Modulation of the Second-Order Nonlinear Optical Properties of the

Article pubs.acs.org/JPCA

Modulation of the Second-Order Nonlinear Optical Properties of the Two-Dimensional Pincer Ru(II) Complexes: Substituent Effect and Proton Abstraction Switch Cun-Huan Wang,† Na−Na Ma,† Xiu-Xin Sun,† Shi-Ling Sun,† Yong-Qing Qiu,*,† and Peng-Jun Liu*,‡ †

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China ‡ College of Chemistry & Chemical Engineering, Hainan Normal University, Haikou 571158, People’s Republic of China S Supporting Information *

ABSTRACT: The static second-order nonlinear optical (NLO) properties on a series of the two-dimensional (2D) pincer Ru(II) complexes with the substituted Tpy and H2SCS tridentate ligands (Tpy = 2,2′:6′,2″-terpyridyl and H2SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl) have been investigated by density functional theory (DFT). Introducing different donor/acceptor substituents to two ligands has an influence on the static first hyperpolarizabilities (βtot) of the 2D systems. Compared to the reference system 1 [Ru(H2SCS)(Tpy)]+, introducing the branches with strong electron acceptor group (p-NO2-phenylethynyl) to the Tpy ligand or the branches with strong electron donor group (pNH2-phenylethynyl) to the H2SCS ligand can effectively improve the βtot values. Time-dependent DFT (TDDFT) calculations indicate that the enhanced βtot values of the substituted systems are dominated by the intraligand charge transfer (ILCT), metalto-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) transitions. Furthermore, the proton abstraction plays an important role in tuning the second-order NLO response. Particularly, for system 5 bearing the branches with NO2 groups on H2SCS ligand, there is a dramatic enhancement in the βtot values for its deprotonated forms. The βtot values of the monodeprotonated system 5-H and the dideprotonated system 5-2H (58.712 × 10−30 and 761.803 × 10−30 esu) are about 7.58 times and 36.4 times larger than their diprotonated system 5, respectively. The second-order NLO responses based on substituent effect and proton abstraction switch are two-dimensional in characteristic with the large off-diagonal tensor values. off-diagonal β tensor components. Up to now, various 2D NLO chromophores have been exploited theoretically and experimentally.12−16 Coe and co-workers16 have reported a series of cis-{RuII(NH3)4}2+-based chromophores with large βzzz and βzyy values. The results reveal that the metal-to-ligand charge transfer (MLCT) is associated with βzyy value. Switchable NLO materials have received a lot of attention due to the novel applications for the electro-optic technologies.17,18 To obtain the effective NLO switching, the molecule must be stable in two or more states displaying distinctly different NLO responses. Switching of molecular second-order NLO response can be successfully achieved by oxidation/ reduction, protonation/deprotonation and photoisomerization, and so on.19−21 The substituted terpyridyl transition-metal complexes have been widely investigated for their excellent electrochemical and optical (linear and nonlinear) properties. Among this kind of complexes, the substituted terpyridyls derivatives are a class of

1. INTRODUCTION The design and synthesis of the second-order nonlinear optical (NLO) materials displaying large first hyperpolarizabilities (β) has been the focus of intensive investigations for several decades owing to their potential applications in the fields of optoelectronics and photonics.1−5 Substantial efforts have been devoted previously to exploit the one-dimensional (1D) NLO chromophores with typical donor−π conjugated−acceptor (D−π−A) dipole configuration. However, in recent years, the 2D NLO chromophores have been regarded as a candidate for the optimization of second-order NLO response due to their potential advantages.6−11 Such as, the 2D chromophores exhibit better phase-matching than 1D species owing to their large offdiagonal β tensor components. The 2D chromophores effectively improve the second-order NLO response without undesirable visible transparency losses. Moreover, the 2D chromophores form transparent and phase-matchable noncentrosymmetric crystal structures that exhibit large secondorder NLO responses owing to the large off-diagonal β tensor components. It has been proved that the electron transition with μ12 (the electron transition dipole moment between states 1 and 2) perpendicular to the dipole axis will lead to significant © 2012 American Chemical Society

Received: June 25, 2012 Revised: October 9, 2012 Published: October 10, 2012 10496

dx.doi.org/10.1021/jp3062288 | J. Phys. Chem. A 2012, 116, 10496−10506

The Journal of Physical Chemistry A

Article

Figure 1. Stepwise and reversible deprotonation−protonation processes of the secondary thioamides moieties for H2SCS ligand.

Scheme 1. Structure Formulas for Systems 1−5 and the Optimized Structure of System 1 Showing the Atom Labeling Scheme

π-delocalized nitrogen heterocyclic chelating chromophores. Notably, when these ligands are coordinated to the metal center, the chelation prompts them to adopt plane arrangement, which leads to a strong enhancement of second-order NLO response by stabilizing the π* levels.22,23 Recently, Teratani and co-workers24 synthesized a new pincer bis-tridentate ruthenium complex [Ru(H2SCS)(Tpy)]+, where H2SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl and Tpy = 2,2′:6′,2″-terpyridyl, and the chemical and electrochemical properties were elucidated. In the complex, the H2SCS ligand is a pincer-like ligand containing S donor atoms in coordination with the metal ion in the tridentate manner.25,26 Besides, when coordinated to the metal, the secondary thioamides moieties of H2SCS ligand underwent the stepwise reversible deprotonation−protonation processes via the −NH− fragment (Figure 1), which provides the possibility for the molecular NLO switch. Moreover, we find that the two tridentate ligands of this pincer complex can act as the building blocks for constructing functionally active Λ-shaped 2D systems. We can build the 2D Λ-shaped structures (A−π− D−π−A/D−π−A−π−D) by introducing the branches with donor/acceptor groups (e.g., p-NH2-phenylethynyl or p-NO2phenylethynyl) at the bilateral terminal positions of the two tridentate ligands, respectively. It should be a novel and intriguing strategy to modulate the second-order NLO response and modify 2D NLO character.

