First Theoretical Framework of Triphenylamine ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2018, 122, 4009−4018

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First Theoretical Framework of Triphenylamine−DicyanovinyleneBased Nonlinear Optical Dyes: Structural Modification of π‑Linkers Muhammad Usman Khan,† Muhammad Khalid,*,‡,§ Muhammad Ibrahim,*,† Ataualpa Albert Carmo Braga,∥ Muhammad Safdar,‡ Abdulaziz A. Al-Saadi,⊥ and Muhammad Ramzan Saeed Ashraf Janjua*,⊥ †

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 § Department of Chemistry, University of Education Lahore, Faisalabad Campus, Faisalabad 38000, Pakistan ∥ Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Avenida Professor Lineu Prestes, 748, São Paulo 05508-000, Brazil ⊥ Department of Chemistry, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Kingdom of Saudi Arabia ‡

S Supporting Information *

ABSTRACT: This work was inspired by a previous report [Janjua, M.R.S.A. 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 donor−π−acceptor dyes had been designed by structural tailoring of π-conjugated linkers and theoretical descriptions of their molecular NLO properties were reported. Density functional theory and timedependent density functional theory calculations were performed on optimized geometries to elucidate the electronic structures, absorption spectra, and NLO properties and also to shed light on how structural modification influences the NLO properties. The simulated absorption spectra results indicate that all of the dyes showed the maximum absorbance wavelength in the visible region. The lowest unoccupied molecular orbital−highest occupied molecular orbital energy gaps of all of the dyes have been found smaller, which results in large NLO response. Calculation of natural bond orbital analysis reveals that electrons successfully migrated from donor to acceptor via π-conjugated linkers and a charge separation state was formed. High NLO response revealed that this class of metal free organic dyes possess eye-catching and remarkably large first hyperpolarizability values, especially D8 with highest ⟨α⟩ and βtot computed to be 771.80 and 139 075.05 au, respectively. Our research presented a vital confirmation for controlling the kinds of π-conjugated linker that was a significant approach for the design of new appealing NLO compounds. This theoretical framework also highlighted 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 The NLO materials are currently attracting considerable interest because of their large applications in telecommunication sector, optics, and optoelectronic devices.3−5 Many efforts have been 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, © 2018 American Chemical Society

frequency shifting, and conversion. Furthermore, other advantages connected with metal free organic materials are as follows: (i) their facile synthesis, (ii) low cost, (iii) easy fabrication, and (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 πReceived: December 14, 2017 Revised: January 27, 2018 Published: January 31, 2018 4009

DOI: 10.1021/acs.jpcc.7b12293 J. Phys. Chem. C 2018, 122, 4009−4018

Article

The Journal of Physical Chemistry C

Figure 1. Sketch map of studied structures.

bond system. It has been considered that the first hyperpolarizability (β, second-order NLO properties) is linked with the intramolecular charge transfer (ICT), which takes place from an electron-donating 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 involves the switching of suitable substituents (D and A groups) at appropriate position and a πconjugated 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 the charge transfer (CT) transitions.29,30 It has been assessed through literature that D and A moieties are accountable for offering essential ground-state charge asymmetry. However, πconjugated systems present a route for the transfer of charge and relocation of charges in an electric field.31−35 The NLO response of materials is strongly influenced by the nature of the D and A moieties and the extent of π-conjugated system.36,37 Both experimental and theoretical explorations have shown that the large second-order NLO response originates from the amalgamation of strong D and A groups placed at the 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 πconjugated 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 thieno[3,2-b]thiophene/thiophene and 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 compounds. In present study, we name the original D−π−A compounds as D1 and D2 and design 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 π-spacers thieno[3,2b]thiophene, thiazolo[5,4-d]thiazole, 2-(thiophen-2-yl)thiophene, benzo[b]thiphene, and benzo[d]thiazole were used as first π-linker. However, two π-spacers, thiophene and thiazole, have been used as second π-linker between the D and A parts. Different combinations of first and second π-linkers have been made to design 10 new TPA−DCV-based D−π−A dyes namely D3−D12 (see Figures 1 and 2). This theoretical study is important not only for the prediction of NLO properties of organic dyes but also for 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 designing novel metal free organic dyes. We believe that this work will provide a springboard to other researchers for the synthesis of proficient NLO dyes. 4010


