CHAPTER 8 REPORT ON HIGHER SHG EFFICIENCY IN BIS (CINNAMIC ACID): HEXAMINE COCRYSTAL

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1 CHAPTER 8 REPORT ON HIGHER SHG EFFICIENCY IN BIS (CINNAMIC ACID): HEXAMINE COCRYSTAL 8.1. Introduction In recent times higher Second Harmonic Generation (SHG) efficiency organic materials receive great interest, because of their applications in telecommunications, optical computing, optical data storage and optical information processing [118, 140, 143]. The advantages of organic NLO materials over inorganic materials lie in their fast response time, facile modification of molecular properties through precise synthetic methods, and high optical damage thresholds. Non centro symmetric nature is the essential criterion for the material to possess SHG along with the phase matching condition [173, 174]. The SHG efficiency mainly depends on the inherent structural attributes of a material. Furthermore, the extent of charge transfer (CT) across the NLO chromophores determines the SHG efficiency - Greater the value of CT greater the SHG [121, 137, ]. The presence of strong intermolecular interactions, such as hydrogen bonds, enhances the CT in the supramolecular assembly, which in turn increases the SHG response. However the presence of CT between molecules through hydrogen bonding generally favours the non centro symmetric arrangement of molecules rather than CT within a molecule. In the absence of steric hindrance, these CT interactions together with the molecular CT effects, give the molecular packing environment either centro symmetric or non centro symmetric structure, hence the stringent condition for choosing this material for high SHG output. 106

2 In the present work, we have synthesized a cocrystal of Bis (cinnamic acid): hexamine (2:1) to attain higher SHG efficiency. The presence of methyelene group in hexamine is involved in the formation of unconventional hydrogen bond C H -- X (X = N, O) along with the conventional N H -- X (X = O, N) hydrogen bonding [253]. Steric hindrance is found to be absent in trans cinnamic acid. These factors have prompted us to choose them for co crystallization to obtain higher NLO efficiency. The CT interaction through hydrogen bonding and the coplanarity of cinnamic acid due to the absence of steric hindrance make this cocrystal a high SHG efficient NLO material. These phenomena are explained in detail through structural, vibrational and UV Vis analysis Materials and methods All the chemicals were purchased from sigma Aldrich and used directly without any further purification. The cocrystal was synthesized by following the procedure as prescribed in the literature [254]. The powder XRD diffraction pattern of the sample was recorded using the instrument Shimadzu XRD 600 diffractometer. The FT IR spectrum was recorded using Shimadzu Fourier Transform Spectrometer in the range cm -1 by KBr pellet method. Raman spectra of powder samples were measured using a Renishaw Invia micro Raman spectrometer with 514 nm laser excitation from 30 cm -1 to 3200 cm -1. No bands were observed above 2000 cm -1. UV Vis absorption spectrum was recorded using UV 1700 series spectrophotometer in the range nm in the solvent ethanol. 107

3 8.3. Results and discussion Powder XRD analysis The recorded powder XRD pattern is depicted in Fig This powder pattern agrees well with the powder pattern calculated using MERCURY software and this confirms that our synthesized material is Bis(cinnamic acid): hexamine cocrystal. The sharp and well defined peaks at specific 2θ angles imply the well crystallinity of the material. The molecular structure of the cocrystal with atom numbering scheme is presented in Fig Fig Powder XRD pattern of Bis (cinnamic acid): hexamine cocrystal. 108

