1 Introduction. ISSN , England, UK World Journal of Modelling and Simulation Vol. 14 (2018) No. 1, pp. 3-11

Transcription

1 ISSN , England, UK World Journal of Modelling and Simulation Vol. 14 (2018) No. 1, pp Molecular Structure, Mulliken charges, HOMO-LUMO, Electrostatic Potential and Nonlinear Optical Properties of Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate molecule by HF and DFT methods Nadia Benhalima 1, Amel Djedouani 2, Rachida Rahmani 1, Abdelkader Chouaih 1 * Fodil Hamzaoui 3, El Hadj Elandaloussi 4 1 Laboratory of Technology and Solid s Properties, Faculty of Sciences and Technology, Abdelhamid Ibn Badis University, BP 227 Mostaganem 27000, Algeria 2 Ecole Normale Superieure de Constantine, Constantine 25000, Algeria 3 LPFM Academie de Montpellier, France 4 Laboratoire de Valorisation des Materiaux, Université Abdelhamid Ibn Badis, B.P. 227, Mostaganem, Algeria (Received December , Accepted May ) Abstract. In this work, we report a oretical study on molecular structure, electrostatic potential and nonlinear optical properties of Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate molecule (monomer and dimer ). The molecular geometry in ground state was investigated by ab initio and density functional method (B3LYP) with 6-31G(d) and 6-31+G(d,p) basis sets. The results show that computed geometrical parameters are in good agreement with experimental values. The molecular electrostatic potential and HOMO-LUMO of title molecule have been also calculated using oretical methods. The calculated HOMO and LUMO energies show that charge transfer occurs within molecule. In order to investigate nonlinear optical behavior, electric dipole moment µ, polarizability α and hyperpolarizability β were computed. All β values that we report here are magnitude of static hyperpolarizability. Theoretically predicted β values exhibit higher nonlinear optical activity. Keywords: oretical calculation, charge transfer, atomic charges, molecular electrostatic potential, HOMO- LUMO 1 Introduction Nonlinear optical (NLO) materials have been extensively studied for many years [2]. Materials with high NLO response are of great technological interest because of ir potential applications in data storage, signalprocessing, and telecommunication technologies [29]. In particular, organic molecules have received large attention due to combination of molecular design flexibility, chemical tenability and choice of syntic strategies [9]. In last two decades, organic molecules have been synsized, some of m showing impressively high hyperpolarizabilities [19]. Theoretical investigations have been of great help in achievement of se results; y gave first hints about possible high NLO responses of organic molecules with extended conjugated π systems [3], and provided guidelines for a rational design of such molecules [23]. The recent synses of zwitterionic molecules, some of m exhibiting extremely high NLO activity, have also been inspired by oretical investigations [8]. Organic molecules with -electron delocalization are currently of wide interest as NLO materials with potential applications in optical switches and or NLO devices [22]. Large Corresponding author. address: achouaih@gmail.com Published by World Academic Press, World Academic Union

2 4 N. Benhalima & A. Djedouani & R. Rahmani et al: Title Suppressed Due to Excessive Length optical non-linearity could be seen in organic conjugated molecules incorporating an electron acceptor group at one moiety and a donor group at opposite one [27]. For prototypical push-pull molecule, latter would exhibit an asymmetric polarization response to a symmetric electric field at intense field strengths since electron intensity is more easily displaced toward an acceptor substituent than toward a substituent donor [13]. Consequently, dipolar aromatic molecules possessing an electron donor group and an electron acceptor group contribute to large optical non-linearity arising from intramolecular charge transfer between two groups of opposite nature. It is also well known that first and second hyperpolarizabilities in π-electron systems with donor-acceptor groups increase with increasing donor and acceptor strengths. Despite relevant role played by ory in development of field