# Concomitant Effects of Transition Metal Chelation and Solvent Polarity on the First Molecular Hyperpolarizability of 4-Methoxyacetophenone Thiosemicarbazone: A DFT Study

Authors: Nyiang Kennet Nkungli; Julius Numbonui Ghogomu
Journal: Journal of Theoretical Chemistry
 (2016)

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## Abstract

Nonlinear optical (NLO) properties of organic and metal-organic materials are of considerable interest to emerging optoelectronic and photonic technologies. Much work has been carried out on the former materials but the latter ones have received less attention till date. Herein, a density functional theory (DFT) study on the combined effects of transition metal chelation and solvent polarity on the first hyperpolarizability (βtot) of 4-methoxyacetophenone thiosemicarbazone (MAPTSC) is reported. MAPTSC exhibits a tautomeric form with higher optical nonlinearity rendering its NLO response in polar solvents potentially switchable. Our results have revealed significant modifications of the first hyperpolarizability of MAPTSC upon complexation with different transition metal chlorides in the presence of solvents with varying dielectric constants. Therefore, its second-order NLO response is highly tunable by the synergy of transition metal chelation and solvent polarity. MAPTSC and its Zn(II) and Pt(II) chloride complexes are promising NLO materials because their gas-phase βtot values are larger than those of the prototype push-pull molecules,para-nitroaniline (PNA) and urea, by factors of about 1.40–1.76 and 19.57–37.24, respectively; these factors greatly increase in polar solvent medium. Moreover, they possess high optical transparencies in the visible region of the electromagnetic spectrum which mitigate transparency/nonlinearity trade-offs, thereby increasing the likelihood of broad band NLO response.


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## Body

## 1. Introduction

Nonlinear optical (NLO) materials have recently received considerable attention because of their potential applications in the optoelectronic and all-optical (photonic) devices of telecommunication, optical switching, optical signal processing, optical transmission, optical data storage, optical computing, and so forth, [1, 2]. They are also essential for photodynamic therapy, biological imaging, and the protection of human eyes and optical sensors (by NLO materials with optical limiting effects) from high intensity LASER beams [3, 4]. NLO effects arise from the interaction of an applied optical field (usually LASER) with the electronic field created by the oscillation of electrons in a NLO material, generating a new optical field that is altered in frequency, amplitude, phase, or any other physical property [5–7]. Very high light intensities typically provided by LASERS are required for optical nonlinearity to be observed [8]. NLO effects include saturable absorption (SA), reverse saturable absorption (RSA), second-harmonic generation (SHG), two-photon absorption (TPA), and optical limiting [8, 9].Recently, the first hyperpolarizability (βtot) has been under rigorous investigation because a large βtot value is a prerequisite for large and fast second-order or quadratic NLO response. Other necessary conditions for significant quadratic NLO properties in molecular materials include noncentrosymmetry, low optical loss, high thermal and mechanical stability, intense low-energy electronic transitions with charge transfer (CT) character, and significant intramolecular charge transfer (ICT) owing to electron cloud movement from an electron donor to an electron acceptor through a π-conjugate bridge [10, 11]. Molecules with excellent NLO properties usually contain an electron-donor group (D) and an electron-acceptor group (A) linked through an intervening π-conjugated framework, forming the so-called D-π-A structure or push-pull system [12, 13]. Effective methods of enhancing the second-order NLO responses of materials involve increasing the donor-acceptor capabilities of D and A and increasing the length of the π-conjugated pathway between them [2, 9, 14].Important candidates in the area of NLO chromophores are organic and metal-organic materials. For the latter, transition metal complexes withπ-conjugated ligands are interesting for the construction of multifunctional NLO chromophores by virtue of their rich photochemical and photophysical properties, greater design flexibility, excellent chemical stability, and intense low-energy metal-to-ligand charge transfer (MLCT), as well as ligand-to-metal charge transfer (LMCT) electronic transitions [2, 15]. From literature survey, the vast majority of molecules investigated for NLO effects are purely organic push-pull systems. Comparatively, transition metal complexes containing π-conjugated organics are less explored in this area despite their potentials of combining the high optical nonlinearity and chemical flexibility of organic materials with the physical ruggedness of inorganic materials. In an effort to bridge this gap, the impact of metal complexation on the NLO responses of purely organic push-pull systems is currently being investigated. In this perspective, we chose 4-methoxyacetophenone thiosemicarbazone (MAPTSC) as our organic push-pull system because thiosemicarbazones and their metal complexes are notable materials for SHG and, as such, are highly applicable in the field of nonlinear optics [16]. The extensive electron delocalization in the thiosemicarbazone (TSC) moiety greatly improves the SHG efficiency of free TSC ligands and their metal complexes [17]. Furthermore, acetophenone thiosemicarbazone (APTSC) has been identified by means of the Z-scan technique as a NLO material with third-order harmonic generation capability [17]. Because MAPTSC has a powerful electron donor (−OCH3) appended to the benzene ring which is not the case in APTSC. The NLO response of the former is expected to be larger and faster than that of the latter. Moreover, MAPTSC like many TSCs readily forms chelates with transition metals. Several of its transition metal complexes, including most of those currently studied, have been successfully synthesized, crystallized, and characterized [18].Herein, we report for the first time on the combined effects of transition metal coordination by MAPTSC and solvent polarity and on its first molecular hyperpolarizability. Specifically, theβtot values of MAPTSC and its Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II) chloride complexes in vacuo and in solvents with different dielectric constants have been calculated and compared. In an attempt to explain the modifications undergone by the βtot value of MAPTSC following its complexation with different transition metal ions and due to changes in solvent polarity, the relationship between βtot values of all molecules studied and their frontier molecular orbital (HOMO-LUMO) energy gaps has been sought. In the same vein, the relationship between these βtot values and photoinduced electronic transitions in the molecules has been investigated. Time-dependent density functional theory (TD-DFT) calculations, natural bond orbital (NBO) analysis, and charge decomposition analysis (CDA) have been carried out in order to determine the charge transfer (CT) capabilities, optical transparencies in the visible region of the electromagnetic (EM) spectrum, electron delocalization, and orbital interaction patterns of the molecules. Owing to the complications encountered during the experimental measurement of NLO activities of materials by the Z-scan technique, quantum chemical calculations remain a fascinating alternative. For this study, the density functional theory (DFT) was preferred overab initio methods because DFT calculations, with pseudopotentials, are the standard approach for computations on transition metal containing systems [19]. Moreover, the DFT is very instrumental in designing molecules for NLO applications [14] because of its remarkable accuracy/computational-time ratio, coupled with the fact that it takes good account of electron correlation effects in calculations.


## 2. Computational Details and Background Theory on Second-Order NLO Response

All theoretical calculations were carried out with the Gaussian 09W suite of programs [20]. The molecular geometries of MAPTSC and its Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II) chloride complexes were optimized without constraints of any kind in gas and solvent phases using Becke’s three-parameter Lee-Yang-Parr hybrid functional [21] in conjunction with the small core Stuttgart-Dresden pseudopotential (SDD) for the transition metals, and the Pople-style basis set 6-31++G(d,p) for every other element. Then, harmonic vibrational frequencies were computed at the same level of theory as that used for geometry optimization to ascertain that each of the optimized geometries is a local minimum on its potential energy surface. For all calculations reported herein, the restricted closed-shell Kohn-Sham model was adopted for the closed-shell molecules (MAPTSC and its Ni(II), Pd(II), Pt(II), and Zn(II) chloride complexes), whereas the unrestricted Kohn-Sham model was used for the open-shell Cu(II) chloride complex.Bulk solvent effects were taken into account by the integral equation formalism polarizable continuum model (IEF-PCM). Three types of solvents, namely, benzene (a nonpolar solvent), ethanol (a polar protic solvent), and dimethyl sulfoxide, DMSO (a polar aprotic solvent), were used for investigation of solvent effects on the first hyperpolarizabilities of the molecules studied. For all compounds investigated, first hyperpolarizability (βtot) values were calculated at the B3LYP/6-31++G(d,p)/(SDD for metal ions) and B3LYP/6-311++G(d,p)/(SDD for metal ions) levels of theory, for comparison. Vertical excitation energies to the first 10 singlet excited states were computed using TD-DFT method at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory. The corrected asymptotic behavior of the CAM-B3LYP (Coulomb-attenuating method applied to B3LYP) functional improves accuracy especially in the higher regions of the electronic spectrum. Natural transition orbital (NTO) analyses were performed in order to describe the singlet “particle-hole” pairs involved in the dominant electronic transitions in all molecules studied. Natural transition orbitals (NTOs) provide a better qualitative description of electronic excitations. They are obtained by transforming the canonical molecular orbitals into a more compact form in which each excited state is most often represented by a single pair of orbitals, the “particle” and the “hole.” An electronic excitation from “particle” to “hole” is weighted with an eigenvalue [22].The first molecular hyperpolarizabilities of MAPTSC and its complexes were calculated by the finite field method as implemented in Gaussian 09W. In a static electric fieldF, the energy of an isolated molecule can be written as a Taylor series expansion with respect to the electric field, as shown in the following equation [5, 23]:(1)E=E0-μiFi-12αijFiFj-16βijkFiFjFk-124γijklFiFjFkFl-⋯.In this equation, E0 is the energy of the unperturbed molecule (the molecular energy without the applied electric field), Fi is the field at the origin, μi is the permanent dipole moment vector, and αij, βijk, and γijkl are the polarizability, first static hyperpolarizability, and second static hyperpolarizability tensors, respectively. The subscripts i, j, and k label the x, y, and z components, since the external electric field is applied to the molecule with coordinates along x, y, and z directions [13, 24]. The first static hyperpolarizability tensor βtot is defined as(2)βtot=βx2+βy2+βz21/2,where βx, βy, and βz are the components of βtot along the x, y, and z axes, respectively [23, 25, 26]. From the components of the βtot tensor, the values of βx, βy, and βz are calculated as follows:(3)βx=βxxx+βxyy+βxzz;βy=βyyy+βyzz+βyxx;βz=βzzz+βzxx+βzyy.Therefore, the complete equation used to calculate the magnitude of βtot is(4)
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.


## 3. Results and Discussion

### 3.1. The Push-Pull Structures and Optimized Geometries of the Molecules Studied

MAPTSC undergoes rapid and facile thione-thiol tautomerization via the exchange of a proton between the hydrazinic nitrogen and the thionic sulfur of the thiosemicarbazone (TSC) moiety [16]. It can act as an N- and S-donor bidentate neutral ligand (thione tautomer) and as an anionic ligand (deprotonated thiol tautomer) [27]. Much attention in this paper has been focused on the thione tautomer of MAPTSC and its complexes because the thione tautomer is the predominant form of thiosemicarbazones [28]. The push-pull CT mechanisms of the thione tautomer (L1) and thiol tautomer (L2) of MAPTSC are depicted in Figures 1(a) and 1(b), respectively. In both tautomers, the electron donor (D) is the methoxy group (−OCH3). The electron acceptor (A) in L1 and L2 is the hydrazinic N–H and the amino (NH2) groups of the TSC moiety, respectively. As such, the π-conjugated pathway linking A and D in L1 is made up of the benzene ring and the appended azomethine group of the thionic TSC moiety, while that in L2 is comprised mainly of the benzene ring and the two attached conjugated azomethine groups of the thiolic TSC moiety. Hence, the π-conjugated bridge for the D-π-A structure is shorter in L1 than that in L2, and this can lead to the tautomers exhibiting different optical nonlinearities. Based on the length of the π-bridge between A and D, the NLO response of L2 should be stronger than that of L1. A large difference in the optical nonlinearities of the tautomers will render the NLO response of MAPTSC switchable, since L1 and L2 are both stable at room temperature and can be interchanged via thione-thiol tautomerization.Figure 1
Schematic representation of the push-pull mechanisms of the thione and thiol tautomers of MAPTSC, whereA is the electron-acceptor group andD is the electron-donor group.The scenario in the transition metal chloride complexes of MAPTSC is different from that in the free ligands, because L2 loses its thiolic hydrogen via deprotonation as it chelates metal ions. The loss of the thiolic hydrogen and the stability of the chelate ring hinder thione-thiol tautomerization of MAPTSC. As a result, a switchable NLO response based on thione-thiol tautomerism is least expected in the complexes. In this paper, the NLO properties of the Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II) chloride complexes of L1 have been studied. To facilitate the presentation and comprehension of our results, the gas-phase optimized geometries of these molecules visualized using Gauss View 5.0.8 [29] are shown in Figure 2.Figure 2
Optimized geometries of (a) ligand L1, (b) [Ni(L1)Cl2], (c) [Pd(L1)Cl2], (d) [Pt(L1)Cl2], (e) [Cu(L1)Cl2], and (f) [Zn(L1)Cl2] at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
(a) (b) (c) (d) (e) (f)Charge decomposition analysis (CDA) was performed in order to determine the nature of the coordination bonds between L1 and the transition metal ions M(II) present in the metal chloride fragments (MCl2). In addition, CDA was used to investigate the impact L1–MCl2 binding may have on the D-π-A charge transfer mechanisms in the complexes. Furthermore, CDA was used to study the influence of any ambivalent donor-acceptor behavior of the transition metals on the push-pull CT capabilities of the complexes investigated. CDA was first proposed by Dapprich and Frenking [30] and has been generalized by Xiao and Lu [31] from its original formulation. Besides CDA, extended charge decomposition analysis (ECDA) proposed by Gorelsky and coworkers [32] was performed. These methods are capable of providing a deep insight into how electronic charge is transferred between L1 (fragment 1) and MCl2 (fragment 2) in the complexes for charge equilibrium to be attained. Typically, CDA defines two components of intermolecular CT: forward donation (σ-bonding) and back donation or back-bonding (π-bonding). This has made CDA a popular method for estimating the amount of electron delocalization in donor-acceptor complexes [33] and in the donor-acceptor metal-ligand bonds in coordination complexes. The generalized CDA and ECDA were carried out using Multiwfn 3.3.8 [34] based on input files obtained from single-point calculations at B3LYP/6-311G(d,p)/(SDD for metal ions) level of theory. The results of CDA and ECDA on the molecules studied are presented in Table 1.Table 1
ECDA and generalized CDA results for transition metal chloride complexes of L1 calculated using Gaussian output files for single-point calculations at B3LYP/6-311G(d,p)/(SDD for metal ions) level on geometries optimized at B3LYP/6-31++G(d,p)/(SDD for metal ions) level.
C o m p l e x e s a [Ni(L1)Cl2] [Pd(L1)Cl2] [Pt(L1)Cl2] [ C u ( L 1 ) C l 2 ] b [Zn(L1)Cl2]  Gas-phase Frag.1→ Frag.2 donation (d) 0.26654 0.14418 0.21067 0.30532 0.26154 Frag.1← Frag.2 back donation (b) −0.00296 0.04153 0.05289 −0.01265 0.00267 Net Frag.1→ Frag.2 donation (d-b) 0.26950 0.10266 0.15778 0.31797 0.25887 Donation to back donation ratio, |d/b| 89.95478 3.47205 3.98336 24.13581 98.02661 CT(1→2) - CT(2→1) 0.60450 0.39720 0.46260 0.56460 0.41070 Benzene Frag.1→ Frag.2 donation (d) 0.28205 0.15715 0.21982 0.33276 0.29419 Frag.1← Frag.2 back donation (b) −0.00731 0.03463 0.04368 −0.01384 0.00387 Net Frag.1→ Frag.2 donation (d-b) 0.28936 0.12253 0.17615 0.34659 0.29032 Donation to back donation ratio, |d/b| 38.56248 4.53860 5.03318 24.05183 76.07706 CT(1→2) - CT(2→1) 0.69220 0.49480 0.55910 0.66880 0.46970 Ethanol Frag.1→ Frag.2 donation (d) 0.29623 0.16990 0.22919 0.35717 0.32350 Frag.1← Frag.2 back donation (b) −0.00861 0.02929 0.03645 −0.01264 0.00671 Net Frag.1→ Frag.2 donation (d-b) 0.30484 0.14061 0.19274 0.36981 0.31679 Donation to back donation ratio, |d/b| 34.41757 5.80025 6.28753 28.25720 48.20414 CT(1→2) - CT(2→1) 0.78610 0.61200 0.67540 0.78440 0.53030 DMSO Frag.1→ Frag.2 donation (d) 0.29691 0.17013 0.22975 0.35872 0.32508 Frag.1← Frag.2 back donation (b) −0.00853 0.02883 0.03608 −0.01240 0.00691 Net Frag.1→ Frag.2 donation (d-b) 0.30544 0.14130 0.19367 0.37112 0.31817 Donation to back donation ratio, |d/b| 34.82839 5.90087 6.36797 28.93394 47.05196 CT(1→2) - CT(2→1) 0.79060 0.61670 0.68310 0.79150 0.53290 F   a r a g. refers to fragment, where Frag.1 is L1 and Frag.2 is MCl2 [M = Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II)]. T   b h e overall results for alpha and beta electrons are reported for the Cu(II) chloride complex of L1.From the donation to back donation ratios, |d/b| in Table 1, it is obvious that the coordination bonds in the Ni(II), Cu(II), and Zn(II) chloride complexes of L1 are predominantly sigma covalent bonds with very little π-character. The transition metal ions in these complexes act mainly as electron acceptors, since back donation is insignificant compared to donation. The electrons donated by L1 are mainly coming from filled π-bonding orbitals and/or the lone electron pair orbitals of its N- and S-donor atoms and are received in empty orbitals of the transition metal ions. The |d/b| values for [Pd(L1)Cl2] and [Pt(L1)Cl2] clearly show that their coordination bonds have a substantial π-character, which helps to extend the π-conjugated pathway for the push-pull systems to the Pd(II) or Pt(II) ions in the respective complexes. The back donated electrons originate from filled d-orbitals of the transition metal ions and are received in empty π∗-antibonding orbitals on L1. The values of electron donation and back donation have shown that the Pd(II) and Pt(II) ions are capable of ambivalent electron donor-acceptor behaviors in their respective complexes studied in this research endeavor. Such ambivalent electron donor-acceptor capabilities can trigger redox processes that may lead to switchable NLO activities in the complexes. As clear from Table 1, back donation of electrons has witnessed a moderate to drastic increment in the bulk solvents, implying that the solvent environment (especially that created by the highly polar solvents) increases the π-character of the metal-ligand coordination bonds in the complexes investigated.