On basis of the above points, we took the synthetic system 1 [Ru(H2SCS)(Tpy)]+ as the NLO-phore and designed different sorts of novel 2D D−π−A−π−D/A−π−D−π−A systems (Schemes 1 and 2). We performed DFT calculations on these 2D systems to tune the second-order NLO response and analyze the 2D NLO character. Moreover, NLO switching by the stepwise reversible deprotonation-protonation processes is also investigated. This work may provide a novel theoretical guidance for searching new switchable high-performance 2D NLO materials.

2. COMPUTATIONAL DETAILS The geometries of the studied systems were optimized using the hybrid Becke3−Lee−Yang−Parr (B3LYP) functional. It is well proved that B3LYP functional has been successfully employed for ruthenium complexes to optimize the geometry structure.27−29 To obtain a more accurate geometry, the basis set effect and solvent effect have been taken into account in optimization. The 6-31g(d), 6-31+g(d), and 6-311+g(d) basis sets for nonmetal atoms were adopted, respectively. The Stuttgart/Dresden double-ζ (SDD30,31) basis set was employed on Ru atom. It is a relativistic effective core potential (ECP), which replaces the inner core 28 electrons, leaving the outer core 14 electrons (4s24p64d6) as the valence electrons for Ru(II). Meanwhile, acetonitrile solution was applied in the optimization using the polarized continuum model (PCM). The calculated geometric parameters and crystallographic data 10497



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Scheme 2. Structure Formulas for Systems 6−8 (D = NH2 and A = NO2)

Figure 2. Calculated bond length variations (1) and bond angle variations (2) of system 1 compared with experimental results. (The atom labeling scheme is shown in Scheme 1.)

of system 1 have been shown in Table S1 of the Supporting Information. Figure 2 presents the variations of calculated bond lengths and bond angles compared with the experimental results for system 1. It can be clearly observed that the deviation of the calculated bond lengths (≤0.023 Å) and bond angles (≤1.468°) for system 1 based on the B3LYP method with different basis sets in vacuum or acetonitrile solution is very small and negligible. Obviously, the addition of diffuse function and polarization function for nonmetal atoms as well as solvent effect has a slight influence on the structure of system 1. Considering the computational cost, the geometry optimizations of all systems were carried out in vacuum at B3LYP/6-31g(d) level (SDD basis set for Ru atom) with real

frequency. The TD-DFT method is one of the most successful and extensively used methods to calculate the excitation energies in quantum chemistry owing to its efficiency and accuracy.32,33 To choose suitable calculated methods, the electron absorption spectra of system 1 and its dideprotonated system 1-2H were simulated using TD-B3LYP and TDPBE1PBE methods with the 6-31+g(d) basis set (SDD basis set for Ru atom) associated with PCM in acetonitrile solution. The calculated absorption wavelengths and relevant experimental results have been listed in Table S2, Supporting Information. The results show that the absorption wavelengths obtained by B3LYP functional are in more reasonable agreement with the experimental data than the PBE1PBE 10498



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Table 1. Static First Hyperpolarizability (×10−30 esu), β, and In-Plane Nonlinear Anisotropy, η1 and η2, of Systems 1−5 system

functional

βzxx

βzyy

βzzz

1

PBE1PBE M06 PBE1PBE M06 PBE1PBE M06 PBE1PBE M06 PBE1PBE M06

−0.015 −0.668 −1.936 −2.891 −0.084 −0.801 86.577 81.781 −25.598 −28.963

11.381 13.164 −74.212 −63.601 95.336 112.454 11.265 12.993 13.765 15.916

8.686 9.112 −2.639 −2.391 6.532 6.113 73.917 71.705 −6.795 −7.88

2 3 4 5 a

η1a

η2b 1.44 −26.6 18.4

1.14 −3.68

βtot 20.052 21.607 78.787 68.883 101.784 117.766 171.759 166.479 18.628 20.926

η1 = (βzxx/βzzz). bη2 = (βzyy/βzzz).

