Effect of Metal Dopant on Ninhydrin — Organic Nonlinear Optical Single Crystals

1 Department of Chemistry, Saraswathi Velu College of Engineering, Sholinghur, Vellore 631 102, India Department of Physics, Veltech Multi Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Avadi, Chennai 600 062, India Department of Physics, SRR Engineering College, Padur, Chennai 603 103, India Department of Chemistry, Vel Tech Dr. RR & Dr. SR Technical University, Chennai 600 062, India Department of Physics, Presidency College, Chennai 600 005, India


Introduction
e search for new materials with high optical nonlinearity is an important area due to their practical applications such as optical communication, optical computing, optical information processing, optical disk data storage, laser fusion reactions, laser remote sensing, colour display, medical diagnostics, and so forth [1][2][3][4].In semiorganic materials, the organic ligand is ionically bonded with inorganic host.Due to this, the new semiorganic crystals have higher mechanical strength and chemical stability.Most of the organic NLO crystals usually have poor mechanical and thermal properties and are susceptible to damage during processing even though they have large NLO efficiency.Also it is difficult to grow larger size optical quality crystals of these materials for device applications.Purely inorganic NLO materials have excellent mechanical and thermal properties but possess relatively modest optical non linearity because of the lack of extended pi-electron delocalization.
Organic NLO materials are oen superior to inorganic in terms of their response speed, optical clarity, and the magnitude of their third order susceptibility and for a material to exhibit NLO activity it should be noncentro symmetric.Hence it may be useful to prepare semiorganic crystals which combine the positive aspects of organic and inorganic materials resulting in useful non linear optical properties.Organic materials with aromatic rings having high non linear optical coefficient, higher laser damage threshold, fast response, low mobility, and large band gap �nd many applications [5][6][7][8].
Ninhydrin C  H 6 O 4 (2,2-Dihydroxyindane-1,3-Dione) is one such organic materials with high melting point and two hydroxyl groups attached to the same carbon atom.It is used to detect ammonia or primary and secondary amines and �ngerprints.It is also used in amino acid analysis of proteins.Ninhydrin molecules play a vital role in many �elds like soil biology, chemistry, agriculture, medicine, forensic, food science, and so on.Also it is a potential material for micromolar determination of human serum albumin based on chemiluminescence and microbial activity [9].Recently, Uma Devi et al. found the suitability of ninhydrin crystals for the nonlinear optical applications and found that second harmonic generation e�ciency is �ve times that of KDP.Ninhydrin crystallizes in a noncentrosymmetric space group of P2 1 and belongs to monoclinic system [10].Uma Devi et al. already reported the growth and characterization of pure ninhydrin [11] and urea with ninhydrin [12] and stated that urea ninhydrin monohydrate crystal crystallizes in centro symmetric space group of P21/c.e addition of some transition metal ions is expected to in�uence the growth kinetics, habit modi�cation, and the large-size single crystals.e presence of small amount of impurities such as Ni 2+ , Cu 2+ , and Mg 2+ plays an important role in the growth rate, habit modi�cation of the crystal, and its properties [13].An impurity can suppress, enhance, or stop the growth of crystal completely.e impurity effect depends on the impurity concentration, super saturation, temperature, and pH of the solution and this can be successfully explained already for many NLO crystals [14].In the present work, we report the growth of Cu 2+ -doped ninhydrin single crystals and studied the effect of Cu 2+ in the system.

Materials and Methods
Single crystals of Cu 2+ doped ninhydrin were grown by slow evaporation method.To the 100 mL saturated solution of ninhydrin, 0.2% of CuCl 2 was added slowly and stirred well for nearly 2 hours, then �ltered, and allowed to cool.�ithin 10 days tiny, pale yellow crystals of dimension 5.07 mm × 3.41 mm × 2.69 mm were harvested.
2.1.Characterization.e grown crystals have been subjected to various characterization studies like single crystal XRD, FT-IR, dielectric, and SHG.e grown crystals of the title compound were subjected to single crystal XRD analysis using ENRAF Nonius AD4/MAC4 X-ray diffractometer with M o K  (  1 Å) radiation.e grown crystals have also been characterized by X-ray powder diffraction technique using Rich Seifert X-ray powder diffractometer with CuK  radiation of   1 Å. e 2 range was analyzed from 10 ∘ to 70 ∘ by employing the re�ection mode for scanning.e detector used was a scintillation counter.e UV-Visible spectrum of the grown crystal was recorded between 200 and 1200 nm using CARY/5E/UV spectrophotometer.A Perkin Elmer Spectrum one FT-IR spectrometer was employed to record the IR spectrum to analyze the functional groups present in the crystals.e sample for this measurement was �nely grounded and mixed with K�r.e dielectric study was carried out using the instrument, HIOKI model 3532-50 LCR HITESTER.Samples of known dimension (5.07 mm × 3.41 mm × 2.69 mm) were silver coated on the opposite faces and then placed between the two copper electrodes to form the parallel plate capacitor.e capacitance of the sample was noted for the applied frequency that varies from 100 Hz to 5 MHz at different temperatures (353 K and 373 K). e grown crystals of ninhydrin were subjected to Kurtz second harmonic generation test by using Nd:YAG Q switched laser beam with input pulse of 0.68J for the non linear optical property.