### 3.2. First Molecular Hyperpolarizabilities of MAPTSC and Its Complexes

The first molecular hyperpolarizabilities (βtot) of MAPTSC and its complexes studied (listed in Table 2) were calculated using (2) based on the x, y, and z components (βx, βy, and βz) of βtot. The values of βx, βy, and βz were computed from the components of the βtot tensor: βxxx, βyxx, βxyy, βyyy, βzxx, βxyz, βzyy, βxzz, βyzz, and βzzz, using (3). The components of the βtot tensor for each molecule studied were obtained via Gaussian 09W calculations at two levels of theory: B3LYP/6-31++G(d,p)/(SDD for metal ions) and B3LYP/6-311++G(d,p)/(SDD for metal ions) in gas and solvent phases. The trends adopted by the βtot values on going from the gas to the solvent phase have been elucidated graphically in Figure 3. The βtot values calculated using the data obtained from Gaussian calculations are usually in atomic units, but those reported in this paper have been converted into electrostatic units using the conversion factor 1 a.u. = 8.6393 × 10−33 esu. Inspection of the values in Table 2 and the graphs in Figure 3 has revealed similar trends adopted by the βtot values of each molecule (calculated at both levels of theory) on passing from the gas to the solvent phase. Except for a few cases, the βtot values calculated at the former level of theory are larger than those calculated at the latter level. Hence, further discussion in this paper regarding the βtot values and associated second-order NLO activities of the molecules is based only on the results obtained at the B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.Table 2
β t o t, βx, βy, and βz values of MAPTSC and the complexes calculated at B3LYP/6-31++G(d,p)/(SDD for metal ions) and B3LYP/6-311++G(d,p)/(SDD for metal ions) levels of theory in different media.
Basis set 6-31++G(d,p)/(SDD for metal ions) 6-311++G(d,p)/ (SDD for metal ions) Molecule β x(a.u.) β y(a.u.) β z(a.u.) β t o t × 10 - 30 (esu) β x(a.u.) β y(a.u.) β z(a.u.) β t o t × 10 - 30 (esu) Gas-phase L1 814.59 198.71 103.02 7.298 731.74 184.26 114.38 6.594 L2 1056.16 −724.81 −60.49 11.079 1016.92 −700.38 −46.18 10.675 [Ni(L1)Cl2] 331.18 −506.92 409.83 6.317 463.27 −528.93 420.36 7.077 [Pd(L1)Cl2] 315.26 −366.04 431.49 5.596 454.30 −382.74 448.43 6.430 [Pt(L1)Cl2] 1460.79 −444.13 502.44 13.887 1635.25 −452.82 521.00 15.334 [Cu(L1)Cl2] −675.03 −363.16 36.41 6.630 −560.28 −376.66 43.37 5.845 [Zn(L1)Cl2] −811.24 −390.90 345.28 8.332 −706.61 −389.50 361.74 7.639 Benzene L1 1282.49 56.10 167.35 11.184 1179.22 45.43 174.07 10.306 L2 1551.36 −1089.99 −89.36 16.398 1502.70 −1060.86 −70.78 15.903 [Ni(L1)Cl2] −823.01 −357.05 543.42 9.062 −666.92 −391.48 557.98 8.239 [Pd(L1)Cl2] 939.71 −327.52 −637.52 10.210 773.32 −357.51 −660.79 9.315 [Pt(L1)Cl2] −573.09 −407.87 −744.78 8.850 −785.46 −425.23 −769.99 10.188 [Cu(L1)Cl2] −2260.96 −238.64 184.96 19.706 −2117.62 −256.13 197.51 18.507 [Zn(L1)Cl2] −1606.20 −364.44 316.52 14.489 −1495.75 −365.53 330.97 13.606 Ethanol L1 2137.32 −269.15 147.01 18.654 2024.98 −291.42 158.79 17.728 L2 2267.08 −1677.29 −143.98 24.395 2205.02 −1645.93 −119.16 23.794 [Ni(L1)Cl2] −3142.76 −179.82 920.82 28.335 −2956.62 −233.21 947.43 26.898 [Pd(L1)Cl2] 3594.07 −415.42 −1047.59 32.541 3379.29 −475.21 −1083.34 30.932 [Pt(L1)Cl2] 1657.19 −276.44 −1112.58 17.409 1397.74 −309.05 −1150.21 15.865 [Cu(L1)Cl2] −7509.45 −198.28 712.29 65.190 −7206.75 −225.86 739.63 62.619 [Zn(L1)Cl2] −3310.72 −198.42 547.88 29.042 −3182.29 −203.79 582.65 28.005 DMSO L1 2206.37 −303.91 144.37 19.282 2094.06 −325.95 156.44 18.359 L2 2315.01 −1719.20 −147.47 24.945 2251.91 −1687.90 −122.31 24.336 [Ni(L1)Cl2] −3329.44 −163.94 955.14 29.958 −3140.90 −219.30 982.93 28.496 [Pd(L1)Cl2] −3565.27 −417.46 935.93 32.049 −3351.90 −483.07 969.63 30.433 [Pt(L1)Cl2] 1848.88 −258.57 −1142.88 18.911 1585.59 −292.58 −1181.51 17.269 [Cu(L1)Cl2] −8098.75 −202.43 757.49 70.295 −7773.97 −230.92 783.55 67.531 [Zn(L1)Cl2] −3437.58 −176.12 553.46 30.119 −3308.23 −181.74 588.93 29.073Figure 3
Trends inβtot values of L1 and its transition metal chloride complexes from gas to solvent phases.The magnitude of theβtot value for MAPTSC calculated in the present work is highly dependent on the tautomer present and on the nature of the surrounding environment. For instance, the βtot value for L1 is 7.298 × 10−30 esu in gas-phase and 19.282 × 10−30 esu in DMSO, while that for L2 is 11.079 × 10−30 esu in gas-phase and 24.945 × 10−30 esu in DMSO. The gas-phase βtot values of the thione-thiol tautomers of MAPTSC are approximately 0.92–1.40 times larger than that ofpara-nitroaniline (7.9 × 10−30 esu [35]), a prototype push-pull molecule [36], and about 19.57–29.71 times larger than that of urea (372.89 ×10−33 esu [37]), indicating that MAPTSC is a potent NLO material. From the βtot values of the thione-thiol tautomers of MAPTSC, it is clear that the second-order NLO response of MAPTSC is potentially switchable especially in polar solvents, due to the presence of a tautomeric form (thiol form) with a higher CT capability. Since thione-thiol equilibrium is readily influenced by factors such as the nature of the solvent, temperature, and concentration [38], the switchable NLO response of MAPTSC can be easily fine-tuned by modifying these factors.The coordination of L1 to different transition metal centers results in significant modifications of itsβtot value. Further modifications of this value are observed in the presence of bulk solvents especially the highly polar ones. Therefore, the first hyperpolarizability vis-a-vis the second-order NLO response of L1 can be highly tuned by coordination to different transition metal ions and by the presence of solvents with different dielectric constants. The complexation of L1 with Zn(II) and Pt(II) chlorides results in a modest to drastic increments of its gas-phase βtot value, respectively. Unexpectedly, the complexation of L1 with Ni(II), Pd(II), and Cu(II) chlorides reduced its βtot value. This is unexpected because electronic transitions involving metal-based orbitals in these complexes are supposed to reinforce their optical nonlinearities.It is clear from theβtot values in Table 2 and the graphs in Figure 3 that the presence of a bulk solvent strengthens the first hyperpolarizability thereby reinforcing the second-order NLO response of each molecule studied to a greater or lesser extent, depending on solvent polarity. In fact, the βtot values of these molecules when present in the nonpolar solvent (benzene) and in the polar solvents (ethanol and DMSO) are larger than the corresponding gas-phase values by factors in the ranges 2.7–13.1 and 3.5–58.6, respectively. Exceptionally, the βtot value for the Pt(II) chloride complex in benzene dropped below the gas-phase value by a factor of about 5. Based on our βtot values, it is clear that all of the molecules investigated are likely to exhibit their greatest second-order NLO activities in DMSO, which has the largest dielectric constant among the solvents used. Consequently, further discussion pertaining to solvent effects on βtot and second-order NLO properties of MAPTSC and its complexes has been restricted to the results obtained in DMSO. As clearly shown by the graphs in Figure 3, the magnitudes of the βtot values of the molecules studied are in the order [Pd(L1)Cl2] < [Ni(L1)Cl2] < [Cu(L1)Cl2] < L1 < [Zn(L1)Cl2] < [Pt(L1)Cl2] in vacuum and [Pt(L1)Cl2] < L1 < [Ni(L1)Cl2] < [Zn(L1)Cl2] < [Pd(L1)Cl2] < [Cu(L1)Cl2] in DMSO.To determine the plausible CT mechanisms responsible for the first hyperpolarizabilities and hence quadratic NLO activities of the molecules investigated, the relationship between the frontier molecular orbitals (HOMO-LUMO) energy gaps and theβtot values has been scrutinized. Since transition metals are capable of inducing low-energy MLCT and LMCT transitions that can greatly contribute to the first hyperpolarizability of MAPTSC, photoinduced electronic transitions in the molecules have been studied via the TD-DFT method. Changes in the gas-phase βtot value for L1 as a result of L1–MCl2 binding are attributed solely to the effects of the different transition metal ions present in the MCl2 fragment. Such changes in the presence of both the MCl2 fragment and the bulk solvent are attributable to the concomitant effects of metallation and solvent polarity.


### 3.3. First Molecular Hyperpolarizabilities and HOMO-LUMO Energy Analysis

The HOMO-LUMO energy gaps (ΔEH-L) of L1 and its transition metal chloride complexes, computed at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory in gas-phase and in DMSO are listed in Table 3. The ΔEH-L values for the Cu(II) chloride complex of L1 in both media have been calculated from the eigenvalues of the beta spin orbitals, because a majority of its electronic transitions (analyzed in the next section) involve these orbitals. The D-π-A or push-pull CT mechanism predominates in molecules when an inverse proportionality relationship exists between their ΔEH-L and βtot values. The push-pull mechanism is strongly dependent on HOMO-LUMO energy gaps because a proper electronic conjugation must exist among the molecular orbitals of a system for ICT to be effective [7]. The relationship between the gas-phase ΔEH-L and βtot values of the molecules studied is far from being inversely proportional, implying that the D-π-A push-pull process is not the dominant CT mechanism in each of the molecules in gas-phase. Photoinduced electronic transitions with CT character (involving metal-based orbitals) are certainly playing the dominant role. In DMSO, a somewhat inverse proportionality relationship is found between the values of ΔEH-Land βtot, suggesting that CT in these molecules is due to the combined effects of photoinduced electronic transitions with CT character and the D-π-A push-pull process. The polar solvent medium therefore enhances charge transfer in each of the molecules investigated.Table 3
HOMO/LUMO energies andβtot values of MAPTSC and its metal chloride complexes computed in gas-phase and DMSO at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Molecule Energies in gas-phase (eV) β t o t × 10 - 30 (esu) Energies in DMSO (eV) β t o t × 10 - 30 (esu) H O M O a energy L U M O b energy Δ E H - L c H O M O a energy L U M O b energy Δ E H - L c L1 −5.481 −1.189 4.292 7.298 −6.134 −1.334 4.801 19.282 [Ni(L1)Cl2] −5.815 −2.397 3.418 6.317 −6.589 −2.797 3.791 29.958 [Pd(L1)Cl2] −5.895 −2.496 3.399 5.596 −6.574 −3.010 3.564 32.049 [Pt(L1)Cl2] −5.715 −2.386 3.329 13.887 −6.272 −2.350 3.923 18.911 [Cu(L1)Cl2] −6.240 −3.575 2.665 6.630 −6.723 −4.043 2.679 70.295 [Zn(L1)Cl2] −6.744 −2.193 4.551 8.332 −6.621 −2.014 4.607 30.119 HOMO   a stands for highest occupied molecular orbital. L U M O   b stands for lowest unoccupied molecular orbital. Δ E H - L   c represents HOMO-LUMO energy gap.


