First Theoretical Framework of Triphenylamine-Dicyanovinylene

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First Theoretical Framework of Triphenylamine-Dicyanovinylene based Nonlinear Optical Dyes: Structural Modification of #-Linkers Muhammad Usman Khan, Muhammad Khalid, Muhammad Ibrahim, Ataualpa A. C. Braga, Muhammad Safdar, Abdulaziz A. Al-Saadi, and Muhammad Ramzan Saeed Ashraf Janjua J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12293 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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The Journal of Physical Chemistry

First Theoretical Framework of Triphenylamine-Dicyanovinylene based Nonlinear Optical Dyes: Structural Modification of π-Linkers Muhammad Usman Khan,1 Muhammad Khalid,*2,3 Muhammad Ibrahim,*1Ataualpa Albert Carmo Braga,4Muhammad Safdar,2 Abdulaziz A. Al-Saadi,5 and Muhammad Ramzan Saeed Ashraf Janjua,*5 1

Department of Applied Chemistry, Government College University, Faisalabad 38000, Pakistan Department of Basic Sciences & Humanities, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan 64200, Pakistan 3 Department of Chemistry, University of Education Lahore, Faisalabad Campus38000, Pakistan 4 Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof.LineuPrestes, 748, São Paulo, 05508-000, Brazil 5 Department of Chemistry, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Kingdom of Saudi Arabia 2

[email protected]; [email protected]; [email protected] Abstract: This work was inspired by a previous report (M.R.S.A. Janjua, Inorg. Chem. 2012, 51, 11306−11314) in which the nonlinear-optical (NLO) response strikingly improved with double heteroaromatic rings. Herein, series of triphenylamine-dicyanovinylene based donorπ-acceptor dyes have been designed by structural tailoring of π-conjugated linkers and theoretical descriptions of their molecular NLO properties are reported. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed on optimized geometries to elucidate the electronic structures, absorption spectra, NLO properties and also to shed light on how structural modification influences the NLO properties. The simulated absorption spectra results indicate that all dyes showed maximum absorbance wavelength in the visible region. The LUMO-HOMO energy gaps of all dyes have been found smaller which results in large NLO response. Calculation of natural bond orbital (NBO) analysis reveals that electrons are successfully migrated from donor to acceptor via π-conjugated linkers and a charge separation state is formed. High NLO response reveals that this class of metal free organic dyes possesses eye-catching and remarkably large first hyperpolarizability values, especially D8 with highest and βtot computed to be 771.80 (a.u) and 139075.05 (a.u) respectively. Our research presents vital confirmation for controlling the kinds of π-conjugated linker that is a significant approach for the design of new appealing NLO compounds. This theoretical framework also highlights the NLO properties of organic dyes that can be valuable for their uses in modern hi-tech applications.

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Introduction In recent years, development of materials with optimized nonlinear optical (NLO) properties has gained much impetus in applied and fundamental research1 owing to their potential uses in diverse disciplines including materials science, medicine, biophysics, chemical dynamics, atomic, molecular, solid-state physics and surface interface sciences.2 NLO materials are currently attracting considerable interest because of their large applications in telecommunication sector, optics and optoelectronic devices.3-5 There have been many efforts made in the designing of different materials including polymer systems, natural and synthetic nanomaterials, organic and inorganic semiconductors and molecular dyes6-13 that exhibit NLO responses. Among these NLO materials, metal free organic NLO materials are extensively explored owing to their optical modulation, optical switching, better signal processing, frequency shifting and conversion. Furthermore, other advantages connected with metal free organic materials are; (i) their facile synthesis (ii) low cost (iii) easy fabrication (iv) tunable absorption wavelength and easy structural tailoring by suitable substituents which make them suitable candidates for the researchers to model their chemical structures for preferred NLO properties. Because of high electrical polarization of π-electrons, these compounds exhibit large molecular NLO response along with higher laser damage threshold, better tailor and process ability, low dielectric coefficients, and fast response time.14-15 Organic dyes involve the delocalization of electronic charge distribution in their π-bond system. It has been considered that the first hyperpolarizability (β, second order NLO properties) is linked with the intramolecular charge transfer (ICT) takes place from electrondonating group (D) to an electron-withdrawing or accepting group (A) through π-conjugated linkers or spacers.16-19 The NLO properties of organic dyes can be governed by considering it as a function of their basic molecular NLO properties. Suitable π-conjugated linkers are such basic molecules. Following this criterion, huge efforts have been made to design highly efficient organic NLO dyes.20-22 The designing of organic dyes involve the switching of suitable substituents (D and A groups) at appropriate position and a π-conjugated system that can improve the asymmetric electronic distribution leading to an increased NLO activity of these molecules.23-24 Literature is flooded with different architectures including D−A, D−π−A, A−π−D−π−A, D−π−A−π−D, D−π−π–A, D−A−π−A and D–D–π–A.25-28 In most cases, D-π-A type structures are commonly designed and studied to enhance CT transitions.29-30 It has been assessed through literature that D and A moieties are accountable for offering essential 2 ACS Paragon Plus Environment