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

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COMPUTATIONAL PROCEDURE The DFT and TDDFT computations were performed for the determination of electronic structures, NLO properties, and absorption spectra of the 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 the dyes was performed in the gas phase using B3LYP level of theory and 6-311+G(d,p) basis set. The frequency analysis was carried out using same functional and basis set to verify the nature of optimized molecules.42,43 No imaginary frequency was found, which represented a stationary point of minimum and the success of geometry optimization. For the calculation of absorption spectra, the selection of larger basis set calculations and high level method were essential for the reliable results of organic dyes with the D−π−A configuration. Therefore, when it came to the calculation of

absorption spectroscopy of organic dyes, we performed the absorption spectral analysis by TDDFT using Coulombattenuated hybrid exchange-correlation (CAM-B3LYP) functional, which is a hybrid functional with improved long-range properties and a long-range corrected version of B3LYP at 6311+G(d,p) basis set.44 Determination of transition energies using this function has been successfully proven in the literature.44 Effect of solvent (chloroform) has been measured.45 Average polarizability ⟨α⟩ was calculated using eq 1 and considering only diagonal elements.46 ⟨α⟩ = 1/3(αxx + αyy + αzz)

(1)

Gaussian output file provided 10 hyperpolarizability tensors along x, y, and z directions: βxxx, βxyy, βxzz, βyyy, βxxy, βyzz, βzzz, 4011


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The Journal of Physical Chemistry C βxxz, βyyz, and βxyz. The magnitude of total first hyperpolarizability (βtot) is calculated using eq 2.46

Table 1. EHOMO, ELUMO, and Energy Gap (ELUMO − EHOMO) of the Studied Dyes in Electronvolt at DFT/B3LYP/6311+G* Level of Theory

βtot = [(βxxx + βxyy + βxzz )2 + (βyyy + βxxy + βyzz )2 1/2

+ (βzzz + βxxz + βyyz )2 ]

(2)

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RESULTS AND DISCUSSION This study is carried out for the 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 πconjugated linkers between TPA and DCV moieties of D1 and D2. Six π-spacers are used as first π-linker, whereas two πspacers are used as second π-linker. Different combinations of first and second π-linkers have been made by uniting first πlinkers 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 are carried out. In this context, following basic parameters are calculated: (i) polarizability (α), (ii) hyperpolarizability (β), (iii) absorption wavelength, and (iv) light harvesting efficiency (LHE).

a

dye

HOMO (EHOMO)

LUMO (ELUMO)

band gap (ELUMO − EHOMO)

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

−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

−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

2.19 2.12 2.14 2.07 2.11 2.02 2.06 1.96 2.32 2.72 2.4 2.27

Experimental values in parentheses are from ref 40.

the thiazole rings to respond as an electron-withdrawing unit, which might lead to the stability of the LUMO. A strong intramolecular charge-transfer interaction occurs from donor to acceptor with a smaller transition energy. Both of these features play a crucial role in the smallest energy gap. The highest energy gap is observed in D10 with 2.72 eV value. The energy gap in D9, D11, and D12 is also computed higher as compared to other investigated dyes. The structural position of first πlinker 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 of the 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 of the 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 the modification of π-conjugated linkers would be an excellent strategy to obtain a decent NLO activity. The pictographic display for the 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 the donor TPA unit and a small portion on the first π-conjugated linkers. However, LUMOs are mostly positioned on the acceptor DCV unit and partially over the second π-conjugated linkers. This indicates that charge transfer is directed from TPA donor toward DCV acceptor group through π-conjugated linkers. This considerable charge transfer confirms that all of the investigated dyes (D1−D12) would be brilliant NLO materials.

