A New Nonlinear Optical Stilbazolium Family Crystal of (E)-1-Ethyl-2

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A New Nonlinear Optical Stilbazolium Family Crystal of (E)-1ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide: Synthesis, Crystal Structure and its Third-Order Nonlinear Optical Properties Nivetha Karuppannan, and Sivaperuman Kalainathan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11884 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

A New Nonlinear Optical Stilbazolium Family Crystal of (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide: Synthesis, Crystal Structure and its Third-Order Nonlinear Optical Properties Nivetha Karuppanana, Sivaperuman Kalainathana,* a,a,*

Centre for Crystal Growth, Vellore Institute of Technology, Vellore, Tamil Nadu 632014, India

ABSTRACT The derivative of an organic stilbazolium single crystal, (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide (hereafter abbreviated as NSPI), was synthesized by Knoevenagel condensation method. Single crystals of NSPI of size 13 x 12 x 3 mm3 were obtained from a 1:1 ratio of methanol and acetonitrile using a slow evaporation technique. X-ray structural analysis has been pointed out that NSPI crystallizes in the centric triclinic system with P-1 as a space group. In addition, Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopic analyses were carried out to confirm the molecular structure of grown crystal. The small energy difference of the frontier molecular orbitals indicates the possible nonlinear optical properties present in the title crystal. Thermal studies confirm that o

the material has good thermal stability of about 151 C. Etching studies provide the layered growth of the NSPI crystal with less dislocations. The optical transmission range and the band gap were assessed by the UV-Vis transmission study. Red emission takes place at 721 nm was identified from photoluminescence study. The NSPI crystal has good resistance to laser radiation against the Nd:YAG laser at 1064 nm. The third-order NLO efficiency of the NSPI crystal was determined via Z-scan technique with a He-Ne laser excitation source operating at 632.8 nm.

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1 INTRODUCTION Design of new organic donor -π- acceptor compounds (D-π-A) has a significant concentration of research, due to their most applications as nonlinear optical (NLO) materials. To date, organic compounds have materialized as key targets for NLO applications because they are highlighted with extensive optical nonlinearity, nonlinear susceptibility and easily incorporate in optical gadgets.1,

2

Most organic molecules show interesting feature of intramolecular

charge transfer (ICT) through the interaction of donor and acceptor groups. By carefully varying the donor and acceptor of the π-conjugate unit for the regulation of the ICT level and can produce the desired molecular properties.1,

3-5

As a result, the optical nonlinearity in

conjugate system increases. Thus, they are sought for differential intensive applications such as terahertz generation ,6 frequency conversion, optical communications, data storage and processing, parametric oscillation and electro-optic modulation.7, 8 Specifically, in the organic molecules wide ICT occurs between the donor and the acceptor resulting in very high molecular polarization and therefore strong third order nonlinearity.9, 10 The requirement of competent third-order NLO materials still exists as a noteworthy test for scientific experts and material explorers; even the significance of second order materials is well understood. Increased attention to the search for new materials of third-order is concerted essentially with their possible applications in optical switching, optical limiting and protection of sensors, that rely on major quantities such as nonlinear absorption (NLA), nonlinear refraction (NLR) and nonlinear susceptibility of the third-order (3).10-12 Some organic compounds, which are considered nonlinear, D-π-A type substituted stilbazolium derivatives are very smart optical candidates for generating THz waves, electrooptic and photonic applications, as it divulges second and third-order NLO responses.13-19 Nonlinearity occurs due to the strong Coulombic interaction, high chromophores density,


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orientation of long term stability and strength of donor and acceptor groups with delocalized π-electrons

that

can

enhance

asymmetric

polarization.20,21

Stilbazolium

cations

(chromophores) are a major source for nonlinear and the variation of the counter ions of these salts proved to be an effective strategy for the development of materials with macroscopic high nonlinearity based on Coulomb interactions.22-27 Using this approach, many stilbazolium derivatives with large optical nonlinearities have been extensively explored by many research teams, such as DAST (4-N,N-dimethylamino-4’-N’-methyl-stilbazolium tosylate), (4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2-naphthalenesulfonate)

27

26

DSNS

and so on.