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ground-state charge asymmetry. Whereas, π-conjugated systems present route for the transfer of charge and re-location of charges in an electric field.31-35 NLO response of materials is strongly influenced by the nature of the D, A moieties and extent of π-conjugated system.36-37 Both experimental and theoretical explorations have shown that the large second order NLO response originates from amalgamation of strong D and A groups placed at opposite ends of an appropriate π-conjugated system. On the other hand, structure–property relationship points out that the selection of optimal length of π-conjugation enhanced the NLO response.38-39 Therefore, in this study, we have modified the π-conjugated system. In 2014, Kim et al.40 synthesized two compounds with electron rich triphenylamine (TPA) as the D unit and dicyanovinylene (DCV) group as the A unit. Subsequently, they introduced the thieno[3,2-b]thiophene/thiopheneand thieno[3,2-b]thiophene/thiazole as the π-conjugated linkers between the TPA and DCV groups forming D-π-A type compounds. A systematic theoretical study of such compounds for NLO properties has not been reported as we know. This drew our attention and therefore, computational design has been proposed for the prediction of NLO properties of this kind of D-π-A type compounds. In present study, we named the original D-π-A compounds as D1, D2 and designed series of different metal free organic dyes by structural modeling of D1 and D2 through modification of π-conjugated linkers/spacers between fixed donor TPA and acceptor DCV units. Six πspacers

thieno[3,2-b]thiophene,

thiazole[5,4-d]thiazoloe,

2-(thiophen-2-yl)thiophene,

benzo[b]thiphene and benzo[d]thiazolewere used as first π-linker. While two π-spacers, thiophene and thiazole have been used as second π-linker between the D and A part. Different combinations of first and second π-linkers have been made to design ten new TPA-DCV based D-π-A dyes namely D3 to D12 (see Figure 1 and 2). This theoretical study is not only important for the prediction of NLO properties of organic dyes, but also to study the effect of different π-conjugated linkers on NLO activity. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations have been carried out to calculate the electronic properties, absorption spectra and first hyperpolarizability values of synthesized (D1, D2) and newly designed dyes (D3-D12). Hopefully, this study can serve as a way for the designing of novel metal free organic dyes. We believe that this work will provide a springboard to other researchers for the synthesis of proficient NLO dyes.



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Computational Procedure DFT and TDDFT computations were performed for the determination of electronic structures, NLO properties and absorption spectra of TPA-DCV based D–π–A dyes. Gaussian 09 program package41 was used to perform whole computations. Geometry optimization for the ground state structures of dyes has been performed in gas phase using B3LYP level of theory and 6-311+G(d,p) basis set. Frequency analysis were carried out using same functional and basis set to verify the nature of optimized molecules.42-43 There was no imaginary frequency found which represents a stationary point of minimum and the success of geometry optimization. For the calculation of absorption spectra, selection of larger basis set calculations and high level method are essential for the reliable results of organic dyes with D–π–A configuration. Therefore, when it comes to the calculation of absorption spectroscopy of organic dyes, we have performed absorption spectral analysis by TDDFT using Coulombattenuated hybrid exchange-correlation (CAM-B3LYP) functional which is a hybrid functional with improved long-range properties and long range corrected version of B3LYP at 6-311+G(d,p) basis set.44 Determination of transition energies using this function has been successfully proven in the literature.44 For the calculations of solvent (chloroform), effect has been modeled using conductor-like polarizable continuum (CPCM) model.45 Average polarizability is calculated using equation 1 and considering only diagonal elements.46