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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 The highest occupied molecular orbital (HOMO) and the 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, whereas 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 the molecules.49 Molecules with higher ELUMO − EHOMO are considered to be chemically hard molecules with a higher kinetic stability and less chemical reactivity. In contrast, less stable, more reactive, and soft molecules are those having a small ELUMO − EHOMO frontier orbital energy gap. Soft molecules with a smaller energy gap are more polarizable and considered to be better entrant for qualitative estimation of NLO response.50−53 Taking all of these considerations into account, the DFT computations have been carried out for the determination of EHOMO, ELUMO, and ELUMO − EHOMO of D1− D12 and the results are tabulated in Table 1. The calculated HOMO−LUMO energy levels of D1 and D2 are found to be −5.50/−3.31 and −5.56/−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−D7 have shown almost similar energy gaps, which implies that π-conjugated linkers involved in these dyes have similar effect on HOMO and LUMO energy levels. The band gap of D8 is found to be 1.96 eV. This is the least value of energy gap among all of the studied dyes. The least band gap of D8 might due to the presence of alternated thiazole rings as πspacers. This extended π-conjugation configuration facilitates

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NATURAL BOND ORBITAL (NBO) ANALYSIS Natural bond orbital (NBO) investigation is recognized as an efficient technique that offers useful insights for studying the interactions among bonds and suitable basis for examining the charge transfer between filled and vacant orbitals.55,56 It is also believed that the charge densities, which transfer from donor to acceptor in the 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 the results are presented in Table 2. The positive value of donor represents the proficient electron-donating aptitude of donor moiety. On the other hand, 4012


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Table 2. NBO Charges for Donor, π-Spacer, and Acceptor of Designed Dyes (D1−D12) dyes

donor

π-conjugated linkers

acceptors

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

0.0492 0.0586 0.0459 0.0854 0.0406 0.0930 0.0795 0.1052 0.0491 0.0399 0.0861 0.0703

0.1195 0.0971 0.0603 0.1281 0.1102 0.1366 0.0701 0.2791 0.0440 0.0286 0.0782 0.0721

−0.1561 −0.1558 −0.1534 −0.1338 −0.1766 −0.1773 −0.1497 −0.3196 −0.1686 −0.1294 −0.1644 −0.1424

provide a path and facilitate the transfer of electron (without trapping them) from D to A unit. The NBO results of D1−D12 point out that all of the donors and π-conjugated linkers show positive values, whereas all of the acceptors show negative values. These results validate that electrons successfully migrate 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 are found for D8, whereas the least NBO charges are observed in D10. All of the remaining investigated dyes have the NBO charge values closely related to each other.

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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 the 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 ⟨α⟩ values are 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 of the studied dyes decreases in the Table 3. Dipole Polarizabilities and Major Contributing Tensors (au) of the Studied Dyes (D1−D12)

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

NBO charges with negative values indicate that all of the acceptors will effectively accept electrons. The positive NBO charge values of π-conjugated linkers represent that they will 4013

dye

αxx

αyy

αzz

⟨α⟩

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

1358.82 1395.58 1389.93 1532.79 1494.69 1543.42 1449.51 1589.19 1206.84 1189.82 1201.72 1239.44

462.73 453.42 437.64 432.14 459.52 472.43 461.15 445.53 479.09 469.48 471.12 449.62

257.02 254.91 267.54 291.04 276.70 274.43 275.00 280.70 263.55 260.11 264.31 270.79

692.85 701.30 698.36 751.99 743.64 763.42 728.55 771.80 649.82 639.80 645.71 653.28


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Table 4. Computed First Hyperpolarizabilities (βtot) and Major Contributing Tensors (au) of the Studied Dyes (D1−D12) dye

