Tuning the push–pull configuration for efficient second-order nonlinear optical properties in some chalcone derivatives

Accepted Manuscript Title: Tuning the Push-Pull Configuration for Efficient Second-order Nonlinear Optical Properties in Some Chalcone Derivatives Author: Shabbir Muhammad Abdullah G. Al-Sehemi Ahmad Irfan Aijaz R. Chaudhry PII: DOI: Reference:

S1093-3263(16)30106-1 http://dx.doi.org/doi:10.1016/j.jmgm.2016.06.012 JMG 6719

To appear in:

Journal of Molecular Graphics and Modelling

Received date: Revised date: Accepted date:

29-4-2016 18-6-2016 20-6-2016

Please cite this article as: Shabbir Muhammad, Abdullah G.Al-Sehemi, Ahmad Irfan, Aijaz R.Chaudhry, Tuning the Push-Pull Configuration for Efficient Second-order Nonlinear Optical Properties in Some Chalcone Derivatives, Journal of Molecular Graphics and Modelling http://dx.doi.org/10.1016/j.jmgm.2016.06.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tuning the Push-Pull Configuration for Efficient Second-order Nonlinear Optical Properties in Some Chalcone Derivatives Shabbir Muhammad a,c*, Abdullah G. Al-Sehemi,b,c Ahmad Irfan b,c, Aijaz R. Chaudhrya,c, a

Department of Physics, College of Science, King Khalid University, Abha 61413, P.O.

Box 9004, Saudi Arabia b

Department of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O.

Box 9004, Saudi Arabia c

Research Center for Advanced Materials Science (RCAMS), King Khalid University,

Abha 61413, P.O. Box 9004, Saudi Arabia

1

Graphical abstract

2

Highlights  The potential of several push-pull chalcones as efficient NLO-phores is assessed  NLO properties are calculated for different chalcone derivatives using DFT methods  Chalcones with trimethoxypyren and nitro groups are found as efficient NLOphores  The structure-NLO property relationship has been extended to several experimental chalcones

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Abstract Using the density functional theory methods, we effectively tune the second-order nonlinear optical (NLO) properties in some chalcone derivatives. Various unique pushpull configurations are used to efficiently enhance the intramolecular charge transfer process over the designed derivatives, which result in significantly larger amplitudes of the first hyperpolarizability as compared to their parent molecule. The ground state molecular geometries have been optimized using B3LYP/6-311G** level of theory. A variety of methods including B3LYP, CAM-B3LYP, PBE0, M06, BHandHLYP and MP2 are tested with 6-311G** basis set to calculate the first hyperpolarizability of parent system 1. The results of M06 are found closer to highly correlated MP2 method, which has been selected to calculate static and frequency dependent first hyperpolarizability amplitudes of all selected systems. At M06/6-311G** level of theory, the permanent electronic dipole moment (μtot), polarizability (α0) and static first hyperpolarizability (βtot) amplitudes for parent system 1 are found to be 5.139 Debye, 274 a. u. and 24.22×10-30 esu, respectively. These amplitudes have been significantly enhanced in designed derivatives 2 and 3. More importantly, the (βtot) amplitudes of systems 2 and 3 mount to 75.78×10-30 and 128.51×10-30 esu, respectively, which are about 3 times and 5 times larger than that of their parent system 1. Additionally, we have extended the structureNLO property relationship to several newly synthesized Interestingly, the amplitudes of dynamic frequency dependent hyperpolarizability μβω (SHG) are also significantly larger having values of 366.72×10-48, 856.32×10-48 and 1913.46×10-48 esu for systems 1, 2 and 3, respectively, at 1400 nm of incident laser wavelength. The dispersion behavior over a wide range of change in wavelength has also been studied adopting a range of wavelength from 1907 to 544 nm. Thus, the present work realizes the potential of designed derivatives as efficient NLO-phores for modern NLO applications. Keywords Nonlinear Optical; Density Functional Theory; Chalcones; Push-pull Configuration