< α >= 1/ 3(α xx + α yy + α zz )

(1)

Gaussian output file provides ten hyperpolarizability tensors along x, y and z directions: βxxx, βxyy, βxzz, βyyy, βxxy, βyzz, βzzz, βxxz, βyyz, βxyz .The magnitude of total first hyperpolarizability (βtot)

is calculated using equation 2.46

βtot = [(βxxx + βxyy + βxzz )2 + (β yyy + βxxy + β yzz )2 + (βzzz + βxxz + β yyz )2 ]1/2

(2)

Results and discussion This study was carried out for theoretical designing of NLO organic dyes. The designed dyes consist of three parts: donor (TPA), π-conjugated linkers and acceptor (DCV) as shown in Figure 1. We have designed TPA-DCV based D-π-A new dyes (D3-D12) by structural tailoring of different π-conjugated linkers between TPA and DCV moieties of D1 and D2. Six π-spacers were used as first π-linker, while two π-spacers have been used as second π-linker. Different combinations of first and second π-linkers have been made by uniting first π-linkers 4 ACS Paragon Plus Environment

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one by one with second π-linkers to design new NLO dyes. The structures of these dyes are provided in Figure 2. To shed light on how different π-conjugated linkers between fixed TPA and DCV units influence the theoretical NLO and spectral responses, DFT and TDDFT calculations were carried out. In this context, following basic parameters were calculated: (i) Polarizability (α), (ii) hyperpolarizability (β), (iii) absorption wave length and (iv) light harvesting efficiency (LHE).



NC *

N

ST

nd

1 linker

2 linker

*

S

s

S

ACCEPTOR

S

N N

Thieno[3,2-b] thiophene

2nd linker

S N

s

S

Thiazolo[5,4-d] thiazole

N

N

S

2-(thiophen-2yl)thiophene

S

5-(thiazol5yl)thiazole N

*

S

*

*

1ST linker

CN

π- Conjugated Linkers

DONOR

Thiophene

S

Thiazole

Figure 1.The sketch map of studied structures


*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Benzo[b]thiphe ne

S

Benzo[d]thiaz ole

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NC

NC

N

S

S

CN

CN S

N

S

N S

S

D1

D2 NC S

N

N

CN N

N

NC

N

S S

CN S

N

N

S

S

D3

D4

N

N

S

S

N

S

S

CN

CN S

S

CN

CN

D5

D6

N

N

N

N

S

S

N

S

S

CN

CN N

N

S

S

CN

CN

D7

D8 NC

NC N

CN

CN S

S N

N S

S

D9

D10 NC

NC

N

CN N

N

S

N

N S

S

D11

D12

Figure 2.Structures of studied dyes (D1-D12)


CN S


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Electronic Structure The frontier molecular orbital (FMO) theory is seen as an outstanding theory in predicting the chemical stability of the molecules under investigation.47 Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are very important quantum orbitals that notably shape the UV/Vis spectra and reactions between molecules. Usually, LUMO expresses the capacity of accepting an electron while HOMO denotes the electron donation ability.48 The HOMO-LUMO energy gap (ELUMO-EHOMO) is an important parameter for predicting the chemical reactivity, dynamic stability, chemical softness and hardness of molecules.49 Molecules with higher ELUMO-EHOMO are considered to be chemically hard molecules with higher kinetic stability and less chemical reactivity. In contrast, less stable, more reactive and soft molecules are those having small ELUMO-EHOMO frontier orbital energy gap. Soft molecules with smaller energy gap are more polarizable and considered to be better entrant for qualitative estimation of NLO response.50-53 Taking all these considerations into account, DFT computations have been carried out for the determination of EHOMO, ELUMO and ELUMO-EHOMO of D1-D12 and results are tabulated in Table 1. Table 1: The EHOMO, ELUMO and energy gap (ELUMO-EHOMO) of studied dyes in eV at DFT/B3LYP/6-311+G* level of theory Dye D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12

