Substituent effects on molecular properties of dicarba

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Substituent effects on molecular properties of dicarba-closo-dodecarborane derivatives

Georgia M. A. Junqueira & Fernando Sato

Journal of Molecular Modeling Computational Chemistry - Life Science - Advanced Materials - New Methods ISSN 1610-2940 Volume 20 Number 7 J Mol Model (2014) 20:1-9 DOI 10.1007/s00894-014-2275-8

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Author's personal copy J Mol Model (2014) 20:2275 DOI 10.1007/s00894-014-2275-8

ORIGINAL PAPER

Substituent effects on molecular properties of dicarba-closo-dodecarborane derivatives Georgia M. A. Junqueira & Fernando Sato

Received: 13 February 2014 / Accepted: 24 April 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract In this paper we study the role played by substituent effects on reactivity and NLO properties of ortho-, meta- and para- dicarba-closo-dodecarborane derivatives at B3LYP=6−31Gðd; pÞ level of theory. In addition correlations with Hammett parameters of the substituents were established. In accordance with obtained results the reactivity properties of derivatives have not been significantly influenced by the isomer type, however the replaced para isomers were the most sensitive to NLO calculations. Moreover, the push-pull para isomers were found to be the most reactive and displayed the largest values of βtot and dipole moment. Keywords Carborane . Hammett parameters . NLO properties . Reactivity indexes

Introduction The quasi-icosahedral dicarba-closo-dodecarboranes isomers 1,2−(ortho−), 1,7−(meta−) and 1,12−(para−) C2B10H12 (see Fig. 1) are the best-known carboranes and have long been investigated for their high structural, chemical, and biological stability and diverse applications [1–4], including as HIV protease (HIV PR) inhibitors [5] and their potential use as a component of nonlinear optics (NLO) material [6–10]. Recently Nakamura et al. [11] reported an ortho-carborane derivative that exhibits significant inhibition of hypoxiainduced HIF−1, a transcription factor that regulates cell survival during cancer growing. Furthermore, carboranes can be bonded to a metal ion to form metalacarboranes, which may This paper belongs to Topical Collection QUITEL 2013 G. M. A. Junqueira (*) : F. Sato Departamento de Física, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil e-mail: [email protected]

contain from four to 14 vertices in a unique polyhedral arrangement. The strong bonds between carborane and metal combined with high electronic delocalization gives great stability and provides diverse properties to these compounds [4]. For example, it has been shown that a coordination compound of dicarba-closo-dodecarborane can inhibit the HIV PR [5]. Thus, it is important to carry out theoretical studies aiming to understand molecular aspects of such compounds and derivatives. Molecular properties obtained by density functional theory (DFT) can be found in literature describing reactions and a series of structure-activity relationship studies (see [12] and references cited therein). Furthermore, the determination of local and global reactivity indexes has significantly helped in the study of reactivity for a wide number of systems for isolated molecules [12–15] and in solution [16–18]. For example, Kobayashi and co-workers [14] studied the interaction between a drug and an enzyme in the actual drugenzyme complex. The authors [14] related the interaction drugenzyme with the absolute hardness ηN and the absolute electronegativity χ. Using these properties a relationship between bioactivity and different reactivity descriptors was then determined [14]. Recently, solvent effects on global reactivity indexes: electronic chemical potential (μ), chemical hardness (η), and electrophilicity (ω) for dicarba-closo-dodecarboranes isomers were reported by one of us [18]. In that study [18], the solvated isomers became softer and the chemical potential increases when comparing to gas phase, indicating the charge transfer from solvent to the solute. In addition some works have reported relationships between reactivity properties and Hammett electronic parameters (see [15] and references cited therein). Linear correlations between frontier orbital energies and Hammett constants were established to substituted osmabenzene complexes [15]. Another interesting property of the boron-based cages, due to the high electronic and thermal stability, delocalized structures and easy functionalization, is the potential

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J Mol Model (2014) 20:2275 H