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Introduction The nonlinear optical (NLO) material is among the most interesting and starling materials of present era. NLO material has the ability to change the wavelength of incident laser light resulting into second harmonic generation (SHG) and third harmonic generation (THG) effects. Soon after the discovery of first functional laser, the NLO material has been in limelight of scientific interest and a huge pile of materials have been reported till now. Nevertheless, NLO material-designing field has got a recent momentum since the replacement of electron by photon as carrier of information. The applications of NLO materials are ranging from efficient data storage to holographic imaging, frequency doubling and mixing, and telecommunication etc.[1, 2] Over the past few years, our group has proposed several designs to efficiently design the NLO materials with significantly larger first hyperpolarizabilities. These include different types of materials including organic,[3-6] inorganic,[7, 8] organic-inorganic hybrid [9, 10] materials, which are explored as efficient NLO materials. Every class of material inherits its own advantages and disadvantages. For example, inorganic materials are relatively stable but usually accompanied with poor amplitudes of first hyperpolarizability. On the other hand, organic materials are not only possess larger amplitudes of hyperpolarizability but also provide a divers variety of structures as well as ease of fabrication and low economic costs. Recently, a number of push-pull π-conjugated organic chromophores are investigated by studying their experimental and theoretical first hyperpolarizabilities to check their possible applications as efficient NLO materials.[11-16] Furthermore, including bithienylpyrrole NLO-phores where enhancement of the SHG is achieved through the introduction of a second thiophene,[17] two series of novel push–pull chromophores with pyrrole as an electron donor group,[18] push–pull bithiophene azochromophores bearing thiazole and benzothiazole acceptor moieties[19] are important to mentioned here. Similarly, semiempirical analysis of azo chromophores, [20] thermally stable thiazole and benzothiazole push–pull chromophores,[21] Push–pull chromophores incorporated in 1,3-dithiol-2-ylidene moiety as new electro-optics materials[22] were proposed. Along the above reported literature of organic classes, chalcones have been frequently trialed for their potential use in NLO applications.[23] For instance, Prasad et

5

al., have reported the synthesis and characterization of 1,5-di-p-tolylpenta-1,4-dien-3-one (DTDO) along with their computationally calculated static first hyperpolarizabity.[24] Menezes et al., have grown pyridine based chalcone single crystals and checked the role of pyridine ring to modulate NLO properties of these molecules.[25] Sajan et al., have grown the single crystal of 3,4-dimethoxy chalcone and studied their electro-optical properties by computational techniques.[26] Among other recent noticeable chalcone syntheses and characterizations are 3-(3-fluorophenyl)-1-[4-(methylsulfanyl) phenyl] prop-2-en-1-one,[27] (2E)-3-(2-methylphenyl)-1-(4-nitrophenyl) prop-2-en-1-one,[28] 1(3-Nitrophenyl)-5-phenylpenta-2,4-dien-1-one[29],

2-cyano-N-(1-phenylethyl)

acetamide[30] and 1-(4-Bromophenyl)-3-(napthalen-2- yl)prop-2-en-1-one (C19H13BrO) etc. Based on techniques used in those studies, the above reported chalcone derivatives have been characterized as good NLO molecules. Nevertheless, a common feature of all the above reported chalcone derivatives is the lack of a prominent push-pull configuration which is a rudimentary principle to design efficient NLO-phores. The motivation of present study is to use modern quantum chemical methods to limelight chalcone based push-pull configurations that can lead to proficient NLO properties. For this purpose, we have selected a realistic approach by selecting one know parent chalcone molecule with average NLO properties.[31] Subsequently derivatives have been made from the parent molecule to tune the NLO properties through push-pull configurations. 2. Computational Details All the optimized and stable geometries for chalcone derivatives have been obtained using B3LYP with 6-311G** basis set, which is considered a gateway approach in contemporary computational chemistry. The stability of all optimized geometries has been further confirmed by calculating their analytical frequencies. There are several studies where B3LYP has successfully reproduced the experimental structures.[9, 32] It is not necessarily true that any method good for studying geometric structure should also be good for calculation of hyperpolarizability. To select a best DFT functional for first hyperpolarizability calculations, we have calculated the polarizability and first hyperpolarizability for system 1 with six different methods including B3LYP,[33] CAMB3LYP,[34] PBE0,[35] BHandHLYP,[36] M06 [37] and MP2 [38] method at 6-311G** basis set. A comparison of calculated static first hyperpolarizabilities has been made in 6

Table S1. It can be seen that the M06 results are in good agreement with highly correlated and a standard reference MP2 method (see Table S1 of supporting information).[39] The M06/6-311G** level of theory is selected for calculation of dipole moment, polarizability and first hyperpolarizability of all systems. The static first hyperpolarizability (βtot) and its components for all systems were calculated using finite field (FF) approach at M06/6311G** level of theory. The FF method, which is originally developed by Kurtz et al.[40] has been widely used to calculate the first hyperpolarizability of several molecules and it has provided very consistent results with experimental relationship.[5, 41-43] and other theoretical approaches like Time-dependent-sum over states (TD-SOS) methods and response theory.[44] In FF approach, usually a static electric field (F) is applied and the energy (E) of the molecule is stated in terms of following Eq. 1

Here E(0) represents the total energy of molecule in the absence of an electronic field,  is the vector component of the dipole moment,  is the linear polarizability,  and  are the second and third-order polarizabilities, respectively, while x, y and z label the

i, j and k components, respectively. It can be seen from Eq.1 that differentiating E with respect to F obtains the μ, α, β, and γ values. In our present investigation, we have calculated the electronic dipole moment, molecular polarizability, polarizability anisotropy and molecular first hyperpolarizability. For a molecule, its dipole moment (μ) is defined as follows: (

)

(2)

The average polarizability (  0 ) can be calculated by following equations: (

)

(3)

For anisotropy of polarizability (Δα)

√

√[(

)

(

)

]

Similarly, the magnitude of the total first static hyperpolarizability (βtot) can also be calculated using following eq.