of NLO organic molecules, re are so far only few papers reported comparison between experimental and computed hyperpolarizability for large size NLO chromophores [11]. Zwitterionic compounds have received interest of chemists and physicists due to ir applications as nonlinear optical materials [18]. There are some reports concerning formation of zwitterionic form in crystal structure of ortho-hydroxy Schiff bases [28]. The present work was assessed to determination of molecular structure, electrostatic potential and nonlinear optical properties of Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate compound. A combined experimental and oretical study was carried out and a detailed description of structural features of studied molecule is reported. The ground state molecular structure was optimized and compared with experimental data obtained by X-ray analysis. In addition, various nonlinear optical properties such as electric dipole moment, molecular electrostatic potential, polarizability and hyperpolarizability were also addressed oretically. The HOMO-LUMO energy gap has been computed and NLO activity of molecule was analyzed. 2 Computational details The molecular geometries of title compound were fully optimized at various ory levels using Gaussian 03 program [17] and Gauss-View molecular visualization program [1]. From X-ray crystal structure data [15], Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate compound crystallizes in monoclinic space group P21/c with four molecules per unit cell (Z = 4) and following cell dimensions: a = (4)A, b = (10)A, c = (3)A. β = (6). The data were collected at low temperature (173 K). The spatial coordinate positions of title compound, as obtained from X-ray structural analysis, were used as initial coordinates for oretical calculations. For title molecule, full geometry optimizations have been carried out at Hartree-Fock (HF) [10] level using 6 31G(d) and 6-31+G(d,p) basis sets, by Berny method [6, 16]. The effects of electron correlation on geometry optimization are taken into account by Becke s three parameter hybrid exchange functions and Lee-Young-Parr correlation functional (B3LYP) [7, 30] level in DFT method. The ab initio and DFT optimized geometries of crystal structure of investigated compound correspond to slightly non-planar conformation having 105 dihedral angle. The optimized structures were n used to calculate electric dipole moments and hyperpolarizabilities of title compound. Nonlinear optical calculations were performed at both Hartree-Fock (HF) ab initio and DFT levels with 6 31G(d) and G(d, p) basis sets. The 6 31G(d) basis set has been found to be more adequate for obtaining reliable trends in β values [14, 33]. 3 Results and discussion 3.1 Optimized geometry The optimized bond lengths, bond and dihedral angles of title compound calculated by B3LYP and HF methods using different basis sets are listed in Tab. 1, in accordance with atom numbering shown in Fig. 1. Our optimized structural parameters are now compared with exact experimental X-ray data [15]. Tab. 1 shows that most of bond lengths are found to be greater than experimental ones. For example, calculated bond lengths (C1 N1, C1 N2, C2 N3, C4 C8, C4 C5, N2 N3) at B3LYP level are slightly larger than corresponding WJMS for contribution: submit@wjms.org.uk

3 World Journal of Modelling and Simulation, Vol. 14 (2018) No. 1, pp experimental values. This overestimation can be explained by fact that oretical calculations belong to isolated molecule in gaseous phase and experimental results belong to similar molecules in solid state. The C = O bond length namely C1 O4 (1.222)A, C5 O1 (1.215A ) and C8 O3 (1.258A ) calculated by means of B3LYP/6-31+G(d,p) method are most close to measured ones. These values agree well with data reported in literature [4, 5]. On or hand, some computed bond angles deviate about 1 2 from experimental data. The bond angles of O4 C1 N2, C2 N3 N2 and N3 C2 C3 were measured to be (17), (15) and (17). The calculated values using B3LYP/6-31G(d) are of order of , and , respectively. According to above comparisons, biggest differences of bond lengths and bond angles mainly occur in groups involved in bonding [i.e., C1 O4, C8 O3 and N1 C1 N2], which can also easily