HOMO(EHOMO) -5.50 (-5.51)[a] -5.56 (-5.57) -5.64 -5.70 -5.43 -5.46 -5.59 -5.62 -5.54 -5.48 -5.66 -5.71

LUMO(ELUMO) -3.31 (-3.48)[a] -3.44 (-3.58) -3.50 -3.63 -3.32 -3.44 -3.53 -3.66 -3.22 -2.76 -3.26 -3.44

Band Gap(ELUMO-EHOMO) 2.19 2.12 2.14 2.07 2.11 2.02 2.06 1.96 2.32 2.72 2.4 2.27

[a] Experimental values in parentheses are from ref.40

The calculated HOMO-LUMO energy levels of D1 and D2 were found to be −5.50/−3.31 and −5.56 eV/−3.44 eV which are in close agreement with the experimentally determined values −5.51/−3.48 and −5.57/−3.58 eV respectively. This good agreement points out that the adopted computational methodology is appropriate to investigate D1-D12. From Table 1, it is evident that D1, D2, D3, D4, D5, D6 and D7 have shown almost similar energy gaps which 8 ACS Paragon Plus Environment

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imply that π-conjugated linkers involved in these dyes have similar effect on HOMO and LUMO energy levels. The band gap of D8 was found to be 1.96 eV. This is the least value of energy gap among all studied dyes. The least band gap of D8 might due to the presence of alternated thiazole rings as π-spacers. This extended π-conjugation configuration facilitates the thiazole rings to respond as an electron withdrawing unit which might leads to the stability of the LUMO. A strong intramolecular charge-transfer interaction occurs from donor to acceptor with smaller transition energy. Both of these features play a crucial role in smallest energy gap. The highest energy gap was observed in D10 with 2.72 eV value. The energy gap in D9, D11 and D12 was also computed higher as compared to other investigated dyes. The structural position of first π-linker in D9-D12 causes the reverse polarity effect which results in transition energy value and energy gap.54 The maximum reverse polarity effect in D10 results in largest energy gap among all studied dyes. The calculated band gap (ELUMO-EHOMO) of D1-D12 increases in the following order: D8 < D6 < D4 < D5 < D7 < D2 < D3 < D1 < D12 < D9 < D11< D10. Overall, all studied dyes (D1-D12) have small energy gaps. Their smaller ELUMO-EHOMO values indicate that D1-D12 would be excellent candidates for NLO properties. So structural tailoring by modification of π-conjugated linkers would be an excellent strategy to obtain decent NLO activity. The pictographic display for distribution pattern of HOMO and LUMO is represented in Figure 3. Such electron density distributions are valuable for proficient charge transfer. From Figure 3, it can be seen that major portion of HOMOs are located over donor TPA unit and small portion on first π-conjugated linkers. Whereas, LUMOs are mostly positioned on acceptor DCV unit and partially over second πconjugated linkers. This indicates that charge transfer is directed from TPA donor toward DCV acceptor group through π-conjugated linkers. This considerable charge transfer confirmed that all investigated dyes (D1-D12) would be brilliant NLO materials.



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HOMO(1)

LUMO(1)

HOMO(2)

LUMO(2)

HOMO(3)

LUMO(3)

HOMO(4)

LUMO(4)

HOMO(5)

LUMO(5)

HOMO(6)

LUMO(6)


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HOMO(7)

LUMO(7)

HOMO(8)

LUMO(8)

HOMO(9)

LUMO(9)

HOMO(10)

LUMO(10)

HOMO(11)

LUMO(11)



HOMO(12)

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LUMO(12)

Figure 3. HOMOs and LUMOs of the studied dyes D1 to D12.