βx, βy, and βz represent the components of the first hyperpolarizability tensor along x-, y-, and z-axis, respectively. The above component is defined by the equation (eq 2):

functional. Therefore, the absorption spectra for all systems were calculated at the TD-B3LYP/6-31+g(d) level in acetonitrile solution (SDD basis set for Ru atom). In this work, the static first hyperpolarizabilities for the studied systems were calculated by analytical third energy derivatives, which is more efficient and less expensive than numerical derivatives.34 To check the consistency of our calculations, the hybrid-type Perdew−Burke−Ernzerhof exchange−correlation functional (PBE1PBE) and the newly developed hybrid meta exchange−correlation functional (M06) were chosen for the calculation of the static first hyperpolarizabilities. The 6-31+g(d) basis set for nonmetal atoms and SDD basis set for Ru atom were used for our studied systems. All calculations were performed using Gaussian09W programs package.35

βi = βiii +

βx 2 + βy 2 + βz 2

∑ [(βijj + βjij + βjji)]

i, j = x, y, z

i≠j

(2)

where βiii is the diagonal tensor. Due to the Cartesian coordinates of the studied 2D systems, βzzz is the diagonal tensor, and βzyy or βzxx are the off-diagonal tensors that are confirmed by the plane of the 2D molecule (“zy”or “zx”plane) (Scheme 1). The βtot values and the dominant tensor components have been listed in Table 1. The βtot values of the studied systems are functional-dependent but both functionals display the same trend in βtot values, that is, 5 < 1 < 2 < 3 < 4. For clarity, we only take the calculated static first hyperpolarizabilities with M06 functional as an example to evaluate qualitatively second-order NLO responses of the studied systems. As shown in Table 1, The calculated βtot value of system 2 with two electron donors at R1 position of Tpy ligand is 3 times as large as the reference system 1 (21.607 × 10−30 esu), and incorporation of two electron acceptors at R1 position of Tpy ligand (system 3) produces an increase of the βtot value about 5-fold higher than that of the reference system 1. Additionally, incorporation of two electron donors at the R2 position of the H2SCS ligand (system 4) leads to the enhancement in the βtot value up to 166.48 (×10−30 esu) and is 8 times as large as that of system 1. However, incorporation of two electron acceptors at the R2 position of the H2SCS ligand (system 5) has a slight influence on the βtot value with respect to system 1. The results suggest that introducing the branches with NO2 groups to the Tpy ligand or the branches with NH2 groups to H2SCS ligand can be helpful to improve the second-order NLO response. Moreover, it is generally believed that low-lying HOMO−LUMO energy gap might be helpful for enhancing molecular second-order NLO response. As shown in Figure 3, the HOMO−LUMO energy gaps of systems 1−5 decrease in the order 5 > 1 > 2 > 3 > 4, whereas the relevant second-order nonlinear optical responses increase in the order 5 < 1 < 2 < 3 < 4. In these studied systems, system 4 has the maximum βtot value because of its lowest HOMO− LUMO energy gap. For the sake of examine the 2D NLO character of the studied systems, we introduce the “in-plane nonlinear anisotropy”, which is defined as the ratio η = (βzyy/βzzz) (assuming the molecule is located in the zy plane).36 The magnitude of the ratio η value is crucial to characterize the 2D second-order nonlinearity property of the molecule. The calculated η (η1 =

3. RESULTS AND DISCUSSION 3.1. Molecular Geometries. The optimized structure for the new synthetic charged system 1 [Ru(H2SCS)(Tpy)]+ was depicted in Scheme 1. It can be seen that two tridentate ligands are perpendicular to each other and Ru atom acts as a central standpoint connecting two ligands. The z axis is oriented through Ru atom and the center of two ligands, and the zy plane is located at the terpyridyl ligand, and zx plane is located at the pincer H2SCS ligand. It is well-known that the large first hyperpolarizabilities depend not only on the nature of the πconjugated bridge but also on the strength of the donor and acceptor groups. To construct obvious 2D D−π−A−π−D/ A−π−D−π−A models optimizing the second-order NLO response, therefore, on the basic backbones of the reference system 1, we first designed systems 2−5 by introducing πelectron spacer groups (alkynyl and phenyl groups) and strong acceptor or donor groups (−NO2 or −NH2) to the R1 and R2 positions of the two ligands, respectively, as shown in Scheme 1. The aim in our work is to check the effect of the introduced donor/acceptor substituents on second-order NLO properties and exploit the 2D NLO character of the systems. In the present work, system 2 adopts C2 symmetry owing to the nature and the location of amino group in system 2, whereas other systems adopt C2v symmetry. 3.2. Static First Hyperpolarizabilities. 3.2.1. Substitution Effect. In the present work, we calculated the static first hyperpolarizabilities (βtot) of the studied systems with PBE1PBE and M06 functionals. The static total first hyperpolarizability βtot of the molecule is defined as βtot =

1 3

(1) 10499



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the studied 2D systems, the z-polarized transition dipole moment elements contribute to the diagonal component tensor βzzz, and the y- (and/or x-) polarized transition dipole moment elements are associated with the off-diagonal β component tensors (βzyy and/or βzxx). The TDDFT calculated excited energy Eeg, oscillator strength fos, transition dipole moment element μige, polarized direction, and corresponding orbital transitions have been listed in Table 2. For system 1, the two low energy transitions are polarized along z axis (parallel transitions), whereas in systems 2 and 3, the low energy transitions are polarized along y axis (perpendicular transitions), thus resulting in larger off-diagonal component βzyy, even βzyy ≈ βtot, which consequently generates bigger η2 values compared with that of system 1. For systems 4 and 5, among three electron transitions, two of them are polarized along the x axis (perpendicular transitions), which contributes to the βzxx tensor component. It is well-known that the oscillator strength is proportional to the transition energy and transition moment ( fos = (8π2me/3e2h)Eegμeg2) and is a key factor in determining the β value. In systems 1−5, the transitions with maximal oscillator strength are polarized perpendicular to the dipole axis, thus resulting in a dominance of βzxx and/or βzyy over βzzz in the five systems, which has been explained in the reported 2D cases previously.14,16 The orbital features involved in the perpendicular transitions with maximal oscillator strengths for systems 1−5 have been shown in Figures 5 and 6. To more intuitively understand the electron density distributions of the corresponding molecular orbitals, the Mulliken population analyses on the studied systems have been supplemented in Table S3 of Supporting Information. From Figure 5 and Table S3, Supporting Information, for system 1, the occupied molecular orbital HOMO−4 mainly localizes on the Tpy ligand and contains some contributions from S atoms and the central benzene ring of the H2SCS ligand (8%), and the unoccupied molecular orbital LUMO+1 localizes on the Tpy ligand and contains a metal contribution (10%), which is assigned to ligand-to-metal and ligand-to-ligand charge transfers (LMCT and LLCT) processes. This implies that the H2SCS ligand acts as the electron donor and the metal atom/Tpy ligand act as the electron acceptor. Furthermore, introducing the branches with donor/acceptor groups to the two ligands respectively, a great difference concerning the direction and the degree of charge transfer has been observed, which thus plays an important role in the modification of the second-order NLO response. For