### 3.4. Electronic Absorption Spectra of L1 and Its Transition Metal Chloride Complexes

In order to determine the optical transparency of each investigated molecule in the visible region of the EM spectrum and to determine the electronic transitions with CT character, the UV-Vis spectra of these molecules were simulated via TD-DFT studies. To study solvatochromic effects, the UV-Vis spectra were calculated in both gas-phase and DMSO. For each molecule, electronic transitions to the first 10 low-lying singlet excited states were studied at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory. The computed electronic excitation energies (ΔE in eV), wavelengths (λ in nm), and oscillator strengths (f) are presented in Table 4. The electronic transitions with oscillator strengths 0.02 and above have been considered, since those with very low oscillator strengths are likely to make insignificant contributions to the first hyperpolarizabilities of the molecules studied. The UV-Vis spectra of these molecules in gas and solvent phases are presented in Figure 4.Table 4
Absorption energies (E, eV), wavelengths (λ, nm), and oscillator strengths (f) of L1 and the complexes, calculated at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory in gas-phase and DMSO by the TD-DFT method.
Molecule Gas-phase DMSO State Δ E (eV) λ (nm) f State Δ E (eV) λ (nm) f L1 S2 4.6892 264.40 0.4821 S2 4.7279 262.24 0.7898 S3 5.0039 247.77 0.0203 S3 5.0309 246.44 0.0231 S4 5.0404 245.98 0.0401 S4 5.2409 236.57 0.0406 S6 5.3192 233.09 0.0435 S5 5.6131 220.88 0.0294 S8 5.6300 220.22 0.0826 S6 5.6917 217.83 0.1009 S10 5.7011 217.47 0.0717 S7 5.7689 214.92 0.0329 S8 5.9143 209.63 0.2104 S9 5.9352 208.90 0.0676 S10 5.9525 208.29 0.0210  [Ni(L1)Cl2] S7 3.8661 320.70 0.0336 S5 3.8202 324.55 0.0377 S9 4.1644 297.73 0.0342 S6 4.0588 305.47 0.0206 S7 4.4403 279.23 0.2780 S8 4.5408 273.04 0.3197 S9 4.6220 268.25 0.0666  [Pd(L1)Cl2] S6 3.9119 316.94 0.0488 S5 3.8273 323.95 0.0376 S8 4.1873 296.10 0.0557 S7 4.4265 280.10 0.3549 S10 4.3174 287.17 0.0320 S8 4.5383 273.20 0.2687 S9 4.7574 260.61 0.0802 S10 4.7830 259.22 0.0345  [Pt(L1)Cl2] S5 3.5049 353.75 0.0293 S5 4.1907 295.85 0.2784 S6 3.8361 323.20 0.1224 S6 4.4592 278.04 0.3691 S7 3.8860 319.05 0.0368 S7 4.5295 273.72 0.0466 S8 3.9611 313.00 0.0243 S8 4.7188 262.75 0.0219 S9 4.0806 303.84 0.0292  [Cu(L1)Cl2] S10 3.4788 356.40 0.0330 S5 2.5892 478.86 0.0268 S8 3.2291 383.96 0.0267 S10 3.4829 355.98 0.0305  [Zn(L1)Cl2] S2 4.7424 261.44 0.5390 S1 4.5399 273.10 0.7015 S4 5.1784 239.43 0.0285 S3 5.1949 238.67 0.0271 S10 5.8132 213.28 0.0202 S5 5.7561 215.39 0.1552 S6 5.8884 210.56 0.1346 S7 5.9773 207.43 0.1919 S10 6.2838 197.31 0.0746Figure 4
The electronic absorption spectra of L1 and its complexes in gas-phase and DMSO computed at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.The modification undergone by theβtot value for L1 upon complexation with different transition metal chlorides appears to be related to the types and intensities (oscillator strengths) of the electronic excitations in the complexes formed. The reduction in the βtot value for L1 following its complexation with Ni(II), Pd(II), and Cu(II) chlorides in vacuo is apparently linked to the reduction in the number of electronic transitions in the resulting complexes, coupled with a decrease in the oscillator strengths of the transitions. Although complexation leads to a slight lowering of the electronic excitation energies of the dominant electronic transitions in the complexes [Ni(L1)Cl2], [Pd(L1)Cl2], and [Cu(L1)Cl2], a corresponding increase in their βtot values is not observed. This is probably due to the significant intensity reduction of these transitions relative to those in L1. On the contrary, the complexation of L1 with the Pt(II) chloride in vacuum results in a dramatic increment of the ligand’s gas-phase βtot value, which can be attributed to the introduction of several low-energy intense electronic transitions.By inspection of Figure4 and Table 4, it can be seen that the number of reasonably strong electronic transitions in the molecules studied increases significantly in DMSO. Moreover, the polar DMSO medium leads to a modest lowering of the excitation energies of the dominant electronic transitions in all complexes investigated, except the Pt(II) chloride complex. Therefore, the [Ni(L1)Cl2], [Pd(L1)Cl2], [Cu(L1)Cl2], and [Zn(L1)Cl2] complexes must possess highly polar excited states that are greatly solvated and stabilized by the polar DMSO molecules, whereas their ground states are less polar and hence less stabilized by solvation. Consequently, excitation energies from the less stabilized ground states to the highly stabilized excited states are reduced, leading to an increase in the number of intense electronic excitations in the complexes. This in turn causes a dramatic increment in the βtot values of the complexes in DMSO as solvent, thus enhancing their quadratic NLO responses. Strong absorption of light in the visible region of the EM spectrum restricts the NLO applications of materials [39] due to significant transparency/nonlinearity trade-offs that often lead to lower device efficiency and reduced photostability. From Figure 4, it can be seen that, apart from the [Cu(L1)Cl2] complex, all investigated molecules have high optical transparencies in the entire visible region, implying that they suffer very low optical losses. Hence, transparency/nonlinearity trade-offs are highly minimized in L1 and its Ni(II), Pd(II), Pt(II), and Zn(II) chloride complexes, confirming their suitability for the fabrication of NLO devices.To decipher the relationship between solvent polarity and the first hyperpolarizabilities of the molecules studied, their UV-Vis spectra in gas-phase and in DMSO have been compared as shown in Figure4. It is clear from Figure 3 that polar solvents substantially increase the βtot value for the Cu(II) chloride complex of L1 but have almost no effect on the βtot value for the Pt(II) chloride complex. The effects of the polar solvents on the βtot values for these molecules have been explained in terms of the solvatochromic shift directions of their strongest electronic transitions on going from the gas-phase to the DMSO solvent medium, as elucidated in Figure 4. The drastic increment in the βtot value for the Cu(II) chloride complex in DMSO is ascribed to its positive solvatochromism (the shift of its strongest CT absorption bands towards lower excitation energies and longer wavelengths). The remarkable negative solvatochromism exhibited by the Pt(II) chloride complex in DMSO (the shift of its strongest CT absorption bands towards higher excitation energies and shorter wavelengths) accounts for the insignificant modification of its βtot value in the polar solvents. The positive solvatochromism (bathochromic or red-shift of the strongest CT absorption bands) exhibited by the [Zn(L1)Cl2] complex is responsible for the increment in its βtot value in DMSO. Solvatochromism as such is not obvious from the comparison of the gas and solvent phase UV-Vis spectra of L1 in Figure 4. The increment in its βtot value in DMSO is certainly linked to the increase in the intensities of its strong electronic excitations. Although the complexes [Ni(L1)Cl2] and [Pd(L1)Cl2] display a certain degree of negative solvatochromism in DMSO (hypsochromic or blue-shift of their strongest CT absorption bands), appreciable increments in their βtot values are observed due to significant contributions from strong low-lying electronic transitions of CT character to their first hyperpolarizabilities.By comparing the gas-phase UV-Vis absorption spectra of ligand L1 with those of its metal chloride complexes, it is clear that metallation results in significant modifications of the ligand’s electronic absorption spectrum. These modifications are caused by the introduction of CT bands induced by the promotion of electrons from orbitals that are predominantly metal in character to orbitals that are predominantly ligand in character (MLCT), or the reverse (LMCT). These CT bands generally give significant contributions to the first molecular hyperpolarizability. Such CT transitions are absent from the UV-Vis spectrum of the Zn(II) chloride complex because thed-orbitals of the Zn(II) ion are fully filled and do not participate in electronic transitions.The dominant electronic transitions of the molecules studied (those with the greatest oscillator strengths) have been characterized via natural transition orbital (NTO) analysis which generates unique “particle-hole” NTO pairs from the canonical molecular orbitals. Only the dominant electronic transitions (those boldly highlighted in Table4) have been characterized because they are capable of remarkable contributions to the first molecular hyperpolarizability. For each “particle-hole” NTO pair, the “particle” shows where in the system electrons come from and the “hole” indicates where the electrons go to, during electronic transitions. The NTOs of the molecules studied (shown in Table 5) were computed using Gaussian 09W and then rendered with VMD 1.9.2 [40] using isosurface cube files generated by Multiwfn 3.3.8. To obtain a quantitative description of the NTOs, orbital contributions from different molecular fragments (reported in Table 6) were calculated using the orbital composition analysis option [41] of Multiwfn 3.3.8. These orbital compositions, together with the electronic distributions of the NTOs (depicted by their isosurfaces in Table 5) were exploited in characterizing the dominant electronic transitions of the molecules studied. These transitions along with their most probable characters are listed in Table 7.Table 5
The dominant NTO “particle-hole” pairs for the dominant electronic transitions of each investigated molecule in gas-phase and in DMSO.
Molecule Gas-phase DMSO Particle Hole Particle Hole L1 [Ni(L1)Cl2] [Pd(L1)Cl2] [Pt(L1)Cl2] [Cu(L1)Cl2] [Zn(L1)Cl2]Table 6
Natural transition orbitals (NTOs) compositions for the thione tautomer of MAPTSC and its complexesin gas-phase and DMSO, calculated at the TD-DFT/CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Molecule NTOs Orbital compositions of fragments (%) Main distribution of NTOs on molecular systems M ( I I ) a M A P b T S C c 2Cl- Gas-phase L1 Hole — 36.496 63.504 — MAP + TSC Particle — 13.856 86.144 — TSC [Ni(L1)Cl2] Hole 4.537 50.724 43.388 1.351 MAP + TSC Particle 46.092 2.627 9.700 41.581 Ni(II) ion + 2Cl- [Pd(L1)Cl2] Hole 3.958 49.695 45.108 1.239 MAP + TSC Particle 32.732 4.901 6.195 56.172 Pd(II) ion + 2Cl- [Pt(L1)Cl2] Hole 5.517 43.596 49.385 1.501 MAP + TSC Particle 56.516 5.890 7.933 29.661 Pt(II) ion + 2Cl- [Cu(L1)Cl2] Hole 59.034 3.915 11.890 25.160 Cu(II) ion + 2Cl- Particle 8.056 3.214 66.050 22.679 TSC + 2Cl- [Zn(L1)Cl2] Hole 1.873 45.358 51.649 1.119 MAP + TSC Particle 1.731 34.905 57.951 5.413 MAP + TSC DMSO L1 Hole — 41.677 58.323 — MAP + TSC Particle — 29.818 70.182 — MAP + TSC [Ni(L1)Cl2] Hole 19.896 37.686 36.486 5.932 Ni(II) ion + MAP + TSC Particle 6.189 40.681 35.610 17.520 MAP + TSC + 2Cl- [Pd(L1)Cl2] Hole 34.365 25.958 24.693 14.984 Pd(II) ion + MAP + TSC + 2Cl- Particle 1.806 61.076 23.100 14.018 MAP + TSC + 2Cl- [Pt(L1)Cl2] Hole 5.065 60.271 33.556 1.108 MAP + TSC Particle 30.291 52.437 12.099 5.173 Pt(II) ion + MAP [Cu(L1)Cl2] Hole 61.952 2.016 17.871 18.162 Cu(II) ion + TSC + 2Cl- Particle 9.275 22.645 20.973 47.107 MAP + TSC + 2Cl- [Zn(L1)Cl2] Hole 1.932 61.449 35.673 0.945 MAP + TSC Particle 0.714 70.615 27.791 0.880 MAP + TSC M   a(II) represents the transition metal ions. M A P   b is the 4-methoxyacetophenone moiety. TSC   c is the thiosemicarbazones moiety.Table 7
Excitation energies and wavelengths, oscillator strengths, NTO eigenvalues, and characters of the dominant electronic transitions for MAPTSC and its complexesin gas-phase and DMSO, calculated at the TD-DFT/CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Medium Molecule Excited state Electronictransitiona NTO eigenvalue f Δ E (eV) λ m a x (nm) C h a r a c t e r b Gas-phase L1 2 P→ H 0.8866 0.4821 4.6892 264.40 n → π ∗ [Ni(L1)Cl2] 9 P→ H 0.9299 0.0342 4.1644 297.73 MLCT/LLCT [Pd(L1)Cl2] 8 P→ H 0.9261 0.0557 4.1873 296.10 MLCT/LLCT [Pt(L1)Cl2] 6 P→ H 0.9731 0.1224 3.8361 323.20 MLCT/LLCT [Cu(L1)Cl2] 10 P→ H 1.6191 0.0330 3.4788 356.40 LMCT/LLCT [Zn(L1)Cl2] 2 P→ H 0.9265 0.5390 4.7424 261.44 ILCT  DMSO L1 2 P→ H 0.9190 0.7898 4.7279 262.24 n → π ∗/π→π∗ [Ni(L1)Cl2] 8 P→ H 0.6493 0.3197 4.5408 273.04 LMCT/ILCT/LLCT [Pd(L1)Cl2] 7 P→ H 0.8746 0.3549 4.4265 280.10 LMCT/ILCT/LLCT [Pt(L1)Cl2] 6 P→ H 0.9330 0.3691 4.4592 278.04 MLCT/ILCT [Cu(L1)Cl2] 10 P→ H 1.7312 0.0305 3.4829 355.98 LMCT/ILCT/LLCT [Zn(L1)Cl2] 1 P→ H 0.9575 0.7015 4.5399 273.10 ILCT P   a stands for “particle” and H stands for “hole.” n   b represents occupied lone pair orbitals.The qualitative and quantitative analyses of the NTOs have revealed photoinduced electronic transitions with CT character in the UV-Vis spectra of the molecules. In gas-phase, the “particle” of L1 is predominantly contributed by lone pair orbitals (n) located on the sulfur and nitrogen atoms in the TSC moiety, while the corresponding “hole” is fairly distributed over the entire molecule and is dominated by π∗-antibonding character. This implies that n→π∗ electronic transitions predominate in the gas-phase ligand. The “particle” and “hole” of L1 in DMSO are both distributed over the entire molecular framework. While the “particle” is comprised ofn and π-type orbitals, the “hole” is dominated by π∗-type orbitals. Therefore, both n→π∗ and π→π∗ electronic transitions are possible in the ligand when present in DMSO as solvent. The DMSO environment thus enhances CT in L1 giving rise to the dramatic increment of its βtot value. The complexation of L1 with different transition metal chlorides introduces new electronic transitions with CT character in the electronic spectrum of L1 and/or modifies the existing ones, owing to the presence of low-lying metal-based orbitals. It is evidenced in Tables 5 and 7 that the dominant electronic transitions in the metal chlorides complexes of L1 studied (except the Zn(II) chloride complex) are dominated by MLCT and/or LMCT electronic transitions, along with LLCT transitions. The introduction of MLCT and LMCT electronic transitions often, but not always, bolsters the first molecular hyperpolarizabilities of organic push-pull systems, as observed in the present study. From the foregoing, the NLO response of MAPTSC can be highly tuned by transition metal chelation.The complexation of L1 with the Ni(II), Pd(II), Pt(II), and Cu(II) chlorides in vacuo leads to “partial resonance” or loss of conjugation in the benzene rings of the “particle” NTOs of the resulting complexes (see Table5). Consequently, ICT in these complexes derived from donor-acceptor electron cloud movement across the π-conjugated pathway is disrupted in vacuo. As such, the D-π-A or push-pull CT process is unlikely to occur in the gas-phase complexes. Nevertheless, the electronic transitions with CT character MLCT, LMCT, and LLCT do occur in these complexes accounting for their first hyperpolarizabilities. The scenario in the gas-phase Zn(II) chloride complex is opposite to that in the aforementioned complexes because π-conjugation is rather reinforced in the benzene rings of both the “particle” and the “hole,” whereas MLCT, LMCT, and LLCT electronic transitions are prohibited. This implies that CT in this complex is almost entirely due to the push-pull mechanism. The absence of the metal-based transitions MLCT and LMCT from the UV-Vis spectrum of the [Zn(L1)Cl2] complex is attributed to the nonparticipation of the fully filledd-orbitals of the Zn(II) ion in electronic transitions. It is clear from the isosurfaces in Table 5 that π-conjugation is unhindered in the benzene rings of the “particle” NTOs of the complexes when present in the DMSO solvent medium. This significantly strengthens their push-pull CT mechanisms, resulting in dramatic first hyperpolarizability increments. It is found that as solvent polarity increases, ICT in the Ni(II), Pd(II), Pt(II), and Cu(II) chlorides complexes of L1 increases, and their first hyperpolarizabilities are increasingly contributed by a synergy of electronic transitions with CT character (MLCT, LMCT, and LLCT) and the donor-acceptor electron cloud movements across intervening π-conjugation paths. Exceptionally, ICT in the Zn(II) chloride complex is found to occur entirely via the push-pull mechanism in both gas and solvent phases.