Natural bond orbital (NBO) analysis Natural bond orbital (NBO) investigation is recognized as an efficient technique that offers useful insights for studying interactions among bonds and suitable basis for examining charge transfer between the filled and vacant orbitals.55-56 It is also believed that the charge densities which transfer from donor to acceptor in D-π-A structures can be elucidated with the help of NBO. Therefore, NBO analysis has been performed on optimized structures of D1-D12 and results are presented in Table 2. The positive value of donor represents the proficient electron donating aptitude of donor moiety. On the other hand, NBO charges with negative values indicate that all acceptors will effectively accept electrons. The positive NBO charge values of π-conjugated linkers represent that they will provide a path and facilitate the transfer of electron (without trapping them) from D to A unit.NBO results of D1-D12 point out that all donors and π-conjugated linkers showed positive values while all acceptors showed negative values. These results validate that electrons are successfully migrated from D to A segments through π-conjugated linkers which results in the formation charge separation state. From Table 2, it is clear that the highest NBO values were found in D8 while least NBO chares have been observed in D10. Remaining all investigated dyes have NBO charge values closely related to each other. Table 2: NBO charges for donor, π-spacer and acceptor of designed dyes (D1-D12) Dyes D1 D2 D3 D4 D5

Donor 0.0492 0.0586 0.0459 0.0854 0.0406

π-conjugated linkers 0.1195 0.0971 0.0603 0.1281 0.1102 12

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Acceptors -0.1561 -0.1558 -0.1534 -0.1338 -0.1766

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D6 D7 D8 D9 D10 D11 D12

0.0930 0.0795 0.1052 0.0491 0.0399 0.0861 0.0703

0.1366 0.0701 0.2791 0.0440 0.0286 0.0782 0.0721

-0.1773 -0.1497 -0.3196 -0.1686 -0.1294 -0.1644 -0.1424

Nonlinear Optical (NLO) Properties NLO materials and dyes are widely employed for optical switches, optical memory devices, communication technology and signal processing. For designing of these materials, good understanding of NLO properties is essential. Electronic properties of entire material are considered to be responsible for the strength of optical response which in turn depends on the linear response (polarizability, α) and nonlinear responses (hyperpolarizabilities, β and γ etc.,). Hence, for the evaluation of NLO properties of D1-D12, these linear and nonlinear responses of D1-D12 should be assessed. To explore the influence of π-conjugated linkers on linear and NLO properties of D1-D12, the values were calculated and results are tabulated in Table 3. Table 3 is equipped with the average polarizability values of D1-D12 along with their major contributing tensors. The average polarizability value of all studied dyes decreases in the following order: D8 > D6 > D4 > D5 > D7 > D2 > D3 > D1 > D12 > D9 > D11> D10. Average polarizability value for D1 and D2 was found to be 692.85 and 701.30 (a.u) respectively. Modification of different π-conjugated linkers influenced the polarizability values in all designed dyes mainly in D4, D6 and D8. This indicate that average polarizability value of TPA-DCV based dyes with thiazolo[5,4-d]thiazole, 2-(thiophen-2yl)thiophene, 5-(thiazol-5-yl)thiazole as first linkers and thiazole as second linker was higher than other studied dyes. The highest value of polarizability was observed in designed dye D8 both in terms of average polarizability and αxx tensor. Table 3: Dipole polarizabilities and major contributing tensors (a.u.) of the studied dyes (D1D12) Dye D1 D2 D3 D4 D5

αxx 1358.82 1395.58 1389.93 1532.79 1494.69

αyy 462.73 453.42 437.64 432.14 459.52 13 ACS Paragon Plus Environment

αzz 257.02 254.91 267.54 291.04 276.70

> 692.85 701.30 698.36 751.99 743.64


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D6 1543.42 472.43 274.43 763.42 D7 1449.51 461.15 275.00 728.55 D8 1589.19 445.53 280.70 771.80 D9 1206.84 479.09 263.55 649.82 D10 1189.82 469.48 260.11 639.80 D11 1201.72 471.12 264.31 645.71 D12 1239.44 449.62 270.79 653.28 Literature suggests that the energy gap between LUMO and HOMO influence the polarizability of a molecule. Small energy gap is requisite to show large linear polarizability. In general, molecules with small energy gap and large linear polarizability value show large hyperpolarizability values.57 Transitions along x and y-directions are generally used to compute the polarizability. For instance, formula for dipole polarizability (along x direction) is explained by equation 3:

(M ) ∝ E

(3)