4 Fig Molecular structure of Bis (cinnamic acid): hexamine with atom numbering scheme. 109

5 Vibrational analysis Vibrational Spectroscopy of protic groups, such as O H, N H and sometimes C H has long been realized as the most important spectroscopic tool to identify hydrogen bonding [211, 255]. This is due to the fact that these vibrations are very sensitive to hydrogen bond structures, and show characteristic frequency shifts upon hydrogen bonding. Conventional hydrogen bonding studies are quite common; however there are a few workers who have concentrated on the unconventional hydrogen bonding [ , 256]. The selected vibrational assignments of IR spectrum (Fig. 7.3) are listed in Table 8.1 and assignments of Raman spectrum (Fig. 8.4) are listed in Table 8.2. In pure trans - cinnamic acid the carboxylic O H vibrational stretching frequency was observed at 3453 cm -1 as a very strong band. In Bis (cinnamic acid): hexamine cocrystal it is shifted to the higher frequency of 3468 cm -1 (blue shift) with very less absorption intensity. This shift indicates the presence of hydrogen bonding of type O H -- N. Correspondingly the characteristic frequency of carboxylic C = O shows a shift from 1681 cm -1 to 1691 cm -1. The C H stretching characteristic vibrational frequency of 2919 cm -1 in pure hexamine [188] is shifted to 2941 cm -1 in the given material. In view of the blue shift in C H stretching frequency we are led to infer that there is an emergence of hydrogen bonding. Generally the hydrogen bond is characterized by a weak interaction involving an electronegative proton donor X, hydrogen, and an electronegative proton acceptor Y [X H -- Y] [212, 257]. The interaction is predominantly electrostatic in nature, although CT interactions are considered to be important [ ]. According to the classical electrostatic model of hydrogen bonding, the electron density of Y exerts an attractive force on the proton, and the approach of Y should always lengthen the X - H bond [262]. Moreover, if significant charge transfer occurs from the proton 110

6 acceptor Y to the proton donor, in particular to the X - H σ*antibonding orbital, the X - H bond should also be weakened upon the hydrogen bond formation and concomitantly elongated [263]. This elongation of the X - H bond results in a decrease in the X - H stretching vibration frequency, and such a red-shift has been accepted in many experimental studies as an evidence of hydrogen bond formation [233, 264]. On the other hand, Hobza et.al [ ] have proposed that blueshifted hydrogen bonding can be explained by charge transfer from the proton acceptor Y to remote (presumably highly electronegative) atoms in X instead of the X - H σ*antibonding orbitals, followed by a structural reorganization of the proton donor framework resulting in contraction of the X - H bond. Y. Fang et. al [265] proposed that the steric effect may also cause the blue shift. Hence we conclude that the blue shifted O H--N hydrogen bond arises due to the charge transfer from N atom of hexamine to O atom of carboxylic acid group and a possible steric hindrance arising due to the substituent of molecule hexamine with carboxylic acid group of cinnamic acid. The blue shift in C H stretching frequencies arise due to the charge transfer from the highly electronegative N atoms to the remote C atoms rather than C H σ* anti bonding orbital. The presence of O H -- N hydrogen bonding implies that there is a strong CT interaction between the cinnamic acid and hexamine molecules. The strong interaction is evident from the blue shifted O H stretching frequency with the contraction of bond length of O H. The presence of unconventional hydrogen bonding in hexamine prevents the main conventional hydrogen bonded (O H -- N) chains in the molecule from folding and twisting [266]. Furthermore from observing the vibrational assignments of FT - IR we conclude that all characteristic vibrational frequencies which are present in the cinnamic acid have no shift in their frequencies (particularly C = C) except O H and C = O. Most 111

7 importantly in the material under investigation we observe that the in plane ring deformation of CCC and stretching of C - C ring from Raman vibrational modes of cinnamic acid [267] have no drastic shift in frequencies in Bis (cinnamic acid): hexamine. This implies that trans cinnamic acid retains its coplanarity, which brings higher π electron conjugations extensively in the molecule. On the other hand, all characteristic vibrational frequencies of hexamine molecule show a marked shift in frequency. This participation of C, H and N atoms in hydrogen bonding is a result of the frequency shift. Fig FT IR spectrum of Bis (cinnamic acid): hexamine 112

8 Fig Raman spectrum of Bis (cinnamic acid): hexamine 113

9 Table 8.1. Selected FT IR vibrational assignments of Bis (cinnamic acid): hexamine with pure cinnamic acid and hexamine Wave number (cm -1 ) Assignments Cinnamic acid Bis (cinnamic acid: hexamine) υ (COC, stretch) υ (C = C, aromatic) υ (C = C, aromatic) υ (C =C, alkene) υ (C = O) υ(oh) Hexamine Bis (cinnamic acid: hexamine) ω (N C N) δ (N C N) υ (N C) υ (N C) ω (CH 2 ) γ (CH 2 ) υ(c H) υ stretching, ω wagging, δ deformation,γ - scissoring 114