### 3.5. Natural Bond Orbital (NBO) Analysis

To gain a deeper insight into the nature of conjugative and hyperconjugative interactions responsible for electron delocalization and ICT in L1 and its complexes, natural bond orbital (NBO) analyses have been carried out on the molecules studied using the NBO 3.1 [42] module embedded in Gaussian 09W. The NBO results are presented in Tables 8 and 9. The delocalization of electron density (ED) between occupied Lewis-type (bonding or lone pair) natural bond orbitals (NBOs) and formally unoccupied non-Lewis NBOs (antibonds or Rydberg) correspond to stabilizing donor-acceptor interactions. The stabilization energy, E(2), that results from the interaction of a donor (i) and an acceptor (j) NBO pair is estimated by means of the second-order perturbation theory as shown in (5). The values of E(2) are proportional to the extent of conjugation. Hence, the larger the E(2) value, the more intensive the donor-acceptor NBO interaction and the larger the ICT that occurs in the entire system [23, 43].(5)Eij2=-qF^ij2εj-εi,where q is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies) of donor and acceptor NBOs, respectively, and F^ij is the off-diagonal NBO Fock matrix element. In molecular systems, π-conjugation results from π-electron delocalization due to π→π∗ interactions. While primary hyperconjugation is due to σ→π∗, n→π∗, n→σ∗, or π∗→π∗ interactions, secondary hyperconjugation occurs as a result of σ→σ∗ interactions [11]. The NBO interactions LP(2)O2→π∗(C6-C7), LP(1)O2→π∗(C6-C11), or LP(2)O2→LP∗(1)C6, with E(2) values in the range 16.04–59.60 kcal/mol have clearly confirmed that the −OCH3 group is the electron donor in all of the D-π-A or push-pull systems studied. Intensive π→π∗ conjugative interactions dominate in the benzene rings of all molecules studied, greatly enhancing their CT capabilities and optical nonlinearities. The π-conjugated pathway for the D-π-A structure of each complex is found to be extended from the benzene ring to the TSC moiety by the intensive hyperconjugative interactions π(C9-C10)→π∗(C16-N21), π∗(C16-N21)→π∗(C9-C10), π∗(C16-N21)→π∗(C8-C9), LP(1)N21→σ∗(C9-C16), and LP(1)C9→π∗(C16-N21). These are boldly highlighted in Tables 8 and 9.Table 8
Significant second-order interaction energies (E(2), kcal/mol) between donor and acceptor NBOs of L1 and its Cu(II) and Zn(II) chloride complexes.
L a E ( 2 ) (kcal/mol) [Cu(L1)Cl2]a E ( 2 ) (kcal/mol) b [Zn(L1)Cl2]a E ( 2 ) (kcal/mol) Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO π(C6-C7)→π∗(C8-C9) 22.85 23.77 π(C6-C7)→π∗(C8-C9) 12.11 (12.21) 12.77 (12.90) π(C6-C11)→π∗(C7-C8) 17.82 16.94 π(C6-C7)→π∗(C10-C11) 16.59 16.60 π(C6-C7)→π∗(C10-C11) 8.15 (8.08) 8.13 (8.03) π(C6-C11)→π∗(C9-C10) 25.30 26.24 π(C8-C9)→π∗(C6-C7) 17.00 16.69 π(C8-C9)→π∗(C6-C7) 8.08 (8.02) 7.94 (7.91) π(C7-C8)→π∗(C6-C11) 22.81 23.24 π(C8-C9)→π∗(C10-C11) 21.53 21.77 π(C8-C9)→π∗(C10-C11) 10.56 (10.61) 10.71 (10.74) π(C7-C8)→π∗(C9-C10) 17.03 16.60 π(C10-C11)→π∗(C6-C7) 21.19 21.28 π(C10-C11)→π∗(C6-C7) 10.54 (10.59) 10.54 (10.60) π(C9-C10)→π∗(C6-C11) 17.79 17.12 π(C10-C11)→π∗(C8-C9) 17.49 17.13 π(C10-C11)→π∗(C8-C9) 8.63 (8.57) 8.36 (8.30) π(C9-C10)→π∗(C7-C8) 23.27 22.73 L P ( 2 ) O 2 → π ∗(C6-C7) 30.64 30.75 π(S25-Cu29)→LP∗(1)C24 (150.08) — π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) 3.05 10.62 L P ( 1 ) N 21 → σ ∗ ( C 9 - C 16 ) 11.68 11.15 σ ∗(S25-Cu29)→LP∗(5)Cu29 (1.22) (24.74) π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 34.19 LP(1) N 22 → π ∗(C16-N21) 25.75 23.29 π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 23.05 (23.93) 14.34 (14.73) L P ( 2 ) O 2 → π ∗(C6-C11) 28.78 29.21 LP(1) N 22 → L P ∗ (1)C24 — 130.14 L P ( 2 ) O 2 → π ∗(C6-C7) 16.04 (16.11) 16.10 (16.20) L P ( 1 ) N 21 → L P ∗(6)Zn29 23.71 31.05 LP(1) N 22 → σ ∗ (C24-S25) 48.42 — L P ( 1 ) N 21 → L P ∗(5)Cu29 (27.57) — L P ( 1 ) N 21 → L P ∗(9)Zn29 28.64 30.63 LP(2) S 25 → σ ∗ (N22-C24) 13.92 11.69 L P ( 1 ) N 21 → L P ∗(6)Cu29 12.23 16.4 (37.24) LP(1) N 22 → L P ∗ (1)C24 128.39 135.41 LP(3) S 25 → L P ∗ (1)C24 — 212.55 L P ( 1 ) N 21 → L P ∗(8)Cu29 10.74 18.42 L P ( 1 ) N 22 → π ∗(C16-N21) 22.05 20.45 LP(2) S 25 → σ ∗ (C24-N26) 11.37 9.33 L P ( 1 ) N 21 → L P ∗(9)Cu29 10.42 7.47 L P ( 1 ) S 25 → L P ∗(7)Zn29 0.55 17.41 LP(1) N 26 → L P ∗ (1)C24 — 137.34 LP(1) N 22 → π ∗ (C16-N21) 10.53 (10.40) 9.82 L P ( 2 ) S 25 → L P ∗(7)Zn29 0.80 21.61 LP(1) N 26 → σ ∗ (C24-S25) 46.09 — LP(1) N 22 → π ∗ (C24-S25) 34.87 — LP(1)S25→LP∗(8)Zn29 17.49 1.20 LP(1) N 22 → L P ∗(1)C24 (58.01) 64.46 (64.40) LP(2)S25→LP∗(6)Zn29 69.54 50.55 LP(2)S25→LP∗(6)Cu29 29.82 28.71 LP(2)S25→LP∗(8)Zn29 51.69 10.44 LP(2)S25→LP∗(7)Cu29 20.94 24.77 LP(3)S25→LP∗(6)Zn29 4.59 66.42 LP(3) S 25 → L P ∗ (1)C24 — 31.27 LP(3)S25→LP∗(7)Zn29 6.22 36.18 LP(2) S 25 → L P ∗ (1)C24 — 10.05 (76.21) LP(2) S 25 → σ ∗ (C24-N26) 10.71 — LP(3)S25→LP∗(6)Cu29 — 26.22 LP(2) S 25 → L P ∗ (1)C24 — 38.26 LP(3)S25→LP∗(7)Cu29 — 12.37 LP(3) S 25 → L P ∗ (1)C24 280.74 37.01 LP(1) N 26 → π ∗ (C24-S25) 32.81 — LP(1) N 26 → L P ∗ (1)C24 115.63 162.67 LP(1) N 26 → L P ∗ (1)C24 (59.12) 84.10 (83.87) Refer   a to Figure 2 for atomic numbering. The   b  E(2) values in parentheses are obtained from NBO data for the beta spin orbitals.Table 9
Significant second-order interaction energies (E(2), kcal/mol) between donor and acceptor NBOs of the Ni(II), Pd(II), and Pt(II) chloride complexes of L1.
[Ni(L1)Cl2] E ( 2 ) (kcal/mol) [Pd(L1)Cl2] E ( 2 ) (kcal/mol) [Pt(L1)Cl2] E ( 2 ) (kcal/mol) Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO π(C6-C11)→π∗(C7-C8) — 16.93 π(C6-C7)→π∗(C8-C9) 24.61 — π(C6-C7)→π∗(C8-C9) 24.64 — π(C6-C11)→π∗(C9-C10) — 26.23 π(C6-C7)→π∗(C10-C11) 16.14 — π(C6-C7)→π∗(C10-C11) 16.09 — π(C7-C8)→π∗(C6-C11) — 23.15 π(C6-C11)→π∗(C7-C8) — 17.03 π(C7-C8)→LP∗(1)C6 — 58.75 π(C7-C8)→π∗(C9-C10) — 16.76 π(C6-C11)→π∗(C9-C10) — 26.13 π(C7-C8)→π∗(C9-C10) — 16.70 π(C7-C8)→LP∗(1)C6 57.55 — π(C7-C8)→π∗(C6-C11) — 23.13 π(C8-C9)→π∗(C6-C7) 15.86 — π(C7-C8) → LP(1)C9 42.41 — π(C7-C8)→π∗(C9-C10) — 16.91 π(C8-C9)→π∗(C10-C11) 21.01 — π(C9-C10)→π∗(C6-C11) — 17.00 π(C8-C9)→π∗(C6-C7) 15.92 — π(C9-C10) → LP(1)C11 — 42.48 π(C9-C10)→π∗(C7-C8) — 22.38 π(C8-C9)→π∗(C10-C11) 21.05 — π(C9-C10)→π∗(C7-C8) — 22.52 π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) — 10.57 π(C9-C10)→π∗(C6-C11) — 16.94 π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) — 10.31 π(C10-C11)→LP∗(1)C6 51.66 — π(C9-C10)→π∗(C7-C8) — 22.41 π(C10-C11)→π∗(C6-C7) 21.19 — π(C10-C11) → LP(1)C9 42.27 — π(C10-C11)→π∗(C6-C7) 21.16 — π(C10-C11)→π∗(C8-C9) 17.03 — σ(C16-N21)→LP∗(5)Ni29 11.55 2.80 π(C10-C11)→π∗(C8-C9) 17.14 — σ(C16-N21)→LP∗(5)Pt29 10.59 11.15 σ(C16-N21)→LP∗(7)Ni29 — 10.09 π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 39.26 — π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 33.48 — σ(S25-Ni29)→LP∗(1)C24 11.77 0.80 π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 28.89 π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 24.58 σ ∗(S25-Ni29)→LP∗(5)Ni29 14.56 — σ(S25-Pd29)→σ∗(C24-N26) 10.17 0.51 σ ∗(S25-Pt29)→LP∗(5)Pt29 36.64 38.36 σ ∗(S25-Ni29)→LP∗(6)Ni29 — 20.44 σ ∗(S25-Pd29)→LP∗(5)Pd29 26.54 219.1 LP(2)O2→LP∗(1)C6 — 59.60 LP(2)O2→π∗(C6-C11) — 29.43 σ ∗(S25-Pd29)→LP∗(6)Pd29 — 16.90 LP(2)O2→π∗(C6-C7) 32.30 — LP(2)O2→LP∗(1)C6 58.88 — LP(2)O2→π∗(C6-C7) 32.32 — L P ∗(1)C6→π∗(C7-C8) — 55.77 LP(1)C9→π∗(C7-C8) 75.01 — LP(2)O2→π∗(C6-C11) — 29.38 LP(1)C11→π∗(C9-C10) — 88.87 LP(1)C9→π∗(C10-C11) 64.80 — LP(1)N21→LP∗(5)Pd29 57.82 69.97 LP(1)N21→LP∗(5)Pt29 89.27 95.65 L P ( 1 ) C 9 → π ∗ ( C 16 - N 21 ) 21.78 — LP(1)N21→LP∗(7)Pd29 — 40.41 LP(1)N21→RY∗(5)Pt29 6.41 14.74 LP(1)N21→LP∗(5)Ni29 89.26 56.13 LP(1)N21→RY∗(20)Pd29 1.66 10.41 LP(1)N21→RY∗(6)Pt29 14.99 12.30 LP(1)N21→LP∗(7)Ni29 — 62.68 LP(1) N 22 → π ∗ (C16-N21) 16.99 16.18 LP(1)N21→RY∗(9)Pt29 12.77 8.23 LP(1) N 22 → L P ∗ (1)C24 96.07 111.03 LP(1) N 22 → L P ∗ (1)C24 — 119.38 LP(1)N21→RY∗(10)Pt29 18.13 17.98 LP(1) N 22 → π ∗ (C16-N21) 16.50 15.82 LP(1) N 22 → π ∗ (C24-S25) 26.13 — LP(1)N21→RY∗(12)Pt29 14.82 16.96 LP(2) S 25 → L P ∗ (1)C24 125.33 148.36 LP(1)S25→LP∗(5)Pd29 — 13.31 LP(1)N21→RY∗(13)Pt29 18.11 18.46 LP(2)S25→LP∗(5)Ni29 3.27 12.32 LP(1)S25→LP∗(6)Pd29 — 18.47 LP(1)N21→RY∗(14)Pt29 12.53 17.21 LP(2)S25→σ∗(S25-Ni29) 19.63 — LP(2) S 25 → L P ∗ (1)C24 — 139.93 LP(1)N21→RY∗(16)Pt29 16.89 25.50 LP(1) N 26 → L P ∗ (1)C24 105.34 168.89 LP(1) N 26 → L P ∗ (1)C24 — 169.32 LP(1)N21→RY∗(17)Pt29 2.90 21.51 L P ∗(1)C6→π∗(C7-C8) 58.81 — LP(1) N 26 → π ∗ (C24-S25) 34.62 — LP(1)N21→RY∗(18)Pt29 21.06 24.04 L P ∗(1)C6→π∗(C10-C11) 55.05 — LP(1)Pd29→RY∗(1)N21 12.36 — LP(1)N21→RY∗(19)Pt29 23.29 1.35 L P ∗(5)Ni29→RY∗(1)N21 8.62 10.06 LP(2)Pd29→RY∗(1)N21 1.70 12.99 LP(1)N21→RY∗(20)Pt29 13.73 10.84 L P ∗(5)Ni29→RY∗(4)N21 8.91 11.29 LP(2)Pd29→RY∗(18)N21 — 10.73 LP(1) N 22 → π ∗ (C16-N21) 16.01 15.64 L P ∗(5)Ni29→RY∗(6)N21 3.76 11.94 LP(2)Pd29→RY∗(20)N21 — 10.82 LP(1) N 22 → L P ∗ (1)C24 — 115.02 L P ∗(5)Ni29→RY∗(7)N21 1.84 11.76 L P ∗(5)Pd29→RY∗(1)N21 7.07 35.81 LP(2) S 25 → L P ∗ (1)C24 — 138.8 L P ∗(5)Pd29→RY∗(2)N21 5.82 28.21 LP(1) N 22 → π ∗ (C24-S25) 43.67 — L P ∗(5)Pd29→RY∗(3)N21 4.03 23.20 LP(1) N 26 → L P ∗ (1)C24 — 168.64 L P ∗(5)Pd29→RY∗(5)N21 5.12 20.25 LP(1) N 2 → π ∗ (C24-S25) 52.15 — L P ∗(5)Pd29→RY∗(6)N21 2.98 38.34 LP(1)Pt29→RY∗(1)N21 43.11 42.56 L P ∗(5)Pd29→RY∗(7)N21 0.86 12.60 LP(1)Pt29→RY∗(2)N21 24.12 26.52 L P ∗(5)Pd29→RY∗(9)N21 0.98 12.50 LP(1)Pt29→RY∗(5)N21 31.50 26.69 LP(1)Pt29→RY∗(6)N21 22.21 27.81 LP(1)Pt29→RY∗(1)N21 43.11 27.98 LP(1)Pt29→RY∗(2)N21 24.12 23.92 LP(1)Pt29→RY∗(5)N21 31.50 17.58 LP(1)Pt29→RY∗(6)N21 22.21 18.69 LP(5)Pt29→RY∗(1)N21 23.70 27.98 Refer to Figure2 for atomic numbering.Strong hyperconjugative interactions (italicized in Tables8 and 9) predominate in the TSC moieties of all investigated molecules, giving rise to extensive electron delocalization that greatly improves the SHG efficiencies of the molecules. On the basis of their E(2) values, the strengths of the NBO interactions in all molecules studied are found to increase upon transition from the gas to the solvent phase. This partially accounts for the significant first hyperpolarizability increments of these molecules in the solvent environment. Strong donor-acceptor NBO interactions are found between the donor atoms (N21 and S25) of L1 and the transition metal ions in the MCl2 fragments. The electrons donated by N21 and S25 are received in vacant metal-based lone pair (LP∗) and Rydberg (RY∗) orbitals. On the other hand, back donated electrons from the metal ions are mainly received in RY∗ orbitals on N21. Back donation of electrons is highly significant in the Pd(II) and Pt(II) chloride complexes of L1 and minimal in the rest of the complexes, confirming the results earlier obtained from CDA analysis. Both CDA and NBO analyses have shown that the Pd(II) and Pt(II) ions can function as ambivalent electron donors and acceptors, while the Ni(II), Cu(II), and Zn(II) ions may only act as electron acceptors in their respective complexes investigated herein. It is also clear from NBO analysis that the π-conjugated pathways of these push-pull systems are comprised of the benzene ring and the attached TSC moieties.