7

(

)

(5)

where (6) (7) (8) The second-order polarizability (β) that is a third rank tensor that can be described by a 3 × 3 × 3 matrix. According to Kleinman symmetry (βxyy = βyxy = βyyx, βyyz = βyzy = βzyy,… likewise other permutations also take same value), the 27 components of the 3D matrix can be reduced to 10 components. These components have been calculated using GAUSSIAN 09. [45] For dynamic (frequency dependent) electric field induced SHG (EFISHG) first hyperpolarizabilities (μβ), the measurement provide information on the projection of the vector part of β on the dipole moment vectors as given by following Eq. ∑

(9)

where μ is the norm of dipole moment vector and μζ and βζ are the components of μ and β vectors. The EFISHG first hyperpolarizability (μβ) values can be finally calculated using following relationship: (10) All the μβω values have been given in electro static units (10-48 esu) within T-convention of reference.[46] The frequency dependent coupled-perturbed Kohn-Sham (CPKS) method with the M06 functional has been applied to calculate the dynamic first hyperpolarizability values. In CPKS method, the matrices of CPKS equation are expanded in Taylor series of external dynamic electric field and are solved analytically order by order. According to experimental setup for EFISHG first hyperpolarizability (μβω) measurement, the frequency dependent calculations are carried out using incident wavelengths from 544 nm to 1907 nm. The TD-DFT with M06 flavor has been used to

8

evaluate the transition energies of all selected systems. We have performed all calculations by using G09 program. [45]

Figure 1. Optimized structure of parent system 1 indicating important geometrical parameters as included in Table 1 3. Results and Discussion 3.1 Molecular Geometries The molecular structure of the parent molecule (E)-1-(4-bromophenyl)-3-(naphthalen-2yl)prop-2-en-1-one (system 1) [31] and its derivatives have been shown in scheme 1. The parent system 1 has been reported to contain a moderate NLO response. [31] In present study, we designed systems 2 and 3, by replacing nephthalen and bromo moieties of parent system with 5,7,9-trimethoxypyren (system 2) as well as 5,7,9-trimethoxypyren and nitro groups (system 3), respectively as given in scheme 1. The inspiration for designed present derivatives is based on several similar syntheses [47, 48] that are practically plausible through simple condensation reaction of trimethoxypyren aldehyde with 4-nitrophenyl-ketone functional group at least for system 3. The methoxy group is not only a good donor group due to the resonance effect of lone pair of electrons on oxygen atoms but also provides divers possibilities for synthesizing a verity of chalcone derivatives as reported by earlier syntheses. [47, 48] Furthermore, recently we have studied the impact of position and number of methoxy group(s) to tune the nonlinear optical properties of chalcone derivatives. It has been found that the methoxy group can

9

play a crucial role to tune the NLO properties of chalcones with para methoxy and o,m,p,-trimethoxy substituents.[49] Table 1. Some important geometrical parameters of parent system 1 according to Figure 1 as calculated at B3LYP/6-311G** level of theory

Bond Length Br1-C7 C7-C8 C8-C10 C10-C12 C12-C13 C13-O20 C13-C14 C14-C16 C16-C18 C18-C19 C19-C21 C21-C22

Cal. (Å) 1.914 1.393 1.387 1.387 1.504 1.223 1.481 1.346 1.457 1.385 1.415 1.419

Exp. (Å) 1.894 1.381 1.383 1.391 1.493 1.222 1.476 1.327 1.458 1.373 1.415 1.416

Bond Angles Br1-C7-C8 C7-C8-C10 C10-C12-C13 C12-C13-O20 C13-C14-C16 C16-C18-C19 C19-C21-C22 C21-C22-C24 Br1-C7-C8-C10 C12-C13-C14-C16 C16-C18-C19-C21 C19-C21-C22-C24

Cal. (Å) 119.41 118.96 117.53 119.54 120.10 118.71 122.22 120.70 179.92 176. 70 179.94 179.96

Exp.[31] (Å) 119.54 118.86 118.09 119.65 120.59 119.50 122.67 120.65 179.09 170.47 178.34 178.35

The optimized structure of parent system 1 has been given in Figure 1 with indication of important parameters that have been included in Table 1. From Table 1 and Figure 1, we can see a comparison of experimental and calculated geometrical parameters including selected bond lengths, bond angles and torsion angles. It can be seen from Table 1 that the maximum differences of bond lengths are 0.02 and 0.019 Å for Br1-C7 and C14-C16, respectively.