In this equation the numerator M denotes the transition moment between ground and mth

excited state, while denominator Egm represents transition energy. M is a complex vector quantity. The phase factors of ground to excited state have also considered in the transition dipole moment. The orientation of transition dipole moment presents the polarization because of electronic transition. Furthermore, it also explain that how the dyes will interact with an electromagnetic wave of given polarization. From equation, it can be seen that α is directly proportional to second power of the transition moment while transition energy has inverse relation with it. It means dipole polarizability value amplifies with raise in transition moment. Second power of transition moment describes the power of interaction owing to the distribution of charge density contained by the system. As a result, in general, it is judged that

a system with larger value of M and smaller value of transition energy will comprise of large hyperpolarizability value. So dipole polarizabilities are quantitative measurement to give an idea for excellent NLO activity of dyes.NLO response of dyes can be calculated in terms of their first hyperpolarizability (β). Second order NLO properties are linked with the ICT. Owing to flow of electrons from D to A moieties via π-bridge, strong ICT is occurred. This type of ICT is observable in D1-D12 as displayed in Figure 3. The interaction of an external electric field with electronic density alters the dipole moment, hence NLO response.58 In this research manuscript, hyperpolarizabilities of D1-D12 have been calculated


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employing CAM-B3LYP functional and 6-311+G(d,p) basis set and the results of βtot values along wither their major contributing tensor are collected in Table 4. Table 4: The computed first hyperpolarizabilities (βtot) and major contributing tensors (a.u) of the studied dyes (D1-D12) βxxx βxxy βxyy βyyy βxxz βxzz βtot Dye D1

-104990.23

2596.08

1118.47

85.61

139.21

156.40

103750.17

D2

-119260.13 -2579.47

1270.87

-28.64

-349.47

170.67

117848.43

D3

-116085.84

1515.45

996.23

49.57

636.52

134.76

114967.46

D4

-129341.22 -1579.04

1095.94

-9.61

-501.60

170.33

128086.05

D5

121468.42

2071.39

-1108.90 289.38

475.66

-124.16

120259.91

D6

-139902.19

2177.44

1264.67

254.84 -188.36

145.35

138514.17

D7

-119929.14

694.37

1107.47

205.39

797.68

116.80

118711.13

D8

-140439.07

1008.37

1183.02

198.04

581.88

187.60

139075.05

D9

-75412.62

2730.30

27.89

584.31

59.94

137.62

75320.06

D10

-59768.95

-725.50

795.98

389.67

27.24

185.52

58788.40

D11

-66669.74

2104.92

314.64

400.25

453.52

128.63

66275.95

D12

-88593.97

2388.98

576.34

301.33 1129.32

79.93

87987.937

The βtot value for all studied dyes decreases in the following order: D8 > D6 > D4 > D5 > D7 > D2 > D3 > D1 > D12 > D9 > D11> D10. βtot value for D1 and D2 was observed as 103750.17 and 117848.43 (a.u) respectively. It was observed that TPA-DCV based dyes with 5-(thiazol-5-yl)thiazole as first linker and thiazole as second linker (D8) have shown the highest value of βtot 139075.05 (a.u) than all other studied dyes. On the other hand, least value 58788.40 (a.u) of βtot was measured for D10 in which benzo[b]thiphene and thiazole were used as first and second π-conjugated linker respectively. This highest and lowest NLO response in case of D8 and D10 respectively can be attributed to the effective charge transfer from D to A through their respective π-conjugated linkers. As shown in Table 4, first hyperpolarizability coefficients of all investigated dyes are much high. For instance, the computed βtot value of D8 was computed 3234 times greater than the first hyperpolarizability value of urea molecule which is frequently used as reference organic molecule.59 Similarly, computed βtot value of D1 was found 2413 times greater, D2 was 2741 times,D3 was 2674 times, D4 was 2979 times, D5 was 2797 times, D6 was 3221 times, D7 was2760 times, D9 was1751 times, D10 was 1367 times, D11 was 1541 times and D12 was 15 ACS Paragon Plus Environment