## 3.1. The Push-Pull Structures and Optimized Geometries of the Molecules Studied

MAPTSC undergoes rapid and facile thione-thiol tautomerization via the exchange of a proton between the hydrazinic nitrogen and the thionic sulfur of the thiosemicarbazone (TSC) moiety [16]. It can act as an N- and S-donor bidentate neutral ligand (thione tautomer) and as an anionic ligand (deprotonated thiol tautomer) [27]. Much attention in this paper has been focused on the thione tautomer of MAPTSC and its complexes because the thione tautomer is the predominant form of thiosemicarbazones [28]. The push-pull CT mechanisms of the thione tautomer (L1) and thiol tautomer (L2) of MAPTSC are depicted in Figures 1(a) and 1(b), respectively. In both tautomers, the electron donor (D) is the methoxy group (−OCH3). The electron acceptor (A) in L1 and L2 is the hydrazinic N–H and the amino (NH2) groups of the TSC moiety, respectively. As such, the π-conjugated pathway linking A and D in L1 is made up of the benzene ring and the appended azomethine group of the thionic TSC moiety, while that in L2 is comprised mainly of the benzene ring and the two attached conjugated azomethine groups of the thiolic TSC moiety. Hence, the π-conjugated bridge for the D-π-A structure is shorter in L1 than that in L2, and this can lead to the tautomers exhibiting different optical nonlinearities. Based on the length of the π-bridge between A and D, the NLO response of L2 should be stronger than that of L1. A large difference in the optical nonlinearities of the tautomers will render the NLO response of MAPTSC switchable, since L1 and L2 are both stable at room temperature and can be interchanged via thione-thiol tautomerization.Figure 1
Schematic representation of the push-pull mechanisms of the thione and thiol tautomers of MAPTSC, whereA is the electron-acceptor group andD is the electron-donor group.The scenario in the transition metal chloride complexes of MAPTSC is different from that in the free ligands, because L2 loses its thiolic hydrogen via deprotonation as it chelates metal ions. The loss of the thiolic hydrogen and the stability of the chelate ring hinder thione-thiol tautomerization of MAPTSC. As a result, a switchable NLO response based on thione-thiol tautomerism is least expected in the complexes. In this paper, the NLO properties of the Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II) chloride complexes of L1 have been studied. To facilitate the presentation and comprehension of our results, the gas-phase optimized geometries of these molecules visualized using Gauss View 5.0.8 [29] are shown in Figure 2.Figure 2
Optimized geometries of (a) ligand L1, (b) [Ni(L1)Cl2], (c) [Pd(L1)Cl2], (d) [Pt(L1)Cl2], (e) [Cu(L1)Cl2], and (f) [Zn(L1)Cl2] at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
(a) (b) (c) (d) (e) (f)Charge decomposition analysis (CDA) was performed in order to determine the nature of the coordination bonds between L1 and the transition metal ions M(II) present in the metal chloride fragments (MCl2). In addition, CDA was used to investigate the impact L1–MCl2 binding may have on the D-π-A charge transfer mechanisms in the complexes. Furthermore, CDA was used to study the influence of any ambivalent donor-acceptor behavior of the transition metals on the push-pull CT capabilities of the complexes investigated. CDA was first proposed by Dapprich and Frenking [30] and has been generalized by Xiao and Lu [31] from its original formulation. Besides CDA, extended charge decomposition analysis (ECDA) proposed by Gorelsky and coworkers [32] was performed. These methods are capable of providing a deep insight into how electronic charge is transferred between L1 (fragment 1) and MCl2 (fragment 2) in the complexes for charge equilibrium to be attained. Typically, CDA defines two components of intermolecular CT: forward donation (σ-bonding) and back donation or back-bonding (π-bonding). This has made CDA a popular method for estimating the amount of electron delocalization in donor-acceptor complexes [33] and in the donor-acceptor metal-ligand bonds in coordination complexes. The generalized CDA and ECDA were carried out using Multiwfn 3.3.8 [34] based on input files obtained from single-point calculations at B3LYP/6-311G(d,p)/(SDD for metal ions) level of theory. The results of CDA and ECDA on the molecules studied are presented in Table 1.Table 1
ECDA and generalized CDA results for transition metal chloride complexes of L1 calculated using Gaussian output files for single-point calculations at B3LYP/6-311G(d,p)/(SDD for metal ions) level on geometries optimized at B3LYP/6-31++G(d,p)/(SDD for metal ions) level.
C o m p l e x e s a [Ni(L1)Cl2] [Pd(L1)Cl2] [Pt(L1)Cl2] [ C u ( L 1 ) C l 2 ] b [Zn(L1)Cl2]  Gas-phase Frag.1→ Frag.2 donation (d) 0.26654 0.14418 0.21067 0.30532 0.26154 Frag.1← Frag.2 back donation (b) −0.00296 0.04153 0.05289 −0.01265 0.00267 Net Frag.1→ Frag.2 donation (d-b) 0.26950 0.10266 0.15778 0.31797 0.25887 Donation to back donation ratio, |d/b| 89.95478 3.47205 3.98336 24.13581 98.02661 CT(1→2) - CT(2→1) 0.60450 0.39720 0.46260 0.56460 0.41070 Benzene Frag.1→ Frag.2 donation (d) 0.28205 0.15715 0.21982 0.33276 0.29419 Frag.1← Frag.2 back donation (b) −0.00731 0.03463 0.04368 −0.01384 0.00387 Net Frag.1→ Frag.2 donation (d-b) 0.28936 0.12253 0.17615 0.34659 0.29032 Donation to back donation ratio, |d/b| 38.56248 4.53860 5.03318 24.05183 76.07706 CT(1→2) - CT(2→1) 0.69220 0.49480 0.55910 0.66880 0.46970 Ethanol Frag.1→ Frag.2 donation (d) 0.29623 0.16990 0.22919 0.35717 0.32350 Frag.1← Frag.2 back donation (b) −0.00861 0.02929 0.03645 −0.01264 0.00671 Net Frag.1→ Frag.2 donation (d-b) 0.30484 0.14061 0.19274 0.36981 0.31679 Donation to back donation ratio, |d/b| 34.41757 5.80025 6.28753 28.25720 48.20414 CT(1→2) - CT(2→1) 0.78610 0.61200 0.67540 0.78440 0.53030 DMSO Frag.1→ Frag.2 donation (d) 0.29691 0.17013 0.22975 0.35872 0.32508 Frag.1← Frag.2 back donation (b) −0.00853 0.02883 0.03608 −0.01240 0.00691 Net Frag.1→ Frag.2 donation (d-b) 0.30544 0.14130 0.19367 0.37112 0.31817 Donation to back donation ratio, |d/b| 34.82839 5.90087 6.36797 28.93394 47.05196 CT(1→2) - CT(2→1) 0.79060 0.61670 0.68310 0.79150 0.53290 F   a r a g. refers to fragment, where Frag.1 is L1 and Frag.2 is MCl2 [M = Ni(II), Pd(II), Pt(II), Cu(II), and Zn(II)]. T   b h e overall results for alpha and beta electrons are reported for the Cu(II) chloride complex of L1.From the donation to back donation ratios, |d/b| in Table 1, it is obvious that the coordination bonds in the Ni(II), Cu(II), and Zn(II) chloride complexes of L1 are predominantly sigma covalent bonds with very little π-character. The transition metal ions in these complexes act mainly as electron acceptors, since back donation is insignificant compared to donation. The electrons donated by L1 are mainly coming from filled π-bonding orbitals and/or the lone electron pair orbitals of its N- and S-donor atoms and are received in empty orbitals of the transition metal ions. The |d/b| values for [Pd(L1)Cl2] and [Pt(L1)Cl2] clearly show that their coordination bonds have a substantial π-character, which helps to extend the π-conjugated pathway for the push-pull systems to the Pd(II) or Pt(II) ions in the respective complexes. The back donated electrons originate from filled d-orbitals of the transition metal ions and are received in empty π∗-antibonding orbitals on L1. The values of electron donation and back donation have shown that the Pd(II) and Pt(II) ions are capable of ambivalent electron donor-acceptor behaviors in their respective complexes studied in this research endeavor. Such ambivalent electron donor-acceptor capabilities can trigger redox processes that may lead to switchable NLO activities in the complexes. As clear from Table 1, back donation of electrons has witnessed a moderate to drastic increment in the bulk solvents, implying that the solvent environment (especially that created by the highly polar solvents) increases the π-character of the metal-ligand coordination bonds in the complexes investigated.