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Scheme 1. The representation of chemical structures of systems 1 - 3 The compared bond angles are in good agreement with experimental data having slight differences with maximum difference of 0.79

o

for C16-C18-C19 angle. Overall the

comparison between experimental and calculated structures shows a good agreement and reliability of structure used for further calculations. This is a well-know fact based on several combined experimental and computational studies that B3LYP reproduces reasonably well the experimental structures. Additionally, the comparison of bonding parameters among all systems 1-3 has been also performed in Table S2 of supporting information. The selected parameters including bond lengths and bond angles for systems 1-3 are somewhat similar to each other with maximum change of 0.045 Å for C16-C18 bond length and 0.88° for C16-C18-C19 bond angle. From Table S2 of supporting information, it can be seen that there are more changes in structures of system 1 and system 3 as compared to systems 1 and 2.

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Table 2. The calculated ground state dipole moments (μg) in Debye for all selected systems

Components

Sys. 1

Sys. 2

Sys. 3

μx

-0.016

0.161

0.000

μy

-0.009

-0.152

0.389

μz

5.140

7.057

9.729

μtot (D)

5.139

7.061

9.800

3.2 Dipole moment and Polarizability The electric dipole moment and the polarizability are the most fundamental electric response properties. The ground state electronic dipole moments of all systems have been showed in Table 2 calculated at M06/6-311G** level of theory. A comparison of dipole moments of all systems shows that the system 3 has the largest dipole moment of 9.80 Debye at M06/6-311G** level of theory. This is perhaps due to the strong donor-acceptor configuration of system 3. Table 3. The calculated polarizability (α, a. u.) and its individual components for all the selected systems at M06/6-311G** level of theory

a

Components

Sys. 1

Sys. 2

Sys. 3

αxx αxy αyy αxz αyz αzz α0 Δα

91 13 270 12 110 460 274 (40.55)a 320 (47.36)a

147 26 440 28 109 687 425 (62.93)a 468 (69.38)a

147 3 399 16 24 722 423 (62.63)a 499 (73.93)a

The parentheses values are in electrostatic units. The conversion of atomic units to

electrostatic units is: α, 1 a. u. ≈ 1.148176 × 10−24 cm−3[50] Conventionally, the ground state electronic dipole moment is directed from negatively

12

charged part of molecule to positive part. The presence of strong donor-acceptor configuration in system 3 results in larger charge separation as well as larger ground state dipole moments. In all the systems, the major components of dipole moments are μz (directed along negative z-axis), which are the main charge transfer axis. The polarizability plays very important role for designing electro-optical materials because if the molecules can be polarized by external electric field, they can somewhat orientate themselves in bulk to change the index of refraction to allow the switching of light passing through it. Table 4. The calculated values of first hyperpolarizability βtot (×10-30 esu)a along with their individual components (a. u.) as for all the studied systems at M06/6-311G** levels of theory

a

β Components

Sys. 1

Sys. 2

Sys. 3

βxxx

0.5

10

-6

βxxy

0.5

-31

-40

βxyy

49

45

8

βyyy

579

509

18

βxxz

21

35

27

βxyz

50

-38

-19

βyyz

668

176

-299

βxzz

-10

-365

-100

βyzz

-7

-1252

-1117

βzzz

-3432

-8941

-14556

βtot

2802 (24.22) a

8770 (75.78) a

14872 (128.51) a

βtot / βtot (PNA) b

(2.25)b

(7.07) b

(12.00) b

The parentheses values are in electrostatic units. The conversion factor for β from a.u. to

SI/esu units is: 1 a. u. = 3.26361x10-53C3m2J-2 = 8.639418x10-33 esu [50] bThe ratio between βtot of the system and PNA The orientationally an average value of polarizability is called anisotropy and is 13

represented as Δα. Recently, it has been recognized that the third-order nonlinear susceptibility, χ(3), obtained by degenerate four wave mixing (DFWM) methodology is directly related to the anisotropy of the polarizability, ∆α, originating from the molecular reorientation.[51] This effect is the dominant contribution to the values observed for the anisotropic molecule. In present calculations, we have calculated polarizability and anisotropy of polarizability of all the systems (1-3) as given in Table 3. The alpha values are 274, 425 and 423 a. u. for systems 1, 2 and 3, respectively. The modification of molecular structure as donor-acceptor configuration generally increases the polarization where as this effect is more prominent for anisotropy of the polarizability, ∆α, as given in Table 3. 3.3 First Hyperpolarizability (βtot) The

