The study of imine-bridged organics has been the hot spot synthesis of second order nonlinear optical (SONLO) and photo-responsive materials in recent years. Herein we present a study of synthesis, growth, and characterization of ([paranitrophenyl]imino)benzene (PNPIB) NLO single crystal. The title compound was synthesized in one step by Schiff base formation. <110> PNPIB single crystal of diameter of 40 mmand length 50 mmwas successfully grown by SR method using a seed as the nucleus. The growth rate formula is derived for the SR method. PNPIB single crystals of 10 mmdiameter and 60 mmheight have been grown at an average growth rate of 3 mmper day from the point seed in a glass crystallizer. Almost 100% stable crystal conversion efficiency was achieved. The as grown PNPIB crystals were characterized using single crystal X-ray diffraction (XRD), X-ray powder diffraction (XRPD), Fourier Transform Infrared (FTIR), Ultraviolet-Visible-Near Infrared (UV-Vis-NIR), 1H & 13C NMR spectral studies, dielectric measurement and NLO studies. Single crystal XRD analysis confirms that the grown ingot belongs to the space group of P 2 of monoclinic system thus exhibiting noncentrosymmetric structure. The crystalline perfection was assessed by XRPD. The powder diffraction pattern of the grown crystal has been indexed. The presence of C=N bond with intramolecular hydrogen bonding and the protonation of ions were confirmed by FTIR analysis. The UV-Vis-NIR spectrum of the crystal shows that the crystal has a cut-off at 298 nm. The 1H & 13C NMR spectra confirm the molecular structure. The dielectric behaviour was measured in the frequency range 1 KHz - 10 KHz for the temperature range from 30℃ to 170℃. The slight decrease in dielectric constant has been observed as the frequency is increased and the dielectric loss is very low for the entire frequency range. The second harmonic generation (SHG) in the crystal was observed by Kurtz powder technique.
Second order non-linear optical (SONLO) materials have recently attracted much attention because of their potential application in emerging optoelectronic technologies [1,2]. Materials with large second order optical nonlinearities are used in the area of laser technology, laser communication and data storage devices [3,4]. The search for new conversion materials over the past decades lead to the discovery of many organic NLO materials with high nonlinear susceptibilities. Organic materials have been demonstrated in recent years to possess superior second order and third order NLO properties compared to the more traditional inorganic materials. The structural flexibility of organic compounds is an asset for materials with optimised second order NLO susceptibility, fast response and tailor made flexibility [
Most of the chemicals in this work were purchased from E-Merck and used without further purification. The title compound was synthesised by one step condensation method. Equimolar ratio of para-nitrobenzaldehyde and aniline were dissolved. The reaction mixture was stirred for about an hour to give a yellow crystalline salt. The as obtained crystalline salt was used for solubility and growth experiments. The reaction scheme is shown in
The synthesized salt was used to measure the solubility of pure PNPIB crystals in Dimethyl formamide (DMF). A 250 ml borosil glass beaker filled with 100 ml DMF was placed inside a constant temperature bath. An acrylic sheet with a circular hole at the middle was placed over the beaker through which a spindle from an electric motor, placed on the top of the sheet was introduced into the solution. A Teflon paddle was attached at the end of the rod for stirring the solution. The synthesized salt was added in small amounts with DMF solvent and stirring was continued till the formation of precipitate, which confirmed the supersaturation of the solution. A 20 ml of the saturated solution was withdrawn by means of a warmed pipette and the same was poured into a clean, dry and weighed petri dish. The solution was kept in a heating mantle for slow evaporation till the whole of the solution got evaporated and the mass of PNPIB in 20 ml of solution was determined by weighing petri dish with salt and hence the solubility, i.e. quantity of PNPIB salt in gram dissolved in 100 ml of the solvent was determined. The solubility of PNPIB crystals in DMF solvent was determined for five different temperatures (30˚C, 35˚C, 40˚C, 45˚C and 50˚C) by adopting the same procedure. The resulting solubility curve of pure PNPIB is shown in
It is well known that the evaporation rate of the solvent DMF into the atmosphere is a function of temperature, humidity and air velocity. It is evident that the evaporation process in the atmosphere is diffusion of DMF molecules coming out of its surface through the air larger covering its surface. To calculate theoretically the absolute evaporation rate, we must know the diffusion coefficient of DMF vapour in air and the thickness of the boundary layer accurately. Kazuo Histake et al. reported a detailed survey on the evaporation rate [
material (g/cm3) and T is the temperature (K). By using the above parameters, the growth rate of the crystal is calculated. The evaporation rate of the solvent in an ampoule was also measured by observing the lowering rate of the top surface of the solution level.
