Physicochemical Studies on Thienyl Chalcone Derivative for Nonlinear Optical Application

Single crystals of 1-(5-chlorothiophen-2-yl)-3-(2,4,5-trimethoxyphenyl)prop-2-en-1-one (CTTMP) having nonlinear optical property have been grown by slow evaporation technique. The functional groups were identified by FT-IR and NMR techniques. The mechanical property of the crystals was studied using Vicker’s microhardness tester and the load dependence hardness was observed. The material is dimensionally stable up to 112C. The dielectric properties of the crystals were determined using semiconductor characterization system. The optical limiting behavior of the crystal was studied using Z-scan technique.


Introduction
Nonlinear optical (NLO) materials play an important role in fields like photonics and optoelectronics [1].Such materials find application in technologies such as optical computing and optical communication [2,3].Recently, much focus has been given to organic NLO materials for applications in field of photonics [4] due to their large nonlinear response, good optical limiting behavior, fast switching time, and convenient optimization routes through molecular design compared to the currently used inorganic materials [5].Chalcone derivatives are one of the interesting groups of materials as they are exhibiting high NLO coefficients and good crystallizability [6].The intramolecular charge-transfer feature available in the chalcone allows us to design new molecules substituted for donor or acceptor groups, which gives an understanding of structure-property relationship [7].Several chalcone compounds are reported to crystallize in noncentrosymmetric crystal pattern and provide necessary configuration for NLO activity with two planar benzene rings connected through a conjugated double bond and a carbonyl moiety [8,9].Most of the work reported on these chalcones is on second order nonlinear efficiency [10,11].The strong delocalization of electrons in these systems is responsible for high molecular polarizability and their remarkable third order nonlinearities.However the third order nonlinear efficiency of Donor--Acceptor--Donor (D--A--D) type of molecules with centrosymmetric crystal structure is not found in the literature.Recently, we have reported crystal structures and second or third order nonlinear optical properties of few chalcone derivatives having thienyl group [12,13].We have also reported synthesis and crystal structure of 1-(5-chlorothiophen-2-yl)-3-(2,4,5-trimethoxyphenyl) prop-2-en-1-one derivative [14].However, physicochemical property of this derivative has not been investigated.In order to find the feasibility of CTTMP crystal for photonic device applications the optical limiting behavior and mechanical and thermal stability have been studied.procedure reported in the literature [14].The chemical structure of the compound is given in Figure 1.

Experimental Procedure
The solubility studies in organic solvents showed the compound is sparingly soluble in ethanol, methanol, DMF, and acetone.To grow large single crystals of CTTMP, we preferred DMF due to its moderate solubility and less volatile nature of the solvent.A known volume of solvent was taken in a conical flask, which was immersed in a constant temperature bath.The powdered sample was added until the dissolution ceased.The solubility of CTTMP at various temperatures, from room temperature to 50 ∘ C, was determined and is shown in Figure 2. The solubility increases almost linearly with the temperature and hence slow cooling or slow evaporation at constant temperature could be a method of choice to grow the single crystals of CTTMP.
Saturated solution of CTTMP in dimethyl formamide (DMF) was prepared at 30 ∘ C and was filtered to remove the suspended particles, if any and kept undisturbed in a dustfree environment.At the period of super saturation, tiny crystals were nucleated.They were allowed to grow to a period of two to three days and then harvested.The laboratory grown crystals of CTTMP are shown in Figure 3.The morphology of the CTTMP single crystal was indexed using the single crystal X-ray diffraction data reported [15] in the literature as input to the morphology indexing computer program (WinXMorph) [16].The indexed morphology of CTTMP is shown in Figure 4.The crystals are prismatic, with large habit face (1 0 0) and distinct cleavage plane (0 1 1).It is clear that the (1 0 0) plane is having larger area due to the reduced growth rate along the crystallographic a-direction compared to b-and cdirections.

Result and Discussion
3.1.FT-IR and NMR Spectral Studies.The FT-IR spectrum of the synthesized material was recorded in the wave number range 400-4000 cm −1 by KBr pellet technique (SHI-MADZU 8400S FT-IR spectrometer).The spectrum shows the characteristic absorption at 1647 cm −1 attributable to C=O group of , unsaturated carbonyl compound confirming the formation of 1 Further, the compound is also characterized using proton NMR.Thus, the two doublets observed at  8.022 and 7.212 with a coupling constant of 15.2 correspond to protons of-CH=CH-group.Other characteristic peaks observed in the spectrum are : 7.543 (d, 1H, H of thiophene ring), 7.008 (d, 1H, H of thiophene ring), 6.914 (t, 1H, H of aromatic ring), 6.444 (d, 2H, H of aromatic ring), and 3.849 (3 closely packed singlet, 9H, 3×OCH 3 ).