nonlinear

response

properties

are

characterized

by

first

and

higher

hyperpolarizabilities. In present investigation, we have calculated total (βtot) first hyperpolarizabilities according to Eq. 4, which have been given in Table 4. The static total (βtot) first hyperpolarizability along with their individual components has been given in Table 4. The order of increasing βtot for all selected systems is system 1 < system 2 < system 3 with βtot amplitudes of 2802, 8770, 14872 a. u., respectively. The increase of βtot amplitudes in systems 2 and 3 are 3.14 and 5.30 times as compared with that of parent system 1. The βtot amplitudes of systems 2 and 3 are also larger than those of some standard NLO molecules and similar chalcones. For instance, the βtot amplitudes of systems 2 and 3 are 7 and 12 times larger than that of para-nitroaniline (PNA = 1240 a. u. at M06/6-31G* levels of theory), which is usually considered as prototype NLO molecule. Similarly, the βtot amplitudes of systems 2 and 3 are 135 and 229 times than that of urea molecule (urea = 65 a. u. at M06/6-31G* levels of theory), which also reported in literature for NLO response as standard molecule. From Table 4, it can be seen that hyperpolarizability values are dominated by their diagonal (βzzz) components among all other components as calculated at M06/6-31G* levels of theory. How are the β values of derivative molecules enhanced by changing donor-acceptor configuration? For the static case (ω = 0.0), a simple two-level expression[52] is often employed in literature to roughly estimate the β values. In two-level approximation, the nonlinear optical response is calculated including the ground and one excited state in sum-over-state 14

expression. However, care should be taken while applying two-level model to the molecules with significantly populated excited states.[53] Despite the fact that the general validity of two-level model has been also questioned for extrapolating β value to zero frequency, it is widely used in experimental studies of nonlinear optical properties of organic molecules. Unlike the sum-over-state approximation, two-level model is simple representation of molecular response including wavelength dependence. This model is also the origin of the most widely applied push-pull technique for designing efficient NLO chromophore. Oudar and Chemla have first used two-level model to study the first hyperpolarizability of nitroanilines.[52] In two-level expression the β value is expressed as: ⁄

(11)

Here Δμ is the dipole moment difference between the ground and crucial excited states, f0 is the oscillator strength and ΔE is the transition energy. We have performed TD-DFT calculations to approximate the relative contributions of oscillator strength and ΔE to tuning the NLO properties of our designed systems. Table 5. The transition energies (ΔE), oscillator strength (fo) and % configuration interaction of crucial transitions at TD-M06/6-311G** level of theory Sys.

Sys. 1

Sys. 2

Sys. 3

Electronic Excitation

ΔE (eV)

fo

Major Contribution

%C.I.

S0S1

3.351

0.000

H-3L+1

70

S0S2

3.503

0.527

HL

70

S0S3

3.826

0.505

H-1L

57

S0S1

2.869

0.009

HL

67

S0S2

3.251

0.411

H-1L

63

S0S4

3.641

0.916

HL+1

51

S0S1

2.536

0.006

HL

68

S0S2

2.908

0.361

H-1L

69

S0S9

3.832

1.233

H-1L+1

47

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From the two-level model, it can be seen that the β value is directly proportional to the oscillator strength f0 and change in dipole moment between ground and first excited state Δμ, while it is inversely proportional to cube of transition energy ΔE, which represents that the transition energy plays a crucial role to modulate the β amplitudes. From Table 5, it can be seen that systems 2 and 3 have lower energy transitions ΔE with larger oscillator strengths (fo) resulting in their larger β amplitudes as compared with that of systems 1. Additionally, the analysis of these transitions indicates an intramolecular charge transfer nature as explained in proceeding section of frontier molecular orbital. Table 6. The collection of some recently synthesized chalcone derivatives along with their theoretically calculated βtot amplitudes Compound Name

βtot (x10-30 esu)

Methodology

Ref./Year

(E)-1-(4-bromophenyl)-3-(naphthalen2-yl)prop-2-en-1-one

24.22

M06 6-311G**

Present Study

(E)-1-(4-bromophenyl)-3-(5,7,9trimethoxypyren-2-yl)prop-2-en-1-one

75.78

M06 6-311G**

Present Study

(E)-1-(4-nitrophenyl)-3-(5,7,9trimethoxypyren-2-yl)prop-2-en-1-one

128.5

M06 6-311G**

Present Study

8.30

B3LYP 6-311++G** B3LYP 6-31G* B3LYP 6-31G* B3LYP 6-31G**

0.899

CAM-B3LYP 6-311G**

[58]/2014

124.95

B3LYP 6-311++G**

[59]/2014

24.8

B3LYP 6-31G**

[60]/2013

4-hexylacetophenone

0.847

3-acetylcoumarin oxime carbonate

1.403

Methyl 4,4’-difluoro-5’-methoxy-1, 1’: 3, 1’-terphenyl-4’-carboxylate (E)-3-(4-chlorophenyl) -1-(4fluorophenyl)prop-2-en-1-one Ethyl-6-(2-methoxyphenyl)-4-(furan2-yl)-2-oxocyclohex-3-ene-1carboxylate (2E)-3-[4-(methylsulfanyl) phenyl]-1(4-nitrophenyl) prop-2-en-1-one 1-(4-Aminophenyl)-3-(3,4dimethoxyphenyl)-prop-2-en-1-one

1.64

[54]/2015 [55]/2015 [56]/2014 [57]/2014

16

3.4 Comparative analysis of First hyperpolarizability Unlike the several previous reports about chalcone NLO systems, we have performed a rigorous literature review and sorted out some similar chalcone and chalcone derivatives to compare with our designed systems. It is important to mention that all the values collected in Table 6 are theoretically reported using the same finite field method as in present study. For B3LYP methodology, we have already discussed that it has tendency to overestimate βtot amplitudes as compared with correlated MP2 method. A comparison of our calculated βtot amplitudes for systems 1, 2 and 3 shows that βtot amplitudes of these systems are significantly larger than several others previously reported similar systems. Especially the βtot amplitudes of systems 1 and 2 are several orders of magnitude larger than those given in Table 6. Thus, the comparison of our present investigated systems indicates that these kinds of push-pull systems might be a potential source for NLO materials.

