derivatives include carboncarbon stretching, CeX in-plane bending (where X is the substituent) 425

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Table 7 Computational Polarizability and Hyperpolarizability Values for M5NA and silver-adsorbed M5NA. Polarizability Terms

MNA

M5NA

M5NA-Ag

B3LYP/cc-pVTZ

B3LYP/cc-pVTZ

CAM-B3LYP/cc-pVTZ

LC-BLYP/cc-pVTZ

B3LYP/LANL2DZ

Electric dipole moment, μtot (Debye)

7.282

6.078

5.955

5.960

2.254

9.521

2.20580

Dipole polarizability, α (10−24 esu) α(0;0)iso α(0;0)aniso α(−w;w)iso α(−w;w)aniso

16.119 13.610 16.458 14.235

15.673 12.275 15.949 12.692

15.153 11.408 15.377 11.727

14.594 10.600 14.783 10.855

31.365 50.301 10.329 18.902

41.365 55.837 18.548 12.769

34.183 33.934 31.907 25.463

First dipole hyperpolarizability, β (10−30 esu) β(0;0,0) 6.457 β(−w;w,0) 7.704 β(−2w;w,w) 11.742

3.359 4.044 6.537

2.444 2.818 3.929

1.854 2.084 2.687

197.005 49.746 82.230

117.641 522.985 7971.32

80.714 1222.64 165.024

Second dipole hyperpolarizability, γ (10−36 esu) γ(0;0,0,0) 3.071 γ(−w;w,0,0) 3.641 γ(−2w;w,w,0) 5.536

2.893 3.323 4.561

2.249 2.482 3.049

1.744 1.875 2.187

31.154 79.203 135.593

199.350 8024.70 4.016 × 104

186.961 3.231 × 105 1.082 × 104

1290–990 cm−1 and 900–650 cm−1 respectively [51]. For M5NA, the CH in-plane bending vibration is observed at 1126 and 1079 cm−1 in the theoretical vibrational spectrum. CH stretching vibrations, termed as normal modes 2, 20a and 20b, will be observed in the region 3000–3120 cm−1. In the computational spectrum, the ring CH stretching vibrations are observed at 3122, 3098 and 3063 cm−1 for the three CH bonds of the phenyl ring and the corresponding experimental peaks are observed at 3228 and 3082 cm−1 in the FTIR and at 3081 cm−1 in the FT-Raman spectra.

M5NA-Ag2

M5NA-Ag3

values of MNA and M5NA molecules using B3LYP functional shows that MNA has better NLO properties than M5NA due to the para positioning of donor and acceptor groups in MNA. Since CAM-B3LYP is reported [39,58,59] to be a better functional for hyperpolarizability calculations, a comparison has been made between the hyperpolarizability values using B3LYP, CAM-B3LYP and LC-BLYP functionals for M5NA molecule. For silver adsorption cases, all the first and second dipole hyperpolarizability components of silver adsorbed M5NA are greater than that of pure M5NA molecule, as can be seen from Table 7.

4.7. Nonlinear optical studies 5. Conclusion Fig. 10(a) shows the measured open aperture z-scans for pure M5NA for laser pulse energy of 75 μJ while Fig. 10(b) gives the nonlinear optical response of M5NA at 20 μJ of laser energy when silver nanocolloid is added to it. M5NA with linear transmission of 92% exhibits a good NL response and M5NA-Ag with 72% linear transmission shows the nonlinear behaviour even for low laser energy. The measured data is numerically fitted with the nonlinear propagation equation which gives the type of nonlinear phenomenon responsible for the behaviour of the molecules, here in this case is a combination of two photon absorption and saturable absorption [53,54]. The effective nonlinear absorption coefficient α(I), which is intensity dependent, can be written as [55]:

α (I ) =

α0 1+

I Is

The theoretical investigation of M5NA has been done using DFT method and the changes in the linear and nonlinear optical properties due to the attachment of silver atoms to M5NA have been studied. The addition of silver nanoparticles to the M5NA solution resulted in the increment of intensity of the pure sample with the appearance of an additional peak in the UV–vis spectrum due to the presence of silver particles. Hirshfeld surface analysis brought out all the interaction sites of M5NA while the charge transfer interactions are studied using NBO and AIM analyses. The third-order optical nonlinearity of M5NA has been explored in the present work and the results claim the use of M5NA crystal as a good optical limiter. M5NA, although a well-reported NLO material, increases its nonlinear optical property by the addition of silver nanoparticles to it and the fact is supported by the open aperture Z-scan curves and hyperpolarizability calculations.

+ βI (3)

where I is the input laser intensity, Is is the saturation intensity, α0 is the linear absorption coefficient and β is the effective two-photon absorption co-efficient. The propagation equation used for fitting the measured data is given by:

α0 = −⎡ + βI⎤ I ⎢ ⎥ (1 + I Is ) dz ' ⎣ ⎦

Acknowledgement The author Jerin Susan John (JSJ) thanks the University Grants Commission (UGC), India, for the award of a Teacher Fellowship under FDP scheme (F. No. FIP/12th Plan/KLKE002 TF04) leading to the Ph.D. degree. The author D.Sajan (DS) thank the Science and Engineering Research Board, Department of Science and Technology, Government of India (DST SERB), New Delhi-110 070, for financial support (SR/ FTP/PS-220/2012). The authors (DS and JSJ) also acknowledge the DST-FIST program (SR/FST/College-182/2013, November 2013 & FIST No. 393 dated 25-09-2014) to the Bishop Moore College Mavelikara for providing the workstation, UV-visible and Photoluminescence measurement facilities. This research (or a portion thereof) was performed using facilities at CeNSE, funded by Ministry of Electronics and Information Technology (MeitY), Govt. of India, and located at the Indian Institute of Science, Bengaluru.

dI

(4)

from which the Is and β values can be found. In Eq. (4), z′ represents the propagation distance within the sample. The values of Is and β obtained for M5NA are 35 × 1011 W/m2 and 1.4 × 10−11 m/W respectively and those for M5NA-Ag are 12 × 1011 W/m2 and 8 × 10−11 m/W respectively. It is clear from these values that the incorporation of silver nanoparticles enhances the NLO behaviour of M5NA. From the hyperpolarizability calculations [56,57], which are more specific to the orientation of the molecular polarizability tensor components, the variation of the nonlinear parameters with the addition of silver atoms can be studied. A comparison of the hyperpolarizability 426

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Appendix A. Supplementary material

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