βxxx

βxxy

βxyy

βyyy

βxxz

βxzz

βtot

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

−104 990.23 −119 260.13 −116 085.84 −129 341.22 121 468.42 −139 902.19 −119 929.14 −140 439.07 −75 412.62 −59 768.95 −66 669.74 −88 593.97

2596.08 −2579.47 1515.45 −1579.04 2071.39 2177.44 694.37 1008.37 2730.30 −725.50 2104.92 2388.98

1118.47 1270.87 996.23 1095.94 −1108.90 1264.67 1107.47 1183.02 27.89 795.98 314.64 576.34

85.61 −28.64 49.57 −9.61 289.38 254.84 205.39 198.04 584.31 389.67 400.25 301.33

139.21 −349.47 636.52 −501.60 475.66 −188.36 797.68 581.88 59.94 27.24 453.52 1129.32

156.40 170.67 134.76 170.33 −124.16 145.35 116.80 187.60 137.62 185.52 128.63 79.93

103 750.17 117 848.43 114 967.46 128 086.05 120 259.91 138 514.17 118 711.13 139 075.05 75 320.06 58 788.40 66 275.95 87 987.937

to A moieties via a π-bridge, strong ICT occurs. 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, and hence, the NLO response.58 In this research work, the hyperpolarizabilities of D1−D12 have been calculated employing CAM-B3LYP functional and 6311+G(d,p) basis set and the results of βtot values along with their major contributing tensor are collected in Table 4. The βtot value for all of the studied dyes decreases in the following order: D8 > D6 > D4 > D5 > D7 > D2 > D3 > D1 > D12 > D9 > D11> D10. The βtot value for D1 and D2 is observed to be 103 750.17 and 117 848.43 au, respectively. It is observed that the TPA−DCV-based dyes with 5-(thiazol-5yl)thiazole as the first linker and thiazole as the second linker (D8) show the highest value of βtot 139 075.05 au than all of the other studied dyes. On the other hand, the least value of 58 788.40 au of βtot is measured for D10 in which benzo[b]thiphene and thiazole are used as first and second π-conjugated linkers, respectively. This highest and lowest NLO responses in case of D8 and D10, respectively, can be attributed to the effective charge transfer from D to A through their respective πconjugated linkers. As shown in Table 4, first the hyperpolarizability coefficients of all of the investigated dyes are much higher. For instance, the computed βtot value of D8 is 3234 times greater than the first hyperpolarizability value of urea molecule, which is frequently used as the reference organic molecule.59 Similarly, computed βtot value of D1 is found to be 2413 times greater, D2 2741 times, D3 2674 times, D4 2979 times, D5 2797 times, D6 3221 times, D7 2760 times, D9 1751 times, D10 1367 times, D11 1541 times, and D12 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 π-electrons. Delocalization of π-electrons decreases the HOMO−LUMO energy gap and stabilizes the molecules.

following order: D8 > D6 > D4 > D5 > D7 > D2 > D3 > D1 > D12 > D9 > D11 > D10. The average polarizability value for D1 and D2 is found to be 692.85 and 701.30 au, respectively. Modification of different π-conjugated linkers influence the polarizability values in all of the designed dyes, mainly in D4, D6, and D8. This indicate that the average polarizability value of TPA−DCV-based dyes with thiazolo[5,4-d]thiazole, 2(thiophen-2-yl)thiophene, and 5-(thiazol-5-yl)thiazole as first linkers and thiazole as second linker is higher than those of the other studied dyes. The highest value of polarizability is observed in designed dye D8, both in terms of average polarizability and αxx tensor. Literature suggests that the energy gap between LUMO and HOMO influence the polarizability of a molecule. Small energy gap is requisite to show a large linear polarizability. In general, molecules with a small energy gap and a large linear polarizability value show large hyperpolarizability values.57 Transitions along x and y directions are generally used to compute the polarizability. For instance, the formula for dipole polarizability (along x direction) is explained by eq 3 α∝