Moreover in recent times, efficient third-order NLO efficiency of stilbazolium derivatives with increasing the significance of ICT between the donor and acceptor has been engineered. 28-34

Therefore, new NLO materials are to be sought in the stilbazolium family, so that they

can be utilized effectively for optical applications. Considering the prominence and applicability, we put effort to synthesize a stilbazolium crystal derivative, namely (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide (NSPI), in which 4-nitrobenzaldeyde was used in the donor and N-ethyl pyridinim iodide as the acceptor by slow evaporation technique. The Knoevenagel condensation was carried out to synthesize the title compound. The present article demonstrates the synthesis, crystal growth, structure, spectroscopic, HOMO-LUMO, thermal, etching, optical, laser damage threshold studies of grown crystal. The title material crystallized with a P-1 triclinic centrosymmetric space group, the result of which is obtained a new material for the third harmonic generation (THG). The NLA and NLR of NSPI crystal was determined using the Z-scan technique and the outcomes have been discussed.



2 EXPERIMENTAL SECTIONS 2.1 Material synthesis All reagents were obtained from analytical grade and utilized as procured from the manufacturer (Alfa Aesar and Sigma Aldrich). To synthesize NSPI, equimolar amounts (30 mmol each) of 1-ethyl 2-methyl pyridinium iodide (7.4 g) and 4-nitrobenzaldehyde (3 mL) in piperidine catalyst under reflux in methanol (20 mL) at 60 ºC (Knoevenagel condensation). After completion of the reaction (4 h), the resulting mixture was cooled to room temperature, filtered and washed with diethyl ether to give a refined product in 76% yield after recrystallization from methanol. The reaction scheme for NSPI is shown in Figure 1. 2.2 Solubility, Crystal growth and Morphology of NSPI Solubility analysis in a specific solvent is an essential criterion for the growth material, such as a single crystal. The solubility of NSPI in pure and mixed solvents has been tested and it has been found that in these solvents 1:1 mixture of methanol: acetonitrile is the most suitable. The solubility estimation was performed by dissolving NSPI in a mixture of methanol and acetonitrile to obtain saturation and the contents were gravimetrically scrutinized at various temperatures ranging from 30-45 °C with a range of 5 °C using a constant temperature bath (± 0.01 °C accuracy) . Figure 2 displays the positive solubility temperature gradient of NSPI. The saturated solution was prepared at 35°C using 5.2 g of purified salt of NSPI dissolved in 100 mL of a mixed solvent of methanol and acetonitrile according to the solubility data. The solution was then stirred for 2 h until a homogeneous mixture was obtained, filtered and wrapped tightly by a piece of aluminum foil with few holes to minimize the evaporation of the solvent. Crystallization by slow evaporation occurs by placing the beaker in a constant temperature bath (± 0.01 °C accuracy) maintained at 35 ºC. A NSPI single crystal of size 13 x 12 x 3 mm3 was collected after a period of 40 days (See Figure 3a).


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The WinXMorph software

35

was used to infer the morphology of the NSPI crystal (Figure

3b) with the help of Crystallographic Information File (CIF) from a grown crystal as an input. The morphology of the NSPI shows 10 developed faces: (001), (011), (101), (-100), (010), (0-10), (100), (-10-1), (0-1-1) and (00-1). Among these, (010) and (0-10) faces have greater morphological prominence. The preferred growth direction of NSPI crystal is identified along ‘a’ axis. In the present work, all the characterizations are made on plane (010). 3 CHARACTERIZATION TECHNIQUES Used Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo-Ka radiation (λ = 0.71075 Å) at 100 K, in order to determine the crystalline structure of NSPI. Various vibrational modes of the NSPI crystal were identified by FTIR spectroscopy using the SHIMADZU IRAFFINITY spectrometer by pelletizing with potassium bromide in the range of 4000 to 400 cm-1. To study the carbon-hydrogen framework, 1H NMR and