be understood taking into account intermolecular bond interactions occurring in crystal. When X-ray structure of title compound is compared with its optimized counterparts (see Fig. 2), slight conformational discrepancies are observed between m. The calculated value of dihedral angle (C1 N2 N3 C2) using both B3LYP/6-31G(d) (108.4 ) and HF/6-31G(d) (105.8 ) methods is slightly different from experimental one (0.00 ). In case of calculated dihedral angle, re is a good agreement between B3LYP and HF methods. In spite of differences observed, calculated geometric parameters are in general in good agreement with X-ray structure. N1 C1 N2 O4 C3 C2 N3 C4 C8 O3 O1 C5 O2 C6 C7 C9 O1w Fig. 1: Optimized structure with B3LYP/6-31G (d) level of title molecule 3.2 Mulliken charge analysis It is clear that Mulliken [24] populations provide simplest picture of charge distribution. Knowledge of charge density function can lead to some important properties of molecule such as charges on different atoms, molecular dipole moment and electrostatic potential around molecule. The Mulliken charges for non-h atoms of title compound were calculated at HF and B3LYP level with 6-31G(d) basis set in gas-phase. The atomic charges show that N1, N2, N3 and O1, O1w, O2, O3, O4 atoms have bigger negative atomic charges in gas phase compared to carbon atoms. This behavior can be result of intramolecular N HO bond and C8 = O3 double bond features. The obtained results by both methods are summarized in Tab Molecular electrostatic potential Molecular electrostatic potential (MEP) is associated to electronic density and has been used primarily for predicting sites and relative reactivities toward electrophilic attack in studies of biological recognition and WJMS for subscription: info@wjms.org.uk

4 6 N. Benhalima & A. Djedouani & R. Rahmani et al: Title Suppressed Due to Excessive Length Table 1: Calculated geometrical parameters for title molecule (Distance, Å; Angle, ) Geometrical X ray α HF DFT parameters 6-31G(d) 6-31+G(d,p) 6-31G(d) 6-31+G(d,p) C1-O (2) C1-N (2) C1-N (2) C2-N (2) C2-C (2) C2-C (2) C4-C (2) C4-C (2) C5-O (2) C5-O (2) C6-C (2) C6-O (2) C6-C (3) C7-C (2) C8-O (2) N2-N (2) O4-C1-N (17) O4-C1-N (17) N1-C1-N (16) N3-C2-C (15) N3-C2-C (17) C4-C2-C (16) C8-C4-C (16) C8-C4-C (15) C5-C4-C (15) O1-C5-O (16) O1-C5-C (18) O2-C5-C (15) C7-C6-O (17) C7-C6-C (18) O2-C6-C (16) C6-C7-C (17) O3-C8-C (16) O3-C8-C (15) C4-C8-C (15) C1-N2-N (15) C2-N3-N (15) C6-O2-C (14) C1-N2-N3-C bonding interactions [25, 32]. Molecular electrostatic potential, V(r), at a point r(x,y,z) is described as following: V (r) = Z A ρ(r)dr R A r r r A ZA is charge on nucleus A located at RA and ρ(r) is electron density. The electrostatic potential plots are shown in Fig. 2 ((a) monomer and (b) dimer). MEP was calculated at B3LYP/6-31G(d) optimized geometry. The negative regions of MEP surface correspond to an attraction and are colored in red. While, positive regions correspond to a repulsion and are colored in blue. The negative regions are related to electrophilic sites and positive ones to nucleophilic sites. The MEP maps of title molecule show that negative regions were found around oxygen atoms and that positive potential sites are around atoms. WJMS for contribution: submit@wjms.org.uk

5 N N N World O1 Journal of Modelling and Simulation, Vol. 14 (2018) No. 1, pp O1w Table O2 2: Mulliken0.070 Charges (q) in non- atoms of title molecule calculated with 6-31G(d) basis set O O q Atoms 3.3. Molecular electrostatic potential HF DFT Molecular electrostatic potential (MEP) is C1 associated to electronic density and has been used primarily C for predicting sites and relative reactivities toward electrophilic attack in studies of biological recognition C and bonding interactions [27, C4 28]. Molecular electrostatic potential, V(r), at a point r(x,y,z) is described as following: C C C C Z C A X ( r) dr ' V (r) N1 A R A r r ' r ZA is charge on nucleus A located at N2RA and (r) is electron density. The electrostatic potential plots are shown in Figure 2 ((a) monomer N3 and (b) dimer) MEP was calculated at B3LYP/6-31G(d) O optimized geometry. The negative regions of MEP surface correspond to an attraction and are colored in O1w red. While, positive regions correspond