Figure 3. Molecular orbital energy diagram, βtot × 10−30 esu, and HOMO−LUMO energy gaps of the studied systems in Scheme 1 (H = HOMO, L = LUMO).

βzxx/βzzz or η2 = βzyy/βzzz) values with the M06 functional have been summarized in Table 1. From Table 1, when the branches are introduced to the Tpy ligand located in the zy plane, the βzyy value becomes the major off-diagonal tensor, whereas attaching the branches to the H2SCS ligand in zx plane induces the βzxx value to be the major off-diagonal tensor. This is an interesting phenomenon that there are the two differentdirectional off-diagonal tensors (β zyy or β zxx ) in the homogonous series of systems by introducing the branches to the perpendicular ligands, respectively. Moreover, the offdiagonal tensors (βzyy or βzxx) are found to surpass the diagonal component βzzz in the studied systems, that is, η1 > 1 or/and η2 > 1, which demonstrates that these systems have good 2D NLO character. Among these systems, systems 2 and 3 have significant η2 values, up to −26.6 and +18.4, respectively, which display the excellent 2D character of the nonlinearity. However, systems 4 and 5 weaken the 2D character of the optical nonlinearity, as indicated by the decreased anisotropy values η1 of +1.14 and −3.36, respectively. This may be due to the fact that the outer bilateral branches in H2SCS ligand deviate 84.9° (4-H2SCS) and 89.3° (5-H2SCS) from the zx plane in systems 4 and 5, respectively, whereas the branches are coplanar with Tpy ligand in systems 2 and 3 (Figure 4). To understand the origin of the second-order NLO responses for the studied 2D systems, the crucial electron transitions in TDDFT calculations have been investigated. In

Figure 4. Dihedral angles between the branches and molecular planes in systems 2−5. 10500



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Table 2. Detailed TDDFT Calculations for Systems 1−5 transition dipole moment (a.u.) μxge

μyge

0.1157 0.0827

0.00 0.00

0.00 0.00

−1.34 −1.04

3.97 1.9 2.41

0.2182 0.2271 0.1460

0.00 0.01 −0.01

−1.5 2.21 −1.57

0.00 0.00 0.00

2.85

1.1334

0.01

4.03

0.00

1.63

0.3147

0.00

2.81

0.00

2.02 3.23

0.2572 1.2781

0.00 0.00

2.28 −4.02

0.00 0.00

2.31

0.0775

1.17

0.00

0.00

2.62

0.1445

0.00

0.00

1.50

3.52

1.0293

3.37

0.00

0.00

2.31

0.0851

1.23

0.00

0.00

2.63

0.1622

0.00

0.00

1.59

3.03

1.0213

−3.71

0.0

0.0

system

Eeg (eV)

fos

1

2.63 3.09

2

3

4

5

μzge

major contributions H−2 H−1 H−0 H−4 H−0 H−2 H−4 H−0 H−3 H−0 H−0 H−2 H−4 H−5 H−1 H−1 H−4 H−2 H−2 H−3 H−4 H−0 H−2 H−2 H−0 H−3 H−4

→ → → → → → → → → → → → → → → → → → → → → → → → → → →

L+1 (+68%) L+3 (+80%) L+5 (+13%) L+1 (+90%) L+0 (+94%) L+1 (+91%) L+1 (+7%) L+4 (+75%) L+0 (+16%) L+0 (+85%) L+2 (+13%) L+1 (+91%) L+0 (+80%) L+1 (+10%) L+0 (+87%) L+2(+10%) L+1 (+46%) L+1 (+21%) L+3 (+53%) L+4 (+19%) L+13 (+13%) L+4 (+59%) L+3 (+20%) L+3 (+67%) L+4 (+20%) L+0 (+50%) L+1 (+47%)

polarized direction z z y y y y y y y x z x

x z x

Figure 5. Molecular orbitals of systems 1−3 involved in the perpendicular transitions with maximal oscillator strengths (y-polarized transitions).

