ABSTRACT
2-{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 detail
Index terms: nonlinear optical material. organic compound.
crystal growth DFT study. dielectric constant. refractive index
INTRODUCTION
Organic 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. 2a
III 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.