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c Fig. 1 B3LYP/6−31G(d,p) optimized structures of ortho− (a) meta− (b) and para− (c) dicarba-closo-dodecarboranes [C2B10H12]. Hydrogen atoms attached to carbon atoms will be replaced by electron donor and acceptor groups, R1 and R2

nonlinear optical (NLO) application [4, 6–10]. NLO properties are of interest due to their importance in the designing of new optical devices. Some of these applications are frequency-doubling devices, optical signal processing and optical computers [19, 20]. Organic molecules with NLO applications, in general, contain electronic systems with electron donors and acceptor groups in its extremities (push-pull type), high values of first-order

hyperpolarizability (β) and permanent dipole moments. The second-order responses can be enhanced by increasing the electronic asymmetry or the conjugation length between the extremity groups [19, 20]. Beside, the electronic effect is not the only important factor for NLO properties. For example, the final structure must be noncentrosymmetric and the molecules must present absorption at wavelength greater than 800 nm for electro-optic

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devices and greater than 400 nm for second harmonic generation (SHG), to avoid re-absorption of the converted light [21]. The synthesis of new molecules and the development of novel molecular architectures are of fundamental significance in NLO research. Aiming at obtaining new molecular-based materials with the best quality and low cost, electronic structure computational methods are an important tool to make predictions of NLO properties of new systems. In literature there are important works of the polarizabilities, first- and second-order hyperpolarizabilities calculations at semi-empirical [22–25] and DFT [26–28] levels of theory, including studies from one of us [29, 30]. In addition some works have reported relationships between hyperpolarizabilities and Hammett electronic parameters [30–34]. Park and Cho [32, 33] calculated first-order hyperpolarizability (β) of the 1,3,5−tricyano−2,4,6 −tris(styryl)benzene [32] and triazine derivatives [33] at HF/6−31G level of theory. There, authors obtained a linear correlation between β and Hammett constants, wherein the β values of these molecules become greater with enhancement in donor strength and as conjugation length increases. In ref [30] were identified structural aspects leading to enhancement of the NLO properties of the oxocarbons derivatives, through correlations between Hammett parameters of the substituents (∑σp) and ln (βtot). Those results suggested the investigated compounds as potential molecules for NLO applications [30]. Since 20 years ago the NLO behavior of icosahedral carboranes had been investigated [6]. Studies in this area involving dodecarboranes include synthesis of novel compounds [6, 10], NLO properties [6, 7, 9, 10], and some theoretical [8, 35] calculations. A new class of dodecarboranes ([B12H11−C2B10H11]2−) with potential to NLO materials was proposed through ab initio calculations [8]. In addition Grüner et al. [10] obtained a derivative [12 −C 7 H +6 −CB 11 H −11 ] with β ten times larger than p −nitroaniline. In the present work, molecular first-order hyperpolarizability (βtot) and dipole moment were obtained at DFT level of theory for dicarba-closo-dodecarborane derivatives. Moreover, with the aim to get insight on the effect of substituent on the electronic properties, correlations between Hammett parameters of the substituents (∑σp) and ln (βtot) were tested. Finally, carborane chemistry has growing interest in fundamental aspects (synthesis, electronic structure) [1, 3, 4, 18, 36], medicine [4, 5, 11, 37], and materials [2, 4, 6–10, 38]. In addition, theoretical studies to contribute to the understanding of the molecular level in these systems are scarse. Thus, the proposal of this work is to systematically study the substituent effects on reactivity indexes and on NLO properties of dicarba-closo-dodecarboranes isomers. The paper is organized as follows: In the next section we describe the computational procedures. Then, the results

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are reported and discussed , and finally, the concluding remarks are given.