13

C NMR

spectra of NSPI crystal were recorded using a 400 MHz Bruker Spectrometer in powder form, dissolved in a deuterated solvent of DMSO-d6. Energy behaviour of the NSPI crystal was examined by HOMO-LUMO analysis of the B3LYP/6-31G (*) basis set level in the gas phase using the Sparton’14 V1.0.1 program. Thermogravimetric and differential thermal analysis of the NSPI crystal were analysed using a NETZSCH STA F3 analyzer with an o

o

initial weight of 9.3 mg, heated to 50-800 C at a rate of 10 C/min under an argon atmosphere. Etching studies for the NSPI crystal were performed using a Carl Zeiss optical microscope. The optical transmission spectrum was taken at a wavelength of 200 to 800 nm employing ELICO SL 218 double beam UV-Visible spectrometer using a NSPI single crystal of thickness 1.5 mm in solid state. Hitachi F-7000 FL spectrophotometer was employed to record the luminescence spectrum of the NSPI crystal at room temperature. The threshold value of the laser damage was performed with a Nd: YAG laser at a repetition of 10 Hz and a



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pulse of 10 ns. The NLO properties of the NSPI crystal were examined by a single beam Zscan technique using a He-Ne laser at 632.8 nm. 4 RESULTS AND DISCUSSION 4.1 Structural analysis The NSPI crystal structure was resolved by direct methods and refinement was done on F2 using the program SHELXL-97 by the full-matrix least-squares methods.36 (E)-1-ethyl-2-(4nitrostyryl) pyridin-1-ium iodide contains C15H15N2O2 + cation and Iˉ anion in its asymmetric unit and crystallizes in the triclinic space group P-1. The title crystal refers to a centrosymmetric group and does not show nonlinearity of the second-order defined by the space group. The cell parameters were calculated as follows: a = 7.5150(5) Å, b = 9.8080(6) o

o

o

Å, c = 10.1132 (5) Å, α= 82.674(2) , β = 75.929(2) , γ = 86.569(2) , and volume, V=716.83 3 (8) Å . The crystallographic data and structure refinement for the NSPI crystal are

enumerated in Table 1. The (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium cation adopts a trans configuration with respect to the C7=C8 double bond, which is determined by the C4-C7-C8o

C9 torsion angle of 178.5 (3) . The selected bond lengths and bond angles of the NSPI crystal are listed in Table S1. The molecular structure represented by the ORTEP diagram and the packing diagram of the NSPI crystal are depicted in Figures 4 and 5. The crystalline packing is stabilized by weak C-H...I and C-H...O interactions. In addition, the packing is also consolidated by C-H...π interactions. The crystallographic data for the NSPI crystal was deposited at the Cambridge Crystallographic Data Centre with CCDC No. 1419336.


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4.2 Vibrational spectroscopy The vibrational frequencies strongly rely on the crystal structure with the bonds associated and can thus obtain information on intermolecular interactions experienced by the molecule. 37

The FTIR recorded for the NSPI crystal is presented in Figure S1. The band located at

3037cm-1 is ascribed to the aromatic C-H stretching vibration. Stretching with C-H alkyl is observed from the peaks appearing at 2912 and 2850 cm-1. The presence of π-conjugated C=C (vinyl group) stretching vibration in stilbazolium cation is noticed from the peak at 1591 cm-1. The absorption band emerges from 1481 and 1230 cm-1 matches to CH2 bending and CN stretching vibration. The peaks for asymmetric and symmetric NO2 vibration modes were revealed at 1512, 1338 and 1303 cm-1, respectively. The peak at 842 cm-1 is attributed to the para-substituted mode of vibration of aromatic rings. The 1, 2 substituted pyridinium ring vibration is established from the peak 960 cm-1. The bending modes of in plane and out plane of the aromatic C-H is viewed in the region between 1100-1200 cm-1 and 675-1000 cm-1. Thus, the vibration modes of the functional groups prominent in the title crystal were identified by the FTIR profile. 4.3 NMR spectral studies The information on molecular structure, dynamics and the different chemical environment were obtained by NMR spectroscopy. The 1H and

13

C NMR spectra of the NSPI crystal are

shown in Figures S2 and S3. In the 1HNMR spectrum, a singlet signal at 2.49 ppm and 3.56 ppm results from DMSO-d6 and the water molecule in the DMSO-d6. The quartet and triplet chemical signal at 4.82 ppm and 1.47 ppm were assigned to the protons in the ethyl group (1CH2-CH3) of the pyridine ring. The doublet signals emerged at 7.83 ppm and 7.98 ppm were attributed to the CH protons at positions 1' and 2', which is evidence for the formation of the title compound. The peak occurred at 8.51 ppm and 8.55 ppm was assigned to protons at



positions 4 and 5 of the pyridine ring. The proton resonates as a doublet at 8.01 ppm and 8.99 ppm bound to the carbon atom at positions 3 and 6 of the pyridine ring. The doublet signal attributed to the protons bound to the carbon atom in the 2", 3", 5" and 6" positions of the aromatic ring resonates as 8.11 ppm and 8.31 ppm. In