The experimental set-up for the grown PNPIB single crystal by S-R method is shown in
The single crystal X-ray diffraction was recorded with Bruker Kappa APEXII diffractometer and the wavelength of X-ray used was 0.7093 Å (Target-Molybdenum). From the single crystal XRD, the lattice parameters were calculated and the crystal belongs to monoclinic system with a space group of P2. The lattice parameters are a = 14.48 Å, b = 10.72 Å, c = 14.58 Å, α = 90˚, β = 101.95˚ and γ = 90˚ and v = 2214 Å3. The lattice parameters were calculated using SHELXL programme.
Powder XRD analysis was carried out using XPERT
Powder diffractometer with CuKα (λ = 1.541 Å) radiation to confirm the structure of the as grown PNPIB crystals. The sample was scanned over the range 10˚ - 60˚ at a rate of 1˚ nm−1. The XRPD and peak indexing are shown in
FTIR spectrum is important evidence that provides more information about the structure of a compound. In this technique, almost all functional groups in a molecule absorb characteristically within a definite range of frequency. The absorption of IR radiation causes the various bonds in the molecule to stretch and bend with respect to one another. The most important range (4000 - 400 cm−1) is of prime importance for the study of an organic compound by spectral analysis [
It is very helpful in the investigation of the NLO materials also making it possible to check, apart from NLO responses and also spectroscopic absorbance in the appropriate wavelength. Therefore the wavelength obtained by UV visible spectral analysis can be helpful in the synthesis of promising NLO materials [
shown in
In order to analyze carbon-hydrogen bonded network, 1H and 13C NMR spectra were recorded using Bruker ARX 300 spectrometer in CDCl3 at 300 K. 1H and 13C spectrum of PNPIB single crystals are shown in Figures 9 and 10 respectively. Signals between 7.2 ppm to 7.5 ppm correspond to aromatic protons attached to imino group. Peak signals between 8 ppm to 8.4 ppm correspond to aromatic protons attached to nitro group. A singlet at 8.6 ppm is attributed to proton attached to nitogen of imino group. Absence of peak around 3.5 ppm suggests the complete removal of aniline the starting compound. The signals between 148 ppm and 152 ppm are assigned to carbons attached to nitro group and imino group. The absence of signal beyond 160 ppm suggests the complete removal of the starting material benzaldehyde in the crystal. The signals between 120 ppm to 130 ppm correspond to the aromatic carbons.
The dielectric study on PNPIB single crystal was carried out <010> face of PNPIB crystal using the instrument HIOKI 3532-5 LCR HITESTER. A sample of dimension 2 × 6 × 2 mm3 having silver coating on the opposite faces was placed between the two copper electrodes and thus a
parallel plate capacitor was formed. The capacitance of the sample was measured by varying the frequency from 500 Hz to 5 MHz.
10 9 8 7 6 5 4 3 2 1 0 ppm
200 180 160 140 120 100 80 60 40 20 0ppm
A preliminary study on the SHG efficiency of the grown PNPIB crystal was measured by Powder Kurtz method. The measured SHG efficiency was compared with KDP and found that the grown PNPIB crystal has nearly 1.5 times higher NLO efficiency than KDP, which is familiar inorganic NLO material.
PNPIB single crystal was synthesized through solventfree reaction mechanism and grown by both slow solvent evaporation technique and S-R method. The lattice parameters and the noncentrosymmetric space group were confirmed by Single crystal XRD analysis. XRPD analysis ascertained the perfect crystalline arrangement in PNPIB structure. FTIR spectral analysis confirms the presence of functional groups and their mode of vibrations in PNPIB crystal. UV-Vis-NIR spectral analysis has reached the conclusion that PNPIB exhibits solvatochromism and intramolecular π ® π* charge transfer transition which proves that PNPIB has a good NLO property. H1 & C13 NMR spectral analysis strongly confirm the carbon-hydrogen network among imino and nitro functional groups. Dielectric measurements were carried over the range 500 Hz - 5 MHz which shows that the dielectric constant decreases slowly with increase in frequency thus PNPIB crystal exhibiting good optical quality.
The authors S. Anbarasu and Prem Anand Devarajan would like to thank University Grants Commission, Bahadurshah Zafar Marg, New Delhi-110 002, India for funding this Major Research Project (File No.: 40-434/ 2011(SR), dt. 04.07.2011).