Computational methods All the calculations were performed using GAUSSIAN03 [39]. The geometries of all systems were fully optimized in the gas phase at density functional theory (DFT) with B3LYP functional employing standard split valence basis-set with inclusion of polarization and diffuse functions [6−31+G(d,p)]. Harmonic frequency calculations were performed to verify that the optimized geometries properly represents a minimal configuration. HOMO and LUMO energies were obtained at the same level of theory. In DFT formalism, the electronic chemical potential μ (the negative of the electronegativity, χ) is a measure of the electron tendency to escape from the electronic cloud, and is related to both ionization potential and electronic affinity properties. The absolute hardness ηN, according to Parr and Pearson [40], is a property derived from the electronic chemical potential μ. The ionization energy and electron affinity can be replaced by the frontier molecular orbitals HOMO and LUMO energies, respectively, using Koopmans’s theorem [41]. It is usually much easier to calculate the orbital energies of a molecule, than to measure its ionization potential and electron affinity. The electrophilicity ω is a measure of the stabilization in energy when the system acquires an additional electronic charge from the environment. The index ω englobes both the ability to acquire additional electronic charge and the resistance of the system to exchange electronic charge with the environment. Therefore, from HOMO and LUMO energies one can obtain global reactivity indexes such as the electronic chemical potential μ (Eq. 1), the hardness η (Eq. 2) and the electrophilicity ω (Eq. 3) in the context of molecular (Kohn-Sham) orbital theory. Electronic chemical potential μ¼

∈LUMO þ ∈HOMO 2

ð1Þ

Chemical hardness η ¼ ∈LUMO −∈HOMO

ð2Þ

Electrophilicity ω ¼ μ2 =2η

ð3Þ

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The dipole moment (d), the linear polarizability (α), and the first- and second-order hyperpolarizabilities (β and γ) play a significant role mainly in the study of

NLO properties [21, 42, 43]. The energy (E) of a molecular system under an external electric field F can be written as [44]:

E ð F Þ ¼ E ð0Þ−d F i −ð1=2!Þαij F i F j −ð1=3!Þβijk F i F j F k −ð1=4!Þγ ijkl F i F j F k F l −⋯

where E(0) stands for the energy of the system in the absence of an external field, and Fi are the components of the applied field. The first-order hyperpolarizability β is a 3X3X3 tensor. The tensor components (βijk with i,j,k=x,y,z) are generally reducible when considering the symmetry properties. Through Kleinman symmetry (βij =βji) [45], the output from GAUSSIAN-03 provides ten components. In the present paper, we obtain the three independent values for β components βi (i=x,y,z) (Eq. 5) and then, solve the complete expression for calculating the magnitude of the βtot, the molecular first-order hyperpolarizability [21].(Eq. 6). X  β i ¼ βiii þ 1=3 β ijj þ β jij þ β jji j≠i

i ¼ x; y; z;

j ¼ x; y; z ð5Þ

ð4Þ

 1=2 ð6Þ βtot ¼ β2x þ β2y þ β2z NLO properties were calculated at B3LYP[6−31+G(d,p)] level of theory by time dependent Hartree fock (TDHF) [46, 47] method within the static approach.

Results and discussion In this paper we studied 39 compounds: 13-ortho, 13-meta, and 13-para isomers with the substituent groups: CH3, NO2, NH2, COH, OCH3, Cl, and C6H5. The analyzed series is depicted in Fig. 2. Relationships among studied properties

Fig. 2 B3LYP/6−31G(d,p) optimized geometries of the 13 ortho−, meta− and para− dicarba-closo-dodecarboranes [C2B10H10R1R2]

Author's personal copy J Mol Model (2014) 20:2275 Table 1 Calculated properties to dicarba-closo-dodecarboranes derivatives. Energies (EHOMO and ELUMO) are in atomic units (au). Reactivity indexes (μ, η, and ω) are in eV. Molecular first-order hyperpolarizability (βtot)×10− 30 cm5/esu and dipole moment are in Debye

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Energies

Reactivity

NLO

EHOMO

ELUMO

μ

η

ω

β

dipole

(∑σp =−1.32) NH2,NH2 (∑σp =−0.66) H,NH2 (∑σp =−0.17) H,CH3 (∑σp =−0.02) ph,ph (∑σp =−0.01) H,ph (∑σp =0) H,H (∑σp =+0.12) NH2,NO2 (∑σp =+0.23) H,Cl (∑σp =+0.42) H,COH (∑σp =+0.44) OCH3,NO2 (∑σp =+0.61) CH3,NO2 (∑σp =+0.78) H,NO2 (∑σp =+1.56) NO2,NO2 META (∑σp =−1.32) NH2,NH2 (∑σp =−0.66) H,NH2 (∑σp =−0.17) H,CH3 (∑σp =−0.02) ph,ph (∑σp =−0.01) H,ph

