ABSTRACT2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}-malonitrile (DAT2) is a potential candidate for generating Terahertz radiation in the range 0.1-3 THz. In the present work, we synthesized DAT2 compound and confirmed it’s molecular structure by proton NMR. Good quality single crystals of DAT2 of size 1.2×0.5× 0.2 cm3 are grown for the first time, using solvent evaporation technique. The grown crystals are tested for crystalline perfection using powder XRD study and the crystal structure is determined to be monoclinic with centrosymmetric space group P 2/C. The electronic band structure was studied by Density Functional Theory and the theoretical band gap was compared with experimental bandgap 1.6 eV. Further,using DFT the dielectric constants [εr (ω ) andεi (ω )], reflectivityR (ω ), refractive index n (ω ), extinction coefficient K (ω ), optical conductivity σ (ω ) and energy loss function L (ω ) for DAT2 crystal has been calculated. The results are discussed in detailIndex terms: nonlinear optical material. organic compound. crystal growth DFT study. dielectric constant. refractive indexINTRODUCTIONOrganic nonlinear compounds are found to be very attractive for ultrafast photonic applications such as optical switching, second harmonic generation and terahertz (THz) wave generation oweing to their high nonlinearities and ultrafast response times1. Organic NLO crystals like DAST,DSTMS, BNA and OH1 etc., possessing very high second order nonlinear coefficients, are presently extensively studied for generation of Terahertz between 0.1-10 THz . 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}-malonitrile (DAT2) is an organic nonlinear optical material studied for terahertz generation (THz) in the region 0.1 – 3 THz1,2. Many explosives like RDX, TNT etc., have their finger prints in this region and hence DAT2 can be used as a suitable candidate for detection of hidden explosives 3. DAT2 has a molecular formula of C21H23N3. It has very high electro-optic coefficient of r 12 = 56 pm/V2.The reason of high NLO response in DAT2 arises from the electron donor in dialkylamino group and the electron acceptor is a dicyanomethylidene, >C—-C(CN)2 group connected by a π-conjugated hexatriene bridge. Compared with other organic crystals, DAT2 posesses high thermal stability and good SHG efficiency. Crystal growth DAT2 by melt and vapour growth methods produced only few millimeter sized crystals, which makes it unsuitable for fabrication of optical devices. Hence, the authors are motivated in the developing large size DAT2 single crystals suitable for Terahertz applications.The computational chemistry is an easy way to study the molecular level properties of complicated structures, their bonding interactions, electronic and optical properties etc.,(Shoba et al. 2014, )5. It is important to know the physical properties such as material’s bandgap, dielectric constant, refractive index, conductivity,reflectivity etc.,to evaluate the performance related to optolectronic applications6,7, Also, no quantum chemical calculations were reported for DAT2 molecule so far. Hence, the authors are interested to study the important properties such as electronic band structure, dielectric constant and linear optical properties of DAT2 crystal. In the present work, the structural, electronic and optical properties of DAT2 crystal are determined by the ab initio pseudopotential density functional method. The results are presented and discussed in detail.II Material Synthesis and Crystal Growth : The raw materials 4-dimethyl aminobenzaldehyde, (3,5,5-trimethylcyclohex-2-enylidene) malononitrile, acetonitrile and other chemicals were purchased from Sigma-Aldrich (99.99% purity) and used for synthesis. DAT2 was synthesized by procedure8 described in Fig. 1. 4-dimethyl aminobenzaldehyde (0.1 mol) and (3,5,5-trimethylcyclohex -2- enylidene) malononitrile (0.1 mol) were dissolved in acetonitrile (100 ml) in a round-bottomed flask. To this mixture piperidine (0.01 mol) was added and the solution was stirred at 40̊ C for 24 h. The product formed was confirmed by TLC plate. The resulting solution was roto-evaporated and dried. The product was purified by column chromatography using n-hexane and ethyl acetate mixture (99.99/1). The yield obtained was 37%. DAT2 was dissolved in deutrated chloroform (CDCl3) and the proton NMR spectrum was recorded using Bruker instrument operating at 500MHz. 1H NMR (400 MHz, CDCl3): δ = 7.38 (d, J = 8.9 Hz, 2H), 7.01 (d, J = 16 Hz, 1H), 6.76 (d, J = 16 Hz, 1H), 6.73 (s, 1H), 6.64 (d, J = 8.9 Hz, 2H), 3.12 (s, 6H), 2.74 (s, 2H), 2.65 (s, 2H), 1.06 (s, 6H).13C NMR (100 MHz, CDCl3): δ = 169.2, 155.3, 151.2, 138.3, 129.5,129.4, 124.4, 121.4,114.1, 113.4, 112.3, 75.7, 42.9, 40.3, 39.2, 31.8, 28.1.For growing crystals of DAT2, the as synthesized salt was recrystallized twice from methanol to increase it’s purity. In the present work, we attempted to grow DAT2 single crystals using methanol. But, it yielded only small sized needle type crystals. Next, the solubility of DAT2 in acetone was determined. At room temperature the solubility was experimentally determined to be 5.2 g/100ml. Saturated DAT2 solution was prepared at 40̊ C and allowed to cool down to room temperature by slow evaporation of the solvent. Dark-red coloured plate like DAT2 crystal of dimension 1.2 ×0.5 × 0.2 cm3 was obtained after two weeks as shown in Fig. 2aIII Crystal structure : To identify the crystal structure and to calculate the lattice parameters of our as grown DAT2 crystals, powder X-ray diffraction analysis was carried out using Rigaku Ultimata III X-ray diffractometer. Cu Kα radiation of wavelength, 1.5405 Å was used as source for diffraction. The glancing angles were varied from 5 to 80◦ at the scan rate of 2° per minute. Fig. 2b shows the Bragg’s diffraction peaks obtained for powdered DAT2 sample. The experimental peaks were compared with Crystallographic Open Database (COD no: 96-710-1371). Using hkl values, the lattice parameters were calculated a = 6.1305Å, b = 7.424 Å, c = 20.258 Å,α = 90 ̵̊, β = 96̊ γ = 90̊. he crystal structure DAT2 belongs to monoclinic with space group P 2/c.