13

C NMR spectrum, the signals appeared at 53.64 and 15.66 ppm is due to the carbon

atoms in ethyl group attached to the pyridine ring. The vinyl carbon atoms in the 1' and 2' position of the title compound generate their signals at 124.05 ppm and 140.51 ppm. The carbon atoms of the pyridine ring gave their signals 126.45ppm, 144. 95 ppm, 145.40 ppm. 147.99 ppm and 150.84 ppm. The signal evolved at 121.31 ppm, 129.56 ppm, 141.00 ppm, was attributed to the carbon atoms of the aromatic ring. The presence of different proton and carbon peaks in 1HNMR and

13

C NMR spectra revealed resonances consistent with the

proposed NSPI structure. 4.4 HOMO-LUMO analysis The most reactive state of the π-electronic frameworks and the different types of reactions involving conjugate structures are described by the electronic densities of Frontier molecular orbitals (FMO). 38 FMO consist of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which acts as electron donor and electron acceptor. It gives information on the chemical stability, reactivity and interactions of the appropriate molecules in charge transfer, which contributes significantly to the NLO response. 39 The surface of the FMO on which the charge density is localized is displayed in Figure 6. HOMO is located on the iodide ion and the LUMO is distributed on the pyridine ring (with the exception of the -CH2-CH3 group), the vinyl group (CH), p-substituted aromatic ring and a smaller electron density distributed on an iodide ion. The energy difference for the NSPI crystal obtained by the HOMO-LUMO interaction is 0.73 eV, which


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is relatively low compared with urea (6.70 eV). 40 Small difference in energy results in higher polarizability,41 which leads to greater delocalization of electrons around the donor and the acceptor (i.e., charge transfer) and thus responds to the good nonlinear optical activity in this compound. Molecular electrostatic potential The molecular electrostatic potential (MEP) provides information on the size and shape of the molecules with potentially positive electrostatic, negative and neutral regions based on the color classification.

42, 43

The MEP surface of the NSPI single crystal is shown in Figure 7.

The electrostatic potential of the surface values increases by about -200 a.u. (dark red) and 200 a.u. (dark blue). The maximum positive area in the MEP plot is the preferred nucleophilic attack site marked with blue. Likewise, the maximum negative area is the favoured location for electrophilic attack, found in red and zero green areas. The negative potential lies in the electronegative iodine atom and the positive potential around the pyridine ring, and the zero potential is located on the remaining carbon atoms. From the MEP surface, we can state that the NSPI crystal has potential reactive sites for nucleophilic and electrophilic attacks, respectively. 4.5 Thermal studies The phase transition and the various decomposition stages and melting point of the NSPI crystal were examined by subjecting its powder sample to concurrent Thermogravimetric (TG) and Differential Thermal Analysis (DTA). The thermograms obtained are depicted in Figure 8. The curve of the TG precisely indicates that grown crystal have good thermal o

stability up to 151 C, without producing a phase transition or decomposition before this temperature. The curve TG then shows its complete decomposition in three thermal stages. o

o

In the first step between the temperature from 151 C to 337 C a significant weight loss of 9 ACS Paragon Plus Environment


o

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o

about 75.8% is observed. At a temperature from 337 C to 527 C, 5.5% of the second weight o

loss is observed and eventually 10.8% of weight loss occurred and continues up to 800 C. These weight losses are certified for the decomposition of the title material. Further, it was o

observed that the DTA curve provides a sharp melting endotherm at 235 C and remains o

o

endothermic peaks at 280 C, 314 C indicates the presence of thermal decomposition, which is complementary to that of the TG trace. The good crystal nature of the NSPI crystal is evident from the sharpness of the peaks observed in DTA. 7.9% of the sample remained as o

residue at 800 C. In addition, it is quite interesting to note that the melting point of NSPI is even better than other stilbazolium derivatives