O2 to a repulsion and are colored in blue. The negative regions are related to electrophilic sites and O3 positive 0.06 ones to nucleophilic sites. The MEP maps of title molecule show that negative regions O4were found around oxygen atoms and that positive potential sites are around atoms. Fig. 2: Molecular electrostatic potential maps (a) and (b) calculated at B3LYP/6-31G(d) level Fig. 2: Molecular electrostatic potential maps (a) and (b) calculated at B3LYP/6-31G(d) level 3.4 Homo-lumo calculation The highest occupied molecular orbitals (HOMOs) and Lowest-lying unoccupied molecular orbitals (LU- MO) are very useful for physicists and chemists because energy difference between se orbitals (energy gap) represents minimum energy required to promote an electron, and is refore often mostfrequent and important energy transfer mechanism within a system. Furr, charge transport properties of molecule [12] are determined by difference of energy between HOMO and LUMO. The orbitals also provide important electron density information which can help determine which part of molecule is most actively participating in an energy transfer event. The HOMO-LUMO energy gap of molecule was calculated using HF/(6 31G(d) and G(d, p)) and B3LY P/(6 31G(d) and G(d, p)) level as presented in Tab. 3. The energy gap between HOMO and LUMO indicates molecular chemical stability. In addition, a lower HOMO-LUMO energy gap explains fact that eventual charge transfer interaction is taking place within molecule. The HOMO-LUMO plots are given in Fig Hydrogen bonding and non linear optical properties Hydrogen bonds are very important dipole interactions in stabilizing structures. In amino acids having zwitterionic form, NH2 moiety is a good donor and carboxylate or nitrate group is an excellent acceptor. In NLO crystals, bonds linked with adjacent molecules, where O-H O bonds are relatively stronger than N-H O bonds [26]. Fig. 4 shows molecular packing diagram of molecule where bond interactions N- H O and O-H O were observed. These interactions are responsible for NLO effect in Zwitterionic WJMS for subscription: info@wjms.org.uk

6 31+G(d,p)) level as presented in Table 3. The energy gap between HOMO and LUMO indicates molecular chemical stability. In addition, a lower HOMO-LUMO energy gap explains fact that 8 eventual charge transfer N. Benhalima interaction & is A. taking Djedouani place & R. within Rahmani et molecule. al: Title Suppressed The HOMO-LUMO Due to Excessive plots Length are given in Figure 3. Fig. Fig. 3: The 3 The HOMO and and LUMO plotof of title title molecule molecule 3.5. Hydrogen bonding and non linear optical properties 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate Hydrogen bonds are very important dipole interactions in stabilizing monohydrate compound. structures. In The amino NLOacids effect having may also be zwitterionic associated form, to push-pull NH 2 moiety type is structure. a good donor As expected, and carboxylate best donors or nitrate interact group with is an excellent best acceptors acceptor. leading to In strong NLO crystals, mentioned bonds bonds. linked These with rigid adjacent molecules, bonds link where symmetry-independent O-H O molecules bonds are into relatively a dimer, stronger which can than be N-H O considered bonds as [30]. building block of structure. Figure It is evident 4 shows that molecular crystal structures packing diagram are generated of molecule and stabilized where by bonds bond but interactions more evidently N that H O y also and O H O participate were considerably observed. These to interactions increase of are hyperpolarizability responsible for of NLO effect bonded in Zwitterionic molecular systems. 