Surface Structure of the Crystals.
The most common habit of the crystal CTTMP grown in DMF is prismatic.The scanning electron micrograph analysis was carried out to visualize growth patterns of single crystals by using the instrument JEOL JSM-6380LA.The SEM of the crystal (Figure 5) shows step-like patterns on the surface, which confirms the layered growth of the crystal.The DSC plot given in Figure 6 shows a sharp endothermic peak at 112 ∘ C, which corresponds to the melting point of CTTMP.
The melting point of the crystals infers that the material may be suitable for device fabrication up to a temperature of 112 ∘ C. Further it indicates that there is no phase transition before melting.The sharpness of the peak shows the good degree of crystallinity and purity of the sample.Density of the compound was measured using specific gravity bottle at ambient temperature (27 ∘ C).The average value of density was found to be 1.356 g/cm 3 .
The hardness of the crystal is an important factor to be considered in device fabrication.The hardness of the crystal was found from Vicker's microhardness tester.Smooth and flat surfaces of the crystals were selected for this study and for each trial indentation time were 10 s and the diagonal lengths of the indented impression were measured for different loads.Vicker's microhardness number was calculated using the relation VHN = 1.8544P/d 2 kg⋅mm −2 , where d is the mean diagonal length of the indenter impression in millimeter and P is the applied load in kg.The load dependence of Vicker's hardness number is shown in Figure 7. Vicker's hardness number initially decreased with increase in load and saturated above the load of 50 g.The decrease in VHN value with load may be due to the plastic flow of the material.The measurement performed beyond a load of 25 g resulted in severe cracks.This is due to the release of internal stress generated locally by indentation.Similar load dependence was observed in other NLO crystals such as 3-(3,4-dimethoxyphenyl)-1-(4methoxyphenyl) prop-2-en-1-one [17], urea, and N-methyl urea [18].

Linear and Nonlinear Optical Studies.
The refractive index of the CTTMP single crystal was determined by Brewster's angle method.It was found to be 1.582 at 632.8 nm and 1.594 at 543.5 nm, respectively.The refractive indices were measured only along one of the crystallographic directions due to its platy type growth.By the measurement of refractive index it is possible to measure the electrooptic r-coefficients.Electrooptic effect is useful in the modulation of light intensity.Since CTTMP has higher value of index of refraction at 632.8 nm and 543.5 nm, it can be used to modulate light intensity in optical limiters.
The SHG conversion efficiency of the compound was measured by powder technique [19] using Nd:YAG laser of wavelength 1064 nm and beam energy was 4 mJ/pulse.The compound did not show SHG efficiency may be attributed to the fact that it crystallizes with inversion symmetry as revealed from single crystal X-ray diffraction study reported by us earlier [15].
Prabhu et al. [14] reported third order optical nonlinearities of CTTMP sample using single beam Z-scan technique.The third order NLO property of CTTMP is found to be higher than that of CTDMP compound [12].From the reported studies it has been observed that the compound exhibits two-photon absorption as well as self-defocusing effect [20,21].The optical limiting behavior of CTTMP is extracted from the open aperture Z-scan curve shown in Figure 8.The samples behave linearly until incident fluence of 10 4 J/m 2 and transmittance decreases for higher incident fluencies, suggesting the occurrence of optical limiting.The limiting threshold of the sample at the concentration of 1 × 10 −3 mol/L is 7 × 10 4 J/m 2 .

Dielectric Properties of the Crystals.
The dielectric studies were carried out using semiconductor characterization system (Kethely 4200SCS) for frequencies ranging from 1 kHz to 5 MHz with an applied voltage 1 mV at laboratory temperature.The graph of the dielectric constant   (solid line) and the dielectric loss factor tan  (dashed line) at various frequencies for the crystal plate of CTTMP is shown in Figure 9.The graph shows that   and tan  values were not varying much with higher frequencies.Also the dielectric loss factor is very low, except at low frequencies.The slightly higher values of   and tan  at low frequencies may be due to factors like grain boundaries and defects.The dielectric constant of CTTMP crystal is comparable to those of wellestablished NLO crystals such as OH1 and DAST as molecular structures of CTTMP are quite comparable with an extended conjugation.
3.6.Structure-NLO Property Relationship.CTTMP is having high third order nonlinearity due to the delocalization of  electrons.The molecular structure of the compound contains chlorine at 5-position of thiophene ring which can act as an electron donating group by mesomeric effect.The OCH 3 group is also a donor group.The electron donating strength of the OCH 3 is greater than Cl.The CTTMP molecule containing Cl and three OCH 3 groups at the ends and an electron acceptor carbonyl (C=O) group at the middle forms a donor--acceptor--donor (D--A--D) system, where charge transfer takes place from the donor end to the acceptor at the middle of the molecule.The charge transfer to the carbonyl group is more effective from the donor attached to the phenyl group, rather than the thiophene group (asymmetric charge transfer), leading to large third order optical nonlinearity.Due to steric hindrance, the substitution of two OCH 3 groups at the second and third positions of the phenyl ring reduces the third order optical nonlinearity [12], whereas substitution of three OCH 3 groups in the phenyl moiety enhances the third order nonlinearity.

Conclusion
CTTMP single crystals were grown by slow evaporation solution growth technique using DMF as the solvent.Scanning electron microscope confirms step-like growth of the crystals.The mechanical and thermal stability of the material shows it as a promising material for NLO applications.Third order NLO studies indicate the molecule exhibits twophoton absorption, good optical limiting behavior, and selfdefocusing effect.Dielectric studies indicate that the values of   and tan  are found to be stable at higher frequencies.Substitution of three OCH 3 groups in the phenyl moiety enhances the third order nonlinearity, because of the net charge transfer, and hence the dipole moment increases.