28-30, 33

presented in Table 2. By this analysis, o

we can make sure that the grown crystal withstand temperature up to 151 C without phase change, and may be suitable for device applications up to this temperature. 4.6 Chemical etching studies Crystalline defects and the mechanism of growth are probed with the help of chemical etching. The imperfections formed during growth affect the physical properties of the crystal, so that effective crystalline perfection is essential for use in optical device applications.44 Therefore it is essential to develop a single crystal with reduced dislocation density. Etching studies were made on NSPI crystal using methanol: acetonitrile (1:1) as an etchant for different durations of 10 s and 20 s and are shown in Figure 9. The surfaces etched were dried by pressing between tissue papers and inspected under an optical microscope (magnified 50 times). Rectangular etch pits were noticed throughout the etching period; and the size of the pit increases with time. Moreover, the etch pits do not disappear, suggesting that they are due to dislocations.45 The calculated EPD (etch pit density) is 2.3 x 103 cm-2 and is comparable to other crystals.

46, 47

The low EPD value indicates that the NSPI crystal contains mimimum


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defects with better crystalline perfection. The rectangular etch pits suggest that the growth mechanism is a two-dimensional layer of the NSPI crystal.48, 49 4.7 UV- visible transmission studies To be practical, the NLO material should have a lower cut-off wavelength and a wider optical transmission window. As can be seen in Figure 10, the NSPI crystal exhibits good transmission with efficiency of about 60% in the 440 and 800 nm wavelength range. It also displays a wavelength cut-off at 428 nm expected to the π-π* transition of the stilbazolium chromophore in the present compound. The wide transparency ranges from visible to near infrared to an interesting feature of the NSPI crystal, which means that it is suitable for the production of optical gadgets. Although the NSPI crystal shows a wavelength cut-off around 428 nm, it is still better than the other stilbazolium derivatives 25, 31, 34 listed in Table 3. The reliance of the optical absorption coefficient (α) on the photon energy (hυ), is helpful for studying the electronic transition that occurs in the material.50 The absorption coefficient and band gap is estimated from the measured transmittance (T) using the expression, 51,52

α=

1 2.3026 log (T)

(1)

t 1/2

αhν = A(hν − Eg )

(2)

where t , A, Eg and ν denote the thickness of the grown crystal, the constant, the band gap and the incident frequency respectively. A graph of (αhν)2 vs hν is plotted and the intersection hν of the extrapolated straight line from (αhν)2 to hν gives Eg (Inset of Figure 10). The direct value of Eg for the NSPI crystal is calculated to be 2.83 eV. The band gap is calculated theoretically using the relation,



Eg =

hc λc

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(3)

The calculated theoretical band gap is 2.89 eV, which is closely related to the value of the band gap obtained from the inset of Figure 10. 4.8 Photoluminescence studies The photoluminescence (PL) study affords information on the emission characteristics of the materials at the molecular level, which include deep and shallow level deformities and energy vacuum states. 53 Presently, crystalline organic materials have extended potential applications to create Light Emitting Devices (LEDs). In view of the need of LED, the luminescence characteristic of the grown crystal is analysed by exciting it at 427 nm. Figure 11 displays the emission spectrum of the NSPI crystal recorded at room temperature. A broad red emission peak was observed at 721 nm, attributed to the π-π* of the stilbazolium chromophore in the NSPI crystal. The broad observed PL signal is due to the several electronic transitions that occur in the energy levels within the energy gap.54 The luminescent nature recommends that the NSPI crystal with the high emission intensity can be used as red light emitter and optical device applications.55 4.9 Laser-induced damage threshold studies Investigate laser damage threshold (LDT) on nonlinear materials considered necessary to design a device, as they must withstand the impact of a high power lasers.56 This parameter confines the possible usage of a material for device applications. In present case, LDT measurements are performed with a high intensity Nd: YAG laser with a wavelength 1.064 μm in a 10 Hz repetition at a pulse of 10 ns. A laser beam diameter of 1mm is used. The energy of the laser pulse is modified by an attenuator and allowed to the crystal positioned near the converging lens (30 cm focal length). The laser radiation is passed, and the energy is 12 ACS Paragon Plus Environment

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increased to the point where visual damage, followed by sound, has been identified on the surface. The input energy causing the damage is monitored by means of a power meter (model no: EPM 2000). The threshold induces damage to the surface is calculated using the most general formula, 57 Power density (Pd) = (E/τA)

(4)

where E, τ and A signify the energy of the laser source (mJ), the pulse width of the laser source (ns) and circular spot size area. The LDT for NSPI crystal is calculated to be 3.2 GW/cm2 , which is higher than KDP, Urea and DAST materials

58, 59

presented in Table 4.