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate For organic NLO materials, oretical and experimental monohydrate studies are performed compound. in The order NLO to understand effect microscopic may also be origin associated of nonlinear to "push pull" behavior [20, type 31]. structure. As expected, best donors interact with best In this acceptors context, leading this study to is strong extended mentioned to determination bonds. of These electric rigid dipole moment bonds µ, link isotropic polarizability symmetry-independent α and first molecules hyperpolarizability into a dimer, β which can be considered as building block of tot of title compound (monomer and dimer). The dipole moment structure. (µ), isotropic polarizability β tot, first-order hyperpolarizability (α) tensor, can be obtained using following It is evident equations: that crystal structures are generated and stabilized by bonds but more evidently that y also participate considerably to increase of hyperpolarizability of bonded molecular systems. For organic NLO materials, oretical and experimental studies are performed in order to understand microscopic origin µ 0 of = nonlinear (µ 2 behavior [31, 32]. x + µ 2 y + µ 2 z) 1/2 In this context, this study is extended to determination of electric dipole moment µ, isotropic polarizability α and first hyperpolarizability α = 1 3 (α xx + of α yy title + αcompound zz ) (monomer and dimer). The dipole moment, isotropic polarizability β, first-order hyperpolarizability tensor, can be tot = (βx 2 + βy 2 + βz 2 ) 1/2. The whole equation for computing magnitude of first hyeprpolarizability (β) from Gaussian 03 output is given below: β tot = [(β xxx + β yyy + β zzz ) 2 + (β yyy + β yzz + β yxx ) 2 ] + (β zzz + β zxx + β zyy ) 2. The first hyperpolarizability is a third rank tensor that can be depicted by a matrices. The 27 components of 3D matrix can be reduced to 10 components because of Kleinman symmetry [21]. The Gaussian 03 output provides 10 components of this matrix as βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz, βzzz respectively. The first hyperpolarizability tensors provided by Gaussian 03 are given in atomic units (a.u.), computed values were converted into electrostatic units. (α : 1a.u. = esu; β : 1a.u. = esu). The dipole moment (µ0), mean polarizability (α) and first hyperpolarizability (β) for monomer and dimer are calculated at 6 31G(d) and G(d, p) basis sets. Tab. 3 gives HF and B3LYP results of electronic dipole moment µ i (i = x, y, z), polarizability αij and first hyperpolarizability βijk for Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate compound (monomer and dimer ). Theoretical calculation plays a significant role in understanding structure-property relationship WJMS for contribution: submit@wjms.org.uk

7 Table 3. The electric dipole moment µ (D), average polarizability α ( esu) and first hyperpolarizability β ( esu) of title molecule. World Journal of Modelling HF and Simulation, Vol. 14 (2018) No. 1, pp DFT 9 Parameters 6-31G* 6-31+G** 6-31G* 6-31+G** which is able to help Monomer in designing Dimer novel NLO Monomer materials. Dimer It is well established Monomer that Dimer higher Monomer values of Dimer dipole moment, µ x molecular 2.43 polarizability, and hyperpolarizability are very important 2.43 for 7.52 active NLO 2.20 properties The highest µ y value of dipole 1.01 moment obtained 1.16 with 0.85 B3LYP /Lanl2dz 1.52 is equal 1.49 to 3.68 D3.92 for monomer 1.75 and D for dimer. µ z The highest 1.46 value of dipole 2.88 moment 1.76 is observed for 1.31 component 1.12 µ x. In this 0.02 direction, 1.68 this value is 1.41 equal µ to 2.67 and D for monomer and dimer respectively. The calculated polarizability (α), is equal to α xx α xy 24 esu and esu for monomer 5.71 and22.45 dimer respectively obtained with 6-31G+(d,p) As it canαbe yy seen in Tab. 3, calculated polarizability αij have non zero values and was dominated by diagonal components. α xz The first hyperpolarizability 4.70 values 7.72 β tot of 3.38 title molecule obtained 3.19 with B3LYP/Lanl2dz are α equal yz to esu for monomer and esu for dimer. The calculated results show that α zz α title (a.u) molecule might have microscopic nonlinear (NLO) behavior with non-zero values. The calculated first order α (esu) hyperpolarizability of title molecule20.29 has a low value of esu obtained23.89 with HF method using β xxx G(d, p) basis set and a high value of esu obtained with DFT method using β same xxy basis set. Thus, calculated low β total value for title molecule ( esu) is found to be 8 β xyy times β yyy greater than β total value of urea ( esu), refore, predicting that43.70 our molecule is a powerful β xxz candidate 5.92 for NLO material β xyz From cited above results, molecule having greatest β tot value, corresponds to low HOMO LUMO β