## 3.2. First Molecular Hyperpolarizabilities of MAPTSC and Its Complexes

The first molecular hyperpolarizabilities (βtot) of MAPTSC and its complexes studied (listed in Table 2) were calculated using (2) based on the x, y, and z components (βx, βy, and βz) of βtot. The values of βx, βy, and βz were computed from the components of the βtot tensor: βxxx, βyxx, βxyy, βyyy, βzxx, βxyz, βzyy, βxzz, βyzz, and βzzz, using (3). The components of the βtot tensor for each molecule studied were obtained via Gaussian 09W calculations at two levels of theory: B3LYP/6-31++G(d,p)/(SDD for metal ions) and B3LYP/6-311++G(d,p)/(SDD for metal ions) in gas and solvent phases. The trends adopted by the βtot values on going from the gas to the solvent phase have been elucidated graphically in Figure 3. The βtot values calculated using the data obtained from Gaussian calculations are usually in atomic units, but those reported in this paper have been converted into electrostatic units using the conversion factor 1 a.u. = 8.6393 × 10−33 esu. Inspection of the values in Table 2 and the graphs in Figure 3 has revealed similar trends adopted by the βtot values of each molecule (calculated at both levels of theory) on passing from the gas to the solvent phase. Except for a few cases, the βtot values calculated at the former level of theory are larger than those calculated at the latter level. Hence, further discussion in this paper regarding the βtot values and associated second-order NLO activities of the molecules is based only on the results obtained at the B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.Table 2
β t o t, βx, βy, and βz values of MAPTSC and the complexes calculated at B3LYP/6-31++G(d,p)/(SDD for metal ions) and B3LYP/6-311++G(d,p)/(SDD for metal ions) levels of theory in different media.
Basis set 6-31++G(d,p)/(SDD for metal ions) 6-311++G(d,p)/ (SDD for metal ions) Molecule β x(a.u.) β y(a.u.) β z(a.u.) β t o t × 10 - 30 (esu) β x(a.u.) β y(a.u.) β z(a.u.) β t o t × 10 - 30 (esu) Gas-phase L1 814.59 198.71 103.02 7.298 731.74 184.26 114.38 6.594 L2 1056.16 −724.81 −60.49 11.079 1016.92 −700.38 −46.18 10.675 [Ni(L1)Cl2] 331.18 −506.92 409.83 6.317 463.27 −528.93 420.36 7.077 [Pd(L1)Cl2] 315.26 −366.04 431.49 5.596 454.30 −382.74 448.43 6.430 [Pt(L1)Cl2] 1460.79 −444.13 502.44 13.887 1635.25 −452.82 521.00 15.334 [Cu(L1)Cl2] −675.03 −363.16 36.41 6.630 −560.28 −376.66 43.37 5.845 [Zn(L1)Cl2] −811.24 −390.90 345.28 8.332 −706.61 −389.50 361.74 7.639 Benzene L1 1282.49 56.10 167.35 11.184 1179.22 45.43 174.07 10.306 L2 1551.36 −1089.99 −89.36 16.398 1502.70 −1060.86 −70.78 15.903 [Ni(L1)Cl2] −823.01 −357.05 543.42 9.062 −666.92 −391.48 557.98 8.239 [Pd(L1)Cl2] 939.71 −327.52 −637.52 10.210 773.32 −357.51 −660.79 9.315 [Pt(L1)Cl2] −573.09 −407.87 −744.78 8.850 −785.46 −425.23 −769.99 10.188 [Cu(L1)Cl2] −2260.96 −238.64 184.96 19.706 −2117.62 −256.13 197.51 18.507 [Zn(L1)Cl2] −1606.20 −364.44 316.52 14.489 −1495.75 −365.53 330.97 13.606 Ethanol L1 2137.32 −269.15 147.01 18.654 2024.98 −291.42 158.79 17.728 L2 2267.08 −1677.29 −143.98 24.395 2205.02 −1645.93 −119.16 23.794 [Ni(L1)Cl2] −3142.76 −179.82 920.82 28.335 −2956.62 −233.21 947.43 26.898 [Pd(L1)Cl2] 3594.07 −415.42 −1047.59 32.541 3379.29 −475.21 −1083.34 30.932 [Pt(L1)Cl2] 1657.19 −276.44 −1112.58 17.409 1397.74 −309.05 −1150.21 15.865 [Cu(L1)Cl2] −7509.45 −198.28 712.29 65.190 −7206.75 −225.86 739.63 62.619 [Zn(L1)Cl2] −3310.72 −198.42 547.88 29.042 −3182.29 −203.79 582.65 28.005 DMSO L1 2206.37 −303.91 144.37 19.282 2094.06 −325.95 156.44 18.359 L2 2315.01 −1719.20 −147.47 24.945 2251.91 −1687.90 −122.31 24.336 [Ni(L1)Cl2] −3329.44 −163.94 955.14 29.958 −3140.90 −219.30 982.93 28.496 [Pd(L1)Cl2] −3565.27 −417.46 935.93 32.049 −3351.90 −483.07 969.63 30.433 [Pt(L1)Cl2] 1848.88 −258.57 −1142.88 18.911 1585.59 −292.58 −1181.51 17.269 [Cu(L1)Cl2] −8098.75 −202.43 757.49 70.295 −7773.97 −230.92 783.55 67.531 [Zn(L1)Cl2] −3437.58 −176.12 553.46 30.119 −3308.23 −181.74 588.93 29.073Figure 3
Trends inβtot values of L1 and its transition metal chloride complexes from gas to solvent phases.The magnitude of theβtot value for MAPTSC calculated in the present work is highly dependent on the tautomer present and on the nature of the surrounding environment. For instance, the βtot value for L1 is 7.298 × 10−30 esu in gas-phase and 19.282 × 10−30 esu in DMSO, while that for L2 is 11.079 × 10−30 esu in gas-phase and 24.945 × 10−30 esu in DMSO. The gas-phase βtot values of the thione-thiol tautomers of MAPTSC are approximately 0.92–1.40 times larger than that ofpara-nitroaniline (7.9 × 10−30 esu [35]), a prototype push-pull molecule [36], and about 19.57–29.71 times larger than that of urea (372.89 ×10−33 esu [37]), indicating that MAPTSC is a potent NLO material. From the βtot values of the thione-thiol tautomers of MAPTSC, it is clear that the second-order NLO response of MAPTSC is potentially switchable especially in polar solvents, due to the presence of a tautomeric form (thiol form) with a higher CT capability. Since thione-thiol equilibrium is readily influenced by factors such as the nature of the solvent, temperature, and concentration [38], the switchable NLO response of MAPTSC can be easily fine-tuned by modifying these factors.The coordination of L1 to different transition metal centers results in significant modifications of itsβtot value. Further modifications of this value are observed in the presence of bulk solvents especially the highly polar ones. Therefore, the first hyperpolarizability vis-a-vis the second-order NLO response of L1 can be highly tuned by coordination to different transition metal ions and by the presence of solvents with different dielectric constants. The complexation of L1 with Zn(II) and Pt(II) chlorides results in a modest to drastic increments of its gas-phase βtot value, respectively. Unexpectedly, the complexation of L1 with Ni(II), Pd(II), and Cu(II) chlorides reduced its βtot value. This is unexpected because electronic transitions involving metal-based orbitals in these complexes are supposed to reinforce their optical nonlinearities.It is clear from theβtot values in Table 2 and the graphs in Figure 3 that the presence of a bulk solvent strengthens the first hyperpolarizability thereby reinforcing the second-order NLO response of each molecule studied to a greater or lesser extent, depending on solvent polarity. In fact, the βtot values of these molecules when present in the nonpolar solvent (benzene) and in the polar solvents (ethanol and DMSO) are larger than the corresponding gas-phase values by factors in the ranges 2.7–13.1 and 3.5–58.6, respectively. Exceptionally, the βtot value for the Pt(II) chloride complex in benzene dropped below the gas-phase value by a factor of about 5. Based on our βtot values, it is clear that all of the molecules investigated are likely to exhibit their greatest second-order NLO activities in DMSO, which has the largest dielectric constant among the solvents used. Consequently, further discussion pertaining to solvent effects on βtot and second-order NLO properties of MAPTSC and its complexes has been restricted to the results obtained in DMSO. As clearly shown by the graphs in Figure 3, the magnitudes of the βtot values of the molecules studied are in the order [Pd(L1)Cl2] < [Ni(L1)Cl2] < [Cu(L1)Cl2] < L1 < [Zn(L1)Cl2] < [Pt(L1)Cl2] in vacuum and [Pt(L1)Cl2] < L1 < [Ni(L1)Cl2] < [Zn(L1)Cl2] < [Pd(L1)Cl2] < [Cu(L1)Cl2] in DMSO.To determine the plausible CT mechanisms responsible for the first hyperpolarizabilities and hence quadratic NLO activities of the molecules investigated, the relationship between the frontier molecular orbitals (HOMO-LUMO) energy gaps and theβtot values has been scrutinized. Since transition metals are capable of inducing low-energy MLCT and LMCT transitions that can greatly contribute to the first hyperpolarizability of MAPTSC, photoinduced electronic transitions in the molecules have been studied via the TD-DFT method. Changes in the gas-phase βtot value for L1 as a result of L1–MCl2 binding are attributed solely to the effects of the different transition metal ions present in the MCl2 fragment. Such changes in the presence of both the MCl2 fragment and the bulk solvent are attributable to the concomitant effects of metallation and solvent polarity.


## 3.3. First Molecular Hyperpolarizabilities and HOMO-LUMO Energy Analysis

The HOMO-LUMO energy gaps (ΔEH-L) of L1 and its transition metal chloride complexes, computed at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory in gas-phase and in DMSO are listed in Table 3. The ΔEH-L values for the Cu(II) chloride complex of L1 in both media have been calculated from the eigenvalues of the beta spin orbitals, because a majority of its electronic transitions (analyzed in the next section) involve these orbitals. The D-π-A or push-pull CT mechanism predominates in molecules when an inverse proportionality relationship exists between their ΔEH-L and βtot values. The push-pull mechanism is strongly dependent on HOMO-LUMO energy gaps because a proper electronic conjugation must exist among the molecular orbitals of a system for ICT to be effective [7]. The relationship between the gas-phase ΔEH-L and βtot values of the molecules studied is far from being inversely proportional, implying that the D-π-A push-pull process is not the dominant CT mechanism in each of the molecules in gas-phase. Photoinduced electronic transitions with CT character (involving metal-based orbitals) are certainly playing the dominant role. In DMSO, a somewhat inverse proportionality relationship is found between the values of ΔEH-Land βtot, suggesting that CT in these molecules is due to the combined effects of photoinduced electronic transitions with CT character and the D-π-A push-pull process. The polar solvent medium therefore enhances charge transfer in each of the molecules investigated.Table 3
HOMO/LUMO energies andβtot values of MAPTSC and its metal chloride complexes computed in gas-phase and DMSO at B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Molecule Energies in gas-phase (eV) β t o t × 10 - 30 (esu) Energies in DMSO (eV) β t o t × 10 - 30 (esu) H O M O a energy L U M O b energy Δ E H - L c H O M O a energy L U M O b energy Δ E H - L c L1 −5.481 −1.189 4.292 7.298 −6.134 −1.334 4.801 19.282 [Ni(L1)Cl2] −5.815 −2.397 3.418 6.317 −6.589 −2.797 3.791 29.958 [Pd(L1)Cl2] −5.895 −2.496 3.399 5.596 −6.574 −3.010 3.564 32.049 [Pt(L1)Cl2] −5.715 −2.386 3.329 13.887 −6.272 −2.350 3.923 18.911 [Cu(L1)Cl2] −6.240 −3.575 2.665 6.630 −6.723 −4.043 2.679 70.295 [Zn(L1)Cl2] −6.744 −2.193 4.551 8.332 −6.621 −2.014 4.607 30.119 HOMO   a stands for highest occupied molecular orbital. L U M O   b stands for lowest unoccupied molecular orbital. Δ E H - L   c represents HOMO-LUMO energy gap.