Table 7. The calculated values of frequency dependent dynamic EFISHG hyperpolarizability μβω (μβ = 5/3 μ. βω, in ×10-48 esu) for all selected systems

Laser Type Energy Wavelength (eV) (nm) 0.650 1907.2 0.670 1850.7 0.689 1800.2 0.708 1750.4 0.775 1600.4 0.826 1500.3 0.885 1400.2 1.551 799.4 1.959 632.8 2.280 543.7

Sys-1 μβω (SHG) (10-48 esu) 283.59 288.62 293.66 299.21 320.53 340.14 366.72 303.43 341.40 141.98

Sys-2 μβω (SHG) (10-48 esu) 650.97 663.11 675.33 688.80 740.91 789.62 856.32 23807.49 7023.60 49074.14

Sys-3 μβω (SHG) (10-48 esu) 1327.93 1359.98 1392.40 1428.60 1572.36 1712.41 1913.46 7202.66 717.15 2289.65

3.5 Frequency Dependent First Hyperpolarizabilities (SHG) In addition to static first hyperpolarizability values, we have also calculated the dynamic (frequency dependent) electric field induced SHG (EFISHG) first hyperpolarizabilities (μβ) using coupled-perturbed Kohn-Sham (CPKS) method. The μβ values are usually 17

used in complement to the experimental first hyperpolarizability values by EFISHG experiment. According to experimental setup for EFISHG first hyperpolarizability (βω) measurement, the frequency dependent calculations are carried out using different optical wavelengths ranging. In present study, we have used several different optical wavelengths ranging from 1907 to 543 nm. From Table 7 and Figure 2, it can be seen that for frequency dependent dynamic hyperpolarizability μβω (-2ω; ω, ω) values show a gradual increase with decrease in the optical wavelengths of laser. The increase in μβω (2ω; ω, ω) values is quite uniform between 1907 to 1400 nm of optical wavelengths of laser. Mostly in many experimental frequency dependent dynamic hyperpolarizability

μβω (-2ω; ω, ω) calculations two fundamental wavelengths 1907 nm and 1064 nm are used where all compounds are usually transparent.[17, 61] Below the 1400 nm of optical wavelength, the dispersion effects arise which cause noticeable variations among μβω (2ω; ω, ω) values that are probably due to absorption in that regions as shown in Figure 2.

18

Figure

2.

The

graphical

representation

of

frequency

dependent

dynamic

hyperpolarizability μβω (-2ω; ω, ω) values at various wavelength of laser 3.6 Frontier Molecular Orbitals (FMOs) The intramolecular charge transfer process from a donor to acceptor moiety of a molecular system is usually characterized by the excitation of an electron from occupied orbital (HOMO-i ) to unoccupied orbital (LUMO+i) where i = 0,1, 2….In order to see the nature of intramolecular charge transfer in our designed systems, we have plotted the FMOs of three selected systems as given in Figure 3. A careful analysis of HOMO and LUMO orbitals shows an intramolecular charge transfer process among systems 1-3, which is different as going from systems 1 through 3 (see the pattern of HOMO and LUMO in Figure 3). An analysis of Figure 3 shows that in system 1, naphthalene acts as weak electron donor while bromophenyl as electron weak acceptor group in their donor acceptor configuration because of their poor electron donating and accepting abilities, respectively.

Figure 3. Three-dimensional representations of the frontier molecular orbitals (HOMO-i and LUMO+i where i = 0,1) of systems 1-3 based on ground-state geometries

19

In system 2, the donor ability of trimethoxypyren is perhaps larger than naphthalene because of larger π-conjugation as well as the presence of trimethoxy groups. This donoracceptor combination has been further strengthened by the introduction of nitro group at phenyl group in system 3, which is indicated in its lowest energy and strongest transitions from donor part to acceptor moiety. Thus the above analysis of FMOs illustrates a very crucial role to judge the nature of intramolecular charge transfer in our designed molecules as well as their reactivity. 3.7. The Absorption Spectra The absorption spectra of our all selected molecules have been illustrated in Figure 4 as calculated at TD-M06/6-311G** level of theory. For system 1, there are two peaks at 250 and 365 nm probably from π to π* and n to π* transitions in U.V. region. The substitution of tri-methoxy groups causes a red shift in both the wavelengths where π to π* transition shows red shifts ~ 97 and ~ 30 nm and ultimately both the absorption maxima at 347 nm and 395 nm appear as strong shoulders of a single peak for system 2. For system 3, it has also two peaks at 334 and 461 nm with red shifts of ~ 84 and 96 nm, respectively, as compared with system 1. The above red shifts in absorption wavelengths as well as increase in oscillator strengths are due to the modification of parent system 1 into more efficient donor-acceptor configurations in systems 2 and 3. As explained above in twolevel expression, these lower energy transitions have boosted the β amplitudes in in systems 2 and 3.