observed due to the introduction of NO2 groups, which causes a larger βtot value, especially a larger βzyy value. From Figure 5, the charge transfer of system 3 mainly arises from H2SCS ligand and metal atom to the NO2 groups across the πconjugated bridge in the zy plane. System 3 can be viewed as the A−π−D−π−A model. However, as shown in Figure 6 and Table S3, Supporting Information, the charge transfers of systems 4 and 5 mainly occur on the zx plane. For system 4, the charge transfer mainly arises from the branches (−C6H4 CCC6H4−) with NH2 groups to the branches (−C6H4 CCC6H4−), S atoms and metal atom, which is assigned to ILCT and LMCT processes. Therefore, system 4 forms D−π− A−π−D model. For system 5, the HOMOs and LUMOs mainly delocalize on the two branches (−C6H4CC

system 2, the HOMO mainly localizes on the H2SCS ligand (18%) and metal atom (51%), and the HOMO−3 mainly delocalizes on the two branches (50%) and has significant contributions from NH2 groups (20%). The LUMOs (LUMO +4 and LUMO) mainly delocalize on the Tpy ligand (71% for LUMO+4 and 72% for LUMO, respectively). Thus, the charge transfer in system 2 mainly originates from the H2SCS ligand and metal atom to the Tpy ligand and from NH2 groups to the Tpy ligand. Both charge transfers are vectorially opposite in direction and have common impact on the off-diagonal component βzyy value. Particularly, the latter is along the negative z direction and has a negative contribution to βzyy value, thus resulting in the negative value of βzyy. Compared with system 2, a larger charge separation for system 3 has been 10501



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Figure 6. Molecular orbitals of systems 4 and 5 involved in the perpendicular transitions with maximal oscillator strengths (x-polarized transitions).

Table 3. Static First Hyperpolarizability (×10−30 esu), β, and In-Plane Nonlinear Anisotropy, η1 and η2, of Systems 6−8 system

functional

βzxx

βzyy

βzzz

6

PBE1PBE M06 PBE1PBE M06 PBE1PBE M06

94.475 89.298 5.9762 5.643 93.812 88.360

112.827 131.937 121.576 142.431 6.6947 7.148

82.413 78.964 81.503 75.732 254.578 269.526

7 8 a

η1a 1.13

η2b 1.67 1.88

0.33

βtot 289.715 300.199 209.087 223.841 355.084 365.034

η1 = (βzxx/βzzz). bη2 = (βzyy/βzzz).

Figure 7. Molecular orbitals transition properties involved in the two crucial excited states for system 8.

C6H4−), but LUMOs have many contributions from NO2 groups (56% for LUMO and 56% for LUMO+1), and thus the charge transfer is ascribed to the ILCT process. The direction of charge transfer is along the negative z axis, thus resulting in the negative value of βzxx for system 5. In conclusion, the above analysis of charge transfer character indicates that the metal Ru has different impact on the first hyperpolarizability when the branches with NH2 /NO2 groups are introduced to the two ligands, respectively. The metal Ru acts as the donor in systems 1−3 and as the acceptor in system 4, but as the bridge

transporting electron in system 5. Moreover, the introduction of the branches with NO2 groups on the Tpy ligand or the branches with NH2 groups on the H2SCS ligand are helpful to enhance the degree of charge transfer, thus leading to a remarkable increase on the second-order NLO response. On the basis of systems 1−5, we are also inspired to come up with a meaningful idea: if the branches (p-NH2-phenylethynyl or p-NO2-phenylethynyl) are simultaneously introduced to the two perpendicular ligands, is there influence on the molecular second-order NLO response and corresponding 2D NLO 10502



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ΔEH+(step 1) and ΔEH+(step 2) values for the three systems have been listed in Table 4. It has been experimentally

character? To solve this question, we have designed systems 6− 8 according to the different paths in Scheme 2. Systems 6 and 8 adopt C2v symmetry, whereas system 7 is Cs symmetry due to the two H atoms of NH2 group in the out of plane. The calculated βtot values and their individual components have been presented in Table 3. We still observed that the two functionals show the same trend in βtot values. According to the M06 functional calculation, the βtot values of systems 6 and 8 are calculated to be 300.199 × 10−30 and 365.034 × 10−30 esu, respectively, which are 1.8 and 2.2 times as large as that of the parental system 5 (166.479 × 10−30 esu) and 13.9 times and 16.9 times as large as that of the reference system 1 (21.607 × 10−30 esu). Similarly, the βtot values of systems 7, 223.841 × 10−30 esu, is 1.9 times as large as that of its parental system 3 (117.766 × 10−30 esu). Therefore, it can be clearly seen that the simultaneous introduction of the branches into the two ligands in the different paths have the synergetic effect on further improving the second-order NLO response. In addition, we also investigate the 2D character of the optical nonlinearity for systems 6−8 at the M06 level shown in Table 3. For system 6, η1 and η2 values are both larger than 1, which demonstrate that there are two different directional off-diagonal tensors βzxx and βzyy in system 6. Viewing the interesting result, we have obtained two different directional off-diagonal β tensors in the same system via the simultaneous introduction of the branches at bilateral end of the two perpendicular ligands, which rarely occurs in the 2D NLO systems reported before.12−16 Moreover, the η values of system 7 (1.88) and 8 (0.33) are far less than that of their parental system 3 and 4, respectively. In view of larger second-order NLO properties of system 8, we have also investigated its orbital transition properties associated with the two crucial excited states, as shown in Figure 7. The Mulliken population analyses of the corresponding molecular orbitals have been supplemented in Table S4 of Supporting Information. The lower energy transition is polarized along the z axis with charge transfer from the branches of H2SCS ligand and the metal atom to the branches of Tpy ligand, which contributes to the βzzz value of system 8. However, the higher energy transition is polarized along x axis, which accounts for the βzxx value. The contribution to the βzxx value is mainly derived from the weak charge transfer from the branches of H2SCS ligand to the branches of H2SCS ligand (ILCT), where the NH2 groups act as the donor. Hence, it can be clearly seen that the stronger charge transfer in the lower energy transition leads to larger βzzz value compared with the βzxx value. 3.2.2. Proton Abstraction Switch. The excellent reversible deprotonation−protonation process of the thioamide moiety on H2SCS ligand encourages us to probe the protonabstraction switching of second-order NLO response. Because the proton abstraction occurs at the H2SCS ligand, we take the reference system 1, systems 4 and 5 with the branches on the H 2 SCS ligand as examples to discuss the effect of deprotonation on the second-order NLO response. The static first hyperpolarizabilities of the monodeprotonated and dideprotonated species in the three systems have been calculated by M06 and PBE1PBE functionals with 6-31+g(d) basis set (SDD basis set for Ru atom). We first study the deprotonation ability of the three systems. The proton binding energy (ΔEH+)37 is a sensitive indicator to detect the ability of removing a proton from the titled molecule. The larger the ΔEH+ value, the more difficult it is to remove the proton from the molecule. The calculated