−0.28 −0.29 −0.32 −0.26 −0.27 −0.31 −0.30 −0.32 −0.31 −0.31 −0.33 −0.32 −0.34

−0.03 −0.03 −0.02 −0.05 −0.05 −0.03 −0.13 −0.05 −0.10 −0.12 −0.13 −0.13 −0.14

−4.19 −4.34 −4.62 −4.30 −4.44 −4.69 −5.82 −4.56 −5.56 −5.89 −6.16 −6.16 −6.49

6.77 7.02 8.00 5.64 5.95 7.59 4.69 6.57 5.67 5.12 5.48 5.27 5.51

1.30 1.34 1.34 1.64 1.66 1.45 3.61 1.58 2.72 3.39 3.46 3.60 3.82

13.95 10.69 4.66 13.45 7.82 5.00 11.27 5.31 19.40 14.62 2.50 2.73 11.56

5.04 4.98 5.01 5.96 5.06 4.44 2.86 3.14 3.65 3.19 2.80 2.02 2.29

−0.27 −0.27 −0.31 −0.26 −0.27

−0.02 −0.02 −0.02 −0.04 −0.04

−3.95 −3.97 −4.50 −4.15 −4.19

6.96 7.01 7.99 5.85 6.07

1.12 1.12 1.27 1.47 1.44

12.24 10.74 2.33 2.14 5.40

3.12 2.30 2.99 3.18 3.01

(∑σp =0) H,H (∑σp =+0.12) NH2,NO2 (∑σp =+0.23) H,Cl (∑σp =+0.42) H,COH (∑σp =+0.44) OCH3,NO2 (∑σp =+0.61) CH3,NO2 (∑σp =+0.78) H−NO2 (∑σp =+1.56) NO2,NO2 PARA (∑σp =−1.32) NH2,NH2 (∑σp =−0.66) H,NH2 (∑σp =−0.17) H,CH3 (∑σp =−0.02) ph,ph (∑σp =−0.01) H,ph (∑σp =0) H,H (∑σp =+0.12) NH2,NO2 (∑σp =+0.23) H,Cl (∑σp =+0.42) H,COH

−0.31 −0.29 −0.32 −0.30 −0.31 −0.32 −0.32 −0.34

−0.02 −0.11 −0.04 −0.08 −0.11 −0.12 −0.11 −0.12

−4.59 −5.46 −4.80 −5.13 −5.69 −6.01 −5..95 −6.31

7.95 4.77 7.68 5.80 5.28 5.52 5.71 5.64

1.33 3.12 1.50 2.27 3.06 3.27 3.10 3.52

2.49 12.69 4.09 11.35 22.44 8.11 11.66 10.36

2.76 3.69 2.24 2.44 4.76 4.16 3.39 1.65

−0.26 −0.27 −0.32 −0.26 −0.26 −0.32 −0.29 −0.32 −0.29

−0.01 −0.01 −0.01 −0.04 −0.04 −0.02 −0.11 −0.03 −0.08

−3.73 −3.91 −4.54 −4.11 −4.12 −4.58 −5.36 −4.82 −5.03

6.79 7.01 8.35 5.86 6.07 8.24 4.87 7.87 5.91

1.03 1.09 1.24 1.44 1.40 1.28 2.95 1.48 2.14

4.55 11.50 0.42 2.14 7.94 0 23.62 3.79 12.41

2.30 1.20 0.48 0.10 0.52 0 4.45 1.62 2.68

(∑σp =+0.44) OCH3,NO2 (∑σp =+0.61) CH3,NO2 (∑σp =+0.78) H,NO2 (∑σp =+1.56) NO2,NO2

−0.30 −0.31 −0.32 −0.33

−0.11 −0.11 −0.11 −0.13

−5.62 −5.66 −5.89 −6.24

5.34 5.51 5.79 5.63

2.95 2.91 3.00 3.45

23.45 9.35 13.72 0.17

4.08 4.63 4.04 0.03

ORTHO

and Hammett constants (σp) [48] were obtained (Table 1, Figs. 4 and 5). The negative values of σp correspond to the substituent electron donor relative to the hydrogen atom and a positive σp is an electron withdrawing group relative to the

hydrogen atom. The cooperative effect of both groups was described by the summation of individual σp(∑σp). In Fig. 2 and Table 1 the set of substituents are gathered in addition to the electronic Hammett constants (σp) [48]. In the present