Consequently, a greater optical damage resistance recommends that the NSPI crystal can be used efficiently in high intensity laser devices. 4.10 Third-order nonlinear optical properties As a centric space group, the NSPI crystal is able to generate a third harmonic generation (THG) revealed from structural analysis. To study the nonlinear properties of the NSPI crystal, which includes nonlinear refractive index (n2), nonlinear absorption coefficient (β) and nonlinear susceptibility (χ(3)), the Z-scan method was employed developed by Sheik Bahae and his collaborators.

60

It has gained swift acceptance by the scientific community

due to the simplicity, high sensitivity and well-elaborated theory. This technique is based on the transformation of amplitude distortion into phase distortion during beam propagation in the sample. In this technique, the sample is moved in the laser propagation axis (Z-axis) of the focused Gaussian beam i.e., -Z to +Z, by a computer-controlled translation stage so that the intensity transmitted from each Z position is measured in a far-field using a digital power meter (Field master GS-coherent). The sample behaves as a thin lens with a variable focal length.



The experiment takes two types of arrangements for measuring the transmitted intensity by means of closed aperture (CA) and open aperture (OA) configuration. When the crystal sample is exposed to the focal plane, the intensity of the beam decreases or increases which directly depends on the refractive index of the material and its nature of absorption. The change in transmittance as a function of the sample position Z in the open aperture and closed aperture mode gives precise information on the nonlinear refraction and nonlinear absorption. By monitoring the change in the transmittance through a small aperture in the far field (CA), the phase shift can be determined. The output transmittance depends on the aperture radius because a large aperture reduces the transmitted intensity change. Intensity dependent absorption is measured by inserting a lens and replacing the aperture in the detector to collect the entire laser beam transmitted through the sample (OA). In present characterization, a CW (continuous wave) 632.8 nm He-Ne laser with almost Gaussian intensity profile is focused by means of a 3 cm focal length lens so as to produce the beam waist (ωo) 12.05 μm at the point of focus, giving on-axis peak intensity of 26.31 MW/m2. The thickness of the sample 0.60 mm is less than the estimated Rayleigh length Zo (πωo2/2), i.e. 0.72 mm. It follows that the approximation of the thin sample is applicable in this case 61. The normalized transmittances recorded for the NSPI crystal in CA and OA Zscan modes are presented in Figures 12 and 13. Transmittance decreases form the valley before the focus, and then increases considerably form a peak after the focus (CA). This feature suggests the self-focusing behaviour (positive n2), which can be caused by reduced transmittance and large divergence of the beam through the far field aperture. Aperture radius of 2 mm is used and kept constant for the entire process. As the optical intensity increases, the transmittance decreases at the focus forming a dip (OA) and this type of nonlinear absorption corresponds to reverse saturable absorption (RSA, positive absorption coefficient). In RSA, the more absorption cross section is in the 14 ACS Paragon Plus Environment

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excited state than in the ground state.62 The standard formulas are followed to calculate the NLR, NLA, nonlinear susceptibility and molecular second-order hyperpolarizability of HMSI crystal according to the literature.32,

63-66

The peak and valley transmittance as a function of on-axis phase shift ( ΔΦO ) is given by, ∆Tp−v = 0.406 (1 − S)0.25 |∆Φo | where S = 1 − exp (

−2r2a ω2a

(5)

) is the linear transmittance of the aperture with ra and ωa indicating

the aperture radius and radius of the beam at the aperture. The NLR is expressed by relation,63

n2 =

∆Φo KIo Leff

(6)

Where K is the wave number (K=2π/λ), Io is the incident laser power at the focus, and Leff = [1-exp (-αL)]/α], is the effective thickness of the sample, α is the linear absorption coefficient and L the thickness of the sample. Using the OA Z-scan, the NLA can be determined by the expression, 64