Table 4. Proton Binding Energy, ΔEH+ (kcal/mol), via the Two-Step Deprotonation Processes of Systems 1, 4 and 5 system

ΔEH+(step 1)

ΔEH+(step 2)

1 4 5

277.235 279.438 271.111

331.267 330.128 321.750

determined that the new synthetic system 1 could undergo the two-step deprotonation processes. Hence, on the basis of the ΔEH+ values of system 1, we have discussed the ability of removing one or two protons for the designed systems 4 and 5. The results show that, compared with systems 4, the ΔEH+(step 1) and ΔEH+(step 2) values of system 5 are lower than those of system 1. This illustrates that introducing the branches with strong electron withdrawing groups to the H2SCS ligand is helpful to further improve the deprotonation ability. However, why does the introducing the acceptor groups induce the decrease in the proton binding energy? Maybe, the ability of attract electron of the NO2 group in system 5 decreases the electron density around the N−H bond, thus decreasing the proton binding energy, and then improving the deprotonation ability. According to the results in Figure S1, Supporting Information, both functionals display the same trend in βtot values as well. As shown in Figure 8, the abstraction of one or

Figure 8. Comparison of βtot values of systems 1, 4, and 5 with their deprotonated systems at the M06/6-31+g(d) level (SDD basis set for Ru atom).

two protons for system 1 has no significant influence on the second-order NLO response. This may be because there is no obvious D−π−A configuration in system 1. Notably, the meaningful switchable results occur at systems 4 and 5 with their deprotonated species. For system 4 with NH2 groups, the stepwise abstraction of the proton causes the decrease in second-order NLO response. Specifically, the βtot value of the diprotonated system 4 is 2.5 and 12 times as large as those of the monodeprotonated system 4-H and the dideprotonated system 4-2H, respectively. However, for systems 5 with NO2 groups, as a function of the stepwise abstraction of the proton, a significant enhancement of βtot value has been observed. The calculated βtot values of 5-H and 5-2H increase to 158.712 × 10503



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10−30 and 761.803 × 10−30 esu, respectively, which are 7.58 and 36.4 times as large as that of the diprotonated system 5. It is well-known that the structure change plays an important role in property change of molecule. Our optimized calculations for systems 4 and 5 provide interesting and similar structural transformations of the two branches on H2SCS ligand in the deprotonation processes step-to-step. For example, in the diprotonated system 5, the outer bilateral branches in the H2SCS ligand deviate 89.3° from the zx plane, whereas the two branches are gradually nearly coplanar with the zx plane (deviate 4.5° from the zx plane) when one and two protons are removed step-to-step (Figure 9). This kind of structure

transitions with maximal oscillator strength are polarized along x axis (perpendicular transitions), and then it demonstrates that the βzxx value is much larger than βzzz value. To get more insight into the difference of the second-order NLO response between protonated and deprotonated forms for systems 4 and 5, we have performed TDDFT calculations on the crucial excited states (Tables S5 and S6, Supporting Information). Additionally, the electron density difference maps (EDDM) of the ground state and excited state associated with the electron transition with maximal oscillator strength have been plotted in Figure 10. The blue represents where the

Figure 9. Comparison of the geometric structures between the diprotonated system 5 and the deprotonated systems (5-H and 5-2H).

transformation in the deprotonation process may be caused by the decreased steric hindrance effect between the introducing branches and the middle part of H2SCS ligand (located in the zx plane), as a result of the removing of the proton of N−H bond. The obvious structure transformations in the deprotonation processes can enhance the molecular conjugation, which has positive effect on improving the second-order NLO response of system 5. However, the enhanced conjugation does not effectively improve the second-order NLO response of system 4. This suggests that NH2 groups may play an important role in modifying the second-order NLO response compared with the enhanced conjugation of system 4 in the deprotonation processes. Single proton abstraction from the diprotonated forms breaks the molecular symmetry structure along z axis (Figure 9), so the study of the 2D NLO character of the monodeprotonated species makes no sense. For comparison, the dominant β components and η1 values of systems 4 and 5 with their dideprotonated forms have been listed in Table 5.