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Fig. 3 Frontier orbital energies versus Hammett constants

work this global parameter covers a broad range of electronic effect with values from -1.32 (strong electron donor effect) to +1.56 (strong electron acceptor effect). Reactivity properties Initially, a correlation between frontier orbital energies (HOMO and LUMO) of all studied compounds and (σp) was established (Fig. 3). It can be noted from Fig. 3 there is a decrease of both HOMO and LUMO energies when increasing the electron acceptor effect of pair substituent. This finding is consistent with linear correlations between frontier orbital energies and Hammett constants established in substituted osmabenzene complexes [15]. Figure 3 shows values of ∑σp from 0 to -1.32. A decrease in the HOMO Fig. 4 Reactivity indexes versus Hammett constants

and LUMO energies upon substitution indicates that the substituted isomers are better acceptors and a consequent enhancement in the electron accepting nature compared to the unsubstituted ones [13]. According to Table 1 and Fig. 4 the calculated reactivity properties have not been significantly influenced by the isomer type, but by the swap of some hydrogens of the cages by acceptor and donor electron groups. Generally it can be observed that a slight decrease of hardness values (η) of isomers as well as chemical potential (μ) when the electron acceptor effect of the pair substituent is increased (∑σp increase) occurs. In addition, as in ref [15], it is noticed that the Hammett correlation for μ is better than for η and ω. The substitution of hydrogens by electron donors and electron acceptors yields changes in electron distribution of the isomers. Chemical hardness (η) measures the resistance to

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Fig. 5 Reactivity indexes versus molecular first-order hyperpolarizability

change. Since the dicarba-closo-dodecarboranes isomers are naturally electron-deficient, when the hydrogen atoms are changed by more electron-withdrawing the substituted isomers become more reactive (decreased η, softer). Analysis of Fig. 4 with ∑σp between 0 and +0.44 shows that the minor values of η are at ∑σp =+0.12 (pair substituent NH2,NO2) and ∑σp =+0.44 (pair substituent OCH3,NO2), i.e., the most reactive compounds of the series. Such isomers can be considered as push-pull compounds, because of strong electron donor and acceptor groups in their extremities. Since the chemical potential (μ) is a measure of the escaping tendency of an electronic cloud, one can expect that the chemical potential (μ) of the analyzed compounds decreases when the power electron acceptor power of substituents is enhanced. The changes in the electrophilicity index ω takes into account the variation of both η and μ and may be used to compensate effects from the individual variations of each index. Analyzing the results one can observe an increase of ω since that species became softer and the electronic chemical potential μ decreases, when the electron-withdrawing effect of substituents is increased. Nonlinear properties The calculated molecular hyperpolarizabilities (βtot) as well as the dipole moments are reported in Table 1. According to the results, NLO properties are found to be sensitive to the type of isomer. Among the unsubstituted isomers, ortho- exhibits the highest βtot (5×10−30cm5/esu) and dipole moment (4.44 Debye, being the experimental value: 4.31±0.08 Debye [49]). The meta- isomer displays βtot(5×10−30cm5/esu) and dipole moment 2.76 Debye (experimental value: 2.78±0.09×10− 30 cm5/esu [49]). The para- isomers appear to be the most affected by substitutions. Besides the para- isomers present