10 Table Selected Raman vibrational assignments of Bis (cinnamic acid): hexamine. Wave number (cm -1 ) Assignments Cinnamic acid Bis (cinnamic acid: hexamine) α (CCC) υ (C - C ring ) υ (C = C) α in plane ring deformation, υ - stretching UV Vis analysis UV Vis analysis is helpful in the investigation of NLO material from their absorption in the appropriate wavelength. The UV Vis absorption spectrum is portrayed in Fig From the spectrum we observe a high intensity absorption peak at 302 nm. Generally, cinnamic acid exists in two isomers. The trans cinnamic acid is found to absorb at longer wavelengths (272 nm) when compared to cis cinnamic acid. This absorption at longer wavelength is a contribution of effective π orbital overlap which is possible in the trans cinnamic acid. This makes it possible to achieve more readily coplanarity of π electron system. When the steric hindrance is small to coplanarity it causes a little effect on the position and intensity of the absorption maximum. If the steric hindrance to coplanarity is more, then there could be a marked decrease in the intensity which may be accompanied by a red or blue shift [195]. In Bis (cinnamic acid): hexamine cocrystal the coplanarity of the 115

11 trans cinnamic acid is retained. This is evident from the occurrence of absorption maximum at 302 nm with higher intensity. There is no decrease in intensity of the absorption maximum, which implies the absence of steric hindrance between hexamine and trans cinnamic acid. Further, the frequency of the maximum absorption peak in the cocrystal has red shifted over 30 nm from trans cinnamic acid. This red shift indicates an increase in conjugation length between the donor and acceptor [268]. In addition the absence of absorbance in the entire visible region indicates a possible occurrence of non zero macroscopic NLO response [129]. Fig UV Vis absorption spectrum of Bis (cinnamic acid): hexamine 116

12 Structural analysis The selected bond lengths and hydrogen bonding parameters are indicated in Tables 8.3 & 8.4. This is obtained from the Crystallographic Information File (CIF) of this material. From Table 8.3 we infer that the bond lengths of C(6) - C(7), C(7) - C(8), C(8) - C(9), C(9) - C(10) and C(11) - C(12) have shorter bonds than other C C bond in the ring. This indicates the possibility of formation of quinoidal character in the material. CT interaction causes the perturbation of aromatic rings towards a quinoidal electronic configuration since this provides a more extended level of π - conjugation in the system [269]. This quinoidal character has been also observed in the material 4-nitro-4 -methylbenzylidene aniline which has SHG output between 16 and 27 times that of urea [270]. Furthermore we have two unconventional hydrogen bonding of C H -- N along with the conventional hydrogen bonding O H -- N. The presence of unconventional hydrogen bonding stabilizes the non centro symmetric packing arrangements whereas the conventional hydrogen bonding causes the CT interaction between the molecules. Therefore we infer that the supramolecular CT effects contribute significantly to the magnitude of the SHG effect. 117

13 Table Selected bond lengths of Bis (cinnamic acid): hexamine Parameter Bond length (Ǻ) C(6) C(7) C(8) C(9) C(9) C(10) C(12) C(13) C(11) C(12) C(5) C(10) C(5) C(6) C(5) C(11) Table Hydrogen bonding parameters in Bis (cinnamic acid): hexamine. D H --A H A (Ǻ) D - A (Ǻ) D H A ( O ) O1 H1 --N C3 H3B -- N C4 H4A -- N