β=

2√2∆T Io Leff

(7)

where ΔT is the minimum transmittance value in the OA trace. The value of β will be negative for saturable absorption and positive for two-photon absorption. The real and imaginary components of nonlinear susceptibility has been deduced from the values of n2 and β and defined by the relations 63-65,

Re χ

(3)

(esu) =

10−4 (ԑo c 2 no 2 n2 ) cm2 ( ) π W

(8)



Im χ

(3)

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10−2 (ԑo c 2 no λβ) cm2 (esu) = ( ) 4π2 W

(9)

where ɛo and c are the free space permittivity and the vacuum velocity of light, and no is the linear refractive index of the crystal. Thus, the nonlinear susceptibility of the third-order (χ(3)) is obtained using the relation, 2

2 1/2

|χ(3) | = [(Re χ(3) ) + (Im χ(3) ) ]

(10)

The molecular second-order hyperpolarizability was obtained from the relation,63,66 Re χ(3) Re [γ] = Nf 4

(11)

Where N is the molecular number density and f is the local field correction factor of Lorentz approximation described by,

L=

(n2o + 2) 3

(12)

The result of the nonlinear parameters of the NSPI crystal is enumerated in the Table 5. As can be seen from Table 6, it seems that the NSPI crystal has a higher order of nonlinear susceptibility (10-7), comparable to some organic crystals.

28, 68-70

This is because of the

electron density transfer from NO2 group to the N-ethyl pyridinium moiety, so that the strong delocalization of the π-electrons makes the molecule highly polarised, which is responsible for the enhanced nonlinearity of the NSPI crystal.

67, 68

The results obtained imply that the

NSPI crystal has a better third-order NLO response and this material can be ideally suited for applications such as optical limiting, optical signal processing, etc .


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5 CONCLUSIONS An organic stilbazolium derivative, (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide (NSPI) was synthesized and grown as single crystals by the slow evaporation technique. The NSPI crystallizes in the centric triclinic system with P-1as a space group. In addition, fourier transform infrared and nuclear magnetic resonance spectroscopic analyses were performed to affirm the molecular structure of NSPI crystal. The charge transfer characteristics of the present compound was identified using a Frontier Molecular Orbital analysis, which plays an important role in generating the nonlinear optical response. The thermal behaviour was evaluated by using thermogravimetric and differential thermal analysis, which guarantees the o

stableness of the NSPI crystal up to 151 C. Etch pit density of 2.3 x 103 cm-2 indicate better crystalline perfection with minimum defects. The high optical transmission ranges from the visible and the near infrared with a cut-off wavelength around 428 nm certifies that grown crystals are reasonable for applications to optical gadgets. The photoluminescence study indicates that the grown crystal has a high emission peak around 721 nm. The high laser damage threshold value of the NSPI crystal indicates that it may be suitable for applications to high power lasers. Z-scan studies were shown that the crystalline material of NSPI possesses self-focusing behaviour with reverse saturable absorption and good third-order nonlinear response. The results from various studies suggest that NSPI material is highly qualified for nonlinear optics and optoelectronic device applications in the future. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.



FTIR spectrum of NSPI (Figure S1), 1H NMR spectrum of NSPI (Figure S2),

Page 18 of 38

13

C NMR

spectrum of NSPI (Figure S3), Selected bond lengths (Å) and bond angles (o) for NSPI crystal (Table S1). AUTHOR INFORMATION Corresponding author *Sivaperuman Kalainathan E-mail: [email protected]. Telephone: +91-416-2202350 ACKNOWLEDGMENTS The authors are thankful to Akita University, Japan for providing the single crystal XRD facility and also the authors thank the management of Vellore Institute of Technology for their constant support and encouragements. REFERENCES 1. Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Electric field poled organic electro-optic materials: state of the art and future prospects. Chem. Rev. 2009, 110, 25-55. 2. Dadsetani, M.; Omidi, A. R. A DFT study of linear and nonlinear optical properties of 2-methyl-4-nitroaniline and 2-amino-4-nitroaniline crystals. J. Phys. Chem. C 2015, 119, 16263-16275. 3. Zhu, M.; Yang, C. Blue fluorescent emitters: design tactics and applications in organic light-emitting diodes. Chem. Soc. Rev. 2013, 42, 4963-4976. 4. Tao, Y.; Yang, C.; Qin, J. Organic host materials for phosphorescent organic lightemitting diodes. Chem. Soc. Rev. 2011, 40, 2943-2970. 5. He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Multiphoton absorbing materials: molecular designs, characterizations, and applications. Chem. Rev. 2008, 108, 12451330.