the highest βtot values; these isomers show the minors values of all studied compounds: the unsubstituted isomer (βtot =0) and the derivative obtained when both hydrogens linked to carbon atoms were substituted by two strong donor groups NO2,NO2(βtot =0.17×10−30cm5/esu)cug. The original paraisomer displays both βtot and dipole moment nulls. However, when one hydrogen atom is replaced by NO2 (strong acceptor electron group), βtot values become almost 14 and the dipole moment 4 Debye. Such behavior does not occurs with the other isomers. Our study shows that replaced ortho- and metaisomers have reasonable βtot values which are one order of magnitude larger than that of unsubstituted isomers. This is in agreement with the study of Fan et al. [35]. In that work [35] the authors have found that halogen and organic group substituted ortho- 12-vertex-closo-carboranes exhibit βtot values enhanced by one order of magnitude compared to original isomers. However, in our study the βtot values in para- push-pull compounds were enhanced by two order of magnitude as compared to unsubstituted carboranes. In reference [35] only carboranes with alkali-metals have βtot values two to three orders of magnitude larger than those from the original isomers. Isomers ortho-, meta- and para- [B12H11−C2B10H11]2− were proposed by Abe et al. [8] as potential NLO material. We have found dicarba-closo-dodecarborane derivatives with βtot values (10−24×10−30cm5/esu) from three to seven times larger than those obtained in the mentioned study (3.5×10−30cm5/esu) [8]. Moreover, according to Table 1, βtot values are close or larger than the corresponding values for molecules with NLO applications such as p-nitroaniline (9.2× 10−30cm−5/esu) [21] and nitro-amine-1,3,5-hexatriene (29.2× 10−30cm5/esu). Thus, our results suggested the investigated dicarba-closo-dodecarborane derivatives as potential molecules for NLO materials, since these special structures exhibit great chemical stability and high optical transparency [35].

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Following previous works from literature [30–33], aiming to improve the study of the influence of substituents on NLO properties, a plot of molecular first-order hyperpolarizability (βtot) as a function of Hammett’s parameters (∑σp) is depicted in Fig. 5. In accordance with Fig. 5, these ortho- and metacompounds present similar behavior. Before σp =0, βtot values generally decrease when the electron withdrawing power of substituents is enhanced (σp increase). After σp =0, βtot values are the largest for all isomers when substituted by NH2,NO2 and OCH3,NO2 pairs (∑σp =+0.12 and +0.44) (push–pull compounds), respectively. Considering para- compounds, it can be noted that their behavior mainly differs from ortho- and meta- isomers for compounds containing strong donor and acceptor groups such as NH2,NH2 and NO2,NO2 (∑σp =− 1.32 and +1.56), respectively. Among the 39 isomers, the largest values of βtot were obtained for para- push–pull compounds NH2,NO2 and OCH3,NO2 with βtot =24 (such compounds have relatively high dipole moments 4.45 and 4.08 Debye), respectively. This finding is different, for example, from Park and Cho [32, 33] studies. In those references [32, 33] the largest βtot values were obtained for compounds with strong electron donor groups. However, similar results were found in 1,3-mono-squarate derivatives [30]. In that study [30], in order to improve βtot values the electron donor and electron acceptor groups must be in 1,3 positions with acceptor strength stronger than in the donor group (∑σp >0).

Conclusions In the present work we studied the role played by substituent effects on reactivity and NLO properties of dicarba-closododecarboranes derivatives at B3LYP/6−31G(d,p) level of theory. According to our results the three isomers are very close in energy. The effects that different substituents exert on the electronic chemical potential μ, chemical hardness η, and electrophilicity ω of ortho-, meta- and para- dicarba-closododecarborane derivatives were discussed in detail. By means of the employed theoretical methodology, it was possible to identify structural and electronic characteristics leading to enhancement of such molecular properties of the analyzed isomers. It can be concluded that calculated reactivity properties have not been significantly influenced by the isomer type, but by the swap of some hydrogens of the cages by acceptor and donor electron groups. When hydrogen atoms are replaced by more electron withdrawing groups, the compounds became softer as compared to the original isomers. Moreover, the obtained NLO properties were affected by isomer type. In addition the para- isomers were found to be the most sensitive by replacements, and had the larger values of βtot. The parapush-pull compounds [NH2, NO2 and OCH3, NO2] with (∑σp =+0.12 and +0.44) appear to be the most reactive [η=

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4.87 and 5.34 eV], respectively, and exhibit the largest values of βtot [24×10−30cm5/esu]. These results suggest the investigated compounds can be considered as potential molecules for NLO applications, since the βtot values are more than twice those reported for p-nitroaniline (9.2×10−30cm5/esu) [21]. Acknowledgments We are indebted to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support.

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