Figure 10. Electron density maps (EDDM) between the ground state and excited state associated with the electron transitions with maximal oscillator strengths for systems 4 and 5 with their deprotonated forms.

electrons are coming from, and the purple represents where the electrons are going. From Figure 10, the crucial electron transitions of the protonated and deprotonated forms for system 4 have a similar transition pattern (ILCT transition), which emanates from the branches. And the NH2 groups act as the donor. However, the degree of charge transfer is decreased with the stepwise deprotonation, which results in the decrease of second-order NLO responses in deprotonation processes. Furthermore, for the diprotonated system 5, the crucial electron transition is mainly attributed to ILCT transition from the branches, and the NO2 groups act as the acceptor. However, with the stepwise deprotonation, the electron density in the purple region moves toward the central benzene ring, Nto-S backbone of H2SCS ligand and the metal atom. Hence, it can be clearly seen that the degree of charge transfer in deprotonated systems are remarkably enhanced, which is the key factor to originate larger second-order NLO responses in the deprotonated forms (5-H and 5-2H).

Table 5. Calculated β Values (×10−30 esu) and η1 Values for Systems 4 and 5 with their Dideprotonated Forms (M06/631+g(d) (SDD Basis Set for Ru Atom)) system

βxxz

βyyz

βzzz

βtot

η1

4 4-2H 5 5-2H

81.781 −33.739 −28.963 −613.319

12.993 16.119 15.916 17.87

71.705 3.699 −7.88 −166.002

166.479 13.933 20.926 761.803

1.14 −9.11 3.68 3.70

4. CONCLUSION In the article, we systemically investigated the second-order NLO properties of a series of 2D pincer Ru(II) complexes with DFT/TDDFT method. Tuning the βtot values for these systems were performed on the basis of substitution effect and stepwise reversible protonation/deprotonation processes. The main conclusions observed are as follows: (1) The introduction of the branches with NO2 groups at the bilateral end of Tpy ligand

The results show the dideprotonated forms (4-2H and 5-2H) have higher η1 values compared with those of the parent diprotonated systems, which shows 2D to 2D NLO switching. In addition, the detailed analysis of the crucial transitions for the dideprotonated forms (4-2H and 5-2H) (Table S5, Supporting Information) shows that the corresponding 10504



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(3) Di Bella, S.; Fragalà, I.; Ledoux, I.; Diaz-Garcia, M. A.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 9550−9557. (4) Allis, D. G.; Spencer, J. T. Inorg. Chem. 2001, 40, 3373−3380. (5) Costes, J. P.; Lamère, J. F.; Lepetit, C.; Lacroix, P. G.; Dahan, F. Inorg. Chem. 2005, 44, 1973−1982. (6) Moylan, C. R.; Ermer, S.; Lovejoy, S. M.; McComb, I.-H.; Leung, D. S.; Wortmann, R.; Krämer, P.; Twieg, R. J. J. Am. Chem. Soc. 1996, 118, 12950−12955. (7) Tomonari, M.; Ookubo, N.; Takada, T. Chem. Phys. Lett. 1997, 266, 488−498. (8) Wortmann, R.; Glania, C.; Kramer, P.; Matschiner, R.; Wolff, J. J.; Kraft, S.; Treptow, B.; Barbu, E.; Langle, D. G.; Gorlitz, G. Chem. Eur. J. 1997, 3, 1765−1773. (9) Wolff, J. J.; Langle, D.; Hillenbrand, D.; Wortmann, R.; Matschiner, R.; Glania, C.; Krämer, P. Adv. Mater. 1997, 9, 138−143. (10) Di Bella, S.; Fargala, I.; Ledoux, I.; Zyss, J. Chem.Eur. J. 2001, 7, 3738−3743. (11) Moylan, C. R.; Twieg, R. G.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599− 12600. (12) Coe, B. J.; Harris, J. A.; Jones, L. A.; Brunschwig, B. S.; Song, K.; Clays, K.; Garin, J.; Orduna, J.; Coles, S. J.; Hursthouse, M. B. J. Am. Chem. Soc. 2005, 127, 4845−4859. (13) Liu, C. G.; Qiu, Y. Q.; Su, Z. M.; Yang, G. C.; Sun, S. L. J. Phys. Chem. C 2008, 112, 7021−7028. (14) Muhammad, S.; Janjua, M. R. S. A.; Su, Z. M. J. Phys. Chem. C 2009, 113, 12551−12557. (15) Liu, C. G.; Guan, X. H.; Su, Z. M. J. Phys. Chem. C 2011, 115, 6024−6032. (16) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Helliwell, M; Raftery, J.; Swanson, C. A.; Brunschwig, B. S.; Clays, K.; Franz, E.; Garín, J.; et al. J. Am. Chem. Soc. 2010, 132, 1706−1723. (17) Coe, B. J.; Houbrechts, S.; Asselberghs, I.; Persoons, A. Angew. Chem., Int. Ed. 1999, 38, 366−369. (18) Coe, B. J. Acc. Chem. Res. 2006, 39, 383−393. (19) Ma, N. N.; Sun, S. L.; Liu, C. G.; Sun, X. X.; Qiu, Y. Q. J. Phys. Chem. A 2011, 115, 13564−13572. (20) Muhammad, S.; Xu, H. L.; Janjua, M. R. S. A.; Su, Z. M.; Nadeem, M. Phys. Chem. Chem. Phys. 2010, 12, 4791−4799. (21) Aubert, V.; Guerchais, V.; Ishow, E.; Hoang-Thi, K.; Ledoux, I.; Nakatani, K.; Le Bozec, H. Angew. Chem., Int. Ed. 2008, 47, 577−580. (22) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.; Cariati, F.; Fantucci, P. C.; Invernizzi, I.; Quici, S.; Ledoux, L.; Zyss, J. Organometallics 2000, 19, 1775−1788. (23) Tessore, F.; Roberto, D.; Ugo, R.; Pizzotti, M. Inorg. Chem. 2005, 44, 8967−8978. (24) Teratani, T.; Koizumi, T.; Yamamoto, T.; Tanaka, K.; Kanbara, T. Dalton. Trans 2011, 40, 8879−8886. (25) van der Boom, M. E.; Misltein, D. Chem. Rev. 2003, 103, 1759− 1792. (26) Okamoto, K.; Kanbara, T.; Yamamoto, T.; Wada, A. Organometallics 2006, 25, 4026−4029. (27) Dumur, F.; Mayer, C. R.; Hoang-Thi, K.; Ledoux-Rak, I.; Miomandre, G. C.; Dumas, E.; Méallet-Renault, R.; Frigoli, M.; Zyss, J.; Sécheresse, F. Inorg. Chem. 2009, 48, 8120−8133. (28) Bomben, P. G.; Roson, K. C. D.; Sedach, P. A.; Berlinguette, C. P. Inorg. Chem. 2009, 48, 9631−9643. (29) Robson, K. C. D.; Sporinova, B.; Koivisto, B. D.; Schott, E.; Brown, D. G.; Berlinguette, C. P. Inorg. Chem. 2011, 50, 6019−6028. (30) Andrae, D.; Häußermann, U.; Dolg, M.; Preuß, H. Theor. Chim. Acta 1990, 77, 123−141. (31) Dolg., M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441−450. (32) Zheng, K.; Wang, J.; Shen, Y.; Kuang, D.; Yu n, F. J. Phys. Chem. A 2001, 105, 7248−7253. (33) Vlček, A., Jr.; Zalis, S. J. Phys. Chem. A 2005, 109, 2991−2992. (34) Chopra, P.; Carlacci, L.; King, H. F.; Prasad, P. N. J. Phys. Chem. 1989, 93, 7120−7130.