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found 2046 times greater than the value of urea. Furthermore, the decreasing order of βtot is in accordance with the decreasing order of average polarizability which in turns precisely contests with the reverse order of the energy gap between HOMO-LUMO orbitals. In a nutshell, in D1-D12, the higher hyperpolarizability value is due to the delocalization of πelectrons. Delocalization of π-electrons decreases the HOMO-LUMO energy gap and stabilizes the molecules. UV–Vis spectra of dyes (D1–D12) In order to have insight into the excited states absorption spectra, TDDFT computations were carried out using CPCM in solvent at CAM-B3LYP level of theory and 6-311+G(d,p) basis set combination. Six lowest singlet-singlet transitions were studied duringTDDFT computations. Computed transition energy (Ege), oscillator strength (fos), nature of transitions and maximum absorption wavelength (λmax) are collected in Table 5 while absorption spectraof D1-D12 are displayed in Figure 4. All the dyes show absorbance in visible region (Figure 4). Table 5: Computed transition energy (eV), maximum absorption wavelengths (λmax/nm) oscillator strengths (fos), light harvesting efficiency (LHE), transition moment ( M a. u.) and transition natures of dyes(H = HOMO, L = LUMO, H-1 = HOMO-1, etc.) Dye

Ege (eV)

D1

2.5429

D2

2.5192

D3 D4 D5 D6 D7 D8 D9 D10 D11 D12

2.5753 2.5517 2.5602 2.4832 2.6097 2.4692 2.8571 2.9599 2.9275 2.8772

λmax(nm) 487.55 (300–750 nm)[a] 486.34 (300–750 nm)[a] 485.18 495.86 494.87 496.27 485.07 498.99 433.93 424.31 430.50 440.90

ƒos

LHE

Mxgm (a.u)

MO transition

1.998

0.989

5.86

H→L (60%), H-1→L (31%)

1.999

0.989

5.95

H→L (59%), H-1→L (30%)

2.0296 1.9549 1.9402 1.9502 2.0622 2.1235 1.9272 0.5982 2.0406 2.0798

0.990 0.988 0.988 0.988 0.991 0.992 0.988 0.747 0.990 0.991

6.19 6.78 6.74 6.82 5.87 6.90 5.84 3.37 5.09 6.18

H→L (61%), H-1→L (28%) H→L (62%), H-1→L (25%) H→L (54%), H-1→L (34%) H→L (53%), H-1→L (32%) H→L (55%), H-1→L (39%) H→L (59%), H-1→L (33%) H→L (46%), H-1→L (41%) H-1→L (43%), H-3→L (11%) H→L (51%), H-1→L (44%) H→L (47%), H-1→L (36%),

[a] Experimental values in parentheses are from ref.40


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2.5 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12

2.0

Oscillator strength

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

1.0

0.5

0.0 300

350

400

450

500

550

Wavelength (nm) Figure 4. Simulated absorption spectra of dyes (D1-D12)

Calculated λmax of D1 and D2 was observed 487.55 and 486.34 nm respectively which is closely related to the experimental absorption spectra showed absorption band in the region of 300–750 nm. The highest absorption wavelength was measured 498.99 nm for D8 which implies that D8 is red shifted as compared to other studied dyes. From Table 5, it is evident that the electron transitions of all systems (except D10) mainly originate from TPA/donor (HOMO) to the DCV/acceptor (LUMO) along x direction. In D10, charge transfer primarily occurs from HOMO-1 to LUMO. Delocalization of HOMOs and LUMOs above whole molecule can be seen from Figure 3. Optical efficiency of dyes is related to another important factor namely light harvesting efficiency (LHE). Generally, systems with large LHE value displayed maximum photocurrent response. The equation used to express LHE of a dye is:60 LHE = 1-10f

(4)

In this equation f represents the oscillator strength. The values of LHE calculated for D1-D12 are tabulated in Table 5. LHE of D8 was found to be highest among all studied dyes.