yyz energy gap These results39.50 show that HOMO-LUMO gap have a substantial influence on first hyperpolarizability. β xzz The high β value and low HOMO-LUMO energy gap show that title molecule is highly NLO β yzz active β zzz material and it might be efficient for optoelectronic applications β Consequently, (a.u) we can finally infer from above discussion of contents of Tab. 3 that introduction of electron β (esu) correlation in method applied1.612 for analysis of hyperpolarizability, such3.343 as DFT method, HOMO will probably predict more reasonable values as opposed to those converged upon use of HF method. LUMO Gap (Hartrees) Gap (ev) O4 N1 O1w Å H1w H1A Å O3 Fig. 4: Fig. N-H4: ON H O and O-H and OO H O interactions interactions calculated calculated at B3LYP/6-31G at (d) (d) level 4. Conclusion 4 Conclusion In this work we have calculated geometric parameters and nonlinear optical properties of Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate molecule by using HF and B3LYP methods with 6-31G(d) and 6-31+G(d,p) basis sets. The optimized bond lengths and bond angles obtained with B3LYP level of calculation show excellent agreement with experimental values. The energy gap between HOMO and LUMO calculated using B3LYP is lower than value calculated using HF level. The energy gap values indicate molecular chemical stability. Results show that first hyperpolarizability β of Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate molecule is directly related to HOMO-LUMO energy gap. Molecular electrostatic potential (MEP) was calculated at B3LYP/6-31G (d) optimized geometry. The MEP map shows that negative potential sites are on electronegative atoms as well as positive potential sites are around atoms. These sites give WJMS for subscription: info@wjms.org.uk

8 10 N. Benhalima & A. Djedouani & R. Rahmani et al: Title Suppressed Due to Excessive Length Table 3: The electric dipole moment µ (D), average polarizability α( 10 24esu) and first hyperpolarizability β( 10 30esu) of title molecule HF DFT Parameters 6 31G G 6 31G G Monomer Dimer Monomer Dimer Monomer Dimer Monomer Dimer µ x µ y µ z µ α xx α xy α yy α xz α yz α zz α(a.u) α(esu) β xxx β xxy β xyy β yyy β xxz β xyz β yyz β xzz β yzz β zzz β ( a.u) β ( esu) HOMO LUMO Gap (Hartrees) Gap (ev) information about region from where compound can have intramolecular interactions. This study reveals that Zwitterionic 6-methyl-2-oxo-3-[1-(ureidoiminio)ethyl]-2H-pyran-4-olate monohydrate molecule has a large first static hyperpolarizabilities and may have potential applications in development of NLO materials. References [1] A. H. A. Frisch, A.B. Neilson. Gauss view 4, gaussian, inc., wallingford ct usa, [2] P. Agarwal, N. Choudhary. Density functional ory studies on structure, spectra (ft-ir, ft-raman, and uv) and first order molecular hyperpolarizability of 2-hydroxy-3-methoxy-n-(2-chloro-benzyl)-benzaldehyde-imine: Comparison to experimental data. Vibrational Spectroscopy, 2013, 64((2013)): [3] G. P. Agrawal, C. Flytzanis. Delocalization and superalternation effects in nonlinear susceptibilities of onedimensional systems. Chemical Physics Letters, 1976, 44(2): [4] M. I. Attia, N. R. El-Brollosy. Synsis, single crystal x-ray structure, and antimicrobial activity of 6-(1,3- benzodioxol-5-ylmethyl)-5-ethyl-2-[2-(morpholin-4-yl)ethyl]sulfanylpyrimidin-4(3h)-one. Journal of Chemistry, 2015, 2014(4): 1 8. [5] M. I. Attia, H. A. Ghabbour et al Synsis and single crystal x-ray structure of new (2e)-2-[3-(1h-imidazol-1-yl)-1- phenylpropylidene]-n-phenylhydrazinecarboxamide. Journal of Chemistry, 2013, [6] R. F. W. Bader. Atoms in molecules: A quantum ory. 1994, 360(1-3). [7] A. D. Becke. Density-functional rmochemistry. v. systematic optimization of exchange-correlation functionals. The Journal of chemical physics, 1997, 107(20): WJMS for contribution: submit@wjms.org.uk