(MXgm)2 Egm

(3)

In this equation, the numerator Mgm X denotes the transition moment between the ground and mth excited state, whereas the denominator Egm represents the transition energy. Mgm X is a complex vector quantity. The phase factors of ground to excited state have also been considered in the transition dipole moment. The orientation of the transition dipole moment presents the polarization because of electronic transition. Furthermore, it also explains that how the dyes will interact with an electromagnetic wave of a given polarization. From the equation, it can be seen that α is directly proportional to the second power of the transition moment, whereas the transition energy has an inverse relation with it. It means the dipole polarizability value amplifies with rise in transition moment. Second power of the 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 a larger value of Mgm X and a smaller value of transition energy will comprise a large hyperpolarizability value. So, dipole polarizabilities are quantitative measurement that give an idea of the excellent NLO activity of the dyes. The NLO response of the dyes can be calculated in terms of their first hyperpolarizability (β). The second-order NLO properties are linked with the ICT. Owing to the flow of electrons from D

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UV−VIS SPECTRA OF DYES (D1−D12) To have an insight into the excited states absorption spectra, the TDDFT computations are 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 are studied during the TDDFT computations. Computed transition energy (Ege), oscillator strength ( fos), nature of transitions, and the maximum absorption wavelength (λmax) 4014


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Table 5. Computed Transition Energy (eV), Maximum Absorption Wavelengths (λmax, nm), Oscillator Strengths (fos), Light a Harvesting Efficiency (LHE), Transition Moment (Mgm X , au), and Transition Natures of Dyes dye D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 a

Ege (eV) 2.5429 2.5192 2.5753 2.5517 2.5602 2.4832 2.6097 2.4692 2.8571 2.9599 2.9275 2.8772

λmax (nm) b

487.55 (300−750 nm) 486.34 (300−750 nm)b 485.18 495.86 494.87 496.27 485.07 498.99 433.93 424.31 430.50 440.90

fos

LHE

Mxgm (au)

1.998 1.999 2.0296 1.9549 1.9402 1.9502 2.0622 2.1235 1.9272 0.5982 2.0406 2.0798

0.989 0.989 0.990 0.988 0.988 0.988 0.991 0.992 0.988 0.747 0.990 0.991

5.86 5.95 6.19 6.78 6.74 6.82 5.87 6.90 5.84 3.37 5.09 6.18

MO transition H H H H H H H H H H H H

→ L (60%), H − 1 → L (59%), H − 1 → L (61%), H − 1 → L (62%), H − 1 → L (54%), H − 1 → L (53%), H − 1 → L (55%), H − 1 → L (59%), H − 1 → L (46%), H − 1 − 1 → L (43%), H → L (51%), H − 1 → L (47%), H − 1

→ L (31%) → L (30%) → L (28%) → L (25%) → L (34%) → L (32%) → L (39%) → L (33%) → L (41%) − 3 → L (11%) → L (44%) → L (36%),

H = HOMO, L = LUMO, H − 1 = HOMO − 1, etc. bExperimental values in parentheses are from ref 40.

Figure 4. Simulated absorption spectra of the dyes (D1−D12).

LHE = 1 − 10 f

are collected in Table 5, whereas the absorption spectra of D1− D12 are displayed in Figure 4. All of the dyes show absorbance in the visible region (Figure 4). The absorption wavelengths of D1 and D2 was calculated to be 487.55 and 486.34 nm respectively which are in good agreement with the experimental absorption wavelength 300− 750 nm. The highest absorption wavelength is measured to be 498.99 nm for D8, implying that D8 is red-shifted as compared to other studied dyes. From Table 5, it is evident that the electron transitions of all of the systems (except D10) mainly originate from TPA/donor (HOMO) to the DCV/acceptor (LUMO) along the x direction. In D10, the charge transfer primarily occurs from HOMO − 1 to LUMO. Delocalization of HOMOs and LUMOs above the whole molecule can be seen from Figure 3. The optical efficiency of the dyes is related to another important factor, namely, light harvesting efficiency (LHE). Generally, systems with a large LHE value display maximum photocurrent response. The equation used to express LHE of a dye is60