Page 18 of 25

A better clarification of structure-property relationship is mandatory for explaining the reason for second order NLO properties. Oudar and Chemla61 formulated the two-state model on the basis of the complex sum-over-states (SOS) expression which formed a connection between charge transfer transition and hyperpolarizability represented by the following equation 5;

β CT =

∆µ gm f gm 3 E gm

(5)

In this equation, ∆µgm represents the difference between excited and ground state dipole moment which is directly proportional to the first hyperpolarizability. fgm represents oscillator strength from the ground state (g) to the mth excited state (m) and it is also directly

proportional to the β. E indicates the cube of transition energy and it is inversely

proportional to the first hyperpolarizability value. From above equation, it is apparent that the product of oscillator strength and transition moment is the decisive factor in the determination of β value. Therefore, NLO material having combination of large oscillator strength and transition moment magnitude with low energy CT excited state is an optimum design which can give up large β value. The value of transition moment, oscillator strength and excitation energy is tabulated in Table

5. From Table 5, it can be seen that for D1-D12, the factors ∆µgm, E and fgm are closely

related with each other and acquire the same general framework. Relationship between the first hyperpolarizability values and the corresponding two level model values of D1-D12 are represented in Figure 5. As we can see from Figure 5, βtot values are proportional to the corresponding ∆µgmfgm/∆E3gm values in good concurrence suggested by the two-level model.


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Figure 5. Relationship between the βtot (red line) values and the corresponding ∆µgmfgm/E (blue line) values for dyes (D1 to D12).

These results suggest that, for the designing of novel appealing D-π-A structures with brilliant NLO properties, controlling the type of π-bridges is a significant approach. We also anticipate that this insight about the effect of π-conjugated linkers on NLO properties will be applied to the design of novel photoelectric and optical tools with fine performance such as optical data processing, modulation and switching. The general approach for designing NLO compounds with large nonlinear optical response has involved pairing both an electron donor (D) and acceptor (A) to an organic framework that provides moderately strong electronic coupling between D and A. While a finite amount of D–A coupling is essential for a significant first hyperpolarizability. The pairing facilitated by the bridge must not be so strong as to remove the electronic asymmetry provided by the donor and acceptor groups.

Conclusions The objective of the present work was to predict the NLO response of quantum chemically designed metal free organic dyes. Quantum chemical methods have been used to elucidate the absorption spectra, electronic structures and first hyperpolarizability values. DFT and TDDFT methods were employed to explore the influence of different π-linkers on difference in spectral and NLO properties of D1-D12. All dyes show maximum absorbance wavelength in the visible region with low transition energy, high transition moment, oscillating strength and LHE values. Maximum red shifted absorption spectrum was observed 498.99 nm in D8.NBO



results depicted that electrons are successfully migrated from D to A via π-conjugated linker which results in the formation of charge separation state. The optical excitation analysis in terms of FMOs illustrates that HOMO is delocalized over TPA moiety and first π-conjugated linker while LUMO is located over DCV group. Thus ICT from TPA to DCV segment through π-conjugated linker acts the crucial role in the large NLO response of D1D12.Overall, all investigated dyes (D1-12) have shown eye-catching and remarkably large NLO response in the range of 139075.05 to 58788.40 (a.u). Among studied dyes, D8 has shown highest and βtot computed to be 771.80 (a.u) and 139075.05 (a.u) respectively. Computed βtot values of D1-D12 were observed 3234 to 1367 times greater than the value of urea molecule. Furthermore, βtot values of D1-D12 were found proportional to the corresponding ∆µgmfgm/∆E3gm values in good concurrence suggested by the two-level model. This work also describes that structural modeling of π-linkers in D-π-A dyes is a significant approach for the design of new appealing NLO compounds. Metal free organic dyes are a very hot area of research and this theoretical framework provide new ways for experimentalists to design high-performance NLO materials for optics and electronics. In the conceptual design of possible high-performance NLO materials, the proposed dyes should be targeted for further synthetic investigations. Conflict of interest No conflicts declared. Supporting Information Optimized Cartesian coordinates of our studied dyes are available in supporting information file. Acknowledgments Muhammad Ibrahim (Grant # HEC/2013/1114) and Muhammad Khalid (grant#1314) gratefully acknowledge the financial support from Higher Education Commission of Pakistan. M.R.S.A. Janjua and A. A. Al-Saadi would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. SR161009.


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TOC Graphic


First Theoretical Framework of Triphenylamine-Dicyanovinylene

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