14 First order hyperpolarizability (β) It is well known that the higher value of hyperpolarizability (β) is important for causing NLO response in materials, because it is β that gets translated into macroscopic SHG (χ2) in the crystals, provided the crystal has non centro symmetric structure. Polarization is a phenomenon of the creation of an induced dipole moment by an external electric field. The total energy U of the material under the influence of an intense external electric field can be expanded as a Taylor Series as, U ( ) = U (0) (1) Where the subscripts i, j, k represent the different components X, Y and Z of the Cartesian coordinates system, U (0) is the energy of the compound in the absence of, is the molecular permanent electric dipole moment along the i th direction, is the i th Cartesian component of the applied electric field,and α, β are the linear, first hyperpolarizabilities respectively. The magnitude of β can be calculated using the following equation β tot = [β x 2 + β y 2 +βz 2 ] 1/2 (2) where, β x 2 = (β xxx + β xyy + β xzz ) 2 (3) β y 2 = (β yyy + β yzz + β yxx ) 2 (4) β z 2 = (β zzz + β zxx + β zyy ) 2 (5) 119

15 Table 8.5. First order hyperpolarizability (β in a.u) parameters Parameter HF/6 31 G(d, p) β xxx β xxy β xyy β yyy β xxz β xyz β yyz β xzz β yzz β zzz First order hyperpolarizability β tot (e.s.u.) 5.33 X Structures of the Bis (cinnamic acid): hexamine molecule was fully optimized using the density functional theory (DFT) B3LYP method with the 6 31 G (d, p) basis set. Hyperpolarizability tensor elements (Table 7.5) were calculated using Hartree Fock (HF) method with the same basis set. All calculations were carried out using the Gaussian03 Program [271]. Since the values of hyperpolarizability (β) of the Gaussian 120

16 output are reported in atomic units (a.u), the calculated values have been converted into electrostatic units (e.s.u.) (1 a.u. = X e.s.u.). The first order hyperpolarizability value is 5.33 X e.s.u. which is nearly 14 times greater than urea at microscopic level [159] SHG measurements The higher SHG output of the material was confirmed by Kurtz Perry Powder method [106], with KDP and urea as reference material. In this method the crystal was ground into powder of known grain size of microns and it was packed densely between two transparent glass plates. An input beam of 1064 nm Q switched laser beam was made to fall normally on the sample cell. The generated second harmonic signal was passed through a 532 nm narrow pass filter and fed to another channel of power meter. The ratio of fundamental to harmonic intensities determines the efficiency of the sample. The input power of the laser beam was measured to be 1.9 mj/ Pulse. Throughout the experiment the laser power was kept constant. Bis (cinnamic acid): hexamine exhibits (output 620 mv) nearly 3 times greater efficiency than urea (236 mv) and 27 times greater efficiency than KDP (22.4 mv) Conclusions Higher SHG efficiency material needs high coplanarity in molecules and hydrogen bonds which favor CT interaction. Bearing this in mind, we have chosen the coplanar molecule trans cinnamic acid and hexamine for unconventional hydrogen bonds which are synthesized into cocrystals. From FT IR analysis we observe the stretching frequencies of O H and C H shift to higher frequencies (blue shift) 121

17 which indicates the presence of conventional ( O H -- N) and unconventional (C H -- N) hydrogen bondings. The CT between the hexamine and cinnamic acid molecules are observed through O H -- N hydrogen bonding, whereas the C H -- O hydrogen bonding play a role to prevent the folding of the main hydrogen bonding which favours non centro symmetric packing arrangements. The coplanarity of the material is confirmed through the high intense absorption peak at 302 nm and it is supported by Raman without any remarkable shift in the ring group frequencies of cinnamic acid in the cocrystal. This coplanarity of trans cinnamic acid is retained in the material leads to effective π orbital overlap. In addition the red shift of this band indicates an increase conjugation length of donor to acceptor. From structural analysis we observe a shortening in the bond lengths of C(6) - C(7), C(7) - C(8), C(8) - C(9), C(9) - C(10) and C(11) - C(12) in the ring which implies the quinoidal character of the material. This provides a more extended level of π conjugation. The first order hyperpolarizability (β) value of the molecule at microscopic level was calculated using HF method with Gaussian 03 program package. From Powder Kurtz Perry method the higher SHG output (3 times greater than urea) of Bis (cinnamic acid): hexamine cocrystal is confirmed. 122