20

Figure 4. The calculated UV-Visible spectra of all selected systems at TD-M06/6-311G* level of theory 3.8. Molecular Electrostatic Potential (MEP) To have molecular level understanding, we have calculated 3-D plots of molecular electrostatic potential of our all studied systems as shown in Figure 5. The MEP is the measurement of electrostatic potential on constant electron density surface. The 3-D plots of MEP surface overlap on the top of total energy density. The MEP is helpful property to investigate the reactivity of molecular species by predicted that either the approaching nucleophile is attracted to a positive region of molecule. In MEP plot, while maximum positive region that is preferred site for nucleophilic attack indicated as blue color. Similarly, a maximum negative region is preferred site for electrophilic attack that is indicated as red surface.

21

Figure 5. Molecular electrostatic potential (MEP) plots of all selected systems with isovalue 0.0400 a. u. The MEP of our designed systems have been drawn in Figure 5 to get simultaneous information about their molecular size, shape along with its positive, negative and neutral electrostatic potential regions in terms of color grading. The MEP of system 1 has more positive potential and no/fewer negative potential regions. While on the other hand, systems 2 and 3 illustrate much more charge separation in the form of highly positive and negative potential regions. The separation of positive and negative potentials on systems 2 and 3 perhaps results in larger molecular polarity, which can lead to significant solvatochromism

with

external

dielectric

environment

as

well

as

larger

hyperpolarizabilities by interacting with external electric fields as evident from our above calculations of polarizability and hyperpolarizability.

22

Table 8. The calculated values of ground state dipole moments (μg) in Debye, polarizability, first hyperpolarizability βtot (×10-30 esu) as well as transition energy and oscillator strength for all the studied systems 1-8 at M06/6-311G** levels Systems

μtot

α0

βtot (Gas)

βtot (Solvent)

ΔE (eV)a

foa

Set-I Sys. 1

5.139

274

24.21

72.61

3.503

0.527

Sys. 2

7.061

425

75.78

184.85

3.251

0.411

Sys. 3

9.800

423

128.51

295.91

2.908

0.361

Set-II

a

Sys. 4

3.544

267

16.88

46.49

3.557

1.299

Sys. 5

3.590

202

28.33

70.77

3.827

0.836

Sys. 6

3.231

228

38.82

100.54

3.477

0.490

Sys. 7

7.317

256

50.49

137.13

3.490

1.114

Sys. 8

1.648

128

1.220

2.490

5.755

0.014

The transition energy and oscillator strengths for systems 4-8 have been taken for

S0S2 excitations similar to systems 1-3 3.9 Extended Structure NLO Property Relationship on Experimental Systems Based on above structure NLO property relationship, we have further added some recently synthesized chalcones from experiments. It will help to provide an added value in the field of NLO by direct comparison of our designed systems (1-3) with experimentally synthesized chalcones (systems 4-8). As discussed in computational details section, the same M06/6-311G**//B3LYP/6-311G** levels of theory has been used to optimized and to calculate polarizability (αo) and first hyperpolarizability (βtot) amplitudes for systems 4-8. All the optimized geometries of systems 4-8 have been shown in Figure S1. It can be seen from Figure S1 that all the added systems possess different geometrical configurations. Systems 4-6 contain distinct π-conjugation with relatively weak donor-acceptor configurations and perhaps illustrate lower βtot amplitudes of 16.88×10-30, 28.33×10-30 and 38.82×10-30 esu, respectively, which are similar to system 1 as discussed in above preceding section. On the other hand, system 7 consists of a clear donor-π-conjugation-acceptor configuration, which ultimately results larger βtot amplitudes of about 50.49×10-30 esu, respectively, which is similar to system 3 with nitro 23

substitution group. Unlike systems 4-7, in system 8 possess neither consistent πconjugation nor donor-acceptor combination that ultimately results in a very poor NLO response with βtot amplitudes of about 1.22×10-30 esu. The TD-DFT calculations have been also performed to get the lowest energy transitions for systems 4-8. It is important to mention here that the crucial excited state is usually considered the lowest energy transition with reasonable oscillator strength as previously reported in many NLOphores.[7, 62, 63] In present investigation, as all the selected systems have very small oscillator strengths (~0.000 − 0.009) for first lowest energy (S0-S1) transitions, therefor the second lower energy transition (S0-S2) with considered oscillator strength is assumed as crucial one for all the systems. It can be seen from Table 8 that systems 3 and 2 with larger βtot amplitudes have the lowest transition energies of 2.908 and 3.251 eV, respectively. On the other hand, larger transition energy of 5.755 eV in system 8 results in its poor NLO response, which is also inline with our above stated two-level model. Nevertheless, in chemical systems containing comparable transition energies, it is difficult to describe transition energy the only crucial factor where the other two factors involved in two-level approximation (the dipole moment difference between the ground and excited state, and the oscillator strength) also play important role to modulate the first hyperpolarizability. For instance, transition energies of systems 6 and 7 are comparable to each other while the oscillator strength of system 7 is almost double than that of system 6, which might have reasonably increased its first hyperpolarizability amplitude.