## 3.4. Electronic Absorption Spectra of L1 and Its Transition Metal Chloride Complexes

In order to determine the optical transparency of each investigated molecule in the visible region of the EM spectrum and to determine the electronic transitions with CT character, the UV-Vis spectra of these molecules were simulated via TD-DFT studies. To study solvatochromic effects, the UV-Vis spectra were calculated in both gas-phase and DMSO. For each molecule, electronic transitions to the first 10 low-lying singlet excited states were studied at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory. The computed electronic excitation energies (ΔE in eV), wavelengths (λ in nm), and oscillator strengths (f) are presented in Table 4. The electronic transitions with oscillator strengths 0.02 and above have been considered, since those with very low oscillator strengths are likely to make insignificant contributions to the first hyperpolarizabilities of the molecules studied. The UV-Vis spectra of these molecules in gas and solvent phases are presented in Figure 4.Table 4
Absorption energies (E, eV), wavelengths (λ, nm), and oscillator strengths (f) of L1 and the complexes, calculated at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory in gas-phase and DMSO by the TD-DFT method.
Molecule Gas-phase DMSO State Δ E (eV) λ (nm) f State Δ E (eV) λ (nm) f L1 S2 4.6892 264.40 0.4821 S2 4.7279 262.24 0.7898 S3 5.0039 247.77 0.0203 S3 5.0309 246.44 0.0231 S4 5.0404 245.98 0.0401 S4 5.2409 236.57 0.0406 S6 5.3192 233.09 0.0435 S5 5.6131 220.88 0.0294 S8 5.6300 220.22 0.0826 S6 5.6917 217.83 0.1009 S10 5.7011 217.47 0.0717 S7 5.7689 214.92 0.0329 S8 5.9143 209.63 0.2104 S9 5.9352 208.90 0.0676 S10 5.9525 208.29 0.0210  [Ni(L1)Cl2] S7 3.8661 320.70 0.0336 S5 3.8202 324.55 0.0377 S9 4.1644 297.73 0.0342 S6 4.0588 305.47 0.0206 S7 4.4403 279.23 0.2780 S8 4.5408 273.04 0.3197 S9 4.6220 268.25 0.0666  [Pd(L1)Cl2] S6 3.9119 316.94 0.0488 S5 3.8273 323.95 0.0376 S8 4.1873 296.10 0.0557 S7 4.4265 280.10 0.3549 S10 4.3174 287.17 0.0320 S8 4.5383 273.20 0.2687 S9 4.7574 260.61 0.0802 S10 4.7830 259.22 0.0345  [Pt(L1)Cl2] S5 3.5049 353.75 0.0293 S5 4.1907 295.85 0.2784 S6 3.8361 323.20 0.1224 S6 4.4592 278.04 0.3691 S7 3.8860 319.05 0.0368 S7 4.5295 273.72 0.0466 S8 3.9611 313.00 0.0243 S8 4.7188 262.75 0.0219 S9 4.0806 303.84 0.0292  [Cu(L1)Cl2] S10 3.4788 356.40 0.0330 S5 2.5892 478.86 0.0268 S8 3.2291 383.96 0.0267 S10 3.4829 355.98 0.0305  [Zn(L1)Cl2] S2 4.7424 261.44 0.5390 S1 4.5399 273.10 0.7015 S4 5.1784 239.43 0.0285 S3 5.1949 238.67 0.0271 S10 5.8132 213.28 0.0202 S5 5.7561 215.39 0.1552 S6 5.8884 210.56 0.1346 S7 5.9773 207.43 0.1919 S10 6.2838 197.31 0.0746Figure 4
The electronic absorption spectra of L1 and its complexes in gas-phase and DMSO computed at CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.The modification undergone by theβtot value for L1 upon complexation with different transition metal chlorides appears to be related to the types and intensities (oscillator strengths) of the electronic excitations in the complexes formed. The reduction in the βtot value for L1 following its complexation with Ni(II), Pd(II), and Cu(II) chlorides in vacuo is apparently linked to the reduction in the number of electronic transitions in the resulting complexes, coupled with a decrease in the oscillator strengths of the transitions. Although complexation leads to a slight lowering of the electronic excitation energies of the dominant electronic transitions in the complexes [Ni(L1)Cl2], [Pd(L1)Cl2], and [Cu(L1)Cl2], a corresponding increase in their βtot values is not observed. This is probably due to the significant intensity reduction of these transitions relative to those in L1. On the contrary, the complexation of L1 with the Pt(II) chloride in vacuum results in a dramatic increment of the ligand’s gas-phase βtot value, which can be attributed to the introduction of several low-energy intense electronic transitions.By inspection of Figure4 and Table 4, it can be seen that the number of reasonably strong electronic transitions in the molecules studied increases significantly in DMSO. Moreover, the polar DMSO medium leads to a modest lowering of the excitation energies of the dominant electronic transitions in all complexes investigated, except the Pt(II) chloride complex. Therefore, the [Ni(L1)Cl2], [Pd(L1)Cl2], [Cu(L1)Cl2], and [Zn(L1)Cl2] complexes must possess highly polar excited states that are greatly solvated and stabilized by the polar DMSO molecules, whereas their ground states are less polar and hence less stabilized by solvation. Consequently, excitation energies from the less stabilized ground states to the highly stabilized excited states are reduced, leading to an increase in the number of intense electronic excitations in the complexes. This in turn causes a dramatic increment in the βtot values of the complexes in DMSO as solvent, thus enhancing their quadratic NLO responses. Strong absorption of light in the visible region of the EM spectrum restricts the NLO applications of materials [39] due to significant transparency/nonlinearity trade-offs that often lead to lower device efficiency and reduced photostability. From Figure 4, it can be seen that, apart from the [Cu(L1)Cl2] complex, all investigated molecules have high optical transparencies in the entire visible region, implying that they suffer very low optical losses. Hence, transparency/nonlinearity trade-offs are highly minimized in L1 and its Ni(II), Pd(II), Pt(II), and Zn(II) chloride complexes, confirming their suitability for the fabrication of NLO devices.To decipher the relationship between solvent polarity and the first hyperpolarizabilities of the molecules studied, their UV-Vis spectra in gas-phase and in DMSO have been compared as shown in Figure4. It is clear from Figure 3 that polar solvents substantially increase the βtot value for the Cu(II) chloride complex of L1 but have almost no effect on the βtot value for the Pt(II) chloride complex. The effects of the polar solvents on the βtot values for these molecules have been explained in terms of the solvatochromic shift directions of their strongest electronic transitions on going from the gas-phase to the DMSO solvent medium, as elucidated in Figure 4. The drastic increment in the βtot value for the Cu(II) chloride complex in DMSO is ascribed to its positive solvatochromism (the shift of its strongest CT absorption bands towards lower excitation energies and longer wavelengths). The remarkable negative solvatochromism exhibited by the Pt(II) chloride complex in DMSO (the shift of its strongest CT absorption bands towards higher excitation energies and shorter wavelengths) accounts for the insignificant modification of its βtot value in the polar solvents. The positive solvatochromism (bathochromic or red-shift of the strongest CT absorption bands) exhibited by the [Zn(L1)Cl2] complex is responsible for the increment in its βtot value in DMSO. Solvatochromism as such is not obvious from the comparison of the gas and solvent phase UV-Vis spectra of L1 in Figure 4. The increment in its βtot value in DMSO is certainly linked to the increase in the intensities of its strong electronic excitations. Although the complexes [Ni(L1)Cl2] and [Pd(L1)Cl2] display a certain degree of negative solvatochromism in DMSO (hypsochromic or blue-shift of their strongest CT absorption bands), appreciable increments in their βtot values are observed due to significant contributions from strong low-lying electronic transitions of CT character to their first hyperpolarizabilities.By comparing the gas-phase UV-Vis absorption spectra of ligand L1 with those of its metal chloride complexes, it is clear that metallation results in significant modifications of the ligand’s electronic absorption spectrum. These modifications are caused by the introduction of CT bands induced by the promotion of electrons from orbitals that are predominantly metal in character to orbitals that are predominantly ligand in character (MLCT), or the reverse (LMCT). These CT bands generally give significant contributions to the first molecular hyperpolarizability. Such CT transitions are absent from the UV-Vis spectrum of the Zn(II) chloride complex because thed-orbitals of the Zn(II) ion are fully filled and do not participate in electronic transitions.The dominant electronic transitions of the molecules studied (those with the greatest oscillator strengths) have been characterized via natural transition orbital (NTO) analysis which generates unique “particle-hole” NTO pairs from the canonical molecular orbitals. Only the dominant electronic transitions (those boldly highlighted in Table4) have been characterized because they are capable of remarkable contributions to the first molecular hyperpolarizability. For each “particle-hole” NTO pair, the “particle” shows where in the system electrons come from and the “hole” indicates where the electrons go to, during electronic transitions. The NTOs of the molecules studied (shown in Table 5) were computed using Gaussian 09W and then rendered with VMD 1.9.2 [40] using isosurface cube files generated by Multiwfn 3.3.8. To obtain a quantitative description of the NTOs, orbital contributions from different molecular fragments (reported in Table 6) were calculated using the orbital composition analysis option [41] of Multiwfn 3.3.8. These orbital compositions, together with the electronic distributions of the NTOs (depicted by their isosurfaces in Table 5) were exploited in characterizing the dominant electronic transitions of the molecules studied. These transitions along with their most probable characters are listed in Table 7.Table 5
The dominant NTO “particle-hole” pairs for the dominant electronic transitions of each investigated molecule in gas-phase and in DMSO.
Molecule Gas-phase DMSO Particle Hole Particle Hole L1 [Ni(L1)Cl2] [Pd(L1)Cl2] [Pt(L1)Cl2] [Cu(L1)Cl2] [Zn(L1)Cl2]Table 6
Natural transition orbitals (NTOs) compositions for the thione tautomer of MAPTSC and its complexesin gas-phase and DMSO, calculated at the TD-DFT/CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Molecule NTOs Orbital compositions of fragments (%) Main distribution of NTOs on molecular systems M ( I I ) a M A P b T S C c 2Cl- Gas-phase L1 Hole — 36.496 63.504 — MAP + TSC Particle — 13.856 86.144 — TSC [Ni(L1)Cl2] Hole 4.537 50.724 43.388 1.351 MAP + TSC Particle 46.092 2.627 9.700 41.581 Ni(II) ion + 2Cl- [Pd(L1)Cl2] Hole 3.958 49.695 45.108 1.239 MAP + TSC Particle 32.732 4.901 6.195 56.172 Pd(II) ion + 2Cl- [Pt(L1)Cl2] Hole 5.517 43.596 49.385 1.501 MAP + TSC Particle 56.516 5.890 7.933 29.661 Pt(II) ion + 2Cl- [Cu(L1)Cl2] Hole 59.034 3.915 11.890 25.160 Cu(II) ion + 2Cl- Particle 8.056 3.214 66.050 22.679 TSC + 2Cl- [Zn(L1)Cl2] Hole 1.873 45.358 51.649 1.119 MAP + TSC Particle 1.731 34.905 57.951 5.413 MAP + TSC DMSO L1 Hole — 41.677 58.323 — MAP + TSC Particle — 29.818 70.182 — MAP + TSC [Ni(L1)Cl2] Hole 19.896 37.686 36.486 5.932 Ni(II) ion + MAP + TSC Particle 6.189 40.681 35.610 17.520 MAP + TSC + 2Cl- [Pd(L1)Cl2] Hole 34.365 25.958 24.693 14.984 Pd(II) ion + MAP + TSC + 2Cl- Particle 1.806 61.076 23.100 14.018 MAP + TSC + 2Cl- [Pt(L1)Cl2] Hole 5.065 60.271 33.556 1.108 MAP + TSC Particle 30.291 52.437 12.099 5.173 Pt(II) ion + MAP [Cu(L1)Cl2] Hole 61.952 2.016 17.871 18.162 Cu(II) ion + TSC + 2Cl- Particle 9.275 22.645 20.973 47.107 MAP + TSC + 2Cl- [Zn(L1)Cl2] Hole 1.932 61.449 35.673 0.945 MAP + TSC Particle 0.714 70.615 27.791 0.880 MAP + TSC M   a(II) represents the transition metal ions. M A P   b is the 4-methoxyacetophenone moiety. TSC   c is the thiosemicarbazones moiety.Table 7
Excitation energies and wavelengths, oscillator strengths, NTO eigenvalues, and characters of the dominant electronic transitions for MAPTSC and its complexesin gas-phase and DMSO, calculated at the TD-DFT/CAM-B3LYP/6-31++G(d,p)/(SDD for metal ions) level of theory.
Medium Molecule Excited state Electronictransitiona NTO eigenvalue f Δ E (eV) λ m a x (nm) C h a r a c t e r b Gas-phase L1 2 P→ H 0.8866 0.4821 4.6892 264.40 n → π ∗ [Ni(L1)Cl2] 9 P→ H 0.9299 0.0342 4.1644 297.73 MLCT/LLCT [Pd(L1)Cl2] 8 P→ H 0.9261 0.0557 4.1873 296.10 MLCT/LLCT [Pt(L1)Cl2] 6 P→ H 0.9731 0.1224 3.8361 323.20 MLCT/LLCT [Cu(L1)Cl2] 10 P→ H 1.6191 0.0330 3.4788 356.40 LMCT/LLCT [Zn(L1)Cl2] 2 P→ H 0.9265 0.5390 4.7424 261.44 ILCT  DMSO L1 2 P→ H 0.9190 0.7898 4.7279 262.24 n → π ∗/π→π∗ [Ni(L1)Cl2] 8 P→ H 0.6493 0.3197 4.5408 273.04 LMCT/ILCT/LLCT [Pd(L1)Cl2] 7 P→ H 0.8746 0.3549 4.4265 280.10 LMCT/ILCT/LLCT [Pt(L1)Cl2] 6 P→ H 0.9330 0.3691 4.4592 278.04 MLCT/ILCT [Cu(L1)Cl2] 10 P→ H 1.7312 0.0305 3.4829 355.98 LMCT/ILCT/LLCT [Zn(L1)Cl2] 1 P→ H 0.9575 0.7015 4.5399 273.10 ILCT P   a stands for “particle” and H stands for “hole.” n   b represents occupied lone pair orbitals.The qualitative and quantitative analyses of the NTOs have revealed photoinduced electronic transitions with CT character in the UV-Vis spectra of the molecules. In gas-phase, the “particle” of L1 is predominantly contributed by lone pair orbitals (n) located on the sulfur and nitrogen atoms in the TSC moiety, while the corresponding “hole” is fairly distributed over the entire molecule and is dominated by π∗-antibonding character. This implies that n→π∗ electronic transitions predominate in the gas-phase ligand. The “particle” and “hole” of L1 in DMSO are both distributed over the entire molecular framework. While the “particle” is comprised ofn and π-type orbitals, the “hole” is dominated by π∗-type orbitals. Therefore, both n→π∗ and π→π∗ electronic transitions are possible in the ligand when present in DMSO as solvent. The DMSO environment thus enhances CT in L1 giving rise to the dramatic increment of its βtot value. The complexation of L1 with different transition metal chlorides introduces new electronic transitions with CT character in the electronic spectrum of L1 and/or modifies the existing ones, owing to the presence of low-lying metal-based orbitals. It is evidenced in Tables 5 and 7 that the dominant electronic transitions in the metal chlorides complexes of L1 studied (except the Zn(II) chloride complex) are dominated by MLCT and/or LMCT electronic transitions, along with LLCT transitions. The introduction of MLCT and LMCT electronic transitions often, but not always, bolsters the first molecular hyperpolarizabilities of organic push-pull systems, as observed in the present study. From the foregoing, the NLO response of MAPTSC can be highly tuned by transition metal chelation.The complexation of L1 with the Ni(II), Pd(II), Pt(II), and Cu(II) chlorides in vacuo leads to “partial resonance” or loss of conjugation in the benzene rings of the “particle” NTOs of the resulting complexes (see Table5). Consequently, ICT in these complexes derived from donor-acceptor electron cloud movement across the π-conjugated pathway is disrupted in vacuo. As such, the D-π-A or push-pull CT process is unlikely to occur in the gas-phase complexes. Nevertheless, the electronic transitions with CT character MLCT, LMCT, and LLCT do occur in these complexes accounting for their first hyperpolarizabilities. The scenario in the gas-phase Zn(II) chloride complex is opposite to that in the aforementioned complexes because π-conjugation is rather reinforced in the benzene rings of both the “particle” and the “hole,” whereas MLCT, LMCT, and LLCT electronic transitions are prohibited. This implies that CT in this complex is almost entirely due to the push-pull mechanism. The absence of the metal-based transitions MLCT and LMCT from the UV-Vis spectrum of the [Zn(L1)Cl2] complex is attributed to the nonparticipation of the fully filledd-orbitals of the Zn(II) ion in electronic transitions. It is clear from the isosurfaces in Table 5 that π-conjugation is unhindered in the benzene rings of the “particle” NTOs of the complexes when present in the DMSO solvent medium. This significantly strengthens their push-pull CT mechanisms, resulting in dramatic first hyperpolarizability increments. It is found that as solvent polarity increases, ICT in the Ni(II), Pd(II), Pt(II), and Cu(II) chlorides complexes of L1 increases, and their first hyperpolarizabilities are increasingly contributed by a synergy of electronic transitions with CT character (MLCT, LMCT, and LLCT) and the donor-acceptor electron cloud movements across intervening π-conjugation paths. Exceptionally, ICT in the Zn(II) chloride complex is found to occur entirely via the push-pull mechanism in both gas and solvent phases.