or the branches with NH2 groups at the end of H2SCS ligand can effectively enhance the second-order NLO response. The TDDFT calculations indicate that the enhanced βtot values for the substituted systems are dominated by the intraligand charge transfer (ILCT), metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) transitions. (2) The simultaneous introduction of the branches with donor/acceptor groups into the two ligands in the different paths has the synergetic effect on further improving the second-order NLO response. Because the obvious charge transfer from the branches on H2SCS ligand (head) to the branches on Tpy ligand (tail) contributes to βtot value. (3) Moreover, the proton abstraction plays an important role in tuning the second-order NLO response. Particularly, for system 5 with NO2 groups on H2SCS ligand, a dramatic increase in βtot value has been obtained from 20.926 × 10−30esu (the diprotonated system 5) to 761.803 × 10−30 esu (the dideprotonated system 5-2H). The EDDM illustrates that the proton abstraction for system 5 increases the degree of charge transfer, which is helpful to the enhancement in βtot value. In addition, according to the calculated ΔEH+ value, introducing the branches with NO2 groups to the H2SCS ligand (system 5) is helpful to further improve the deprotonation ability compared with system 4. (4) The studied systems possess the 2D character with the large off-diagonal tensor values. Interestingly, the major off-diagonal tensor values of these systems are different in direction. When the branches are introduced to the Tpy ligand, the βzyy value is the major off-diagonal tensor. However, introducing the branches to H2SCS ligand, the βzxx value is the major offdiagonal tensor. To sum up, the studied pincer ruthenium complexes may become the promising candidate for good 2D NLO switchable materials at the external stimulus of proton manipulation.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

The first hyperpolarizabilities obtained by M06 and PBE1PBE functionals in the deprotonated forms of 1, 4, and 5 (Figure S1), the optimized geometric parameters of 1 (Table S1), the calculated absorption wavelengths of 1 and 1-2H (Table S2), Mulliken population analysis of 1−5 and 8, and the detailed TDDFT calculations for 4 and 5 with their deprotonated forms (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Fax: (+86) 431 85098768. E-mail: Y.-Q.Q., qiuyq466@nenu. edu.cn; P.-J.L., [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21173035) and the Natural Science Foundation of Jilin province (20101154).

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REFERENCES

(1) Zyss, J. Molecular nonlinear optics: Materials, physics, and devices; Academic Press: Boston, 1994. (2) Nalwa, H. S., Miyata, S., Eds. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. 10505



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(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (36) Di Bella, S.; Fragala, I. New. J. Chem. 2002, 26, 285−290. (37) Rustad, J. R.; Dixon, D. A.; Rosso, K. M.; Felmy, A. R. J. Am. Chem. Soc. 1999, 121, 3234−3235.

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Modulation of the Second-Order Nonlinear Optical Properties of the

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