24

Figure 6. The graphical comparison of calculated static first hyperpolarizability βtot amplitudes for systems 1-8 in gas and solvent environments using SCRF-PCM model at M06/6-311G** level of theory Solvent Effect on First Hyperpolarizability The solvent effect has been also checked on βtot amplitudes using SCRF-PCM model for systems 1-8 taking methanol as solvent. It is also important to mention that similar solvent effects have been already considered while calculating the dynamic (frequency dependent) first hyperpolarizabilities (μβ) using coupled-perturbed Kohn-Sham (CPKS) method for systems 1-3. The SCRF-PCM model assumes that the solvent is a continuum dielectric, which generates a reaction field interacting with the solute charge distribution. There are several reports about the solvent effects on static first hyperpolarizability. Over the past many years, many reports are presented to describe various levels of approximation used combined with SCRF-PCM model to see the solute and solvent interactions, to study the structure-NLO property relationship as well as the effect on linear and nonlinear optical properties of push-pull molecules by varying the dielectric constant of the solvent etc.[64-68] In our present study, the first hyperpolarizability for systems 1-8 as calculated in gas and solvent environments have been given in Table 8. A 25

graphical analysis has been also performed in Figure 6. It can be seen from Table 8 and Figure 6 that the first hyperpolarizability increases significantly while going from gas to solvent phase. A careful analysis of Figure 6 reveals that the increase in β amplitudes basically depend on the gas phase values for each systems. System 3 and system 8 have shown the highest and the lowest increase in β amplitudes while varying from gas to solvent media, which are related to their largest and the shortest β amplitudes in gas phase among all systems, respectively. The solvent effect usually enhances the β amplitudes. There are many types of intermolecular interaction operating between solute and surrounding solvent environment. For instance, Huyskens 
 et al.,[64] have studied para-nitroaniline (p-NA) which is a prototype NLO molecule with fifty different types of solvents and found that the formation of specific H-bonds between the solute and the solvent always increases the value of first hyperpolarizability. Li et al., [65] studied the organometallic tungsten-carbonyl complexes and found that solvent effects increase static first hyperpolarizability as compared with their previously reported gas-phase results. Similar results have been also observed for homologous,[66] zwitterionic merocyanine dyes[67] and retinal derivatives.[68] The above-mentioned literature indicates that solvent effects have not only enhances the first hyperpolarizability but also reproduced closer results to experiments in case of frequency dependent first hyperpolarizability. Thus, it is important to have the proper solvent effects especially during the comparison of theoretical and experimental values wherever available. 4. Conclusions Thus present quantum chemical study highlights the importance of push-pull on chalcone derivatives to enhance their optical and nonlinear optical properties. Different quantum chemical methods including B3LYP, CAM-B3LYP, PBE0, M06, BHandHLYP and MP2 are tested with 6-311G** basis set to effectively calculate the first hyperpolarizability. The M06 has reproduces the βtot amplitudes as calculated by highly correlated MP2 method for systems 1. The calculated βtot amplitudes for systems 1, 2 and 3 are found to be 2802, 8770 and 14872 a. u., respectively, at M06/6-311G** level of theory. A comprehensive comparative analysis of βtot amplitudes calculated for our designed systems has been performed with standard molecules of paranitro-aniline and other similar types of chalcone derivatives for their real time applications. Based on the 26

designed three systems, we have further extended the NLO-structure property relationship on some recently synthesized chalcone systems. The TD-DFT analysis has been performed to trace the origin of NLO in lower transition energy and higher oscillator strengths. The solvent effect has also calculated using SCRF-PCM model, which showed a significant increase in first hyperpolarizability of all the systems 1-8. Furthermore, an analysis of dynamic frequency dependent hyperpolarizability μβω (SHG) also shows a significant potential of our designed derivatives for efficient NLO properties over a wide range of incident laser wavelengths. The FMOs analysis and MEPs of designed derivatives have been used to highlight its structure-NLO property relationship. Acknowledgement Authors would like to acknowledge the support of the International Coordination Unit, King Khalid University (KKU) for this research through a Grant (RCAMS/KKU/001-16) under the Research Center for Advanced Materials Science (RCAMS) at KKU, Kingdom of Saudi Arabia.

27

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