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Caption of the figures

Figure 1. Reaction scheme of NSPI


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Figure 2. Solubility plot of NSPI

Figure 3a. As grown NSPI crystal 3b. Morphology diagram of NSPI crystal



Figure 4. ORTEP diagram of NSPI

Figure 5. Molecular packing in the unit cell of NSPI


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Figure 6. Surfaces of Frontier Molecular Orbitals for NSPI crystal

Figure 7. MEP surface of NSPI crystal



Figure 8. TG/DTA thermograms of NSPI crystal

Figure 9. Micrograph images of surface (a) 0s; (b) 10s and (c) 20s. 32 ACS Paragon Plus Environment

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Figure 10. Optical transmission spectrum and band gap (inset) of NSPI crystal.

Figure 11. Emission spectrum of NSPI crystal



Figure 12. Closed aperture plot of NSPI crystal

Figure 13. Open aperture plot of NSPI crystal


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Caption of the tables Table 1. Crystallographic data and structure refinement for NSPI crystal Empirical Formula

C15H15IN2O2

CCDC

1419336

Formula weight

382.20

Temperature

100 K

Wavelength

0.71075 Å

Crystal system, Space group

Triclinic, Pī

Unit cell dimensions

a = 7.5150(5) Å

α = 82.674(2) o

b = 9.8080(6) Å

β = 75.929(2) o

c =10.1132(5) Å

γ = 86.569(2) o

Volume

716.83(8) Å3

Z

2

Density (calculated)

1.771 g/cm3

Absorption coefficient

2.237 mm-1

F (000)

376

Crystal size

0.200 X 0.200 X 0.200 mm3

Theta range for data collection

0.0 to 54.9o

Completeness to theta =54.9o

99 %

Refinement method

Full- matrix least-squares on F 2

Reflections Measured / Unique

7113/3252 (Rint = 0.0217)

Variable parameters

189

R indices (all data)

R1 =0.0173, wR2=0.0660

Goodness of Fit Indicator

1.270



Table 2. Comparative melting point of NSPI crystal and other stilbazolium derivatives Crystal

Melting point

References

(ºC) VMST

184.3

[28]

MSTB

193

[29]

3ETSI

201

[33]

VSNS

204.3

[30]

NSPI

235

Present work

Table 3. Comparative optical studies of NSPI crystal and other stilbazolium derivatives Crystal

Cut-off

References

wavelength (nm) NSPI

428

Present work

TASI

437

[34]

HSPI

467

[31]

DAST

475

[25]

DSMOS

476

[25]

Table 4.Comparison of laser damage threshold value for NSPI and some NLO crystals Crystal

Laser damage threshold

References

(GW/cm2) KDP

0.2

[58]

Urea

1.5

[58]

DAST

2.8

[59]

NSPI

3.2

Present work


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Table 5. Calculated third-order nonlinear optical parameters for NSPI crystal Nonlinear absorption coefficient (β)

1.78 x10-5 mW-1

Nonlinear refractive index (n2)

3.99 x10-11 m2W-1

Third-order nonlinear optical susceptibility (χ(3))

9.09 x10-7 esu

Second order molecular hyperpolarizability (γ)

6.33 x 10-34 esu

Table 6. Comparison of χ(3) value of NSPI and some organic NLO crystals Crystal

Laser source

Wavelength (nm)

χ (3) esu

References

VMST

He-Ne

632.8

9.69 x 10-12

[28]

QN

He-Ne

632.8

4.07 x 10-12

[69]

4MBS

He-Ne

632.8

11.04 x 10-8

[70]

DSMOS

Nd: YAG

1064

5.05 x 10-8

[68]

NSPI

He-Ne

632.8

9.09 x10-7

Present work



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A New Nonlinear Optical Stilbazolium Family Crystal of (E)-1-Ethyl-2

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