## 3.5. Natural Bond Orbital (NBO) Analysis

To gain a deeper insight into the nature of conjugative and hyperconjugative interactions responsible for electron delocalization and ICT in L1 and its complexes, natural bond orbital (NBO) analyses have been carried out on the molecules studied using the NBO 3.1 [42] module embedded in Gaussian 09W. The NBO results are presented in Tables 8 and 9. The delocalization of electron density (ED) between occupied Lewis-type (bonding or lone pair) natural bond orbitals (NBOs) and formally unoccupied non-Lewis NBOs (antibonds or Rydberg) correspond to stabilizing donor-acceptor interactions. The stabilization energy, E(2), that results from the interaction of a donor (i) and an acceptor (j) NBO pair is estimated by means of the second-order perturbation theory as shown in (5). The values of E(2) are proportional to the extent of conjugation. Hence, the larger the E(2) value, the more intensive the donor-acceptor NBO interaction and the larger the ICT that occurs in the entire system [23, 43].(5)Eij2=-qF^ij2εj-εi,where q is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies) of donor and acceptor NBOs, respectively, and F^ij is the off-diagonal NBO Fock matrix element. In molecular systems, π-conjugation results from π-electron delocalization due to π→π∗ interactions. While primary hyperconjugation is due to σ→π∗, n→π∗, n→σ∗, or π∗→π∗ interactions, secondary hyperconjugation occurs as a result of σ→σ∗ interactions [11]. The NBO interactions LP(2)O2→π∗(C6-C7), LP(1)O2→π∗(C6-C11), or LP(2)O2→LP∗(1)C6, with E(2) values in the range 16.04–59.60 kcal/mol have clearly confirmed that the −OCH3 group is the electron donor in all of the D-π-A or push-pull systems studied. Intensive π→π∗ conjugative interactions dominate in the benzene rings of all molecules studied, greatly enhancing their CT capabilities and optical nonlinearities. The π-conjugated pathway for the D-π-A structure of each complex is found to be extended from the benzene ring to the TSC moiety by the intensive hyperconjugative interactions π(C9-C10)→π∗(C16-N21), π∗(C16-N21)→π∗(C9-C10), π∗(C16-N21)→π∗(C8-C9), LP(1)N21→σ∗(C9-C16), and LP(1)C9→π∗(C16-N21). These are boldly highlighted in Tables 8 and 9.Table 8
Significant second-order interaction energies (E(2), kcal/mol) between donor and acceptor NBOs of L1 and its Cu(II) and Zn(II) chloride complexes.
L a E ( 2 ) (kcal/mol) [Cu(L1)Cl2]a E ( 2 ) (kcal/mol) b [Zn(L1)Cl2]a E ( 2 ) (kcal/mol) Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO π(C6-C7)→π∗(C8-C9) 22.85 23.77 π(C6-C7)→π∗(C8-C9) 12.11 (12.21) 12.77 (12.90) π(C6-C11)→π∗(C7-C8) 17.82 16.94 π(C6-C7)→π∗(C10-C11) 16.59 16.60 π(C6-C7)→π∗(C10-C11) 8.15 (8.08) 8.13 (8.03) π(C6-C11)→π∗(C9-C10) 25.30 26.24 π(C8-C9)→π∗(C6-C7) 17.00 16.69 π(C8-C9)→π∗(C6-C7) 8.08 (8.02) 7.94 (7.91) π(C7-C8)→π∗(C6-C11) 22.81 23.24 π(C8-C9)→π∗(C10-C11) 21.53 21.77 π(C8-C9)→π∗(C10-C11) 10.56 (10.61) 10.71 (10.74) π(C7-C8)→π∗(C9-C10) 17.03 16.60 π(C10-C11)→π∗(C6-C7) 21.19 21.28 π(C10-C11)→π∗(C6-C7) 10.54 (10.59) 10.54 (10.60) π(C9-C10)→π∗(C6-C11) 17.79 17.12 π(C10-C11)→π∗(C8-C9) 17.49 17.13 π(C10-C11)→π∗(C8-C9) 8.63 (8.57) 8.36 (8.30) π(C9-C10)→π∗(C7-C8) 23.27 22.73 L P ( 2 ) O 2 → π ∗(C6-C7) 30.64 30.75 π(S25-Cu29)→LP∗(1)C24 (150.08) — π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) 3.05 10.62 L P ( 1 ) N 21 → σ ∗ ( C 9 - C 16 ) 11.68 11.15 σ ∗(S25-Cu29)→LP∗(5)Cu29 (1.22) (24.74) π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 34.19 LP(1) N 22 → π ∗(C16-N21) 25.75 23.29 π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 23.05 (23.93) 14.34 (14.73) L P ( 2 ) O 2 → π ∗(C6-C11) 28.78 29.21 LP(1) N 22 → L P ∗ (1)C24 — 130.14 L P ( 2 ) O 2 → π ∗(C6-C7) 16.04 (16.11) 16.10 (16.20) L P ( 1 ) N 21 → L P ∗(6)Zn29 23.71 31.05 LP(1) N 22 → σ ∗ (C24-S25) 48.42 — L P ( 1 ) N 21 → L P ∗(5)Cu29 (27.57) — L P ( 1 ) N 21 → L P ∗(9)Zn29 28.64 30.63 LP(2) S 25 → σ ∗ (N22-C24) 13.92 11.69 L P ( 1 ) N 21 → L P ∗(6)Cu29 12.23 16.4 (37.24) LP(1) N 22 → L P ∗ (1)C24 128.39 135.41 LP(3) S 25 → L P ∗ (1)C24 — 212.55 L P ( 1 ) N 21 → L P ∗(8)Cu29 10.74 18.42 L P ( 1 ) N 22 → π ∗(C16-N21) 22.05 20.45 LP(2) S 25 → σ ∗ (C24-N26) 11.37 9.33 L P ( 1 ) N 21 → L P ∗(9)Cu29 10.42 7.47 L P ( 1 ) S 25 → L P ∗(7)Zn29 0.55 17.41 LP(1) N 26 → L P ∗ (1)C24 — 137.34 LP(1) N 22 → π ∗ (C16-N21) 10.53 (10.40) 9.82 L P ( 2 ) S 25 → L P ∗(7)Zn29 0.80 21.61 LP(1) N 26 → σ ∗ (C24-S25) 46.09 — LP(1) N 22 → π ∗ (C24-S25) 34.87 — LP(1)S25→LP∗(8)Zn29 17.49 1.20 LP(1) N 22 → L P ∗(1)C24 (58.01) 64.46 (64.40) LP(2)S25→LP∗(6)Zn29 69.54 50.55 LP(2)S25→LP∗(6)Cu29 29.82 28.71 LP(2)S25→LP∗(8)Zn29 51.69 10.44 LP(2)S25→LP∗(7)Cu29 20.94 24.77 LP(3)S25→LP∗(6)Zn29 4.59 66.42 LP(3) S 25 → L P ∗ (1)C24 — 31.27 LP(3)S25→LP∗(7)Zn29 6.22 36.18 LP(2) S 25 → L P ∗ (1)C24 — 10.05 (76.21) LP(2) S 25 → σ ∗ (C24-N26) 10.71 — LP(3)S25→LP∗(6)Cu29 — 26.22 LP(2) S 25 → L P ∗ (1)C24 — 38.26 LP(3)S25→LP∗(7)Cu29 — 12.37 LP(3) S 25 → L P ∗ (1)C24 280.74 37.01 LP(1) N 26 → π ∗ (C24-S25) 32.81 — LP(1) N 26 → L P ∗ (1)C24 115.63 162.67 LP(1) N 26 → L P ∗ (1)C24 (59.12) 84.10 (83.87) Refer   a to Figure 2 for atomic numbering. The   b  E(2) values in parentheses are obtained from NBO data for the beta spin orbitals.Table 9
Significant second-order interaction energies (E(2), kcal/mol) between donor and acceptor NBOs of the Ni(II), Pd(II), and Pt(II) chloride complexes of L1.
[Ni(L1)Cl2] E ( 2 ) (kcal/mol) [Pd(L1)Cl2] E ( 2 ) (kcal/mol) [Pt(L1)Cl2] E ( 2 ) (kcal/mol) Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO Donor(i) → acceptor(j) Gas-phase DMSO π(C6-C11)→π∗(C7-C8) — 16.93 π(C6-C7)→π∗(C8-C9) 24.61 — π(C6-C7)→π∗(C8-C9) 24.64 — π(C6-C11)→π∗(C9-C10) — 26.23 π(C6-C7)→π∗(C10-C11) 16.14 — π(C6-C7)→π∗(C10-C11) 16.09 — π(C7-C8)→π∗(C6-C11) — 23.15 π(C6-C11)→π∗(C7-C8) — 17.03 π(C7-C8)→LP∗(1)C6 — 58.75 π(C7-C8)→π∗(C9-C10) — 16.76 π(C6-C11)→π∗(C9-C10) — 26.13 π(C7-C8)→π∗(C9-C10) — 16.70 π(C7-C8)→LP∗(1)C6 57.55 — π(C7-C8)→π∗(C6-C11) — 23.13 π(C8-C9)→π∗(C6-C7) 15.86 — π(C7-C8) → LP(1)C9 42.41 — π(C7-C8)→π∗(C9-C10) — 16.91 π(C8-C9)→π∗(C10-C11) 21.01 — π(C9-C10)→π∗(C6-C11) — 17.00 π(C8-C9)→π∗(C6-C7) 15.92 — π(C9-C10) → LP(1)C11 — 42.48 π(C9-C10)→π∗(C7-C8) — 22.38 π(C8-C9)→π∗(C10-C11) 21.05 — π(C9-C10)→π∗(C7-C8) — 22.52 π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) — 10.57 π(C9-C10)→π∗(C6-C11) — 16.94 π ( C 9 - C 10 ) → π ∗ ( C 16 - N 21 ) — 10.31 π(C10-C11)→LP∗(1)C6 51.66 — π(C9-C10)→π∗(C7-C8) — 22.41 π(C10-C11)→π∗(C6-C7) 21.19 — π(C10-C11) → LP(1)C9 42.27 — π(C10-C11)→π∗(C6-C7) 21.16 — π(C10-C11)→π∗(C8-C9) 17.03 — σ(C16-N21)→LP∗(5)Ni29 11.55 2.80 π(C10-C11)→π∗(C8-C9) 17.14 — σ(C16-N21)→LP∗(5)Pt29 10.59 11.15 σ(C16-N21)→LP∗(7)Ni29 — 10.09 π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 39.26 — π ∗ ( C 16 - N 21 ) → π ∗ ( C 8 - C 9 ) 33.48 — σ(S25-Ni29)→LP∗(1)C24 11.77 0.80 π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 28.89 π ∗ ( C 16 - N 21 ) → π ∗ ( C 9 - C 10 ) — 24.58 σ ∗(S25-Ni29)→LP∗(5)Ni29 14.56 — σ(S25-Pd29)→σ∗(C24-N26) 10.17 0.51 σ ∗(S25-Pt29)→LP∗(5)Pt29 36.64 38.36 σ ∗(S25-Ni29)→LP∗(6)Ni29 — 20.44 σ ∗(S25-Pd29)→LP∗(5)Pd29 26.54 219.1 LP(2)O2→LP∗(1)C6 — 59.60 LP(2)O2→π∗(C6-C11) — 29.43 σ ∗(S25-Pd29)→LP∗(6)Pd29 — 16.90 LP(2)O2→π∗(C6-C7) 32.30 — LP(2)O2→LP∗(1)C6 58.88 — LP(2)O2→π∗(C6-C7) 32.32 — L P ∗(1)C6→π∗(C7-C8) — 55.77 LP(1)C9→π∗(C7-C8) 75.01 — LP(2)O2→π∗(C6-C11) — 29.38 LP(1)C11→π∗(C9-C10) — 88.87 LP(1)C9→π∗(C10-C11) 64.80 — LP(1)N21→LP∗(5)Pd29 57.82 69.97 LP(1)N21→LP∗(5)Pt29 89.27 95.65 L P ( 1 ) C 9 → π ∗ ( C 16 - N 21 ) 21.78 — LP(1)N21→LP∗(7)Pd29 — 40.41 LP(1)N21→RY∗(5)Pt29 6.41 14.74 LP(1)N21→LP∗(5)Ni29 89.26 56.13 LP(1)N21→RY∗(20)Pd29 1.66 10.41 LP(1)N21→RY∗(6)Pt29 14.99 12.30 LP(1)N21→LP∗(7)Ni29 — 62.68 LP(1) N 22 → π ∗ (C16-N21) 16.99 16.18 LP(1)N21→RY∗(9)Pt29 12.77 8.23 LP(1) N 22 → L P ∗ (1)C24 96.07 111.03 LP(1) N 22 → L P ∗ (1)C24 — 119.38 LP(1)N21→RY∗(10)Pt29 18.13 17.98 LP(1) N 22 → π ∗ (C16-N21) 16.50 15.82 LP(1) N 22 → π ∗ (C24-S25) 26.13 — LP(1)N21→RY∗(12)Pt29 14.82 16.96 LP(2) S 25 → L P ∗ (1)C24 125.33 148.36 LP(1)S25→LP∗(5)Pd29 — 13.31 LP(1)N21→RY∗(13)Pt29 18.11 18.46 LP(2)S25→LP∗(5)Ni29 3.27 12.32 LP(1)S25→LP∗(6)Pd29 — 18.47 LP(1)N21→RY∗(14)Pt29 12.53 17.21 LP(2)S25→σ∗(S25-Ni29) 19.63 — LP(2) S 25 → L P ∗ (1)C24 — 139.93 LP(1)N21→RY∗(16)Pt29 16.89 25.50 LP(1) N 26 → L P ∗ (1)C24 105.34 168.89 LP(1) N 26 → L P ∗ (1)C24 — 169.32 LP(1)N21→RY∗(17)Pt29 2.90 21.51 L P ∗(1)C6→π∗(C7-C8) 58.81 — LP(1) N 26 → π ∗ (C24-S25) 34.62 — LP(1)N21→RY∗(18)Pt29 21.06 24.04 L P ∗(1)C6→π∗(C10-C11) 55.05 — LP(1)Pd29→RY∗(1)N21 12.36 — LP(1)N21→RY∗(19)Pt29 23.29 1.35 L P ∗(5)Ni29→RY∗(1)N21 8.62 10.06 LP(2)Pd29→RY∗(1)N21 1.70 12.99 LP(1)N21→RY∗(20)Pt29 13.73 10.84 L P ∗(5)Ni29→RY∗(4)N21 8.91 11.29 LP(2)Pd29→RY∗(18)N21 — 10.73 LP(1) N 22 → π ∗ (C16-N21) 16.01 15.64 L P ∗(5)Ni29→RY∗(6)N21 3.76 11.94 LP(2)Pd29→RY∗(20)N21 — 10.82 LP(1) N 22 → L P ∗ (1)C24 — 115.02 L P ∗(5)Ni29→RY∗(7)N21 1.84 11.76 L P ∗(5)Pd29→RY∗(1)N21 7.07 35.81 LP(2) S 25 → L P ∗ (1)C24 — 138.8 L P ∗(5)Pd29→RY∗(2)N21 5.82 28.21 LP(1) N 22 → π ∗ (C24-S25) 43.67 — L P ∗(5)Pd29→RY∗(3)N21 4.03 23.20 LP(1) N 26 → L P ∗ (1)C24 — 168.64 L P ∗(5)Pd29→RY∗(5)N21 5.12 20.25 LP(1) N 2 → π ∗ (C24-S25) 52.15 — L P ∗(5)Pd29→RY∗(6)N21 2.98 38.34 LP(1)Pt29→RY∗(1)N21 43.11 42.56 L P ∗(5)Pd29→RY∗(7)N21 0.86 12.60 LP(1)Pt29→RY∗(2)N21 24.12 26.52 L P ∗(5)Pd29→RY∗(9)N21 0.98 12.50 LP(1)Pt29→RY∗(5)N21 31.50 26.69 LP(1)Pt29→RY∗(6)N21 22.21 27.81 LP(1)Pt29→RY∗(1)N21 43.11 27.98 LP(1)Pt29→RY∗(2)N21 24.12 23.92 LP(1)Pt29→RY∗(5)N21 31.50 17.58 LP(1)Pt29→RY∗(6)N21 22.21 18.69 LP(5)Pt29→RY∗(1)N21 23.70 27.98 Refer to Figure2 for atomic numbering.Strong hyperconjugative interactions (italicized in Tables8 and 9) predominate in the TSC moieties of all investigated molecules, giving rise to extensive electron delocalization that greatly improves the SHG efficiencies of the molecules. On the basis of their E(2) values, the strengths of the NBO interactions in all molecules studied are found to increase upon transition from the gas to the solvent phase. This partially accounts for the significant first hyperpolarizability increments of these molecules in the solvent environment. Strong donor-acceptor NBO interactions are found between the donor atoms (N21 and S25) of L1 and the transition metal ions in the MCl2 fragments. The electrons donated by N21 and S25 are received in vacant metal-based lone pair (LP∗) and Rydberg (RY∗) orbitals. On the other hand, back donated electrons from the metal ions are mainly received in RY∗ orbitals on N21. Back donation of electrons is highly significant in the Pd(II) and Pt(II) chloride complexes of L1 and minimal in the rest of the complexes, confirming the results earlier obtained from CDA analysis. Both CDA and NBO analyses have shown that the Pd(II) and Pt(II) ions can function as ambivalent electron donors and acceptors, while the Ni(II), Cu(II), and Zn(II) ions may only act as electron acceptors in their respective complexes investigated herein. It is also clear from NBO analysis that the π-conjugated pathways of these push-pull systems are comprised of the benzene ring and the attached TSC moieties.


## 4. Conclusion

Transition metal complexes containingπ-conjugated organic ligands are less investigated for NLO applications despite their potentials of combining the high optical nonlinearity and chemical flexibility of organic materials with the physical strength of inorganic materials. Against this backdrop, the linked effects of transition metal chelation and solvent polarity on the first molecular hyperpolarizability (βtot) of MAPTSC have been investigated herein via DFT calculations. Our results have revealed significant modifications of the βtot value for MAPTSC upon complexation with different transition metal chlorides in the presence of solvents with varying dielectric constants. Therefore, its second-order NLO response is highly tunable by the synergy of transition metal chelation and solvent polarity. In gas-phase, the βtot value for the thione tautomer of MAPTSC (L1) increases upon complexation with Pt(II) and Zn(II) chlorides, but decreases following its complexation with Ni(II), Pd(II), and Cu(II) chloride complexes. Nevertheless, the complexation of L1 with these transition metal chlorides in polar solvents results in modest to drastic first hyperpolarizability increments in nearly all cases studied. Based on these findings, it can be concluded that the complexation of a purely organic push-pull system with a transition metal center may not always improve upon its first hyperpolarizability vis-a-vis the NLO response, especially in the absence of a polar solvent. MAPTSC and its Zn(II) and Pt(II) chloride complexes have been identified as promising NLO materials because their gas-phase βtot values are larger than those of the prototype push-pull moleculespara-nitroaniline (PNA) and urea by factors of about 1.40–1.76 and 19.57–37.24, respectively; these factors greatly increase in polar solvent medium. Moreover, they have high optical transparencies in the visible region of the electromagnetic spectrum which greatly overcomes transparency/nonlinearity trade-offs that often lead to lower device efficiency and reduced photostability.



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