9 World Journal of Modelling and Simulation, Vol. 14 (2018) No. 1, pp [8] A. Capobianco, A. Esposito, T. Caruso. Tuning wavefunction mixing in pushcpull molecules: From neutral to zwitterionic compounds. European Journal of Organic Chemistry, 2012, 2012(15): [9] R. Centore, A. Concilio, F. Borbone. Quadratic nonlinear optical and preliminary piezoelectric investigation of crosslinked samples obtained from a liquid chromophore. Journal of Polymer Science Part B Polymer Physics, 2012, 50(9): [10] H. D. Cohen, C. C. J. Roothaan. Electric dipole polarizability of atoms by hartree-fock method. Journal of Chemical Physics, 1965, 43(10): S34 S39. [11] L. R. Dalton, S. J. Benight. Systematic nanoengineering of soft matter organic electro-optic materials. Chemistry of Materials, 2011, 23(3): [12] W. B. Davis, W. A. Svec, M. A. Ratner. Molecular-wire behaviour in p -phenylenevinylene oligomers. Nature, 1998, 396(6706): [13] B. L. Davydov, L. D. Derkacheva, V. V. Dunina. Connection between charge transfer and laser second harmonic generation. Zhetf Pisma Redaktsiiu, 1970, 12(1): 24. [14] C. Dehu, F. Meyers, et al Solvent effects on second-order nonlinear optical response of. pi.-conjugated molecules: A combined evaluation through self-consistent reaction field calculations and hyper-rayleigh scattering measurements. Journal of American Chemical Society, 1995, 117(40): [15] A. Djedouani, S. Boufas. Djedouani, a., boufas, s., allain, m., bouet, g., khan, m.: Zwitterionic 6-methyl-2-oxo-3- [1-(ureidoiminio)ethyl]-2h-pyran-4-olate monohydrate. Acta Crystallogr Sect E Struct Rep Online, 2008, 64(Pt 9): [16] R. Fletcher, M. J. D. Powell. A rapidly convergent descent method for minimization. Computer Journal, 1963, 6(6): [17] M. Frisch, G. Trucks, et al J. Cheeseman, et al. Gaussian 03, revision b. 04. gaussian, inc., pittsburgh, [18] M. P. Hill, E. C. Carroll et al Rapid photodynamics of vitamin b6 coenzyme pyridoxal 5 -phosphate and its schiff bases in solution. Journal of Physical Chemistry B, 2008, 112(18): [19] H. Kang, A. Facchetti, H. Jiang. Ultralarge hyperpolarizability twisted pi-electron system electro-optic chromophores: synsis, solid-state and solution-phase structural characteristics, electronic structures, linear and nonlinear optical properties, and computational studies. Journal of American Chemical Society, 2007, 129(11): [20] P. Kerkoc, M. Zgonik, K. Sutter. 4-(n,n-dimethylamino)-3-acetamidonitrobenzene single crystals for nonlinearoptical applications. Journal of Optical Society of America B, 1990, 7(3): [21] D. A. Kleinman. Nonlinear dielectric polarization in optical media. Physical Review, 1962, 126(6): [22] M. Lanata, C. Bertarelli et al Molecules with quinoid ground state: a new class of large molecular optical nonlinearities. Syntic Metals, 2003, 138(1-2): [23] S. R. Marder, B. Kippelen, et al Design and synsis of chromophores and polymers for electro-optic and photorefractive applications. Nature, 1997, 388(6645): 845. [24] R. S. Mulliken. Electronic population analysis on lcao mo molecular wave functions. i. The Journal of Chemical Physics, 1955, 23(10): [25] J. S. Murray, K. D. Sen. Molecular electrostatic potentials : concepts and applications. Elsevier, [26] H. S. Nalwa, M. Hanack. Third-order nonlinear optical properties of porphyrazine, phthalocyanine and naphthalocyanine germanium derivatives: Demonstrating effect of -conjugation length on third-order optical nonlinearity of two-dimensional molecules. Chemical physics, 1999, 245(1): [27] H. S. e. a. Nalwa. Nonlinear Optics of Organic Molecules and Polymers [28] A. Ozek, C. Albayrak. Three (e)-2-[(bromophenyl)iminomethyl]-4-methoxyphenols. Acta Crystallographica Section C-crystal Structure Communications, 2007, 63(3): [29] A. Petrosyan. Salts of l-histidine as nonlinear optical materials: a review. J Cryst Phys Chem, 2010, 1(1): [30] G. Rauhut, P. Pulay. Transferable scaling factors for density functional derived vibrational force fields. J.phys.chem, 1995, 99(10): [31] D. Sajan, H. J. Ravindra, N. Misra. Intramolecular charge transfer and bonding interactions of nonlinear optical material n -benzoyl glycine: Vibrational spectral study. Vibrational Spectroscopy, 2010, 54(1): [32] E. Scrocco, J. Tomasi. Electronic molecular structure, reactivity and intermolecular forces: An euristic interpretation by means of electrostatic molecular potentials. Advances in Quantum Chemistry, 1978, 11: [33] K. S. Thanthiriwatte, K. M. N. D. Silva. Non-linear optical properties of novel fluorenyl derivativesłab initio quantum chemical calculations. Journal of Molecular Structure Theochem, 2002, 617(1-3): WJMS for subscription: info@wjms.org.uk