Results and Discussion
3.1.Single Crystal XRD.e single crystal XRD data of the title crystal indicates that it crystallizes in the monoclinic system with noncentrosymmetric space group P2 1 with lattice parameters   112 Å,    Å,   1 Å,  = 90 ∘ ,   ,  = 90 ∘ , and   1 (Å)  , which agrees very well with the reported value [10].It is observed that there is a slight change in unit cell parameters which re�ects the lattice distortion due to the substitution of Cu 2+ [15].

Powder X-Ray Diffraction Analysis
. e crystal structure of ninhydrin was reported by Medrud [10].e title crystal crystallizes in monoclinic with space group P2 1 .Figure 1 shows the indexed X-ray powder diffraction pattern of the grown title crystal.e results agree well with XRD pattern of ninhydrin molecule [11].Appearance of sharp and strong peaks con�rms the good crystallinity of the grown crystals.e prominent peaks have been indexed.A small change in the intensity level of the peaks and peak position at the higher angle side is due to the addition of copper.F 2: Optical transmission spectrum Cu 2+ doped ninhydrin.

Optical Transmission Spectral
state to higher energy state.e UV transmission spectrum of grown crystal is shown in Figure 2. e determination of UV transparency and cutoff wavelength is very important since these crystals are mainly used in optical application.e lower cut off wavelength is found to be 314 nm and upper cut off wavelength is 441 nm.Between 450 and 1200 nm, there is no absorption of wavelength which clearly indicates that grown crystals can be used as window material in optical instruments.e small peak at 339 nm is due to n- * transition [16].High transmittance % observed from 450 nm indicates that the crystal possesses good optical transparency for SHG of Nd:YAG laser.

FTIR Analysis.
Fourier Infrared spectrum was recorded using KBr pellet technique in the range 400-4000 cm −1 and the recorded FTIR spectrum is shown in Figure 3. Vibrational spectroscopy provides an important tool to understand the chemical bonding.e strong absorption peak at 3298 cm −1 is due to O-H symmetric stretching [12] and 3087 cm −1 is due to aromatic C-H stretching.e carbonyl (C=O) peak is observed at 1747 cm −1 and 1717 cm −1 .A medium intense peak observed at 1592 cm −1 is attributed to aromatic ring vibration [17].Peaks at 1292 cm −1 and 741 cm −1 are attributed to in-plane bending modes of aromatic C-H bonding and out-of-plane aromatic C-H bonding, respectively [18].e peaks at 1063 cm −1 , 1153 cm −1 , 1186 cm −1 , 1255 cm −1 , and 1292 cm −1 are attributed to the plane bending modes of aromatic C-H bonds.
3.5.Dielectric Studies.e dielectric properties are associated with the electro-optic property of materials, particularly when they are nonconducting materials [19].Microelectronics industry needs low   materials as an interlayer dielectric.Figures 4 and 5 show the variations of dielectric constant and dielectric loss of the title crystal at two different temperatures, that is, 353 K and 373 K, as a function of frequency.e dielectric constant is calculated using the formula where C is capacitance (farad), t is the thickness (metre), A the area (m 2 ),   is the absolute permittivity in the free space having a value of 8.854 × 10 −12 Fm −1 .It is seen that the value of dielectric constant is found to decrease and attain constant values in the higher frequency region.e decrease in dielectric constant of the title crystal at low frequencies may be attributed to the contribution of the electronic, ionic, orientation, and space charge polarizations which depend on the frequencies.At low frequencies all the four contributions are active [20].e low value of   at high frequencies are important for these materials in the construction of photonic and NLO devices which suggests that the sample possesses enhanced optical quality with low level defects [20][21][22].Dielectric loss decreases with increase in frequency.e larger value of   and tan  at low frequency arises due to the presence of space charge polarization near the grain boundary interfaces which depends on the purity and perfection of the sample [22].
3.6.SHG Test.Second harmonic generation test was performed to �nd the NLO property of the grown crystal by using Kurtz-Perry technique [23].Nd:YAG laser using the �rst harmonics output of 1064 nm with pulse width of 8 ns and repletion rate 10 Hz was passed through the sample.e green signal was emitted which con�rms the second harmonic generation efficiency.

Conclusion
Single crystals of Cu 2+ doped ninhydrin were grown by slow solvent evaporation technique.Single crystal XRD con�rmed the unit cell parameters with the reported value.In powder Xray diffraction pattern, appearance of sharp and strong peaks con�rms the good crystallinity of the grown crystals also the prominent of peaks has been indexed.UV-Vis-NIR study reveals the suitability of the crystal for NLO applications and the lower cut off wavelength is found to be 314 nm and upper cut off wavelength is 441 nm.ere is no absorption of wavelength in the entire visible region.FT-IR spectrum gives the various functional groups present in the structure.A medium intense peak observed at 1592 cm −1 is attributed to aromatic ring vibration.e carbonyl (C=O) peak is observed at 1747 cm −1 and 1717 cm −1 .e other stretching modes have also been assigned.From the dielectric study, it is found that both dielectric constant and dielectric loss of the crystal decrease with increase in frequency.e emission of green signal con�rms the second harmonic generation efficiency of the crystal.us, the moderate SHG efficiency and encouraging dielectric properties of the crystal indicate the suitability of this crystal for photonic device fabrication.
Analysis.e UV-Vis spectrum gives information about the structure of the molecule that the absorption of UV and visible light involves in the promotion of electrons in  and  orbital from the ground

F 4 :
Variation of Dielectric constant with frequency for different temperatures.

F 5 :
Variation of dielectric loss with frequency for different temperatures.