(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 is found to be highest among all of the studied dyes. A better clarification of the structure−property relationship is mandatory for explaining the reason for the second-order NLO properties. Oudar and Chemla61 formulated the two-state model on the basis of the complex sum-over-states expression, which forms a connection between charge transfer transition and hyperpolarizability represented by the following eq 5

βCT =

Δμgm fgm Eg3m

(5)

In this equation, Δμgm represents the difference between excited and ground state dipole moment, which is directly proportional to the first hyperpolarizability. fgm represents the oscillator strength from the ground state (g) to the mth excited state (m) and is also directly proportional to β. E3gm indicates the cube of transition energy and is inversely proportional to 4015


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Figure 5. Relationship between the βtot (red line) values and the corresponding Δμgm fgm/E3gm (blue line) values for the dyes (D1−D12).

the first hyperpolarizability value. From the 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, the NLO material having a combination of large oscillator strength and transition moment magnitude with lowenergy CT excited state is an optimum design that can give large β value. The values of transition moment, oscillator strength, and excitation energy are tabulated in Table 5. In Table 5, it can be seen that for D1−D12, the factors Δμgm, E3gm, and fgm are closely related with each other and acquire the same general framework. The relationship between the first hyperpolarizability values and the corresponding two-level model values of D1−D12 are represented in Figure 5. It is obvious from Figure 5, that βtot values are in good agreement with the formula (Δμgm fgm/ΔE3gm) values suggested by the two-level model. 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 into the effect of π-conjugated linkers on the 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 involves pairing both an electron donor (D) and acceptor (A) to an organic framework that provides moderately strong electronic coupling between D and A. However, 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.

difference in spectral and NLO properties of D1−D12. All of the dyes showed maximum absorbance wavelength in the visible region with low transition energy, high transition moment, oscillating strength, and LHE values. Maximum redshifted absorption spectrum was observed to be 498.99 nm in D8. The NBO results depicted that electrons successfully migrated from D to A via π-conjugated linker, which resulted 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, whereas LUMO is located over the DCV group. Thus, the ICT from TPA to DCV segment through π-conjugated linker plays a crucial role in the large NLO response of D1−D12. Overall, all of the investigated dyes (D1−12) showed eye-catching and remarkably large NLO response in the range of 139 075.05−58 788.40 au. Among the studied dyes, D8 has shown the highest ⟨α⟩ and βtot computed to be 771.80 and 139 075.05 au, respectively. The computed βtot values of D1−D12 were observed to be 3234−1367 times greater than the value of urea molecule. Furthermore, the βtot values of D1−D12 were found to be proportional to the corresponding Δμgm fgm/ΔE3gm values in good concurrence suggested by the two-level model. This work also described that the structural modeling of π-linkers in D−π−A dyes was 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 provided new ways for experimentalists to design high-performance NLO materials for optics and electronics. In the conceptual design of the possible high-performance NLO materials, the proposed dyes should be targeted for further synthetic investigations.

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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. The DFT and TDDFT methods were employed to explore the influence of different π-linkers on the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12293. Optimized Cartesian coordinates of the studied dyes; normalized absorption spectra of D1 and D2 (Figure S1) (PDF) 4016


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.K.). *E-mail: [email protected] (M.I.). *E-mail: [email protected] (M.R.S.A.J.). ORCID

Muhammad Ramzan Saeed Ashraf Janjua: 0000-0002-43231736

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

M.I. (Grant No. HEC/2013/1114) and M.K. (Grant No. 1314) gratefully acknowledge the financial support from Higher Education Commission of Pakistan. M.R.S.A.J. and A.A.A.-S. 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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